Phytoplankton Functional Groups in a Tropical Estuary: Hydrological Control and Nutrient Limitation

Hydrology and nutrients have been indicated as the main driving factors acting on phytoplankton biomass and composition in estuarine systems, although grazing may occasionally have some influence. In order to identify these factors over temporal and spatial scales, we analyzed physical, chemical, and biological properties of a tropical river-dominated estuary during the dry and rainy seasons.As far as we know, this is the first time that the functional groups approach has been used to analyze the changes in phytoplankton composition in an estuary. This recent framework is based on the tolerances and sensitivities in relation to environmental conditions of groups of species, which are labeled by alpha-numeric codes (Reynolds et al., J. PL Res. 24: 417 428, 2002). In the estuary of Paraiba do Sul River, all phytoplankton groups were represented by freshwater organisms, indicating the strong influence of the river. However, remarkable shifts in composition and biomass occurred from the low to high flushing seasons, due much more to the river discharge than to nutrient availability.The overall results showed no nitrogen, phosphorus, or silica limitation to phytoplankton growth (mean values: dissolved inorganic nitrogen= 30.5μM, soluble reactive- phosphorus = 1.45 μM, and silica= 208.05 μM). The higher river flow supports a lower phytoplankton biomass composed mainly of nanoplankton (< 20 μM) fast-growing functional groups, which are able to maintain biomass even in high flushing conditions (X₁) or large heavy organisms, such as some heavy diatoms of group P, which are able to be in suspension in shallow and turbulent systems. The lower river flow led to the coexistence of large organisms (> 20 μM) of the groups P and F, which include slowgrowing populations typically found in mesotrophic lakes.Although the functional group approach was originally developed for temperate lakes, our data support this approach for a tropical estuarine environment.