Limnetica 32

Disturbance is a concept used to explain the structure of communities. This concept emphasizes the effects of disturbance on the “break” of biomass stability in stable environments and the opportunities for the development of other species. The major disturbances in natural warm monomictic lakes include the contrasting effects of seasonal changes and long-term in thermal stability, as well as short-term changes in the mixing layer. This study investigated the effects of disturbance and abiotic environmental factors on the biomass of desmids, based on samples collected monthly from January 2002 through December 2006 in the limnetic region of two natural lakes, Carioca (small and shallow) and Dom Helvécio (large and deep), located in Rio Doce State Park, Minas Gerais State, southeast Brazil. At Lake Dom Helvécio, the extensive banks of aquatic macrophytes, the lake’s dendritic shape and the period of clear water produced higher richness values and facilitated the occurrence of relatively large (maximum linear dimension > 20 µm) desmid species, primarily species belonging to Staurastrum and Staurodesmus. In contrast, less richness and a dominant group of relatively small (maximum linear dimension < 20 µm) desmid species, especially species belonging to the genus Cosmarium, were observed at Lake Carioca in conjunction with a sparse macrophyte cover. The lakes showed a seasonal thermal stratification characterized by high temperature (25-32 ◦C), thermal stability and higher desmid biomass. The stability of the epilimnetic desmid biomass was an indicator of the adaptive flexibility of the desmid species and the capacity of the biomass to recover after the mixing period.