How free-floating macrophytes influence interactions between planktivorous fish and zooplankton in tropical environments? An in-lake mesocosm approach

In tropical climates, free-floating aquatic plants may dominate shallow lakes and affect trophic interactions between zooplankton and their predators have been presented as an important subject to be investigated. The present study used mesocosms within a tropical lake in rural São Paulo-Brazil to test the hypothesis that free-floating macrophytes (FFM) reduce predation pressure on prey, as these aquatic plants provide refuge and shade. Fish predation by Astyanax lacustris was observed with the presence and absence of the FFM Eicchornia crassipes over a period of 5 weeks. FFM promoted a more diverse community structure by providing new habitats, especially for small- to medium-sized littoral cladocerans. Tropocyclops prasinus meridionalis were more abundant with the presence of macrophytes. Furthermore, the size distribution of zooplankton differed between treatments at the end of the experiment, in which the larger organisms were less abundant in the treatment with FFM. The present study suggests that FFM affected the interactions between zooplankton and predators, but this influence occurred due to the macrophytes acting as refuge to smaller organisms rather than large-sized individuals. The results also indicated that the macrophytes influenced the zooplankton community by contributing to an increased species richness.