Size fractionated phytoplankton production in two humic shallow lakes with contrasting coverage of free floating plants

Abstract

We conducted a 1-year survey in two humic shallow lakes from the floodplain of the Lower Paraná River, Laguna Grande Lake (LGL) and a relictual oxbow lake (ROL). We aimed to test two hypotheses: (1) the efficiency in light use of picoplankton (0. 2-3 μm) is greater as light restriction increases and (2) the contribution of picoplankton to the total productivity is higher when the total photosynthetic biomass is lower. We performed P-E curves for picoplankton and nano- and microplankton (>3 μm) using the 14C assimilation technique. The light environments of the water bodies differed mainly owing to the development of free floating plants on the surface of the ROL and the dominance of phytoplankton in LGL. Primary productivity patterns in LGL were seasonality driven whilst in the ROL they were related to the coverage of floating macrophytes, which promoted light limitation and a lower productivity. In LGL, nano- and microplankton were in general more productive and the relative contribution of picoplankton to the total phytoplankton production decreased with the increase in total photosynthetic biomass. Hence, our study extends previously observed patterns to subtropical shallow lakes, where seasonality and free floating plants may influence the dynamics of phytoplankton production.