Phytoplankton community composition and size-abundance spectra in areas of the Alborán sea with different hydrological characteristics

UNIVERSIDAD DE LAS PALMAS DE GRAN CANARIA DEPARTAMENTO DE BIOLOGÍA OCEANOGRAFÍA BIOLÓGICA PHYTOPLANKTON COMMUNITY COMPOSITION AND SIZE-ABUNDANCE SPECTRA IN AREAS OF THE ALBORÁN SEA WITH DIFFERENT HYDROLOGICAL CHARACTERISTICS Memoria de Investigación presentada por la Lcda. Dña. Iria Sala Martínez Directores: Dra. Dña. Lidia Yebra Mora y Dr. D. Jesús M. Mercado Carmona. Tutora: Dra. Dña. María Milagrosa Gómez Cabrera. Las Palmas de Gran Canaria, a 4 de Febrero de 2011 A mis directores, Lidia y Jesús, por su ayuda, orientación y apoyo a lo largo de estos meses. A mis compañeros de despacho que me han hecho sentir como en casa. En especial a Soluna, por todos esos conocimientos fitoplanctónicos que ha compartido conmigo. A mis vecinas al otro lado del pasillo, en especial a Elvira, por estar siempre dispuestas a alegrame el día. A mis amigos, en especial a Ángela y a Fer, por estar siempre ahí, contra viento y marea. A mi madre y a mi hermana, por creer en mi más que yo misma, por ayudarme a no rendirme. Os quiero! A mi padre. 1 Phytoplankton community composition and size-abundance spectra in areas of the Alborán Sea with different hydrological characteristics Iria Sala*, Lidia Yebra, Jesús M. Mercado and Soluna Salles Instituto Español de Oceanografía, Centro Oceanográfico de Málaga, Apdo. 285, 29640 Fuengirola, Málaga, Spain Corresponding author: E-mail: [email protected] Tel. +34-952-460-205, fax +34-952-463-808 ABSTRACT Size-abundance spectra (SAS) were analyzed in different phytoplankton assemblages of the Alborán Sea collected in areas with contrasting hydrological features (upwelling areas and anticyclonic gyre). Abundance of micro-plankton cells decreased following the hydrological gradient from the most productive stations towards offshore stations. This size-fraction of phytoplankton was dominated by diatoms. Pico-plankton followed an opposite gradient. Concordantly, the slope of the SAS trended to decrease from coastal areas (upwelling) towards the anticyclonic gyre. However, phytoplankton assemblages featuring similar SAS values presented a quite different taxonomical composition of their diatom communities. According to the previous available information about the taxonomical composition of the phytoplankton communities in the Alborán Sea, these differences in diatom composition are indicative of differences in productivity of the phytoplankton. Consequently, the utility of phytoplankton SAS as an indicator of changes in the phytoplankton communities of the Alborán Sea is discussed. Key words: size-abundance spectra, phytoplankton composition, Alborán Sea. 2 INTRODUCTION The size structure of the phytoplankton communities has been proposed by Legendre and Le Févre (1991) as a tool to determine whether the energy in the pelagic trophic web flows through a “microbial” or an “herbivore” pathway. It is normally considered that the phytoplankton community is constituted predominantly by small autotrophic organisms in stratified and oligotrophic waters where the microbial food web the predominant pathway to the energy flow (Azam et al., 1983). In contrast, in turbulent and richer areas, where the large phytoplankton cells are comparatively more abundant, the herbivore pathway is assumed to be predominant (Steele, 1974; Delgado, 1990). Numerous subsequent studies demonstrate that this paradigm is an over-simplification of the factual situation normally found in the ocean. Different studies indicate that changes in larger size fractions of phytoplankton are responsible for most of the variability in total phytoplankton biomass while the abundance of picophytoplankton is relatively constant (Raimbault et al., 1988; Rodríguez et al., 1998 among others). Consequently, the microbial food web is in a continuous functioning while the herbivorous food web is relevant when nutrients are injected into upper layers of the water column (Thingstad and Sakshaug, 1990; Arin et al., 2002). If large cells contribution is responsible of the enhancements in productivity, changes in total primary production should always be associated with modifications in size distribution and food-web structure. The Alborán Sea is the westernmost basin in the Mediterranean Sea, being the first basin for inflow of Atlantic water trough the Gibraltar Strait. The superficial inflow of Atlantic water through the Strait of Gibraltar produces a system with two quasi-persistent anticyclonic gyres (Parrilla and Kinder, 1985; Minas et al., 1991; Tintoré et al., 1991), which occupy the entire central part of the Alborán Sea. Besides, the Atlantic water jet produces an intensive geostrophyc front in the north-western sector of the basin (Parrilla 3 and Kinder, 1987; Sarhan et al., 2000) that causes intermittent events of upwelling of deep Mediterranean water enriched in inorganic nutrients off the Málaga coast (one of the most productive areas in the western Mediterranean Sea), in contrast with the more oligotrophic waters found at the centre of anticyclonic gyre. The remarkable hydrodynamism found in the Alborán Sea offers a suitable scenario for the study of the effect of hydrological variability on the size structure of the phytoplankton assemblages (Arin et al., 2002; Ramírez et al., 2005; Mercado et al. 2005, 2006 and 2007; among others). In this study we compared the phytoplankton composition and the size-abundance spectra of the phytoplankton communities growing under contrasting hydrological conditions found in different areas of the Alborán Sea, in order to evaluate the contribution of different phytoplankton groups to the size structure of the communities and to assess the hypothesis that changes in size-structure are mainly due to larger cells. MATERIAL AND METHODS The data analysed in this work were collected in the North Alborán Sea (35º 54’ – 36º 33’ N and 4º 11’ – 4º 54’ W) during a oceanographic survey performed in the framework of Project Nitroalboran that was conducted between 9th and 14th of May 2008 on board R/V García del Cid. Ten stations distributed into two transects following a gradient from the coast to the open sea were analysed (Fig. 1). At each sampling station, a SeaBird25 CTD was deployed between 0 and 100 m to obtain vertical profiles of salinity, temperature and fluorescence. Water samples were obtained at superficial and maximum subsurface fluorescence (MSF) depths using 12 L Niskin bottles. The surface sampling depth was established at 5 m for all stations, while the MSF depth was determined after examining the fluorescence profiles. Nutrients (NO3 and PO4) and chlorophyll a concentrations were analysed as described in 4 Ramiréz et al. (2005). Abundance of picoplankton and nanoplankton was determined by a Becton Dickinson FACScan flow cytometer (FC) from samples fixed with glutaraldehyde (1% f.c.) and frozen in liquid nitrogen (Vaulot et al., 1989). The biovolumes of Synechococcus, Prochlorococcus, nanoeukaryotes and picoeukaryotes were calculated using the values given in Ribes et al. (1999) for samples from the Northwestern Mediterranean Sea. Additional water samples were fixed in dark glass bottles with Lugol’s solution (2% f.c.) for analysis of abundance and taxonomy of microplankton. In the laboratory, 100 mL of the fixed sample were sedimented in a composite chamber for 48 h following the technique developed by Utermöhl (1958). Cells were counted at 200x and 400x with an inverted microscope Leica DMIL connected to a Leica DFC video-camera as described in Mercado et al. (2005). The taxonomic composition of the phytoplankton communities in each sample was determined from the image analysis (IA) performed with Leica Application Suite software. The species nomenclature was validated following Tomas (1997). Cell biovolumes were calculated from cell-size and shape by using the most appropriate geometric formulas (Table 1). In total, 1,040 cells of phytoplankton were measured. A mean value of biovolume was calculated for each species or group listed in Table 1. For some species (Pleurosigma spp., Disposals spp., Oxitoxum spp. and Dyctiocha spp.) only one cell could be measured. For the genus Nitzschia and Ceratium a mean biovolumen was calculated from the values obtained for the different species identified. In order to elaborate the size-abundance spectra (SAS), the taxonomic groups analysed (including those determined by cytometry and optical microscopy) were classified in octave (log2) size classes. Abundance of each size class in logarithm scale was represented versus the log10 of the corresponding octave size. Similarities among SAS and physical, chemical and biological variables were analysed by principal component analysis in each 5 transect (PCA; Savenkoff et al. 1995, Packard et al. 2000). Two PCAs were performed. In the first case, the variables were SAS and micro-, nanoand picoplankton groups. In the second case, the variables were SAS and the hydrological variables (temperature, salinity, nutrients concentration and total chl a). Before performing the PCA, variables were transformed by subtracting the mean and dividing by the SD. The software STATISTICA 7.0 software (Stat. Soft., Inc.) was used for all the analysis. RESULTS Surface temperature in transect A increased from inshore towards offshore stations, varying between 15.65 and 17.58 ºC (Fig. 2A). Opposite to this, salinity exhibited lower values at the offshore stations, ranging from 37.03 at station 1 to 36.60 at station 5, with the minimum value being obtained at station 3 (36.38), which matches with a subminimum of temperature. This horizontal distribution of temperature and salinity suggests the presence of different water masses between coastal and open ocean stations and the presence of a geostrophic front located at station 3. Hydrological features in transect B were similar since the lowest temperature and highest salinity were obtained at station 1 (Fig. 2B). Besides, the horizontal distribution of hydrology suggests the presence of the geostrophic front at station 2. In transect A, NO3 and PO4 concentrations showed similar variability patterns, increasing from station 1 towards station 3, where they reached the higher values at surface (thereafter SUP; 1.81 and 0.17 μmol·L-1, respectively) and at MSF depth (3.59 and 0.25 μmol·L-1, respectively; Fig. 3). Nutrient concentration decreased from station 3 to 5, reaching minimum values at station 5 (note that MSF depth increased towards open ocean). However, the variation range of NO3 concentration was higher than that of PO4. Moreover, in the case of NO3, its concentration decreased below the detection limit at stations 4 and 5 6 at SUP depth, and at station 5 at MSF depth. The highest Chl a concentration was obtained at station 1 (1.69 μg·L-1). This maximum matched with low values of nutrient concentration. The low values of Chl a at stations 4 and 5 coincided with the lowest values of NO3 concentration. In transect B, NO3 maximum concentration was obtained at station 1 (3.15 μmol·L-1 at SUP depth, and 3.38 μmol·L-1 at MSF depth; Fig. 3B), with a submaximun at station 3. Surface concentration at the other stations was below the detection limit. PO4 decreased from inshore towards offshore stations, varying between 0.19 and 0.05 μmol·L-1 at surface and between 0.17 and 0.11 μmol·L-1 at MSF depth. Chlorophyll a concentration presented a peak in station 2 at both surface (2.23 μg·L-1) and MSF depths (2.62 μg·L-1), coinciding with low values of NO3. Much lower concentrations of Chl a were obtained at the offshore stations (Fig. 3B). Diatom abundance presented similar distributions in both transects and depths, with higher values at station 2, and minimum values at station 4, except in the transect A at MSF depth, where the minimum values were obtained at station 3 (Fig. 4). Flagellates abundance presented a similar variation pattern, with the highest value being found at station 2 in both transects (Fig. 4). In contrast, dinoflagellates abundance was homogeneous along transects and depths, except in transect B at MSF depth where it followed a pattern similar to diatoms and flagellates (Fig. 4). The abundance of nanoand picoplankton was more variable than that of microplankton. Prochlorococcus abundance in transect A, increased from station 1 to 3, where it reached the maximum value (12.96·103 cells·mL-1 at SUP depth, and 23.38·103 cells·mL-1 at MSF depth), and then decreased towards offshore stations (Fig. 5). In transect B, the pattern was similar at both depths, but with a lower range of variability (Fig. 5). Synechococcus in transect A decreased from station 1 to 3 with values very similar at surface and MSF 7 depths. Its abundance peaked at station 4 SUP but decreased at station 5 SUP to match the value at MSF depth. In transect B, Synechococcus abundance increased from station 1 towards station 5, with a lower variability than in transect A. Picoeukaryotes abundance peaked at stations 2 and 3 at SUP depth of transect A. However, at MSF depth, the variation pattern was different since a maximum value was obtained at station 1 (28.60·103 cells·mL-1). Nanoeukaryotes abundance in transect A showed a distribution similar to that obtained for diatoms and flagellates, with a maximum at station 2 (6.48·103 cells·mL-1 at SUP depth, and 6.48·103 cells·mL-1) and lower values at the offshore stations. However, in transect B, diatoms and flagellates decreased from inshore to offshore stations. Diatoms was the dominant group of microplankton (>45%) in transect A, followed by flagellates and dinoflagellates, except in the surface sample of station 4 and in the MSF sample of station 5 where dinoflagellates was the dominant group (Table 2). Contrarily, in transect B, diatoms dominance decreased from station 1 towards station 5, while the percentage of flagellates increased until being the dominant group in the samples collected at station 4 SUP and station 5 MSF (Table 2). Synechococcus was the dominant group within the picoplankton in transect A, followed by picoeukaryotes, Prochlorococcus and nanoeukaryotes. Only in the sample collected at station 4 MSF, Prochlorococcus was the dominant group. In transect B, the relative abundance of Synechococcus increased from inshore towards offshore stations (from 33.38% to 91.92% at SUP depth, and from 29.90% to 79.02% at MSF depth). This group was dominant in all the samples except at station 1 where picoeukaryotes were dominant (Table 2). Globally, SAS ranged from -0.70 to -0.88 in transect A and from -0.60 to -0.81 in transect B. There was not a homogeneous variation pattern of SAS with depth. The differences between surface and MSF depths were not statistically significant (Student t-test, p>0.05). Irrespective of these differences, SAS tended to decrease following the gradient coast- 14 REFERENCES Armbrust, E.V., Chisholm, S.V., 1992. Patterns of cell-size change in a marine centric diatom. Variability evolving from clonal isolates. Journal of Phycology, 28:146-156. Azam F., Fenchel T., Field J.G., Gray J.S., Meyer-Reil L.A. and Thingstad F., 1983. The ecological role of water-column microbes in the sea. Marine Ecology Progress Series 10, 257–263. Chisholm SW, 1992. Phytoplankton size. In: Falkowski PG, Woodhead AD (eds) Primary productivity and biogeochemical cycles in the sea. Plenum Press, New York, p 213–217 Cortés, D., Gil, J. and García, A, 1985. General distribution of chlorophyll, temperature and salinity in the north-western sector of Alboran Sea. Communication from the XXIX Congres-Assemblee pleniere CIESM. Lucerne, 11-19. Delgado, M., 1990. 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Biovolume determination of phytoplankton guilds in transitional water ecosystems of Mediterranean Ecoregion. TWB, Transit. Waters Bull. 2(2007), 83-102. Vaulot, D., Courties, C., Partensky, F., 1989. A simple method to preserve oceanic phytoplankton for flow cytometric ananlysis. Cytometry 10, 629–635. 18 FIGURE LEGENDS Figure 1. Sampling area: NW Alborán Sea. Figure 2. Temperature (ºC) and salinity variations along transects A and B at surface depth (5 m). Figure 3. NO3 (μmol·L-1), PO4 (μmol·L-1) and Chl a (μg·L-1) concentrations along transects A and B at surface (open circles) and maximum subsurface fluorescence (MSF, filled circles) depths. The numbers near the filled circles indicate the depth in meters of MSF at each station. Figure 4. Abundance distribution of diatoms, dinoflagellates and flagellates (cells·mL-1) along transects A and B at surface (open circles) and MSF (filled circles) depths. Figure 5. Abundance distribution of Prochlorococcus, Synechococcus, picoeukaryotes and nanoeukaryotes (103 cells·mL-1) along transects A and B at surface (open circles) and MSF (filled circles) depths. Figure 6. Size-abundance spectra (SAS) variations along transects A and B at surface (open circles) and MSF (filled circles) depths. Figure 7. Principal component analysis showing the relationship between SAS and the phytoplankton groups abundance (A); and between SAS and physical and biogeochemical variables (B). Figure 8. Community diatoms composition at each station and depth along transects A and B. 19 Figure 1. 20 Figure 2. 21 Figure 3. 22 Figure 4. A B 1400 10 50 ~ E 100 , o .¡ o 350 .. O 120 ~ • ~ , ' ;; 80 , , , 'ii , • m , , • ~ ;;: 40 ,. o .. , , ." , , ' . ... ' ". O ... '-;¡Z- ••• O 300 • " ~ 200 , ~ ;; , 'ii m 1 00 .. .!! , _. .... , , • • • .' O 1 2 3 • 5 1 2 3 • 5 StatioM A B 1400 10 50 ~ " E 1 00 , , o • , . ~ • , o , 350 O 120 ~ • ~ , ' ;; 8ll , , , '¡¡ , • m , , • V\>X ;;: 40 • o . 0 , . . " , • '_ ",,' 'O . O ... --;¡¡: . -• O 300 • " , , ~ 100 , , ~ ;; '¡¡ ~ 1 00 .. !! u.. , , -. • O 1 2 3 • 5 1 2 3 • 5 Stal io ns 23 Figure 5. 30 Table 2. Relative abundances of microplankton and nanoand picoplankton groups along transects A and B. Microplankton abundance (%) Nanoand Picoplankton abundance (%) Transect Station Diatoms Dinoflagellates Flagellates Prochlorococcus Synechococcus Picoeukaryotes Nanoeukaryotes A SUP 1 67.16 11.54 21.30 3.55 80.39 11.16 4.90 2 74.42 3.16 22.42 15.26 51.53 21.59 11.62 3 52.84 12.06 35.11 27.90 44.61 24.32 3.17 4 25.00 22.46 54.24 2.02 95.49 1.38 1.11 5 67.66 13.06 19.29 1.17 94.81 2.51 1.51 A MSF 1 61.10 7.54 31.36 2.81 52.47 37.84 6.88 2 86.36 2.67 10.97 8.69 55.24 21.48 14.59 3 53.44 9.52 37.04 46.44 34.47 14.26 4.84 4 45.13 10.47 44.40 55.96 5.91 31.81 6.32 5 22.95 20.49 56.56 3.67 84.00 8.82 3.50 B SUP 1 71.45 1.96 26.60 8.88 33.28 48.41 9.43 2 84.90 2.47 12.64 10.95 48.85 34.00 6.20 3 51.58 15.76 32.66 22.36 53.45 19.68 4.52 4 36.73 22.04 41.22 0.55 94.18 4.58 0.69 5 26.40 21.60 52.00 2.41 91.92 4.72 0.94 B MSF 1 69.99 3.90 26.12 14.93 29.20 43.53 12.33 2 73.78 6.95 19.27 14.06 45.21 34.63 6.09 3 38.06 15.79 46.15 21.49 52.76 19.76 5.99 4 20.14 24.10 55.76 3.79 84.82 6.35 5.04 5 55.92 10.47 33.61 9.66 79.02 7.91 3.41 31 Table 3. Regression coefficients of the size-abundance spectra (SAS) model log along transects A and B at surface (SUP) and MSF depths. (*: p<0.05, **: p<0.01, ***p<0.001). Transect Station a SAS (b) R2 A SUP 1 4.087 -0.877 0.76*** 2 4.047 -0.736 0.72*** 3 3.389 -0.736 0.59** 4 3.397 -0.830 0.48* 5 3.948 -0.873 0.63** A MSF 1 3.669 -0.703 0.59** 2 3.983 -0.757 0.54** 3 3.566 -0.869 0.65** 4 3.610 -0.786 0.72*** 5 4.248 -0.885 0.79*** B SUP 1 3.704 -0.627 0.73*** 2 3.974 -0.601 0.71*** 3 3.622 -0.714 0.62** 4 3.743 -0.810 0,66** 5 3.794 -0.761 0.67** B MSF 1 3.706 -0.622 0.69** 2 3.850 -0.529 0.64** 3 3.413 -0.750 0.65** 4 3.184 -0.661 0.47* 5 3.576 -0.712 0.71***