Qualidade da água no reservatório de Lajes-RJ: ocorrências de Cianobactérias e Cianotoxinas

Abstract

The Lajes Reservoir deserves special attention due to its significant contribution to the supply system through the State Water and Sewage Company – CEDAE, with 5,5 m3/s, through two catchment points for the supplying of 1.6 million inhabitants distributed in 7 municipalities: Itaguai, Japeri, Paracambi, Queimados, Rio de Janeiro, Seropedica and Pirai. The growth of cyanobacteria is becoming more and more frequent, indicating the need for constant monitoring. Therefore, this study aimed to evaluate the factors and variables that favor the flourishing of cyanobacteria at the point near the Lajes Reservoir dam through weekly collection in four depths (surface of 5m, 10m and 20 m) during the period of january/18 – january/19. The parameters studied were: counting and identification of the species of cyanobacteria, nitrite, nitrate, ammonia, orthophosphate, total phosphorus, chlorophyll-a, dissolved oxygen, water temperature and to evaluate the occurrence of cyanotoxins, mycrocystin, saxitoxin and cylinderspermopsin. The reservoir of Ribeirão das Lajes presented, during the study, the occurrence of high densities of cyanobacteria having occurrences of approximately 150,000 to 250,000 cells.mL-1 and with peaks of 400,000 cell.ml-1, violating the limits established by CONAMA 357/05 for class 1 waters (20,000 cel.m L-1) in practically all year, with significant differences between climatological seasons, having presented lower values in winter. Of the identified genus of cyanobacteria, Aphanocapsa, Limnococcus, Lemmermanniella/Epigloeosphaera and Planktolyngbya limnetica alternated among the dominant genus, being influenced by weather seasons. Despite the occurrence of the genus Aphanocapsa throughout the study period, the presence of the evaluated cyanotoxins was not detected. The results obtained for the dissolved oxygen parameters and water temperature corroborate studies developed in the reservoir that observed a thermal stratification during the summer, autumn and spring, occurring de-stratification in the winter. The reservoir was classified as ultra-oligotrophic to oligotropic, presented low concentrations of chlorophyll-α, nitrogen and phosphorus, practically no ortophosphate concentrations were quantified, possibly being limiting to phytoplanktonic organisms. Although the classification of low degree by the Trophic State Index – EIT Despite the low trophic grade classified by the Tropical State Index – IET developed by the São Paulo State Company – CETESB, the reservoir was framed as moderately degraded by the Reservoir Water Quality Index developed by Paraná Environmental Institute – IAP. Although the water of the Lajes dam is considered to be of excellent quality and is located in a region where there are no signs of strong anthropogenic contamination, there is a need for a permanent concern with the occurrences of cyanobacteria, due to the increase in density over time and the presence of potentially cyanotoxin-producing genus.