Drivers shaping phytoplankton diversity and composition in a humid Pampean floodplain lake (Natural Reserve)

We aimed to identify the driver/s of phytoplankton diversity (gamma, beta, alpha) and community composition in a Pampean floodplain wetland (Otamendi), using species and functional groups. We performed a seasonal regional survey (2004-2006, phytoplankton regime) across the different aquatic systems in the wetland. Gamma diversity was 254 species. Beta diversity was 2.53 in late spring, 2.49 in winter, and was lowest in summer (2.05) when the wetland was over flooded. Alpha diversity (mean richness) ranged between 29 and 50 species. Multiple regressions showed that phytoplankton alpha diversity (richness, Shannon-Wiener, evenness, and Simpson diversity index) responded to dissolved inorganic nitrogen, suspended solids, light attenuation, and pH. Nutrients also explained well the distribution of phytoplankton functional groups. Dissolved inorganic nitrogen concentrations were usually limiting in the shallow lakes, whereas phosphate concentration was always high. Light attenuation was mostly caused by high suspended solids and high humic substances. We also re-analyzed data from a survey performed during a free-floating plant regime (1998-2000); the dense macrophyte cover attenuates most incoming light into the water column. Compared with the phytoplankton regime, the free-floating plant regime had markedly lower chlorophyll, lower alpha, higher beta (high heterogeneity among habitats with and without macrophytes), and similar gamma diversity.