Hydrological conditions determine shifts of plankton metacommunity structure in riverine floodplains without affecting patterns of species richness along connectivity gradients

Abstract

In riverine floodplains, the hydrological connectivity of the aquatic habitats with the main channel is paramount for aquatic metacommunities, as it influences organism dispersal and the local environmental conditions. We assessed phytoplankton, rotifer and microcrustacean metacommunity structures and species richness along an empirical gradient of hydrological connectivity of the aquatic habitats with the main channel and compared them between high and low water levels of the main river. In addition to exploring the relative influence of hydrological connectivity, local environmental and spatial factors on metacommunity composition and the relationship between species richness and environmental and hydrological connectivity variables during both hydrological conditions. During high water levels, when high nutrient and suspended solid concentrations occurred in sites with higher hydrological connectivity and sites with opposite features and high macrophyte cover occurred in sites with lower connectivity, metacommunities showed Quasi-Nested structures (species-poor sites were subsets of species-richer sites). During low water levels, the environmental variation of the water bodies was not associated to the connectivity gradient and metacommunities showed Clementsian or Quasi-Clementsian structures: discrete groups of species replaced each other along the floodplain. Spatial factors generally were the most important for community composition; during high water levels the flood contributed to the spatial effect by transporting organisms among neighbor water sections, and during low waters it indicated dispersal limitation and/or unmeasured variables that are spatially structured. Species richness was not associated to the gradient of hydrological connectivity, no matter the hydrological condition, and showed stronger association with local factors during low than during high water levels, indicating a better match between organisms and the environment in the absence of recent floods. Our results show that the discharge regime of the river influences plankton metacommunities and their relationship with local environmental factors. This highlights the relevance of natural river water level fluctuations for aquatic communities in the present context of increasing river flow regulation worldwide.