Seasonal succession of cladocerans and phytoplankton and their interactions in a shallow eutrophic lake (Lake Vela, Portugal)

Lake Vela is a polymictic shallow lake exhibiting some characteristics typical of an advanced trophic state, namely, the permanently turbid water, the reduction in biodiversity, and the recurrent occurrence of Cyanobacteria blooms, which occasionally lead to large fish kills. This study was carried out in order to understand the seasonal variation of Lake Vela’s phytoplankton and cladoceran communities and their interactions under advanced eutrophic conditions. When comparing our results with general models of plankton succession observed in other temperate and tropical lakes we found some coherence between several seasonal events. Our results suggest that phytoplankton is mainly regulated by nutrients (“bottom-up” effect). However, the warm mediterranean temperatures had an important role in the phytoplankton succession, being responsible for the rapid and intense Cyanobacteria development in spring and summer. Our work also demonstrated that phytoplankton is one of the main factors responsible for the seasonal structure of the community of cladocerans, which are well related to changes in algae diversity and abundance, being the Cyanobacteria having major impact. Moreover, the occurrence of a massive Cyanobacteria bloom during the study, which induced anoxia and consequent fish kill, enhanced the structuring role that fish have on the cladoceran seasonal succession and the effects of this episode in the normal seasonal succession of plankton.