Environmental modulation of the plankton community composition and size structure along the eutrophic intertidal coast of the Río de La Plata estuary, Argentina

MAXIMILIANO D. GARCIA; NICOLÁS BONEL

JOURNAL OF LIMNOLOGY

CNR IST ITALIANO IDROBIOLOGIA

Lugar: Pallanza; Año: 2014 vol. 73 p. 562 - 573

1129-5767

In this study we investigated the spatial distribution of the plankton community (bacterio-, phyto- and zooplankton) in relation to environmental conditions along the intertidal coast of the Río de la Plata estuary, Argentina. Plankton was analyzed in terms of species composition, abundance, biomass (carbon content) and size structure. We aimed to evaluate the potential effects of anthropic impacts (nutrient enrichment) and physicochemical gradients along the shore (salinity, turbidity) on the composition and functioning of the plankton. We asked whether the natural structuring of the plankton by salinity and turbidity, known to be true of estuaries, is modified by eutrophication along the shoreline studied. We found that the density and biomass of the bacteria and phytoplankton were strikingly enhanced by high eutrophication levels along the intertidal southwest coast of the Río de la Plata estuary. We also found that the highest zooplankton density in the most polluted area, but that the biomass showed a different distribution pattern. Nevertheless, when the zooplankton was analyzed according to size fraction, we found that the microzooplankton biomass was positively associated with smaller-sized phytoplankton groups and the most polluted study sites. Copepods were the major taxonomic groups that best represented the mesozooplankton biomass. We therefore expected that their distribution was modulated by the presence of their food items (large cells) which were, in turn, more abundant in the middle-outer zone. In contrast, we found that the highest biomass of copepods occurred at the innermost site of the estuary and with no significant association with other planktonic groups. Overall, this study highlights the noteworthy impacts of human activities modifying the functioning of this coastal ecosystem. The differences found in the taxonomic composition and size structure of the planktonic-community assemblage between the most polluted and least polluted sites constitutes an excellent baseline for considering plankton as an ecological indicator of water quality.