This study aimed to evaluate the biodiversity and temporal and spatial distribution of planktonic diatoms, as well as of diatoms present in the surface sediments of the water supply reservoirs of the Upper Cotia System (Pedro Beicht and Cachoeira da Graça reservoirs), to contribute with characterization of the ecological quality of these ecosystems. It also aimed to increase the ecological knowledge of diatoms from tropical and low antropogenic impacted reservoirs. The Upper Cotia Water Supply System is located in the Biological Reserve of Morro Grande (RFMG) and is considered as an environmental reference system for the Metropolitan Region of São Paulo (RMSP). Samples were taken in seven locations (five in the Pedro Beicht Reservoir and two in the Cachoeira da Graça Reservoi). Water samples were taken along a vertical profile during the winter (dry season) and summer (rainy season) and in the surface sediments during the winter. The analysis included physical, chemical and diatom planktonic and in sediment. The reservoirs were classified as mesotrophic, and characterized by the slight acidic waters, low conductivity, and low nutrients and chlorophyll-a concentrations. Geochemical information from surface sediments (2 cm, ~3,7 years) demonstrate high organic matter content originated from aloctonous vascular plants. The water and sediment characteristics suggest a natural eutrophication process. A total of 63 specific and infraspecific plankton taxa and 98 sediment taxa were found. The structural organization of planktonic diatoms was affected by the seasonal scale, as well by the reservoirs depth. During the winter, and mainly at the deep locations, three Aulacoseira species were favored by the water circulation. In the summer, and mainly in shallow locations, community was more diverse, with higher abundance of benthic species (Brachysira brebissonii, B. neoxilis, Navicula herbstiae, Stenopterobia delicatissima and S. curvula). Two Aulacoseira species (A. granulata and A. ambigua) were the most abundant and well distributed in the reservoirs, indicating the prevalence of turbulence and the water circulation. The distribution of species in the sediment was more sensitive to the reservoirs depth. Aulacoseira species prevailed, contributing with 21-47%, while Eunotia species were the second in importance, reaching at most 25% abundance. The comparison between planktonic communities and those present in the sediments revealed that the sediments integrate information from both climate periods, and incorporate planktonic and benthic species. Diatom communities highlight the good ecological quality of the reservoirs. However, we recommend a more intensifying and detailed monitoring program in the pumping station of Pedro Beicht, due to the high planktonic biomass reached during summer season. Finally, we emphasize the importance of the RFMG conservation for the protection of this water supply system and the maintenance of a reference site in the RMSP.
News published in Agência FAPESP Newsletter about the scholarship: