Role of atelomixis in replacement of phytoplankton assemblages in Dom Helvecio Lake, South-East Brazil

Scale and frequency of changes in a lake's physical structure, light dynamics, and availability of nutrients are closely related to phytoplankton ecology. Since phytoplankton assemblages were first described, phytoplankton ecologists concluded that these assemblages provide insight into phytoplankton responses to environmental changes. Objectives of this study were to investigate ecology of phytoplankton during a complete hydrological cycle in the deepest natural lake in Brazil, Dom Helvecio, and to sort species into the list of assemblages, checking its accordance with environmental changes in a tropical system within the middle Rio Doce Lake district, South-East Brazil. Canonical Correspondence Analysis, t-test, Mann-Whitney U-test, and Kruskal-Wallis test were used to analyze climatological, environmental, and plankton data, which were obtained monthly in 2002. A new phytoplankton assemblage, N-A (atelomixis-dependent desmids), is suggested because atelomixis (robust movement of water occurring once a day) contributed to replacement of species in Dom Helvecio Lake. Stability of stratification, water chemistry, and composition of phytoplankton assemblages characterized two periods. The first period occurred in six rainy months (Jan-Mar and Oct-Dec) when the lake was stratified and phytoplankton was dominated by two assemblages: N-A and F. The second period occurred in six dry months (Apr-Sep) when the lake was nonstratified and phytoplankton was dominated by four assemblages: S2, X1, A, and L-O. Results suggest that phytoplankton in Dom Helvecio Lake was shaped by seasonal and daily changes of water temperature, even with its lower amplitude of variation within 2002 (El Nino year). These changes promoted water column stratification or mixing, reduced light, and increased nutrient availability. Temperature, therefore, is similarly important to phytoplankton ecology in tropical regions as it is in temperate ones. Sorting phytoplankton species into assemblages matched well with environmental changes and periods identified so it is also suggested that this can be further used as an appropriate tool to manage water quality when evaluating tropical lakes.