Spatial heterogeneity of spring phytoplankton in a large tropical reservoir: could mass effect homogenize the heterogeneity by species sorting?

Reservoirs are river–lake hybrid ecosystems characterized by a marked longitudinal zonation and variable flushing rates depending on the use of stored waters. The structure of their phytoplankton is therefore subjected to the interplay between the environmental conditions of the different zones (species sorting) and the strength of the unidirectional flow (mass effect). The spatial distribution of spring phytoplankton was investigated in a tropical reservoir across its different zones. Phytoplankton displayed heterogeneous spatial patterns from the turbulent, nutrient-rich riverine zones to the relatively stable lacustrine zone. The analysis of this spatial heterogeneity revealed the relative importance of species sorting and mass effect in this morphologically complex reservoir. Different taxonomic groups showed different spatial patterns due to their specific physiological and morphological features, and as a result of the local environmental filtering. In the studied reservoir, the strength of the homogenizing effect of water flow did not smooth the spatial heterogeneity and played a weak role in constraining spring phytoplankton. Actually, water flow contributed to the spatial heterogeneity of phytoplankton by enhancing the local environmental differences caused by an operational drop in water level, and species sorting resulted as the primary forcing in determining spring phytoplankton spatial distribution.