Influence of physicochemical parameters on the spatial distribution of pelagic copepods in the Fatala River estuary (Boffa, Republic of Guinea)

 Corresponding author: S. Diakité. Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution Liscense 4.0. Influence of physicochemical parameters on the spatial distribution of pelagic copepods in the Fatala River estuary (Boffa, Republic of Guinea) Sory Diakité 1, *, Mamadou Djouldé Sow 1, Daouda Konaté 2 Ahmed Guissé 1 and Taliby Dos Camara 3 1 Department of Hydrobiology, Marine and Coastal Research Center of Guinea (CEREMAC-G), BP 1615. 2 Department of Fisheries and Aquaculture, Higher Institute of Veterinary Sciences and Medicine (ISSMV) of Dalaba. 3 Department of Biology, Faculty of Sciences, Gamal Abdel Nasser University of Conakry (UGANC). World Journal of Advanced Research and Reviews, 2025, 27(03), 229-243 Publication history: Received on 23 July 2025; revised on 29 August 2025; accepted on 02 September 2025 Article DOI: https://doi.org/10.30574/wjarr.2025.27.3.3107 Abstract This study, carried out between July and October 2020 in the Fatala River estuary (Boffa prefecture, Republic of Guinea), aimed to assess the specific diversity of pelagic copepods and to analyze their spatial distribution in relation to the physicochemical parameters of the environment. This estuary, subject to significant anthropogenic pressures linked to mining, agricultural, fishing and domestic activities, presents a high vulnerability to water quality degradation. Copepod samples were collected during four field campaigns using a zooplankton net and then identified in the laboratory. Physicochemical parameters (temperature, salinity, conductivity, dissolved oxygen, pH) were measured in situ, while suspended solids, turbidity, nitrates and phosphates were analyzed in the laboratory. The taxonomic analysis identified 59 species, distributed in two orders: Calanoida (89.83%) and Cyclopoida (10.17%). These species belong to 17 families and 27 genera. The community is largely dominated by Paracalanus aculeatus (25.14%), Aetideus armatus (18.93%), Paracalanus scotti (15.07%) and Paracalanus parvus (9.83%), together representing nearly 69% of the total abundance. The results show a clear spatial heterogeneity. In the upstream stations (S1 to S5), copepod densities vary between 0.00 and 499.34 individuals/m³, with an average of 258.352 individuals/m3, a reduced specific diversity (Shannon index H' ≤ 0.19) and the total absence of copepods at some stations. These low densities are explained by unfavorable environmental conditions, characterized by an acidic pH (6.3–6.7), low salinity, reduced mineralization, as well as very high phosphate concentrations (up to 28.6 mg/L). Conversely, the downstream stations (S6 to S14) present much higher densities, between 1733.51 and 4565.77 individuals/m³, with an average of 3248.89 individuals/m3, a higher specific richness and more favorable physicochemical conditions: alkaline pH, increased salinity and conductivity, moderate turbidity. The dominance of a limited number of euryecious species reflects an ecological imbalance typical of estuarine environments subject to multiple anthropogenic pressures. Keywords: Zooplankton; Copepods; Species; Estuary; Pelagic; Density; Physicochemical Parameters 1. Introduction Estuarine ecosystems play a fundamental ecological role as transition zones between freshwater and marine environments. Their high productivity makes them essential habitats for many aquatic species, including zooplankton, of which copepods represent a dominant group (Mouny, 1983; Chaalali, 2013). These small crustaceans play a key role in aquatic food webs, ensuring the transfer of energy from primary producers (phytoplankton) to secondary consumers (fish, invertebrates, etc.) (Rolland, 2004). The spatial distribution of copepods is strongly influenced by physicochemical factors of the environment, such as temperature, salinity, dissolved oxygen, pH or nutrients such as nitrates and phosphates (Moison, 2009). These World Journal of Advanced Research and Reviews, 2025, 27(03), 229-243 230 parameters condition not only the structure of planktonic communities, but also their seasonal and spatial dynamics (Trégouboff & Rose, 1957). In tropical estuaries, often subject to strong anthropogenic pressures (urbanization, agriculture, mining activities, industrial discharges), changes in water quality can lead to significant ecological imbalances, particularly affecting copepod communities (CCLME, 2014). However, in the Guinean context, few studies have focused on the relationship between environmental parameters and the distribution of zooplankton in estuaries, particularly that of the Fatala River, a strategic system for local biodiversity and human activities. This study aims to fill this knowledge gap by analyzing the influence of physicochemical parameters on the spatial distribution of pelagic copepods in the Fatala River estuary (Boffa prefecture). It will help assess the ecological state of the environment and provide a solid scientific basis for the sustainable management of this ecosystem. 2. Methodology Copepod sampling was carried out on board a 15 hp motorized boat, during four campaigns, in fourteen (14) fixed stations (Figure 1). Samples were taken using a Hensen-type plankton net, with a 55 µm mesh and an opening diameter of 70 cm. Samples were collected by vertical lines, from the bottom to the surface. During each campaign, three samples were taken per station, or 42 samples per campaign, for a total of 168 samples over the four missions. The samples were fixed in 5% formalin. Taxonomic identification of copepods was carried out under a binocular microscope, using the determination keys of Rose (1933) and Trégouboff & Rose (1978). In parallel with biological sampling, in situ measurements were made on temperature, salinity, conductivity, dissolved oxygen and pH, using multi-parameter probes. The concentrations of suspended matter, turbidity, nitrates and phosphates were determined in the laboratory according to standard physicochemical analysis protocols. Figure 1 Sampling map World Journal of Advanced Research and Reviews, 2025, 27(03), 229-243 231 2.1. Statistical analyses To average copepod densities at a station, descriptive statistics techniques were used to calculate the arithmetic mean of the density values collected at that station. The collected data were processed using Excel 2010 spreadsheet. The Shannon diversity index (1948) also called Shannon-Wiener index was calculated using the following formula 𝐻′= − ∑𝑝𝑖 𝑆 𝑖=1 . log2(pi) Where H' corresponds to the Shannon index ; S is the number of species; pi the relative frequency of the ith species: pi = ni/N where ni = the number of individuals counted for a species present; N is the total number of individuals counted, all species combined. 3. Results and discussion Figure 2 Temperature variation depending on the stations Figure 3 Variation of pH depending on the stations The results of the physicochemical analyses carried out to assess water quality are presented in Figures 2 to 10. These figures successively show the measured values of temperature, pH, conductivity, suspended matter, turbidity, nitrates, phosphates, salinity and dissolved oxygen. Thus, they provide an initial overview of water quality during the sampling period. World Journal of Advanced Research and Reviews, 2025, 27(03), 229-243 232 Figure 4 Variation in conductivity depending on the stations Figure 5 Variation in the quantity of suspended solids depending on the stations Figure 6 Variation of turbidity depending on the stations World Journal of Advanced Research and Reviews, 2025, 27(03), 229-243 233 Figure 7 Variation in the quantity of nitrate depending on the stations Figure 8 Variation in the quantity of Phosphate depending on the stations Figure 9 Variation of Oxygenenot dissolved according to the stations World Journal of Advanced Research and Reviews, 2025, 27(03), 229-243 234 Figure 10 Variation of salinity depending on the stations The analysis of the physicochemical parameters measured along the longitudinal gradient of the Fatala estuary highlights several indicators of degradation of water quality, reflecting a combination of anthropogenic pressures and natural factors. Water temperature ranged from 28.6 °C at station 1 (upstream) to 31.7 °C at station 9 (downstream), significantly exceeding the average value of 25 °C recommended by AGRBC (2015) and the biological suitability thresholds of 25 to 27 °C defined by the International Office for Water (2003). The lowest temperatures were recorded upstream, notably at stations 1 (28.6 °C) and 2 (29.2 °C), while the highest were located downstream, at stations 8 (31.6 °C) and 9 (31.7 °C). Spatial variation remained moderate (3.1 °C difference), which is comparable to the observations of N'Guessan et al. (2015) in the Sassandra estuary. However, these relatively high temperatures can reduce the solubility of dissolved oxygen, promote the proliferation of algae and opportunistic microorganisms, and thus alter the ecological quality of the environment. The pH shows a significant longitudinal variation, with a tendency to increase from upstream to downstream. Stations 2, 3, 4 and 5, located near inhabited areas and the port of Boffa, show values below 7, probably related to domestic and port discharges and mining activities. The lowest pH (6.3) was recorded at station 3. On the other hand, more alkaline values, reaching 7.7, were observed at stations 8 and 9, downstream, which could be explained by the presence of limestone rocks on the seabed.Despite some episodes of slight acidity, all measurements are within the tolerance range (6–9)recommended by the International Office for Water (2003) and AGRBC (2015), which is consistent with the trends observed by N'Guessan et al. (2015) in the Sassandra River estuary. For comparison, the waters of the Beht basin in Morocco have a significantly higher pH, ranging between 8 and 8.8 (Lakhili et al., 2015). Electrical conductivity values show strong spatial heterogeneity, ranging from 86.3 µS/cm at station 1 to 6750 µS/cm at station 13, close to the mouth. Stations located upstream (stations 1 to 5) record conductivities lower than the reference average of 800 µS/cm set by AGRBC (2015), indicating the predominance of low-mineralized freshwater. On the other hand, the very high values measured downstream reflect the direct influence of saline marine waters, as well as the effects of runoff loaded with dissolved salts and the dissolution of coastal rocks. These results are consistent with the observations of Lakhili et al. (2015) in Morocco and N'Guessan et al. (2015) in Ivory Coast, who also reported a significant increase in electrical conductivity in estuaries under the influence of marine waters. Suspended solids (SS) also exhibit high spatial variability, with concentrations ranging from 2.3 mg/L (station 1) to 53 mg/L (station 10). The lowest levels were recorded at the upstream stations (S1 to S5), respectively 2.3; 3.0; 4.0; 7.0 and 13 mg/L, where the influence of the tide is reduced. Downstream, particularly at station 10, the maximum concentration is attributable to the effect of high tide promoting the resuspension of particles, increased erosive potential, as well as anthropogenic activities, particularly those related to the nearby artisanal fishing port of Marara. This value exceeds the threshold of 50 mg/L set for the “good” class by the International Office for Water (2003). All other stations remain below this limit, indicating a localized but significant degradation of water quality downstream. World Journal of Advanced Research and Reviews, 2025, 27(03), 229-243 235 Turbidity measurements follow a similar trend to that of suspended solids. They vary between 1.8 NTU (station 1) and 51.2 NTU (station 10), with a clear increase from upstream to downstream. This increase is the result of a combination of factors including human activities, sediment inputs by runoff, tidal resuspension, and proximity to the artisanal fishing port. Despite this increase, all stations comply with the thresholds of the International Office for Water (2003), which sets the limit for “average” quality at 70 NTU. Nutrient concentrations reveal significant pollution. Nitrate levels range from 2.8 mg/L at station 8 to 5.0 mg/L at station 2, far exceeding historical levels recorded in Guinean estuaries (0.1 to 0.5 mg/L according to Yanchilenko et al., 1988) as well as the 2 mg/L threshold corresponding to “very good” water quality according to the criteria of the International Office for Water (2003). This increase is probably linked to the intensive use of nitrogen fertilizers in agricultural areas of the watershed. The leaching of these fertilizers leads to a transfer of nitrates into the hydrographic network, promoting ecological imbalances such as eutrophication, with increased risks of algal proliferation and toxicity for aquatic fauna. In addition, nitrification, the process of transforming ammonia into nitrates, consumes a significant proportion of dissolved oxygen, which can worsen the chronic hypoxia already reported in other estuaries such as the Seine (GIP Seine-Aval, 2009). Phosphate concentrations are of particular concern, with values ranging from 13.9 mg/L (station 6) to 28.6 mg/L (station 1), well above environmental standards. According to the International Office for Water (2003), water is considered to be of "very good quality" when phosphates are less than 0.1 mg/L. The values observed in the Fatala estuary therefore classify these waters in the "poor quality" category. This contamination is probably linked to the excessive use of phosphate fertilizers, the residues of which, not absorbed by crops, are carried into aquatic environments by runoff or infiltration. Possible untreated domestic discharges may also contribute to this high phosphate load. Dissolved oxygen concentrations ranged from 6.6 mg/L to 7.3 mg/L. Stations located upstream, particularly stations 3, 4, and 5, had the highest levels (7.1, 7.2, and 7.3 mg/L), likely due to high photosynthetic activity associated with riparian vegetation. The lowest values (6.6 mg/L) were observed downstream at stations 7 and 11, areas subject to higher temperatures, thermal stratification, and a higher organic load. According to the International Office for Water (2003), these concentrations indicate overall "good" water quality, although below the threshold of 8.0 mg/L recommended by the Canadian Council of Ministers of the Environment (CCME, 1996) for marine and estuarine waters. This discrepancy highlights a certain vulnerability of the ecosystem to increasing environmental stress. The highest salinities were recorded at the mouth of the estuary, particularly at stations 7 to 11, where they reached up to 1000 mg/L. This high salinity results from the direct influence of marine waters, with seawater containing an average of 35 g of dissolved salts per kilogram of water (Hervé, 2017). In contrast, the upstream areas have significantly lower salinities, with the lowest value being measured at station 2 (68.4 mg/L). This low salinity is likely linked to the continuous input of continental freshwater, whose salinity is generally less than 0.5 g/L (IFREMER, 2004). The longitudinal salinity gradient observed in the Fatala estuary is typical of estuarine systems, marked by saline intrusion downstream and progressive dilution upstream, although the values remain significantly lower than those observed in other estuaries such as the Loukkos estuary in Morocco (El Morhit et al., 2012), which could be explained by a higher river discharge, less marine intrusion or a specific geomorphological configuration of the estuary. 4. Qualitative study of copepods The systematic inventory of copepods revealed the presence of 59 species belonging to 2 orders (figure 11), 17 families (figure 12) and 27 genera (figure 13). World Journal of Advanced Research and Reviews, 2025, 27(03), 229-243 236 Figure 11 Proportion of the different orders of copepods Figure 12 Proportion of the different copepod families Figure 13 Proportion of the different genera of copepods World Journal of Advanced Research and Reviews, 2025, 27(03), 229-243 237 The taxonomy of the collected copepods highlights a clear dominance of the order Calanoida, which represents 89.83% of all identified individuals, compared to only 10.17% for the order Cyclopoida (Figure 11). This predominance reflects a community structure typical of pelagic environments, generally associated with open marine or coastal environments with high productivity. These results broadly confirm the observations of Sow et al. (2024), who also reported a strong dominance of the order Calanoida (73.67%) in the Tinguilinta River estuary. At the family level, 17 families were identified, among which Paracalanidae (15.25%), Calanidae (11.86%) and Oncaeidae (8.47%) stand out for their abundance (Figure 12). On the other hand, the families Calocalanidae, Metridinidae, Oithonidae and Spinocalanidae with 1.69% each, as well as Clausocalanidae and Euchaetidae (5.08% each), are among the least represented. This composition reflects a relatively balanced family diversity, dominated by a few families well adapted to local ecological conditions. At the generic level, 27 genera were identified. The most abundant were Paracalanus (11.86%), followed by Calanus (8.47%), then Acartia, Candacia, Centropages and Eucalanus, each with 6.78% (Figure 13). These genera, all from the order Calanoida, confirm its supremacy both taxonomically and functionally in the structuring of the planktonic community. Conversely, several genera appear poorly represented, with an abundance of 1.69% each, including Aetideopsis, Calanoides, Calocalanus, Clausocalanus, Euchirella, Metridia, Nannocalanus, Oithona, Pseudocalanus, Scolecithricella, Scolecithrix, Scottocalanus, Spinocalanus, Triconia and Undinula. This low representation could be explained by a more marked ecological specialization or by unfavorable environmental conditions (salinity, nutrient availability) in the area studied. 4.1. Quantitative study of copepods Figures 14a and 14b, Figure 15, Figure 16 and Table 2 present respectively the proportions of the different copepod species identified, the species densities per station, the Shannon index per station, as well as the overall copepod diversity index. Figure 14a Proportion of copepod species