Natural and anthropogenic control of water quality of an Amazon estuary : the Caeté estuary (Brazil)

Universitat Politècnica de Catalunya BarcelonaTech Departament d'Enginyeria Civil i Ambiental Laboratori d'Enginyeria Marítima Programa de Doctorat en Ciències del Mar NATURAL AND ANTHROPOGENIC CONTROL OF WATER QUALITY OF AN AMAZON ESTUARY. THE CAETÉ ESTUARY (BRAZIL) PhD Thesis presented by Marcela Cunha Monteiro for the degree of Doctor Supervisor: Dr. José A. Jiménez Dra. Luci Cajueiro Carneiro Pereira Barcelona, February 2017 "For I know the plans I have for you, declares the LORD, plans for welfare and not for evil, to give you a future and a hope". Jeremiah 29:11 To Helena and Neuza, with all my love i Acknowledgements As a dedicated Christian, I could not begin these acknowledgements without thanking God, above all, for concluding yet another stage in my academic career. I am eternally grateful to Him for guiding and supporting me along this journey. I am also grateful to Erasmus Mundus and the Brazilian National Council for Scientific and Technological Development (CNPq) for the scholarship that have allowed me to complete my studies in Spain. Both these institutions have been fundamental to the fulfilment of a dream. I thank my supervisors, Dr. José A. Jiménez and Dra. Luci Cajueiro, for sharing their knowledge with me during the development of my thesis, for helping me to maintain my focus on this research during all these years, and for the final revision, which was so essential to the conclusion of my thesis. To Professor Dr. Manuel de Jesus Flores-Montes, coordinator of the project that made this research possible. I would also like to thank the participants of the Laboraroti di Engenharia Maritime (LIM) at Universitat Politècnica de Catalunya, for providing me with such an agreeable working environment. Mercè Casas-Prat, Eva Bosom, Monica Leano, Caridad Ballesteros, Marta Alomar, Manuel Griffol, Tiago Oliveira and Jacqueline Albino, my time together with you all made my journey all the more pleasurable. To the members of the Coastal and Estuarine Oceanographic Laboratory (LOCE) at the Federal University of Pará – Danielle Guimarães, Ádila Costa, Rosigleyse Félix, Natália Sousa, Nayra Silva, Emarielle Pardal, Suellem Oliveira, Wellington Trindade and Ruben Pessoa. Thank you so much for helping me with the collection of samples in the field and the analyses in the laboratory. Despite the distance that separates us, you have been present in my daily life throughout the development of my thesis. I am also grateful to the members of the Coastal Geology Laboratory (LAGECO) coordinated by Professor Dr. Nils Edwin Asp, and the Coastal Studies Institute (IECOS), both of the Federal University of Pará, as well as Sergeant Fábio Sadraque of ii the municipal Fire Department of Santa Maria do Pará, for their assistance during the oceanographic campaigns. I would also like to acknowledge all the institutions that provided me with the data I used in this thesis – the Bragança office of the Brazilian Institute of Geography and Statistics (IBGE), the Bragança Municipal (PMB) Secretariat for Infrastructure and Planning, and the GPesca Indústria Ltda fish processing plant. I thank Professor Aninha Moreira M.Sc of the Bragança campus of the Federal Institute of Teaching and Technology of Pará and the environmental administrators Marcelo Mendonça and Marcos Ronielly, who helped me produce the maps used to illustrate this thesis. Finally, thank my family, especially Helena and Renan for their support. You have been present during each moment of this journey, despite the 4000 miles that separate us. I share with you the merits of this conquest! iii Abstract In the Amazon region, few data are available on the impacts caused by the urban settlements found in the proximity of estuaries. In the estuary of the Caeté River, the focus of the present study, the nutrient input is controlled by both natural features and anthropogenic disturbances generated by local communities. In this context, the principal aim of the study was to analyze the quality of the water of the Caeté estuary, and the relative contribution of natural and anthropogenic forcings. To this end, climatological, hydrodynamic and hydrological features were monitored, and potential sources of pollution were identified in the different sectors of the Caeté estuary. Potential future scenarios for the estuary are also described, based on the analysis of anthropogenic and natural processes, which may contribute to the quality of its waters. The results indicate higher levels of nutrient input in the upper sector of the estuary, where 90% of the local population is concentrated, and most of the region’s commercial activities (e.g., public markets, ice factories, and docking facilities) are found. As a consequence, eutrophic waters with high concentrations of faecal coliforms (up to 1100 MPN/100 ml) were observed during spring tides in the dry season when the transport and dilution of the estuary’s waters are less effective. Eutrophication also occurred to a lesser extent in the other (middle and lower) estuary sectors, although in this case, the results indicate the influence of natural processes, reflecting the high nutrient concentrations of this Amazonian region. During neap tides, eutrophication was less pronounced, and water quality was improved in both dry and rainy seasons. A comparative analysis showed that, under similar conditions of the flood cycle, the trophic status of the estuary varied little between spring and neap tides. As the population of the region surrounding the Caeté estuary is increasing by 10–20% per decade, resulting in a significant increase in human pressures and impacts on the study area. The current eutrophication status of the estuary may have permanent effects, which may be aggravated during the dry season or drought events, when the estuary is more vulnerable to the retention of nutrients. The water quality of the Caeté Estuary can x xi List of Figures Figure 3.1: Amazon coast (A) and subdivisions of Amazon coastal region including Coast of Amapá (B), Coast of Pará (C), Reentrâncias Pará-Maranhão (D) and Coast of Maranhão (E) 24 Figure 3.2: Mean annual temperatures and rainfall in three Amazon cities: Macapá (Amapá), Belém (Pará) and São Luís, Maranhão (CPTEC, 2016) 26 Figure 3.3: Annual displacement of the Intertropical Converge Zone on the northern coast of Brazil (Cavalcante, 2007) 28 Figure 3.4: Positive and negative phases of the ENSO (A) and AMO events (B) recorded between 1950 and 2015 30 Figure 3.5: Positive and negative phases of the NAO (North Atlantic Oscillation) events recorded between 1950 and 2010 31 Figure 3.6: Tidal range on the Brazilian coast, according with DHN data 33 Figure 3.7: The Caeté Hydrographic Basin (A) and some of the economic activities engaged in the communities: fishing (B, C and D), poultry (E) and pig farming (F), and subsistence agriculture (G) (modified from Gorayeb, 2008) 39 Figure 3.8: Location of study area on the northeast coast of Brazil (A and B) emphasizing the Caeté estuary (C) 40 Figure 3.9: Bathymetry at 5 stations along the Caeté estuary*. Upper estuary (A and B) and middle estuary (C and D) (unpublished data) 42 Figure 3.10: Annual rainfall in the Bragança region based on a 34 year data series. *No data were recorded in 1981, 1983, 1986 and 198 44 Figure 3.11: Minimum, mean and maximum monthly rainfall in the Bragança region, based on a 34 year data series 44 Figure 3.12: Mean annual discharge of the Caeté river based on a 22 year data series 45 xii Figure 3.13: Minimum, mean and maximum monthly discharge of the Caeté river, based on a 22 year data series 45 Figure 3.14: Tidal wave during the spring and neap tides in the coastal area adjacent to Caeté estuary, according to DHN data 46 Figure 4.1: Tide chart showing the tidal oscillations in the lower sector of the estuary during the months of the campaigns conducted during phase I. The red arrow indicates the estimated tide height during data 55 Figure 4.2: Location of northeast coast of Brazil (A and B) and the sampling stations in the Caeté estuary (phase I) St1: Upper estuary; St2: Middle estuary, and St3: Lower estuary (C). 56 Figure 4.3: Tide chart showing the tidal oscillations in the lower sector of the estuary during the sampling months of phase II. The red arrow indicates the estimate of tide height, during the data collection period 58 Figure 4.4: Location of northeast coast of Brazil (A and B) and the sampling stations in the Caeté estuary (phase II) St1 and St2: Upper estuary; St3, St4 and St5: Middle estuary, and St6, St7 and St8: Lower estuary (C) 59 Figure 4.5: Oceanographic campaign. Type of vessel used during data collection (A), CTDO sensor (B) and Niskin oceanographic bottle (C) 59 Figure 5.1: Population growth in the Bragança region (urban and rural area) between 1940 - 2013. Source: IBGE (2015). (*) estimated value 71 Figure 5.2: Urban communities located along the Caeté estuary 72 Figure 5.3: Diagram showing the history of the occupation of the Caeté between the first European colonisation and the present day 74 Figure 5.4: Effluent input into the Caeté estuary: sewer being channelled into the estuary (A), sewage outlets seen during low tide B and C, and effluents channelled through open-air drains that flow into the estuary (D) 76 Figure 5.5: Concentrations of dissolved nutrients and faecal coliforms recorded in 77 xiii the settlements located in the upper estuary (Mar: Maranhãozinho, Faz: Fazendinha, Brag: Bragança, and Vila: Vila q Era) Figure 5.6: Ammonia cylinder employment during the refrigeration process 78 Figure 5.7: Different stages in the treatment of the residual water produced by GPesca Ltda. Removal of the solid residues (A), filtering of the solid residues (B), tank for the metabolisation of the organic matter (C), the pipeline that discharges the residual water into the Caeté estuary (D and E), effluent piped through the mangrove to a tidal creek in the Caeté estuary (F) 79 Figure 5.8: Illegally fish processing plants in the upper sector of Caeté estuary 80 Figure 5.9: Areas in which waste accumulates along the margins of the estuary. Residues discarded directly onto the ground (A) and container for the collection of waste (B). 81 Figure 5.10: Fishing boats on the Bragança waterfront, where the crews normally discharge the faeces accumulated during trip 82 Figure 5.11: Pollution in the Cereja River. Solid waste in the river bed (A) and typical wastewater piping found in the local households, which directs effluents directly into the river (B) 83 Figure 5.12: Sources of contamination identified in the upper Caeté estuary: upper and middle estuary (A), concentration of pollution sources in the town of Bragança (B), and Cereja River (C) 84 Figure 6.1: Long-term data and monthly rainfall level in Bragança. Source: INMET. (*) Total rainfall until the field campaign in 07 February 2007 86 Figure 6.2: Long-term data and monthly river discharge data measured in the upper Caeté estuary during the study period. Source: ANA. (*) Data not available for 2007 87 Figure 6.3: Wind speeds and directions measured at the Salinópolis station during the study period 88 xiv Figure 6.4: Tidal prism estimated for the Caeté estuary from the DHN tidal data under spring tide conditions 90 Figure 6.5: Current velocities in the Caeté estuary during the spring tide 91 Figure 6.6: Current displacement in the upper sector of the estuary over a 25-hour period 93 Figure 6.7: Current displacement in the middle sector of the estuary over a 25-hour period. (*) No current data 94 Figure 6.8: Current displacement in the lower sector of the estuary over a 25-hour period 95 Figure 6.9: Physical and physical-chemical variables in the Caeté estuary between April 2006 and February 2007. (*) Not sampled 97 Figure 6.10: Dissolved nutrients in the Caeté estuary between April 2006 and February 2007. (*) Not sampled 99 Figure 6.11: Mixing diagrams along the salinity gradient in the Caeté estuary 100 Figure 6.12: Trophic status of the Caeté estuary during the wet and dry seasons. (*) This classification has been discussed in the Chapter 8 102 Figure 6.13: Dissolved inorganic nitrogen in the Caeté estuary during the wet and dry seasons. (*) Not ammonium data 102 Figure 6.14: Redfield ratios, chlorophyll a and faecal coliform concentrations recorded in the Caeté estuary between April 2006 and February 2007. (*) Not data 104 Figure 6.15: Long-term data and monthly rainfall level in Bragança. 106 Figure 6.16: Long-term data and monthly river discharge data measured in the upper Caeté estuary during the study period. (*) Data not available for 2011 107 Figure 6.17: Wind speeds and directions recorded at the Salinópolis station during the study period 108 xv Figure 6.18: Tidal prism estimated for the Caeté estuary from the DHN data under neap tide conditions 110 Figure 6.19: Current velocities in the Caeté estuary during the wet and dry seasons. The tidal range was obtained from the DHN 111 Figure 6.20: Vertical profile of the current velocities in the Caeté estuary 112 Figure 6.21: Physical and physical-chemical variables in the Caeté estuary between September 2010 and October 2011 114 Figure 6.22: Physical and physical-chemical variables in the Caeté estuary between September 2010 and October 2011 115 Figure 6.23: Dissolved nutrients in the Caeté estuary between September 2010 and October 2011. (*) Not sampled 117 Figure 6.24: Mixing diagrams along the salinity gradient of the Caeté estuary 118 Figure 6.25: Trophic status of the Caeté estuary during the wet and dry seasons. (*) This classification has been discussed in the Chapter 8 119 Figure 6.26: Dissolved inorganic nitrogen concentrations in the Caeté estuary between September 2010 and October 2011. (*) Data not collected 120 Figure 6.27: Redfield ratios, chlorophyll a and faecal coliforms concentrations recorded in the Caeté estuary between September 2010 and October 2011. (*) Not collected 122 Figure 6.28: Physical and physical-chemical variables in the Caeté estuary during spring and neap tides 124 Figure 6.29: Dissolved nutrient concentrations recorded in the Caeté estuary during spring and neap tides 126 Figure 6.30: Trophic status of the Caeté estuary during spring and neap tides. (*) This classification has been discussed in the Chapter 8 128 xvi Figure 6.31: Dissolved inorganic nitrogen concentrations in the Caeté estuary during spring and neap tides 129 Figure 6.32: Redfield ratios and chlorophyll a concentrations in the Caeté estuary 129 Figure 6.33: Coliform concentrations recorded in the Caeté estuary 130 Figure 7.1: Estimated population growth in the area of the Caeté estuary from 2020 to 2050. The dashed line indicates the population in 2010 132 Figure 7.2: Mean rainfall recorded per quarter during normal and drought years for the period between 1974 and 2011 (except for 1981, 1983 and 1989) 137 Figure 7.3: The relationship between river discharge and nitrate concentrations observed during monitoring phases I (A) and II (B) 138 xvii List of Tables Table 2.1: The most common sources of pollution 6 Table 2.2: Emissions of total nitrogen (NT) and phosphate (PT) from wastewater in hydrographic basin and estuarine systems around the world (t yr-1) 8 Table 2.3: Indices commonly used to evaluate the trophic state of estuaries 17 Table 3.1: Population and activities developed along the estuary and their potential impacts in relative terms. Key: *** high; ** moderate; * low 48 Table 4.1: Summary of physical, physical-chemical, and microbiological data collected during phases I and II in the Caeté estuary (U: upper estuary; M: middle estuary, and L: lower estuary) 60 Table 4.2: Rainfall classification 62 Table 4.3: Classification of oxygen saturation levels 64 Table 5.1: Estimate (tons) of the BOD and dissolved nutrients discharged daily by GPesca Comercial Ltda. into the Caeté estuary 80 Table 6.1: Rainfall classification according to the monthly rate 86 Table 6.2: Tidal heights recorded in the upper and lower sectors of the Caeté estuary 89 Table 6.3: Tidal asymmetry in the Caeté estuary and the classification based on the asymmetry index (AIDV) 90 Table 6.4: Frequency of current velocity values recorded in the upper, middle and lower sectors 92 Table 6.5: Spearman correlation matrix for the variables monitored during the spring tide period 105 Table 6.6: Rainfall classification according to the monthly rate 106 xviii Table 6.7: Tidal heights recorded in the upper, middle and lower sectors of the Caeté estuary during the wet and dry seasons 109 Table 6.8: Spearman correlation matrix for the variables monitored during the neap tide periods 125 Table 7.1: Total amount of effluents, dissolved nutrients, and faecal coliforms produced by the total population of the upper Caeté estuary from 2020 to 2050 based on a population growth rate of 10% 134 Table 7.2. Total amount of effluents, dissolved nutrients, and faecal coliforms produced by the total population of the upper Caeté estuary from 2020 to 2050 based on a population growth rate of 20% 134 Table 7.3: Total of effluents, dissolved nutrients, and faecal coliforms produced by the total population in the middle and lower sectors of the Caeté estuary, and the values projected for future decades, based on a population growth rate of 10% 135 Table 7.4: Total of effluents, dissolved nutrients, and faecal coliforms produced by the total population in the middle and lower sectors of the Caeté estuary, and the values projected for future decades, based on a population growth rate of 20% 135 xix Thesis Organization This thesis is divided into eight chapters. The first chapter provides an overview of the importance of estuarine environments and the disturbances that affect them, the objectives of the study, and the hypotheses considered at the beginning of the research. The second chapter considers the possible impacts caused by natural and/or human disturbances on the quality of estuarine environments. This chapter also provides information on the different water quality monitoring programs and the indices used to define the trophic status of estuarine environments. The third chapter describes the characteristics of the study area. The fourth chapter presents the methods adopted for the collection of data during the different phases of the study. The fifth chapter describes the anthropogenic pressures and population growth within the area of the Caeté estuary, and provides information on the sources of contamination that affect this area. The sixth chapter presents the hydrodynamic, hydrological, and microbiological characteristics of the Caeté, derived from the oceanographic campaigns conducted during the study. The seventh chapter predicts the future scenarios for the Caeté estuary considering current trends for the increase in the production of effluents up until the year 2050, together with the occurrence of climatic events. Finally, the eighth chapter discusses and compares the findings on the Caeté estuary in the context of regional and global patterns. To conclude this work, the principal findings of the study are compiled and summarised to provide a succinct overview of the results, and management strategies for the Caeté estuary are proposed. In addition, are proposed a number of challenges for the improvement or extension of the database established for the Caeté estuary. Natural and anthropogenic control of water quality of an Amazon estuary 6 Table 2.1: The most common sources of pollution. Studies conducted by the Joint Group of Experts on the Scientific Aspects of Marine Pollution (GESAMP) and scientific investigators from a number of different countries around the world indicate that there are six primary pathways through which pollutants enter estuarine environments: (1) nonpoint run-off from land; (2) direct pipeline discharge; (3) riverine inflow; (4) atmospheric deposition; (5) maritime transportation; and (6) the dumping of waste at sea (McIntyre, 1992, 1995; Goldberg, 1995; Kennish, 1997). On the other hand, Ridgway and Shimmield (2002) concluded that the most common source of pollution in estuaries is the effluents from sewage treatment plants. In fact, the impact of the discharge of treated or untreated wastewater into estuaries is of great concern in most countries, and has been widely reported (Von Sperling and Chernicharo, 2002). These pressures result in negative impacts such as (i) the introduction of pathogens (e.g. certain bacteria, viruses, and parasites) and other infectious agents often associated with sewage effluents; (ii) the introduction of heavy metals, and (iii) an increase in nutrient input Sources Common pollutants categories Point sources Municipal sewage tretament plants BOD, bacteria, nutrients, ammonium, toxic chemicals Industrial facilities Toxic chemiclas, BOD Combined sewer overflows BOD, bacteria, nutrients, turbidity, total dissolved solids, ammonium, toxic chemicals Non-point sources Agricultural run-off Nutrients, turbidity, total dissolved solids, toxic chemmicals Urban run-off Turbidity, bacteria, nutrients, total dissolved solids, toxic chemicals Construction run-off Turbidity, nutrients, toxic chemicals Mining run-off Turbidity, acids, toxic chemicals Septic systems Bacteria, nutrientes Landfills/spilss Toxic chemicals, miscellaneous susbtance Silvicultural run-off Nutrients, turbidity, toxic chemicals Chapter 2: Water quality in estuaries 7 into the estuary, which will be investigated here. Table 2.2 shows the total amount of nitrogen and phosphate introduced per year by wastewater into some estuarine systems. The input of nutrients and consequent eutrophication of aquatic environments rank among the most common environmental problems in estuaries. Eutrophic environments are “wellnourished”, and this process can be defined as “the enrichment of water by nutrients, especially compounds of nitrogen and/or phosphorus causing an accelerate growth of algae and higher forms of plant life to produce and undesirable disturbance to the balance of organisms present in the water and to the quality of the water concerned’’ (OSPAR, 2013). While eutrophication is considered to be a disturbance rather than a form of pollution, the trophic status of aquatic ecosystems has been considered a good indicator of environmental health, and has been used as a diagnostic tool to characterize the water quality status of many aquatic ecosystems, including polluted estuaries (Kennish et al., 2013). Natural and anthropogenic control of water quality of an Amazon estuary 8 Table 2.2: Emissions of total nitrogen (NT) and phosphate (PT) from wastewater in hydrographic basin and estuarine systems around the world (t.yr-1). 1Treated wastewater 2 Untreated wastewater *Estimated by Lacerda (2006) Thus, a eutrophication may occur as a natural process over the course of a period of thousands of years, when the waters gradually age and become more productive due to the accumulation of nutrients and organic biomass or through the anthropogenic input of nutrients from point and nonpoint sources, referred to as cultural eutrophication (Mannion, 2014). This process is a global phenomenon, on different scales of intensity and impacts, ranging from the Baltic, Adriatic and Black Seas, to the estuaries and coastal waters of Japan, China, Australia, the N P N P United States1 Brazil2* Hudson 49,0 9,6 Timonha 50,1 14,0 Missisipi 9,9 Acaraú 238,9 66,9 Coreaú 233,0 65,2 Baltic1 Aracatiaçu 56,6 15,9 Randers Fjord 84,5 0,7 Aracatimirim 52,5 14,7 Curu 98,3 27,5 United Kingdon1 Mundaú 250,0 70,0 Glaslyn-Dwyryd 69,9 Pirangi 76,5 21,4 Mawddach 17,3 Jaguaribe 152,0 42,4 Dyfi 32,8 Icapui 27,5 7,7 Apodi 400,5 112,5 China1 Açu 96,0 27,0 Haihe 20,0 1,6 Guamaré 17,0 4,5 Ceará Mirim 32,0 9,0 Costa Rica2 Guaraíras 96,5 27,0 Tárcoles 2000,0 300,0 Curimataú 57,0 16,0 Reventazón 110,0 Colombia2 Cauca-Magdalena 130,0 Chapter 2: Water quality in estuaries 9 United States and Mexico (Fisher et al., 2006; Martin et al., 2008; Santiago et al., 2010; Aleksandrov, 2010; De et al., 2011; Cheng et al., 2012). The principal consequences of eutrophication include (i) an increase in particulate organic matter and water turbidity; (ii) the occurrence of phytoplankton and microphyta blooms, which may be responsible for the reduction of the dissolved oxygen concentrations; (iii) a decrease in local biodiversity; (iv) a reduction in the aesthetic quality of the water and the potential for human use, and (v) the loss of economic value (Roberts and Pierce, 1974; Karlson et al., 2002; Kennish, 2002; Anderson et al., 2002; Burford et al., 2012). Overall, then, eutrophication results in a loss of water quality and impacts on the socio-economic and ecological environments (Roberts and Pierce, 1974; Anderson et al., 2002; Karlson et al., 2002; Burford et al., 2012). In order to mitigate these effects, countries are increasingly adopting measures to control anthropogenic sources of pollution and to reduce the input of dissolved nutrients and bacteria into the estuaries. One of the most widespread measures is the construction of sewage treatment plants (Greening and Janicki, 2006). The input of nutrients and faecal bacteria (including Escherichia coli) are considered good indicators of the level of human disturbance, due to their presence in human faeces. Estuaries impacted by untreated effluents from urban areas usually have high concentrations of these bacteria, as observed in Zanzibar in East Africa (Mohammed, 2002), the Minho estuary in Portugal (Anne et al., 2006), and the Ganges estuary in India (Batabyal et al., 2014). Studies have shown that faecal coliform concentrations correlate with population density in the surrounding hydrographic basin and especially with land occupation within the watershed. However, the most important anthropogenic factor associated with faecal coliform concentrations is the percentage of impervious surface cover, consisting of roofs, paved roads, and other infrastructure. This cover alone may account for 95% of the variability in mean estuarine faecal coliform concentrations (Mallin et al., 1998). In some cases, wastewater treatment plants have reduced significantly the amount of bacteria present in sewage effluents, exceeding 95% of E. coli in Denmark (Bonde, 1967), and reaching 99.9% in Gdańsk-Wschód and Gdynia Dębogórze in Poland (Szumilas et al., 2001). In some regions, however, treatment is insufficient, and the gap between desirable levels of coliform pollution and local practice continues to expand (Von Sperling et al., 2002). Natural and anthropogenic control of water quality of an Amazon estuary 10 2.2 Natural disturbance to estuaries On the Atlantic coast of Brazil, estuarine areas are typically associated with mangrove forests (Herz, 1991). These forests are characterised by a complex interaction of physical, chemical and biological process, which sustain high rate of primary productivity (~24 tons/hectare/year), and support an enormous diversity of marine, freshwater and terrestrial species (Macintosh and Asthon, 2002). In order to understand the interrelationship between estuaries and mangroves, some authors have focused on the relationship between the exportation of nutrients from areas of mangrove and the high productivity of adjacent waters. In 1962, the first studies of the tidal exchange of materials were conducted by Golley et al. and Teal, who studied a mangrove forest and a salt marsh, respectively. Both studies concluded that there is an outwelling of dissolved nutrients to adjacent marine waters. In 1968, the outwelling hypothesis, which refers to the exportation of nutrients from mangrove forests to the adjacent waters was proposed by Frederic Odun. This author, tried to relate the exportation from mangrove systematically with the high productivity these waters. From this point onwards, studies focused on the exportation or importation of particulate matter, litter, particulate organic carbon, dissolved nutrients or metabolic nutrient demands from mangroves to others ecosystems until Twilley (1988), through a study of mangrove mass balance, suggested that the exportation of organic matter is a common feature of most mangrove ecosystems, given that their natural characteristics – intense tidal exchange, regular rainstorms, and floating litter – favoured this process. Simultaneously, other studies found that mangrove forests also import organic carbon and retain large amounts of the litter produced for in situ consumption, mainly as a result of the restricted inundation regime (e.g., Twilley et al., 1986; Flores-Verdugo et al., 1987; Lee, 1990). The resulting controversies – exportation vs. importation – were resolved by the general conclusion that, while there may be a net importation of inorganic nutrients into mangrove wetlands, there is a net exportation of organic matter from these forests (Twilley, 1988). Up until now, however, data on mangrove outwelling are available from only approximately 15 countries, corresponding to 10% of the world’s mangrove forests, and most of these data Chapter 2: Water quality in estuaries 11 refer to carbon, rather than nutrients. Revising these data in detail, Adame and Lovelock (2011) concluded that mangrove forests tend to export carbon in the form of litter and POC (particulate organic carbon) during tidal inundation. Exportation may range from 0.1 g C m2year-1, as recorded in Hong Kong by Lee (1989) to 498.8 g C m-2year-1 in Mexico (FloresVerdugo et al., 1987). In the case of DOC (dissolved organic carbon), exchange rates ranged from the importation of 67.3 g C m-2 year -1 in Florida (Davis et al., 2003) to the exportation of 138.0 g C m-2 year -1 in the Caeté estuary, Brazil (Dittmar et al., 2006). The exchange of dissolved nitrogen is also highly variable, ranging from the exportation of 5.0 gN m-2 year-1 in the Caeté estuary (Dittmar and Lara, 2001) to the importation of 1.6 gN m2 year -1 in Australia (Ayukai et al., 1998). Both these values were recorded in environments dominated by extreme tidal forces, which favour the exchange of dissolved nitrogen. Dissolved phosphate ranged from the exportation of 0.61 gP m-2 year -1 in Conn Creek, Australia (Boto and Bunt, 1981) to the importation of 1.4 gP m-2 year -1 in the Taylor River, in the USA (Davis et al., 2001). Through the high degree of deviation from conservative mixing, Dittmar and Lara (2001) concluded that outwelling from the mangroves of the Caeté estuary in northern Brazil (Amazon region) was higher than the freshwater input into the ocean, and thus that the mangrove rather than the river supports production in the marine environment. This study also showed that the net exports of dissolved nutrients from the Caeté mangrove system exceed those of other mangroves anywhere in the world. Annual exportation was estimated (mol yr-1) to be 30 x 109 for POC, 2 x 109 for DOC , 0.4 x 109 for ammonium, 15 x 109 for silicate and 0.04 x 109for phosphate. Adame and Lovelock (2011) divided the factors affecting material exchange into: (i) Global scale: climate and latitude; (ii) Regional scale: geomorphological setting and hydrology; (iii) Local scale: dominant tree species, litter fall, area of forest and nutrient concentrations. Many studies have reported the influence of these parameters in their observations. For example, Thong et al. (1993) demonstrated that heavy rains which inundate the mangrove, may increase the nutrient export from adjacent bodies of water and Light and Dineen (1994) Natural and anthropogenic control of water quality of an Amazon estuary 12 concluded that the subtropical climate of southern Florida plays a major role in controlling material transportation. Dittmar and Lara (2001) highlighted a number of factors that influenced the high nutrient concentrations recorded in the Caeté estuary, such as the pore water flux, sedimentation-erosion, and litter exportation. Other studies in similar environments to that of the Caeté estuary, dominated by a macrotidal regime, have confirmed that tidal forcing controls the net flux (Kjerfve and McKellar, 1980). Despite the growing number of studies on nutrient cycling in mangrove systems, phenomena such as primary production, the magnitude of nutrient transport and the dispersal of the material in the adjacent coastal zone has still not been elucidated adequately. Adame and Lovelock (2011) concluded that the main problem for the understanding of the outwelling hypothesis is the diversity of spatial scales and methods, which limits comparisons among studies and sites. Other authors attribute the lack of understanding of this process to the variation in the local factors both among and within mangrove systems, such as geomorphology, tidal amplitude, local climate, the type of vegetation and abiotic factors, which all complicate broader extrapolations (Twilley et al., 1997; Nordhaus et al., 2006). Adame and Lovelock (2011) nevertheless observed certain tendencies in the data. For example, studies that have investigated the exportation of nutrients in the form of litter and POC have concluded that mangroves export nutrients. On the other hand, studies of the exchange of dissolved nutrients have shown both importation and exportation. Finally, studies that have estimated material exchange by measuring the forest’s metabolic demand for nutrients have shown that mangroves import nutrients. While the outwelling hypothesis has not yet been fully elucidated, it is clear that the exchange of carbon between tidal wetlands, such as mangrove forests or salt marshes, and coastal waters is a fundamental component of the carbon budget of the oceans (Twilley et al., 1992) and represents an important contribution on a global scale. Similar process also occur with nitrogenous and phosphorous macronutrients. Chapter 2: Water quality in estuaries 13 2.3 Monitoring water quality in estuaries The monitoring of the quality of the water of estuaries is necessary to guarantee adequate conditions for the current and future use of these environments (Ustin et al., 2014), and to determine whether the water is adequate for specific uses. Many water authorities around the world have created water quality monitoring programmes, such as the National Estuary Program created by the United States Environmental Protection Agency (USEPA), the Water Framework Directive in Spain, and the Nanaimo Estuary Management Plan in Canada. Water quality monitoring in estuaries has a number of common objectives, including (i) the evaluation of estuary conditions and their potential effects on human activities, (ii) the establishment of guidelines for the regulation of these activities in order to prevent or minimise the negative impacts of these activities on the quality of the water, (iii) the definition of a programme of monitoring for the evaluation of ongoing impacts, (iv) the restauration of degraded habitats, and (v) the establishment of a management plan for the estuaries. In Brazil, the monitoring of estuarine water is the responsibility of state environmental protection agencies, and should follow the guidelines proposed by the Brazilian Environment Council (CONAMA, resolution nº375/2005). This resolution was introduced in 1986 and revised in 2005 to establish standards and limits for physical-chemical, microbiological, and other pollutant indicators for surface waters in Brazil (including estuaries), as well as sewage discharge levels. The water quality levels are determined according to the specific uses, which are divided into: - human supply, - protection and preservation of aquatic communities, - irrigation, - aquaculture and fisheries, - animal watering, - navigation and - landscape harmonisation. Natural and anthropogenic control of water quality of an Amazon estuary 14 Some developed nations have already achieved basic levels of water pollution management (Von Sperling and Chernicharo, 2002), and are currently fine-tuning the control of micropollutants and the impact of pollutants in sensitive areas or pollution caused by storm water drainage. Developing nations, by contrast, are under constant pressure, because, while they attempt to adopt increasingly strict international limits and standards, they are generally unable to reverse ongoing trends of environmental degradation. Unfortunately, few Brazilian environmental protection agencies prioritize the monitoring of estuaries, and government initiatives for estuarine water quality monitoring are scarce or nonexistent in some regions, especially northern Brazil. This situation is derived from a lack of resources for most state agencies, reinforced by the complexity of the region’s estuarine environments, which hampers effective monitoring. 2.3.1 Water quality index The trophic status of an estuary can be characterised by a number of different indices or indicators. A water quality index provides a numerical value that expresses the quality of the water for multiple purposes. This value is obtained from the systematic integration of chemical, physical, and bacteriological data (Tyagi et al., 2013). Water quality indicators are measurements, usually quantitative, that can be used to illustrate complex phenomena simply, including tendencies over time, that can provide important insights into the state of an environment (EEA, 2005). The first water quality index was proposed by Horton (1965), a German investigator who developed the index to evaluate the reduction of pollution in environments monitored by government programs. This initiated interest in the development of different approaches to the assessment of water quality, with the primary intention of simplifying procedures (Liou et al., 2004). Water quality indices have been developed to facilitate the study of coastal zones, including estuaries, by permitting the evaluation of anthropogenic pressures on local conditions, and the assessment of ecosystem integrity. These indices summarize a large amount of data into a single, simple and consistent concept accessible to policymakers and the general public (Saeedi et al., 2010). These indices are considered to be one of the most effective ways to describe the quality of aquatic environments. Chapter 2: Water quality in estuaries 15 However, water quality status cannot be considered to be a static parameter, given that the coastal zone is a very dynamic environment affected by terrestrial inputs (natural and anthropogenic), and the interaction of inshore and offshore waters, winds and weather conditions. Water quality indices must therefore evaluate both temporal and spatial dimensions in order to characterise the mean or typical conditions found in the target area (Giovanardi and Vollenweider, 2004). The trophic state of an aquatic environment is currently considered to be an effective measure of the water quality of an aquatic environment, and a large number of trophic indices have been created. Karydis (2009) proposed that a good indicator of trophic state should: - be able to detect trends that cannot be easily observed from the raw data; - be used as an early warning system in decision-making when management practices are applied; - assess the performance of decision-making when management practices are applied; and finally - assess the degree of severity or remediation in areas with established problems of eutrophication. Early trophic indices (e.g. Trophic State Index and TRIX) were based on a set common indicators, such as chlorophyll a and/or nutrient concentrations in the aquatic environment, in particular N and P, which are transported by the runoff of rainfall from agricultural zones or released from urban sewer systems to estuaries. In addition to these parameters, the monitoring of variables such as the transparency of the water, dissolved oxygen and primary productivity have become increasingly widely used to classify aquatic environments, given their vulnerability to pollution. More recent indices consider other biotic indicators, such as phytoplankton species parameters, primary production, seagrass and harmful algal bloom, which may reflect the severity of pollution levels (Karydis, 2009). Ideally, biotic and abiotic variables should be considered together with the direct and indirect effects of pollution on water quality (Borja et al., 2012). These indices usually classify the trophic state in scales (e.g., high, good, moderate, poor and bad) or in one three general trophic states – (i) oligotrophic (low nutrient levels, high clarity), Natural and anthropogenic control of water quality of an Amazon estuary 22 3.1.1 Divisions of the Amazon coast The coastal area of Brazilian states that constitute the Amazon coast (Amapá, Pará and Maranhão) were subdivided here, according to their dominant depositional environments and geomorphological characteristics. These subdivisions, and other geological features, are presented below. The coast of Amapá is 698 km long and is divided into estuarine (236 km) and Atlantic (462 km) sectors (Figure 3.1B). The Atlantic sector encompasses the transition zone between the ocean and the continent. Geomorphologically, it includes the AmapáGuiana mud fields and the North Cape, a wetland area of approximately 18,000 km², in addition to mangrove forests, lagoons, estuaries, salt marshes, and terra firme forest (Souza-Filho et al., 2005). The estuarine sector encompasses fluvial-marine environments, such as dry grasslands, ‘várzea’ and secondary forests, rivers, streams, lakes, and seasonally-flooded grasslands. Geomorphologically, this sector includes the mouth of the Amazon and the Gulf of Marajó, and is influenced primarily by the fluvial discharge of the Amazon River, which forms an extensive plume, influenced by the trade winds and coastal currents (Souza-Filho et al., 2005). This plume may extend 100500 km out into the Atlantic Ocean, and more than 1000 km to the northwest, into the North Atlantic Ocean (Gibbs, 1970; Lentz, 1995; Silva et al., 2005; Ffield, 2007). The coast of Pará is about 600 km long, between the mouth of the Amazon River and that of the Gurupi River (Figure 3.1C). Franzinelli (1992) divides this coastline into two primary features: (i) the flat emergence coast, represented by Marajó Island; and, (ii) the submergence coast between Marajó Bay and Gurupi Bay. In this second sector, the coastal plateau reaches the shore, forming terraces and active cliffs constituted by tertiary sediments of the ‘Barreiras’ and ‘Pirabas’ formations. This region is known as the ‘Reentrâncias’, a highly indented coastline which contains the greatest diversity of depositional environments found anywhere on the Amazon coast, including beaches, dunes, estuaries, cheniers, tidal mudflats, and coastal plains (Souza-Filho et al., 2005). The dominant type of environment are the estuaries, which are typically associated with tidal mudflats (covered with mangrove) and tidal sandbanks that form the region’s sandy macrotidal beaches. Both emergence and submergence coasts can be found in this region, with estuaries of between 60 km and 80 km in length. Chapter 3: The study area 23 The coast of Maranhão can be divided morphologically into three provinces, two of which are part of the Amazon coast: (i) the western coast (part of the ‘Reentrâncias’) extends for 520 km from the mouth of the Gurupi River to the Gulf of Maranhão (Figure 3.1E). This province is composed of about 20 funnel-shaped estuaries, separated by low-lying mangrove-dominated peninsulas bordered by dynamic barrier islands and tide-modified beaches (Pereira et al., 2016) and (ii) the central section with 490 km occupied by the four main channels of the funnel-shaped Gulf of Maranhão. This gulf is 90 km wide at its mouth and extends 130 km inland, and encompasses mangroves, beaches, dune fields and an estuarine complex that includes major local rivers such as the Mearim, Itapecuru, and Munim (Pereira, op. cit.) The ‘Reentrâncias’ coast of Pará and Maranhão is also known as the Amazon Macrotidal Mangrove Coast (AMMC) (Figure 3.1D), which extends 650 km and covers an area of 7591 km², representing approximately 57% of Brazilian mangroves (Oliveira and Maneschy, 2014). The Caeté estuary is located in this sector, and is a low-lying, emerging coastline (0–80 m), with an ample coastal plain, up to 70 km wide, and an extensive continental shelf, stretching as much as 330 km into the ocean (Szlafsztein and Lara, 2002; Souza-Filho et al., 2005). Natural and anthropogenic control of water quality of an Amazon estuary 24 Figure 3.1: Amazon coast (A) and subdivisions of Amazon coastal region including Coast of Amapá (B), Coast of Pará (C), Reentrâncias ParáMaranhão (D) and Coast of Maranhão (E). Chapter 3: The study area 25 3.1.2 Climatological features The Amazon region has a hot, humid equatorial climate (CPTEC, 2016). Mean monthly temperatures are between 24 and 28°C, reaching a maximum of more than 30°C, and a minimum of less than 22°C (Martorano et al., 1993). The evapotranspiration rate tends to be high, with little interannual variability and an annual average of approximately 813 mm (Cohen et al., 1998). Annual rainfall may be up to 3,300 mm in some parts of the Amazon Coastal Zone. Figure 3.2 show the long-term mean monthly rainfall data between 1961 and 2015 provided by CPTEC (2016) where highlight the marked seasonal variation in rainfall patterns in the different sectors of the Amazon coast (Macapá-Amapá, Belém-Pará and São Luís-Maranhão), as well as the discreet variation in air temperatures in this region. The main drivers of annual climate variability in this region are large-scale circulation patterns, including the confluence of the tropical Atlantic trade winds and the displacement of the Inter-Tropical Convergence Zone - ITCZ (Marengo, 1995). The ITCZ consists of a belt of low pressure, which is formed by the hot air of the equatorial region rising up through the atmosphere. The displacement of the ITCZ across the equator is associated with bands of convective clouds and rainfall, which provide a major source of diabatic heat to the troposphere and freshwater to the ocean (Grodsky and Cartyon, 2003). This process depends on the intensity of the northeasterly and southeasterly trade winds, which in turn is associated with the meridional gradient in pressure and sea surface temperatures in the tropical North and South Atlantic (Marengo et al., 2008). Natural and anthropogenic control of water quality of an Amazon estuary 26 Figure 3.2: Mean annual temperatures and rainfall in three Amazon cities: Macapá (Amapá), Belém (Pará) and São Luís, Maranhão (CPTEC, 2016). The position of the ITCZ in the Atlantic ranges from 10º North to 10º South (Figure 3.3). In the first half of the year, the ITCZ shifts to the Southern Hemisphere, over the coastal area of Amapá, Pará and Maranhão, triggering the formation of intense convective currents, which cause heavy rainfall (up to 90% of the annual total) and decreasing winds in this coastal region. During the second half of the year, the ITCZ Chapter 3: The study area 27 moves to the Northern Hemisphere, to about 10º North (Figure 3.3), causing rainfall to decline in the Brazilian Amazon Coastal Zone (Figueroa and Nobre et al., 1990). The rainfall patterns observed in the second half of the year are related to large mesoscale convective systems or squall lines. Squall lines are conglomeration bands of cumulonimbus clouds and form on the coast because of the sea breeze. These bands of convective cloud cover extend from latitudes 10º North to 5º South, and from the Guianas to Maranhão, including Amapá and Pará (Cavalcanti, 1982). When the ITCZ is well established, squall lines are common, although they can also occur independently of the presence of the ITCZ (Cohen et al., 1998). Other systems, such as sea breezes, cyclonic vortexes of air, and easterly waves contribute on a minor scale to the rainfall patterns observed on the Amazon coastal zone during the second half of the year (Figueroa and Nobre, 1990; Marengo, 1995). Natural and anthropogenic control of water quality of an Amazon estuary 28 Figure 3.3: Annual displacement of the Intertropical Converge Zone on the northern coast of Brazil (Cavalcante, 2007). The inter-annual variability of rainfall in the Amazon region, including northeastern Pará, is strongly coupled to low-frequency large-scale oceanic and atmospheric phenomena occurring over the Pacific (El Niño Southern Oscillation - ENSO) and Atlantic (North Atlantic Oscilation - NAO and Multidecadal Atlantic Oscilation - AMO) oceans (Fernandes et al., 2011; García-García and Ummenhofer, 2015). The ENSO is characterized by events of anomalous warming (El Niño) or cooling (La Niña) of the surface waters in the central and eastern Pacific. El Niño is the primary cause of many drought episodes around the world, including in Amazon region (Zeng et al., 2008; Trenberth et al., 2014). This creates intense air mass sinking over the Amazon region and an anomalously northward displacement of the ITCZ over the tropical Pacific and Atlantic Oceans, inhibiting rainfall in central and western Amazonia. El Niño presents a periodicity ranging from 2 to 7 years (Li et al., 2011) and a typical duration between 8 and 15 months (Mo, 2010). Events with strong intensities were registered in 1982-1983, 1997-1998 and 2002-2003 (NOAA, 2016a). Opposite it, during La Niña events rainfall rates tend to increase (Marengo and Espinoza, 2015). This event present a mean periodicity of 5 years and duration highly variable ranging from 5 months to as many as 30 months (Ray and Giese, 2012). Chapter 3: The study area 29 Previous studies have shown that the northern Atlantic surface sea temperature (SST) plays a secondary role in the variability of the water budget in the Amazon region (Yoon and Zeng, 2010; Coelho et al., 2012; García-García and Unmenhofer, 2015). Abnormally high SST have been associated with reduced rainfall and drought events in the Amazon. By contrast, abnormally low SST are associated with an increase in rainfall (Coelho et al., 2012). The physical mechanism linking the northern tropical Atlantic to reduced precipitation rates in the Amazon is related to the migration of the ITCZ away from the northern coast of South America, which results in net water vapor divergence and anomalous subsidence in the Amazon, leading to reduced precipitation (Yoon and Zeng, 2010). The drought recorded in 2005, widely considered to be the worst event of the century, was caused by an increase in the SST of the Atlantic Ocean (Marengo et al., 2008). Subsequently, in 2010, SST of the tropical Atlantic reached their highest historical values, resulting in an even more intense drought event than that recorded in 2005 (Coelho et al., 2012). The periodicity of these events over the Atlantic ocean has been analyzed in a number of studies, which have shown that the AMO follows a 60-year cycle, with a tendency to remain in the same phase during a number of years (Peings and Magnusdottir, 2014). In the case of the NAO, Coelho et al. (2012) concluded that the cycles may be of 2.2, 2.4, 5.8 or 8.0 years, although Olsen et al. (2012) defined cycles of between 3 and 6, 4 and 8 or 8 and 10 years. However, other NAO analysis have identified longer oscillations, of between 50 and 60 years, 55 and 70 years or 65 and 90 years, linked to the AMO. Figures 3.4 and 3.5 show the ENSO, AMO and NAO indices recorded since 1950. The possible effects of droughts caused by El Niño, NAO or AMO events on the Caeté estuary (the focus of this study) are described in Chapter 7. Natural and anthropogenic control of water quality of an Amazon estuary 30 Figure 3.4: Positive and negative phases of the ENSO (A) and AMO events (B) recorded between 1950 and 2015 (NOAA, 2016a, 2016b). Chapter 3: The study area 31 Figure 3.5: Positive and negative phases of the NAO (North Atlantic Oscillation) events recorded between 1950 and 2010 (NOAA, 2016c). Natural and anthropogenic control of water quality of an Amazon estuary 38 animal consumption, irrigation, leisure activities, and so on (Gomes et al., 2009; Silva et al., 2009; Pereira et al., 2010). Other environmental problems observed in the region include the illegal capture of ornamental fish, quarrying, the over-exploitation of fish and crab stocks, deforestation, and oil spills (Guimarães et al., 2011). Chapter 3: The study area 39 Figure 3.7: The Caeté hydrographic basin (A) and some of the economic activities engaged in the communities: fishing (B, C and D), poultry (E) and pig farming (F), and subsistence agriculture (G) (modified from Gorayeb, 2008). Natural and anthropogenic control of water quality of an Amazon estuary 40 3.3 Caeté estuary The Caeté estuary (Figure 3.8), located in northeastern Pará, on the Bragança Peninsula, around 150 km southeast of the mouth of the Amazon River, occupies the lower portion of the Caeté hydrographic basin. This estuary is funnel-shaped which is the result of the coastal evolution of mangrove deposits. Its channel is classified as permanently open and has a uniform cross section which receives the discharge from two main tributaries, with minor contributions from smaller creeks flowing in through the mangrove forest (Cavalcante et al., 2013). Figure 3.8: Location of study area on the northeast coast of Brazil (A and B) emphasizing the Caeté estuary (C). According to existing bathymetric data upper sector of the estuary is the narrowest, with a mean width of 160 m (varying from 60 m to 266 m), and a mean depth of 3.4 m (range 1.0–6.3 m) during low tide of neap tides. The estuary widens to a mean of 270 m in its mid-sector (194–809 m), with virtually the same mean depth (3.6 m), although deeper channels of up to 11 m in depth can be found, approximately 15 km from the mouth of the estuary (Unpublished data, Figure 3.9). The total width of the estuary at the river mouth is about 4600 m (Guerra and Cunha, 1998). Chapter 3: The study area 41 3.3.1 Mangroves The drainage area of the Caeté estuary is composed mainly of mangrove forest, with a total area of approximately 180 km² (Krause et al., 2001). This tide-dominated allochthonous system (according to the Thom, 1984) is formed by various creeks and channels, the dynamics of which are determined mainly by tides and rainfall. The forest comprises a well-developed environment with trees reaching 10 m to 25 m in height, however, dwarf forms of the same species, which are no taller than 1 m, can be observed in the intertidal zone. The predominant three species found in this mangrove are Rhizophora mangle (Rhizophoraceae), Avicennia germinans (Avicenniaceae), and Laguncularia racemosa (Combretaceae). Avicennia dominates the higher ground, while Rhizophora and Avicennia occur together in the intermediate areas, and Rhizophora dominates the lowest ground, including the borders of the tidal creeks and channels (Menezes et al., 2003). Laguncularia is widely distributed, but only predominates at disturbed sites, such as the borders of channels (Thullen and Berger, 2000). Cohen and Lara (2003) showed that the elevated flats in the inner part of the peninsula are flooded much less frequently (<28 days per year) than the lower mangrove. These areas are only flooded during the highest spring tides and constitute a hyper-saline habitat (salinity between 90 and 100), found mainly during the dry season. This area ends in a wide area of mudflats covered by mangroves, extending down to the mid-tide mark for 3–6 km. These mud flats have a gradient of around 1:3000, and are dissected by creeks which are much deeper than those in the elevated flats. The upper mud flats are flooded only during normal spring tides (28–78 days/yr) with pore water salinity of between 50 and 90. Considering the local tidal regime, Cohen et al. (2004) concluded that around 70% of the study area is flooded at high tide during neap tides (~4.9 m tide), and more than 92% during spring tides (~5.7 m tidal range). Natural and anthropogenic control of water quality of an Amazon estuary 42 Figure 3.9: Bathymetry at 5 stations along the Caeté estuary*. Upper estuary (A and B) and middle estuary (C and D) (unpublished data). _________________________ *Bathymetric survey was collected in the upper and middle sectors of the estuary, at five stations (St1–St5). At each station, 10 transversal profiles were taken perpendicular to the estuary channel, each separated by a distance of approximately 100 m, covering a 1 km stretch of the estuary at each station. To reduce the effects of the tide on the bathymetric readings, tide height was measured in the middle and upper sectors of the estuary. Chapter 3: The study area 43 3.3.2 Meteo-oceanographic characteristics Based on a 34 year data series (from 1974 to 2011, except 1981, 1983, 1986 and 1989) provided by the National Meteorology Institute (INMET, 2014), total annual rainfall in the study ranged historically between 1,400 mm and 4,100 mm. Figure 3.10 show annual total rainfall emphasizing years which rainfall rate should be affected by ENSO, AMO or NAO events Overall, more than 80% of the total rainfall occurs during the first half of the year, the wet season, which typically extends from January to July, with February, March, April, and May being the wettest months. The dry season typically lasts between August and December, and normally accounts for less than 20% of total annual rainfall. September, October and November are normally the driest month of the year (Figure 3.11). The Caeté River and its 22 tributaries are the main source of the freshwater discharge into the Caeté estuary. Based on a 22 year data series (1965 to 1971 and 2000 to 2014) provided by the Brazilian National Water Agency (ANA, 2015), from a gauge station approximately 20 km from study area, the average annual river discharge of the Caeté ranges between 23.9 m3.s-1 and 59.7 m3.s-1 (Figure 3.12). Typically, around 70% of annual discharge is recorded during the first half of the year (wet season), with the highest monthly levels being observed in March, April and May. Discharge during the dry season rarely exceeds 30% of the annual total, however. Discharge tends to be lowest in October, November and December. The minimum, mean and maximum discharge rates recorded during the wet and dry are shown in the figure 3.13. Natural and anthropogenic control of water quality of an Amazon estuary 44 Figure 3.10: Annual rainfall in the Bragança region based on a 34 year data series. *No data were recorded in 1981, 1983, 1986 and 1989. Figure 3.11: Minimum, mean and maximum monthly rainfall in the Bragança region, based on a 34 year data series. Chapter 3: The study area 45 Figure 3.12: Mean annual discharge of the Caeté river based on a 22 year data series. Figure 3.13: Minimum, mean and maximum monthly discharge of the Caeté river, based on a 22 year data series. Natural and anthropogenic control of water quality of an Amazon estuary 46 The Caeté estuary, like the rest of the northern Brazilian coast, is dominated by the local macrotides (Asp et al., 2012). The tidal range oscillates between 2 and 4 m during the neap tide, and is from 4 to 6 m during spring tides (Figure 3.14), which is consistent with a macrotidal regime in the classification of Davies (1964). The tidal cycle is semi-diurnal with asymmetric phases. The M2 astronomic component dominates tidal patterns within the Caeté estuary, and is thus the predominant water circulating force within the estuary. Together with the N2 and S2 components, it accounts for 85% of total tidal variation in the region. Tidal currents dominate this region and can reach up to 1.2 m.s-1 in the wet season and 0.7 m.s-1 in the dry season (Gomes et al., 2013). Circulation patterns in the Caeté estuary are driven by the interaction between the local river discharge and coastal tide forces, combined with moderate easterly winds and waves (Cavalcante et al., 2010). Figure 3.14: Tidal wave during the spring and neap tides in the coastal area adjacent to Caeté estuary, according to DHN data. 3.3.3 Socio-environmental features Nine communities (Maranhãozinho, Fazendinha, Bragança, Camutá, Vila q Era, Bacurituea, Caratateua, Vila dos Pescadores and Ponta do Urumajó) are found on the margins of the Caeté estuary, with a total of 80,880 inhabitants (Guimarães et al., 2011; IBGE, 2013), of which approximately 90% are found in the upper, more urbanized Chapter 3: The study area 47 sector. This sector includes the town of Bragança, considered to be the region’s most important urban and commercial center. Fishing is considered the most important economic activity in the region. At the present time, more than 6000 t of fish are landed annually in the Caeté estuary. This estuary rank as the third most important fishing center in Pará and the largest extractive fishing producer of Brazil (IBAMA, 2005; Isaac et al., 2008). Although industrial development in the region is limited, installations such as ice factories, fish processing plants, fish markets, and dry docks for boat repairs can be found in many parts of the estuary, mainly in the town of Bragança. The middle and lower sectors of the estuary have a low population density. These sectors have approximately 7,627 and 587 inhabitants, respectively (Guimarães et al., 2011). The resident population is dependent on the public services provided in the upper sector of the estuary. In the middle sector, subsistence agriculture is intensive. Cassava, rice, and beans are the principal local crops. In the lower sector, fishing is the main subsistence activity (Krause and Glaser, 2003; Glaser and Diele, 2004; Gorayeb, 2008). Table 3.1 shows the number of inhabitants per sector and the expected level (qualitative) of impact caused by unplanned urban growth (e.g., sewage discharge) and by economic activities (e.g., fishing and commerce), based on the studies of Gorayeb (2008), Guimarães et al. (2009, 2011), Pereira et al. (2010), Monteiro et al. (2011) and IBGE (2013). Natural and anthropogenic control of water quality of an Amazon estuary 54 ________________________________________ *Limits among different the sectors were established by Barletta-Bergan et al. (2002) according to the salinity gradient of estuary During this phase, a total of 72 samples were taken for each parameter, except for faecal coliforms (36 samples), which were collected only during the first ebb/flood cycle. Once collected, the water samples were stored in 250 ml polyethylene flasks. For the analysis of dissolved oxygen, the samples were stored in amber-coloured flasks and to faecal coliforms were used glass flasks. All flaks were cleaned according the procedures described by APHA (1992). To fix the dissolved oxygen in the field, 1 ml of a solution of manganese sulphate (MnSO4) and 1 ml of an alkali-iodide-azide solution which contains sodium hydroxide, NaOH, sodium iodide, NaI, and sodium azide, NaN3. All the water samples were stored at 4ºC prior to processing in the laboratory. Chapter 4: Data and methods 55 Figure 4.1: Tide chart showing the tidal oscillations in the lower sector of the estuary during the months of the campaigns conducted during phase I. The red arrow indicates the estimated tide height during data. Natural and anthropogenic control of water quality of an Amazon estuary 56 Figure 4.2: Location of northeast coast of Brazil (A and B) and the sampling stations in the Caeté estuary (phase I) St1: Upper estuary; St2: Middle estuary, and St3: Lower estuary (C). - Neap tide condition (phase II) During phase II, the oceanographic campaigns were conducted during the neap tide, every two months between September 2010 and September 2011, with an additional campaign in October of 2011. The tide charts for the campaign months of phase II are shown in Figure 4.3, and the red arrow indicates the day data were collected during the neap tide period. During this phase, tidal height was measured in the upper and middle estuary, using tide data loggers (TWR 2050), which were moored in each sector. Data were recorded every minute for approximately 13 hours, encompassing a full ebb and flood cycle. As the analyses run in phase I proved the homogeneity of the variables between the tidal cycles (ebb and flood) and the sampling stations, in phase II, physical-chemical and microbiological data were collected only during flood tides. In order to detect potential spatial differences within the estuary, the distance between stations was reduced in B A C Chapter 4: Data and methods 57 comparison with phase I. The upper estuary was represented by two fixed stations (St1 and St2), and the middle (St3, St4 and St5) and lower (St6, St7 and St8) by three fixed stations each, with a total of eight sampling points (Figure 4.4). To detect possible vertical differences in the data, samples were taken at 1.0 m below the surface of the water and 1.0 m from the bottom of the estuary. Vertical salinity, turbidity, dissolved oxygen and oxygen saturation were recorded every 5 min by CTDOs equipped with dissolved oxygen and turbidity sensors (RBR XR-420). Niskin oceanographic bottles of 5 L (General OceanicsTM) were used at each station to obtain vertical water samples (surface and bottom). These samples were used to determine pH, dissolved nutrients (nitrate–NO3-, nitrite–NO2-, ammonium–NH4+, phosphate–PO4and silicate–SiO2), and chlorophyll a and thermotolerant coliform concentrations. A total of 128 water samples were collected during this phase, except for thermotolerant coliforms, which were sampled only in the surface layer, with a total of 76 samples being collected. Figure 4.5 shows different moments during the collection of the data, and data obtained in the oceanographic campaigns of phases I and II were summarized in Table 4.1. Natural and anthropogenic control of water quality of an Amazon estuary 58 Figure 4.3: Tide chart showing the tidal oscillations in the lower sector of the estuary during the sampling months of phase II. The red arrow indicates the estimate of tide height, during the data collection period. Chapter 4: Data and methods 59 Figure 4.4: Location of northeast coast of Brazil (A and B) and the sampling stations in the Caeté estuary (phase II) St1 and St2: Upper estuary; St3, St4 and St5: Middle estuary, and St6, St7 and St8: Lower estuary (C). Figure 4.5: Oceanographic campaign. Type of vessel used during data collection (A), CTDO sensor (B) and Niskin oceanographic bottle (C). B A C Natural and anthropogenic control of water quality of an Amazon estuary 60 Table 4.1: Summary of physical, physical-chemical, and microbiological data collected during phases I and II in the Caeté estuary (U: upper estuary; M: middle estuary, and L: lower estuary). Station Methods Tidal cycle Depth Periods Total samples phase I phase II phase I phase II phase I phase II phase I phase II phase I phase II phase I phase II Current data U, M and L U and M Current meter ADCP 1 flood and 1 ebb 3.0 m from surface from the surface to the bottom, each 0.5 m November of 2010 and April of 2013 425 hrs 26 hrs Tidal range U and L U, M and L Tidal gauge and DHN Tide logger and DHN - - - 450 hrs 312 hrs Salinity Niskin bottle CTDO pH Dissolved oxygen U and L Niskin bottle CTDO Saturated oxygen - - CTDO Turbidity - - CTDO Dissolved nutrients Chlorophyll a Faecal coliforms 1 flood and 1 ebb 1.0 m from the surface 36 samples 76 samples April, June, August, October and December of 2006 and February 2007 September and November of 2010; January, March, May, July, September and October 2011 72 samples 128 samples U, M and L Niskin bottle U, M and L Niskin bottle 2 flood and 2 ebb 1 flood 3.0 m from surface 1.0 m from the surface and 1.0 from the bottom Chapter 4: Data and methods 61 Additional campaigns (i) Currents Due to logistic problems, data on current speed and direction were not collected during phase II. To overcome this problem, two specific campaigns were conducted to obtain these data under similar climatic and oceanographic conditions, that is, during neap tides in the dry (October 2010) and wet (April 2013) seasons. In each campaign, 30 transversal currents profiles (15 during the ebb cycle and 15 during the flood cycle), separated by a distance of about 1 km were conducted from a boat along the Caeté estuary from the upper (St1) to the middle (St6) sectors. Current profiles were not compiled in the lower sector due to the wind-wave action combined with the small size of the vessel used to collect data. In each profile, data on current speed and direction were obtained in the water column with a vertical beam of 0.5 m. This data were recorded with an Acoustic Doppler Current Profile (ADCP) Workhorse 1200 kHz model by Teledyne® RDI™. (ii) Survey of sources of contamination Sources of contamination, including the outfalls of domestic waste water, waste dumps, septic tanks, etc., were identified in situ in the study area in March 2013. In December 2014, the GPesca Indústria Ltda. fish processing plant, located in the middle estuary, was visited for the measurement of the types and quantities of effluents produced by this industrial installation. All sources of pollution were photographed with a Sony 21 MP digital camera and georeferenced (GPS Garmin 72) for the production of a map of the zones most affected by these pollution sources This map was drawn up using aerial photographs and satellite images (Landsat 8 TM and Spot 5 HGR). The magnitude of sewage effluents produced by the local population was estimated by assuming that each inhabitant produced a minimum of 150 l per day of sewage effluents containing 54.0 g of DBO, 8.0 g of Nitrogenous and 2.5 g Phosphorous compounds (ABNT, 1993; Von Sperling, 1996). The total of inhabitants in each sector was estimated based on previous studies carried by Guimarães et al. (2009) and IBGE (2015). Natural and anthropogenic control of water quality of an Amazon estuary 62 4.4 Laboratory procedures 4.4.1 Rainfall classification Rainfall classification was established according to the monthly rate recorded as shown in Table 4.2. This classification was based on month rate recorded by long-term data series in the study area. Table 4.2: Rainfall classification 4.4.2 Tidal and current analysis In order to evaluate the water volume flows in and out in the estuary and to understand the residual flow were calculate the tidal prism and asymmetry index, respectively. In this study, the tidal prism is defined as the volumetric flux passing a cross-section during a flooding cycle. The tidal prism can be estimated by relationship: P = H * A Where H is the average tidal range and A is the average surface area of the basin (220 km2 according Wolff et al., 2000). Residual flow of the estuary was obtained through Asymmetry Index (AIDV) proposed by Mantovanelli et al. (2004). This dimensionless index was based on parameters proposed by Lincoln and Fitzgerald (1988) and is represented by the sum of duration asymmetries (AD) and velocity (AV) between the ebb and flood periods. AIDV= AD + AV; AD= (te– tf)/( te+tf); Rainfall (mm) Classification Period >400 Extreme rainfall wet 200-400 Heavy rainfall 100-200 Moderate rainfall dry <100 Light or no rainfall Chapter 4: Data and methods 63 AV= (Ūe– |Ūf|)/(Ūe+ |Ūf|) Where: te is the duration of ebb period; tf is the duration of flood period; Ūe is the average ebbing time; Ūf is the average flooding time. In accordance with the results obtained, the tides can be characterized as: AIDV = 0, the tidal wave is symmetrical for time and velocity; AIDV > 0, the residual circulation and water transport are directed to ebb; AIDV < 0, the residual circulation and water transport are directed to flood. 4.4.3 Water sample analysis The water samples taken during the ocenanographic campaigns (phase I and phase II) were analysed at the Coastal and Estuarine Oceanography Laboratory (LOCE) at the Federal University of Pará (UFPA) in Brazil. In phase I, salinity was measured with a salinity meter and dissolved oxygen concentrations were estimated using Winkler’s iodometric method, as modified by Strickland and Parsons (1968). In phases I and II, pH values were determined using an electronic pH meter (Labmeter, modeloPH2/PHS– 3B). Water samples were vacuum-filtered through glass-fibre filters (Milipore GF/F 0.7 µm, 47 mm), and the samples and filters were then freeze-dried for the analysis of the dissolved nutrients and chlorophyll a content, respectively. Dissolved inorganic nutrient concentrations (nitrate–NO3-, nitrite–NO2-, ammonium–NH4+, phosphate–PO4 and silicate–SiO2) were determined by spectrophotometric methods, following the procedures described by Strickland and Parsons (1972) and Grasshoff et al. (1983). Chlorophyll a was extracted from the acetate filters with 90% acetone v.v. and determined spectrophotometrically, according to Parsons and Strickland (1963) and UNESCO (1966). Equations were applied to obtain the saturated oxygen and chlorophyll a concentrations of each sample. The analysis of thermotolerant coliforms was based on the multiple tube technique described in the Standard Methods for the Examination of Water and Wastewater Natural and anthropogenic control of water quality of an Amazon estuary 70 During this period, the land surrounding the estuary was occupied by farms. The produce supplied Bragança and the state capital, Belém, approximately 220 km to the west, as well as other towns in the Brazilian Amazon basin, which had expanded rapidly during the Rubber Boom, when locally-produced latex was exported to markets in Europe and the United States. At the beginning of the 20th century, significant transformations occurred in the urban environment of the town of Bragança, with the construction of a railway that linked the town to Belém. This railway, inaugurated in 1908, greatly facilitated the development of Bragança and the expansion of its commerce, although its principal objective was the transportation of agricultural produce. This period led to the founding of the first industries in Bragança, and by 1955, ten factories had been established (mainly for the processing of agricultural produce, such as rice and other cereals) and a total of 200 shops. In the 1970s, the Brazilian Amazon region, including the town of Bragança, was, once again, a focus of colonisation, this time stimulated by official government incentives. As a result, the local population, which had been growing by 3% per decade, increased by 35% between 1970 and 1980. The population of the Bragança region doubled between 1940 and 2010 (Figure 5.1). In around 1970, Bragança underwent a process of transition, during which farming became less significant, while artisanal and semi-industrial fisheries became the region’s principal economic activity. In 1983, the construction of the 36 km long PA458 highway, parallel to the estuary, provided a new, terrestrial route of access to the small communities located in the middle and lower estuary. During this period, immigrants arrived from other Brazilian states, principally Ceará and Maranhão, arrived in Bragança, attracted by the local fisheries. New land use patterns began to emerge, and Bragança expanded due to the increase in the population in the outskirts of the town, and in neighbouring communities. Chapter 5: Anthropogenic driving forces on the water quality of the Caeté estuary 71 Figure 5.1: Population growth in the Bragança region (urban and rural area) between 1940 - 2013. Source: IBGE (2015). (*) estimated value. At the present time, the nine communities (Maranhãozinho, Fazendinha, Bragança – urban area, Camutá, Vila q Era, Bacuriteua, Caratateua, Vila dos Pescadores and Ponta do Urumajó) established along the margins of the Caeté estuary have a total of 80,800 residents (Guimarães et al., 2009; IBGE, 2010), of which, 90% live in Bragança, in the upper sector of the estuary (72,621 inhabitants). Some 26,222 residential properties are found in this sector, of which, approximately 15% are located on the margin of the estuary, in the neighbourhoods of Riozinho, Centro and Aldeia (IBGE, 2010; Figure 5.2). According to the data available from IBGE (2010), 620 of the residences have no sewage disposal system whatsoever, and only 331 of the 26,222 residences are connected to the town’s public sanitation system, which consists only of the sewers that carry the effluents to their final destination, on bare ground or in bodies of water, including the Caeté estuary. A further 3,538 residences have septic tanks, for the retention of solid waste, whereas the other 21,733 or so households depend on alternative systems, typically the piping of effluents directly into local bodies of water, including the Caeté estuary, vacant lots or the open street. Natural and anthropogenic control of water quality of an Amazon estuary 72 Figure 5.2: Communities located along the Caeté estuary. Chapter 5: Anthropogenic driving forces on the water quality of the Caeté estuary 73 In addition to the residential properties, 1,613 retail businesses are found in the upper sector. Most of these businesses trade in supplies for the local fishery industry, including ice factories, fuelling stations, dry docks, and landing piers. At the present time, 140 new commercial establishments are installed each year (data provided by the Bragança Town Council). The other estuary sectors are characterised by much smaller settlements, with a total population of 7,672 inhabitants in the middle sector, and 587 in the lower estuary (Guimarães et al., 2009; IBGE, 2015). The principal processes that contributed to the colonisation of the estuarine sector are shown in Figure 5.3. Despite the current political incentives for the occupation of the region, as mentioned above, and the recent growth of its population, there has been no investment in the consolidation and expansion of the urban infrastructure to meet the basic needs of the growing population. As a result, the town still lacks an efficient public water supply, sewage system or storm drains, and refuse collection services are irregular and inadequate. Clearly, then, the unplanned urban growth of the region has been accompanied by a widespread increase in anthropogenic impacts, and environmental problems that occurred previously on a small or local scale, such as the pollution of the estuary’s waters, have taken on major proportions. Given this, the present study considered population growth in the estuary to be the principal factor influencing the quality of its water. Natural and anthropogenic control of water quality of an Amazon estuary 74 Figure 5.3: Diagram showing the history of the occupation of the Caeté between the first European colonisation and the present day. Chapter 5: Anthropogenic driving forces on the water quality of the Caeté estuary 75 5.2 Inventory of pollution sources in the Caeté estuary A number of different sources of pollution, including sewage outfalls, ice factories, fish processing plants, refuse disposal areas, fishing boats and the Cereja River, were identified in the area surrounding the Caeté estuary, primarily in the upper estuary, on the Bragança waterfront. No quantitative data are available on the amounts of dissolved nutrients and faecal coliforms discharged into the estuary by these sources. Despite this, we surveyed all existing sources to estimate their respective contributions to the decline in the quality of the water of the Caeté estuay. The contribution of each source is described below. Sewage outfalls A total of 12 sewage outfalls were identified on the Bragança waterfront (upper estuary). These outlets are fed through underground pipelines from residences, markets, hospitals, and industries, and the effluents are discharged directly into the Caeté estuary. An IBGE survey (2010) showed that only around 13% of the 26,222 residences in Bragança are connected to septic tanks. These tanks retain part of the solid residues and eliminate the excess water by percolation into the soil. All other households are either connected to the public sewer system (which discharges untreated effluent directly into the estuary) or discharge their effluents directly onto the ground or into other bodies of water. In all cases, part of the effluents produced reach the estuary in the water that does not percolate into the ground, through the bodies of water that traverse the town and discharge into the estuary, or through the channelling of effluents through open-air drains (Figure 5.4). Natural and anthropogenic control of water quality of an Amazon estuary 76 Figure 5.4: Effluent input into the Caeté estuary: sewer being channelled into the estuary (A), sewage outlets seen during low tide (B and C), and effluents channelled through open-air drains that flow into the estuary (D). According to NBR 7229 (ABNT, 1993) and Von Sperling (1996), the residents of Brazilian urban centres produce at least 150 l per day of sewage per day, containing 54 g of DBO, and 8 g of Nitrogenous and 1 g Phosphorous compounds. Assuming that this estimate is valid for the Bragança region, the population in the area surrounding the Caeté estuary (80,880 inhabitants) would produce approximately 12,000 m3 of sewage per day. This corresponds to the production of 4.3 tons of DBO, 0.6 ton of nitrogenous and 0.08 ton of phosphorous compounds per day. Each inhabitant also produces 100– 400 billion faecal coliforms per day, with a total production of 81 x 1012 to 323 x 1012 MPN/100 ml. As mentioned above, most of these effluents reach the upper sector of the Caeté estuary. The analyses of the water from this sector sampled during the spring (April 2012) and neap (May 2012) tides indicate water rich in dissolved nutrients. The high concentrations of faecal coliforms (≥ 1100 MPN/100 ml) recorded at the sampling Chapter 5: Anthropogenic driving forces on the water quality of the Caeté estuary 77 stations reflects the influence of anthropogenic impacts in the vicinity of these settlements (Figure 5.5). Figure 5.5: Concentrations of dissolved nutrients and faecal coliforms recorded in the settlements located in the upper estuary (Mar: Maranhãozinho, Faz: Fazendinha, Brag: Bragança, and Vila: Vila q Era). Ice factories Four of the five ice factories located on the Caeté estuary are found in the upper sector and one in the middle sector. These facilities support the fishing industry, and produce large quantities of ice to conserve incoming catches, and the fishery exported to other regions. All the ice factories located on the Caeté estuary consume large amounts of gas and liquid ammonium during the refrigeration process (Figure 5.6). As there are no septic Natural and anthropogenic control of water quality of an Amazon estuary 78 tanks in which to treat these compounds adequately, the fraction of liquid that comes into contact with the sewage pipeline flows into the Caeté estuary, which is the final destination of most of the effluents produced in the town of Bragança. In its gaseous form, ammoniacal nitrogen (NH3-) is readily biodegradable and water soluble. When dissolved in water, it reacts by ionization to form ammonium (NH4+), which is assimilated by bacteria such as the species of the genus Nitrosomona, which then converts it into nitrites (NO2-) through the ammonification process (Schmidt et al., 2004; Cébron et al., 2005; Ward et al., 2008). Although ammonia is a nutrient required by these bacteria, the NH3and NH4+ forms are both toxic to aquatic life in excessive amounts, and can lead to eutrophication and to metabolic changes and death by inhibiting and/or accelerating algae growth (Widiastuti et al., 2011), as well as the eutrophication of estuaries. Figure 5.6: Ammonia cylinder employment during the refrigeration process. Fish processing plants A fish processing plant (GPesca Comercial Ltda.), located in the middle sector of the Caeté estuary, has been in operation since 2002. This plant, one of the most important in the state of Pará, produces an average of 130 m3 of liquid effluents per day. All the effluents produced by the GPesca plant are treated before being discharged into the Caeté estuary. This treatment has four stages, the first three of which involve the retention of solid residues using filters. The first stage has three filters tanks, the second stage, two tanks, and the third stage, one filter tank. Once all the solid residues have been removed from the effluent, it passes to the fourth stage, which consists of the metabolisation of the organic matter through aerobic filters. By the end of this process, Chapter 5: Anthropogenic driving forces on the water quality of the Caeté estuary 79 the pollutants have been removed and the organic content of the liquid has been reduced considerably. The effluent is then piped through the mangrove to a tidal creek in the Caeté estuary (Figure 5.7). The solids residues, such as fish heads, entrails, fins, and scales (Figure 5.7), which are by-products of the production process are stored and donated for the production of animal feed. While no systematic record is kept of the amount of solid waste produced each day, an estimate based on the weight and number of containers discarded indicates that the total may be as much as 1 ton. Figure 5.7: Different stages in the treatment of the residual water produced by GPesca Ltda. Removal of the solid residues (A), filtering of the solid residues (B), tank for the metabolisation of the organic matter (C), the pipeline that discharges the residual water into the Caeté estuary (D and E), effluent piped through the mangrove to a tidal creek in the Caeté estuary (F). Based on the quantity of effluents produced and the concentrations of dissolved nutrients found per litre of water released in the estuary, it is possible to estimate the volume of BOD, nutrients, and faecal coliforms discharged into the Caeté estuary per day (Table 5.1). Unfortunately, it was not possible to obtain data from other local fish processing plants, which operate illegally during the peak harvesting periods of commercially-valuable species such as the Southern red snapper, Lutjanus purpureus (Figure 5.8). However, it can safely be assumed that these plants produce large Natural and anthropogenic control of water quality of an Amazon estuary 86 Figure 6.1: Long-term data and monthly rainfall level in Bragança. (*) Total rainfall until the field campaign in 07 February 2007. A general characterization of the rainfall data allows to group the monitored months into different categories, as shown in the table 6.1. Table 6.1: Rainfall classification according to the monthly rate. (*) Periods influenced by equinoctial spring tides As rainfall levels have a direct effect on fluvial discharge, the long-term data available on the discharge of the Caeté River were also considered in this study. The mean discharge of the Caeté in 2006 (36 m3.s-1) was similar to the historical mean of 40 m3.s-1 (Figure 6.2). Approximately 81% of the total discharge recorded in 2006 was registered during the wet season, when the monthly mean was 82.5 m3.s-1. The highest monthly discharge was recorded in April 2006, as a consequence of the peak in the rainfall recorded in March 2006. The remaining 19% of the total discharge was recorded during Rainfall (mm) Months >400 March and April* of 2006 Extreme rainfall 200-400 February and May* of 2006 Heavy rainfall 100-200 January, June and July of 2006 Moderate rainfall <100 August, September*, October* and November of 2006 and January and February of 2007 Light or no rainfall dry wet Classification Chapter 6: Environmental aspects 87 the dry season, when the mean monthly discharge was 19.3 m3.s-1. Reflecting the trend in rainfall, the discharge of the Caeté in January 2007 was only 18% of the historical mean. Figure 6.2: Long-term data and monthly river discharge data measured in the upper Caeté estuary during the study period. (*) Data not available for 2007. Winds also presented a seasonal pattern. The highest wind speeds were recorded during the dry season, when maximum average velocities of 4.0 to 8.0 m.s-1 were recorded frequently. Exceptionally, brief gusts with velocities above 12.0 m.s-1 were recorded. By contrast, wind speeds were low during the wet season, with a predominance of maximum average velocities of 1.0 m.s-1 or less (Figure 6.3). North-westerly winds (270º < θ < 315º) predominated during the wet season, principally between April and June 2006. North-easterly winds were recorded during only 20% of the time during this season (Figure 6.3). During the dry season, by contrast, northeasterly winds (0º < θ < 45º) predominated, accounting for approximately 75% of the records between October 2006 and February 2007. Natural and anthropogenic control of water quality of an Amazon estuary 88 Figure 6.3: Wind speeds and directions measured at the Salinópolis station during the study period. Chapter 6: Environmental aspects 89 6.1.2 Hydrodynamic aspects Tidal heights in the upper and lower Caeté estuary are shown in table 6.2. An amplification between 0.10 and 0.40 m being recorded going upstream. This amplification was more pronounced during the dry season when river discharge was reduced. Table 6.2: Tidal heights recorded in the upper and lower sectors of the Caeté estuary. The tides were asymmetric. High water reaches the upper sector with a delay of 1.1–2.6 hours, whereas there is a delay of 2.0–3.0 hours at low water. The asymmetry was less pronounced in the lower sector, with minimal differences between the flood (5.3-6.0 hours) and ebb (6.3-7.2 hours) phases (Table 6.3). As expected, tidal asymmetry was less pronounced during the dry season when river discharge was reduced. Based on the asymmetry index (AIDv), the ebb phase dominated in the upper sector during both the wet and the dry seasons. However, this dominance pattern was observed only during the wet season in the lower sector, whereas the flood tide predominated during the dry season (Table 6.3). The estimated mean tidal prism during the study period was 93 x 106 m3 (Figure 6.4), with peak of 103 x 106 m3 in August 2006. Apr'06 4.3 4.3 Jun'06 4.2 4.1 Aug'06 4.6 4.6 Oct'06 4.4 4.2 Dec'06 4.6 4.2 Feb'07 4.3 4.1 Tidal range (m) lower upper dry wet Natural and anthropogenic control of water quality of an Amazon estuary 90 Table 6.3: Tidal asymmetry in the Caeté estuary and the classification based on the asymmetry index (AIDV). Figure 6.4: Tidal prism estimated for the Caeté estuary from the DHN tidal data under spring tide conditions. High current velocities were recorded in all three estuary sectors, reaching a maximum of 1.2 m.s-1 in the upper sector, 2.0 m.s-1 in the middle, and 1.7 m.s-1 in the lower sector (Figure 6.5). The increased river discharge during the wet season resulted in the highest ebb current velocities recorded in the upper sector (around 1.2 m.s-1). During the dry season, the increased influence of marine forces and the reduced fluvial discharge contributed to stronger flood tide currents in both the middle (from August to October ebb flood ebb flood Apr'06 8.3 3.8 7.2 5.3 ebb dominance (AIDV 0.6) ebb dominance (AIDV 0.3) Jun'06 8.0 4.2 6.3 5.5 ebb dominance (AIDV 0.5) ebb dominance (AIDV 1.8) Aug'06 7.3 5.0 7.0 5.3 ebb dominance (AIDV 0.4) ebb dominance (AIDV 0.2) Oct'06 7.5 4.3 6.7 5.3 ebb dominance (AIDV 0.3) flood dominance (AIDV -0.1) Dec'06 7.5 4.7 6.8 5.7 ebb dominance (AIDV 0.5) flood dominance (AIDV -0.2) Feb'07 7.5 4.6 6.3 6.0 ebb dominance (AIDV 0.5) symmetrical wave (AIDV 0.0) lower upper wet dry Asymmetry (hours) Asymmetry index upper lower Chapter 6: Environmental aspects 91 2006) and lower sectors (October of 2006) (Figure 6.5). The data indicate the occurrence of a strict slack tide in the Caeté estuary (up to 0.5 hours), while peak velocities tend to occur in the middle of the tidal phase. Figure 6.5: Current velocities in the Caeté estuary during the spring tide. Natural and anthropogenic control of water quality of an Amazon estuary 92 In general terms, the highest current velocities (> 1.6 m.s-1) were recorded in the middle estuary (Table 6.4), which is probably due to the presence in this sector of sand banks that narrow the main channel of the estuary, as observed in Figure 3.9 (Chapter 3). Table 6.4: Frequency of current velocities values recorded in the upper, middle and lower sectors. The progressive current vector indicated the maximum displacement was 41 km during period of extreme rainfall which coincided with peak of fluvial discharge (April) in the upper estuary. In the middle estuary, the maximum was 36 km during the driest month and equinoctial spring tides (October), and in the lower estuary the maximum displacement was 17 km in periods influenced by equinoctial sprig tides (April and October, Figures 6.6, 6.7 and 6.8). sector 0-0.4 m.s-1 >0.4-0.8 m.s-1 >0.8-1.2 m.s-1 >1.2-1.6 m.s-1 >1.6-2.0 m.s-1 upper 37.5 48.2 27.4 0.2 0.0 middle 37.5 27.4 22.5 10.8 2.9 lower 36.7 27.1 24.5 11.2 0.5 Frequency (%) Chapter 6: Environmental aspects 93 Figure 6.6: Current displacement in the upper sector of the estuary over a 25-hour period. Natural and anthropogenic control of water quality of an Amazon estuary 94 Figure 6.7: Current displacement in the middle sector of the estuary over a 25-hour period. (*) No current data. Chapter 6: Environmental aspects 95 Figure 6.8: Current displacement in the lower sector of the estuary over a 25-hour period. Natural and anthropogenic control of water quality of an Amazon estuary 102 Figure 6.12: Trophic status of the Caeté estuary during the wet and dry seasons. (*) This classification has been discussed in the Chapter 8. Figure 6.13: Dissolved inorganic nitrogen in the Caeté estuary during the wet and dry seasons. (*) Not ammonium data. The molar DIN:DIP ratios recorded in the three sectors during the study period is shown in Figure 6.14. Low values, below the threshold proposed by Redfield (16:1), predominated in all months, indicating a deficit of nitrogen, except in the period marked by the absence of rainfall (October 2006) in the upper sector, when an excess of nitrogen was recorded. By contrast, the DSi:DIP ratios were higher than the Redfield threshold, indicating an excess of silicate in the Caeté estuary, primarily during the dry season, when a maximum of 322:1 was observed in the upper sector (Figure 6.14). Chapter 6: Environmental aspects 103 Chlorophyll a did not present a seasonal pattern, with peaks being observed in both the wet and dry season, with concentrations ranging from 4.4 to 16.5 mg.m-3 in the wet season and from 3.92 to 17.6 mg.m-3 in the dry season. The highest chlorophyll a values (above 15.0 mg.m-3) were recorded in the upper sector, which presents the highest dissolved nutrient concentrations. Concentrations were lower in the others sectors, but still reflected the high productivity of the estuary (Figure 6.14). Faecal coliforms are an important indicator of water quality in environments affected by anthropogenic impacts, like the Caeté estuary. The low faecal coliforms concentrations recorded during the wet season (maximum of 110 MPN/100 ml) confirmed the capacity of the fluvial discharge of dilute contaminants during this period. The increase in faecal coliform concentrations observed during the dry season indicate higher levels of contamination and a poor water quality, as indicated by the trophic indices (Figure 6.14). As expected, faecal coliform concentrations presented a marked spatial gradient from the upper to the lower estuary, with increasing closer to the pollution sources. Concentrations of over 500 MPN/100ml were recorded only in the upper sector, where they corresponded to 40% of the samples, whereas in the other sectors, values were invariably lower than 500 NMP/100 ml (Figure 6.14). This supports the highest level of anthropogenic impacts found in this sector, as shown in Chapter 5. The correlation matrix for the variables analysed in the present study (included only those with a full dataset) is shown in Table 6.5. As observed above, salinity correlated positively with pH and phosphate concentrations. This reflects the increase in pH observed during the dry season due to the increased influence of marine waters, and also indicates that they may represent an important source of phosphate for the estuary, contributing to the increased concentrations recorded during this period in the middle and upper sectors of the estuary as shown in the Figure 6.9 and 6.10. Natural and anthropogenic control of water quality of an Amazon estuary 104 Figure 6.14: Redfield ratios, chlorophyll a and faecal coliform concentrations recorded in the Caeté estuary between April 2006 and February 2007. (*) Not data. Chapter 6: Environmental aspects 105 Table 6.5: Spearman correlation matrix for the variables monitored during the spring tide period. 6.2 Neap tide condition (Phase II) 6.2.1 Rainfall, freshwater discharge and winds The rainfall recorded during the oceanographic campaigns of 2010 (August–December, 363 mm) was 100% higher than that of the 39-year historical average for the same period (180 mm). The increase was due to the high rate of rainfall recorded in December of 2010, approximately 2 times higher than registered by long-term data series (Figure 6.15) due do the La Niña recorded in this period. The total rainfall recorded between August and December corresponds to 18% of the annual total recorded in 2010 (1987 mm). September, October, and November 2010 was considered the driest quarter, with month rates lower than 28 mm (Figure 6.15). In 2011, the total rainfall recorded during the oceanographic campaigns (January–October, 2617 mm) was similar enough to the historical mean for the region (2369 mm for the same period) for the year to be considered typical (Figure 6.15). Approximately 95% of this total was recorded during the wet season (January–July, 2483 mm). April was the rainiest month, with a total of 503 mm. During the dry season, however (August–October) a total of only 135 mm was recorded, corresponding to only 5% of the annual total. Salinity pH Nitrate Nitrite Phosphate pH 0.76* Nitrate -0.11 0.02 Nitrite -0.06 -0.22 0.22 Phosphate 0.43* 0.12 0.15 0.38 Chl a-0.03 -0.06 -0.05 0.06 -0.07 Significant correlations in bold *Significance level (p < 0,00001) Natural and anthropogenic control of water quality of an Amazon estuary 106 Figure 6.15: Long-term data and monthly rainfall level in Bragança. A general characterization of the rainfall data allows to group the monitored months into different categories, as shown in the table 6.6. Table 6.6: Rainfall classification according to the monthly rate. (*) Periods influenced by equinoctial spring tides. The average fluvial discharge during the dry season of 2010 (August–December, 9.1 m3.s-1) was 45% lower than the historical mean (16.3 m3.s-1) recorded over a 19-year period (Figure 6.16). The mean discharge recorded during the 2011 oceanographic campaigns (January– October) was 73.6 m3.s-1 during the wet season (January–July), which corresponds to a rate 30% higher than the long-term series recorded to this same period, and 17.1 m3.s-1 during the Rainfall (mm) Months >400 January and April* of 2011 Extreme rainfall 200-400 February, March, May*, June and July of 2011 Heavy rainfall 100-200 August and December of 2010 Moderate rainfall <100 September*, October* and November of 2010 and August, September*, October*, November and December of 2011 Light or no rainfall Classification wet dry Chapter 6: Environmental aspects 107 dry season (August–October), which is similar to the long-term series recorded. Monthly rates varied from 4.6 m3.s-1 to 103.0 m3.s-1 (Figure 6.16). Figure 6.16: Long-term data and monthly river discharge data measured in the upper Caeté estuary during the study period. (*) Data not available for 2011. Little seasonal variation was observed in wind speed or direction (Figure 6.17). Wind speeds typically ranged from 3.0 to 6.0 m.s-1 in both seasons. The highest speeds, between 9.0 and 12.0 m.s-1, were recorded mainly during the dry seasons of 2010 and 2011, although they corresponded to little more than 10% of the records during this period. Only 2% of the records collected during the wet were these high. North-easterly winds (0º < θ < 45º) predominated during both seasons, with a frequency of approximately 95% during the dry season and 80% during the wet season. Natural and anthropogenic control of water quality of an Amazon estuary 108 Figure 6.17: Wind speeds and directions recorded at the Salinópolis station during the study period. Chapter 6: Environmental aspects 109 6.2.2 Hydrodynamic aspects Tidal heights recorded in the Caeté estuary are shown in table 6.7. Tidal amplification of 0.1 to 0.7 m is observed in the middle sector due to the reduction of the width of the channel, as observed through bathymetric data described in Chapter 3 (St4, Figure 3.9). This is followed by damping of up to 0.9 m as the tide is propagated to the upper sector. As expected, the tidal prism was lower during the neap tide than the spring tide, ranging between 50 and 72 x 106 m3, with the highest values being recorded in November 2010 and July 2011, when the oceanographic campaign was conducted during the transition between neap and spring tides (Figure 6.18). The reduction of the tidal prism observed during this phase may have also contributed to the reduction in the tidal current velocities observed during this period, which ranged between 0.2 and 0.7 m.s-1 in both wet and dry seasons (Figure 6.19). Table 6.7: Tidal heights recorded in the upper, middle and lower sectors of the Caeté estuary during the wet and dry seasons. Sep'10 2.7 2.7 2.6 Nov'10 3.4 3.6 3.3 Jan'11 3.1 3.2 2.6 Mar'11 2.2 3.1 2.6 May'11 3.0 3.3 2.8 Jul'11 3.8 3.8 3.3 Sep'11 2.9 3.0 2.3 Oct'11 2.3 2.6 2.4 middle lower dry Tidal range (m) dry wet upper Natural and anthropogenic control of water quality of an Amazon estuary 110 Figure 6.18: Tidal prism estimated for the Caeté estuary from the DHN data under neap tide conditions. A seasonal pattern similar to that recorded during the spring tides was also observed here, with current velocities being influenced strongly by local climatological variables. During the wet season, the highest values (between 0.6 to 0.7 m.s-1) were observed during the ebb phase, whereas during the dry season, they were recorded in the flood phase (Figure 6.19). The vertical current profile of the Caeté estuary was homogeneous, with differences of typically little more than 0.1 m.s-1 between the surface and the bottom (Figure 6.20). This indicates a lack of gravitational circulation and a predominance of mixing between the freshwater and marine waters (as confirmed by the vertical salinity data in the next section, see section 6.2.3). The greatest differences between surface and bottom velocities were recorded during the wet season (0.5 m.s-1), indicating weaker mixing and a certain degree of stratification, which was also confirmed by the vertical salinity. This pattern was observed primarily during heavy rainfall periods (May 2011), when the higher fluvial discharge was recorded (mean of 85.9 m3.s-1). Chapter 6: Environmental aspects 111 Figure 6.19: Current velocities in the Caeté estuary during the wet and dry seasons. The tidal range was obtained from the DHN. Natural and anthropogenic control of water quality of an Amazon estuary 118 Figure 6.24: Mixing diagrams along the salinity gradient of the Caeté estuary. Chapter 6: Environmental aspects 119 During the neap tides, eutrophication was less intense than that observed during spring tides. Karydis index reported a predominance of mesotrophic conditions in the Caeté estuary (Figure 6.25). During the dry season, when the dilution and transportation of nutrients towards the mouth of the estuary are less intense (low fluvial discharge), the trophic conditions deteriorated in the upper and middle sectors of the estuary. In the wet season (high discharge), on the other hand, the more effective transportation of nutrients from the upper sector to the mouth of the estuary resulted in an increase in the trophic status of the lower estuary. According to the TRIX, the water of the Caeté was good and eutrophication medium (Figure 6.25). The DIN values recorded during the present study indicate the active nitrification of the estuary. The concentrations of the more oxidated forms of nitrogen (nitrate and nitrite) were higher during the dry season, when the estuary’s waters were more oxygenated (Figure 6.26). During the period of reduced oxygenation (wet season), however, the concentrations of ammonium (the least oxidated form of nitrogen) increased, corresponding to 50% of the total inorganic nitrogen. Once again, high concentrations of ammonium were recorded in the upper sector, adjacent to the sources of pollution. Figure 6.25: Trophic status of the Caeté estuary during the wet and dry seasons. (*) This classification has been discussed in the Chapter 8. Natural and anthropogenic control of water quality of an Amazon estuary 120 Figure 6.26: Dissolved inorganic nitrogen concentrations in the Caeté estuary between September 2010 and October 2011. (*) Data not collected. The molar DIN:DIP ratios recorded during the present study indicated an excess of nitrogen only in the upper estuary during almost all months, with values ranging from 15:1 to 133:1 (Figure 6.27). In the middle sectors, the ratios were balanced (except in March 2011), while in the lower estuary, they were low. The DSi:DPI ratio indicated an excess of silica over phosphate in all sector, especially during the period of high fluvial discharge (wet season). Peaks of chlorophyll a were recorded more frequently during the wet season, when the concentrations of nutrients (except ammonium) declined in the upper and middle sectors, indicating that the reduction in the availability of nutrients does no limit phytoplankton growth. In broad terms, the values decrease between the upper estuary (values frequently above 10.0 mg.m-3) and the lower sector (values are typically below 10.0 mg.m-3) (Figure 6.27). In addition to dissolved nutrients, rainfall also appears to contribute to the dilution and transportation of faecal coliforms from upper estuary. As result, slight increase in concentrations in the middle and lower sectors of the estuary during the period of increased fluvial discharge, between January 2011 and May 2011 (Figure 6.27). The contamination of the upper sector was consistently high, however, with 90% of the samples returning values of over 1100 MPN/100 ml. As described in Chapter 5, sources of contamination – household and urban effluent outlets, and the fishing fleets – are concentrated in this sector. The best conditions were observed in the lower estuary, where anthropogenic impacts are reduced (Figure 6.27). Chapter 6: Environmental aspects 121 The correlation matrix indicated that most of the parameters analyzed (except nitrate) were correlated positively with salinity (Table 6.8), although chlorophyll a presented a negative relationship, decreasing during the dry season, when salinity increased. The chlorophyll a correlations were also correlated positively with nitrate, which may be assimilated by the phytoplankton, increasing its biomass. The other correlations observed here (silicate vs. ammonium and silicate vs. phosphate) may also be accounted for by the increase in the biomass of the phytoplankton (which are made up of frustules that contain silica) during the periods when these nutrients (ammonium and phosphate) are most abundant (Table 6.8). Natural and anthropogenic control of water quality of an Amazon estuary 122 Figure 6.27: Redfield ratios, chlorophyll a and faecal coliforms concentrations recorded in the Caeté estuary between September 2010 and October 2011. (*) Not collected. Chapter 6: Environmental aspects 123 Table 6.8: Spearman correlation matrix for the variables monitored during the neap tide periods. 6.3 Comparative analysis between phase I and phase II This comparative analysis encompasses the hydrological data obtained during the flood tide cycle in the oceanographic campaigns of phases I (spring tides) and II (neap tides). The lowest rainfall rates were recorded in phase I (2006-2007) when the rainfall rate recorded in the dry season was only 40% of that recorded during the same period in 2010, and only 18% of the total rainfall recorded during the wet and dry seasons of 2011. However, wind speeds (10% increase in the occurrence of winds of 8.0 m s-1), current velocities (peaks twice as high) and the tidal prism (40% greater) were all more pronounced during phase I. The increase in the tidal prism observed during phase I favoured the intrusion of saline waters into the estuary, causing an increase in salinity and pH in this phase. The reduced rainfall, and consequently, the lower discharge of the Caeté, may also have favoured this process. In both phases, in fact, peaks in salinity and pH were recorded during periods with low or absence of rainfall, when the discharge of the Caeté was reduced (Figure 6.28). Dissolved oxygen also peaked during phase I, coinciding with the greater wind and current velocities observed during this period, although no clear seasonal pattern was recorded (Figure 6.28). Sal pH Nitrite Nitrate Ammonium Phosphate Silicate pH 0.49**** Nitrite 0.21* -0.02 Nitrate -0.17 0.02 0.35*** Ammonium 0.21* 0.17 0.40*** 0.52*** 0.51 Phosphate 0.58**** 0.29** 0.57**** 0.02 0.37 Silicate 0.33*** 0.19 0.53**** 0.04 0.44**** 0.59**** Chl a-0.20* 0.13 -0.09 0.21* 0.19 -0.12 0.00 Significant correlations in bold Significance level * = p < 0,05; ** = p < 0,001; *** = p < 0,0001; **** = p < 0,00001 Natural and anthropogenic control of water quality of an Amazon estuary 124 Figure 6.28: Physical and physical-chemical variables in the Caeté estuary during spring and neap tides. Chapter 6: Environmental aspects 125 High concentrations of dissolved nutrients were recorded during both phases, indicating a highly productive environment. There was an increase in these concentrations during the dry season in the upper and middle sectors. During the phase II the concentrations of nitrogenous nutrients (nitrate and ammonium) and silicate were higher. The decrease in the tidal prism and the lower current velocities recorded during this phase may have favoured the accumulation these dissolved nutrients (Figure 6.29). Phosphate was the only nutrient that varied significantly between phases, with higher concentrations being recorded during spring tides (F = 8.88, p = 0.05 This reflects the contribution of marine waters which act as a phosphate source for the estuary, as showed by the salinity vs. phosphate diagram (Figure 6.11) and would account for the higher concentrations recorded in the upper and middle sectors during the dry season, when the marine intrusion increases (Figure 6.29). In addition, the predominantly alkaline conditions (pH > 7.0) recorded during the spring tides favour the release of the phosphorus absorbed by the sediments into the water column. Natural and anthropogenic control of water quality of an Amazon estuary 126 Figure 6.29: Dissolved nutrient concentrations recorded in the Caeté estuary during spring and neap tides. Chapter 6: Environmental aspects 127 In general terms, the trophic indices did not vary systematically between spring and neap tide periods. Only the Karydis index indicated a marked increase in eutrophication in the upper estuary during the dry season (Figure 6.30). In the other sectors, the estuary was mesotrophic to oligotrophic. The upper and middle sectors presented more intense trophic conditions (meso and eutrophic) during the dry season, when nutrient transportation towards the coast is reduced. In the lower sector when nutrient transportation was reduced, oligotrophic conditions were observed, and during the wet season, when the fluvial discharge increases, the trophic state become mesotrophic. The TRIX index reflected similar seasonal and spatial patterns within the estuary, with a predominance of bad water quality and high eutrophication during phases I and II, although slightly better conditions were observed during the neap tides of the wet season (Figure 6.30). This index, based on the concentrations of nitrogen, phosphate, and saturated oxygen, reflects the pattern observed previously for these variables. In both phase the dissolved inorganic nitrogen (DIN) was constituted primarily of nitrate, followed by ammonium (Figure 6.31). The relative scarcity of nitrite (unstable form of inorganic nitrogenous) indicates that nitrification and denitrification were occurring during flood phase in both monitored phases. The DIN:DIP ratios indicated a deficit of nitrogen during the spring tides in all sectors of the estuary, whereas an excess of nitrogen was observed in the middle and upper sectors in the neap tides during the wet season (Figure 6.32). An excess of silicate was recorded in all both phases in all sectors throughout the study period. Again, an excess was observed during neap tides, mainly in the wet season (Figure 6.32). In general, the chlorophyll a concentrations were highest during the neap tides (except for the middle sector during the dry season and the lower sector during the wet season), coinciding with the period when ammonium was most available for assimilation by the phytoplankton. During the spring tides, the increased intrusion of marine waters, generally characterized by lower productivity, may have contributed to the lower values recorded (Figure 6.32).