Ecological indexes and phytoplankton structure of urban reservoirs supplied by estuarine waters and under the effects of seasonality (Belém, Pará, Brazil)

Ecological indices are used in the study of phytoplankton because they are related to their productivity and can describe the stresses caused by anthropogenic impacts on the aquatic environment. The aim of this study was to evaluate the phytoplankton structure and the application of ecological indices in the monitoring of water quality in public supply reservoirs in Belém (Pará). Monthly sampling occurred in four stations in the Bologna and Água Preta reservoirs from January to October/2019, to determine the abiotic and biotic factors and the calculation of ecological indices. Turbidity, transparency, precipitation and dissolved oxygen were the main abiotic variables that differentiated the reservoirs, evidencing the estuarine, seasonal and anthropogenic influence on species distribution. A total of 187 phytoplankton species were identified with representation of diatoms. In January, Água Preta reservoir presented lower diversity (2.14 ± 0.58 nats.ind-1), equitability (0.57 ± 0.14) and higher dominance (0.25 ± 0.19) with the species Trachelomonas volvocina in higher density, contrary to June that presented higher diversity and equitability and no species dominance. The Bolonha reservoir showed lower richness (8.3 ± 1.9), diversity (1.2 ± 0.5 nats.ind-1) and equitability (0.32 ± 0.12) with dominance of the species Gonyostomum sp. (0.55 ± 0.19). In this sense, the Água Preta reservoir was considered diverse with species well distributed in the environment, while the Bolonha reservoir showed low diversity and dominance of a phytoplankton species. Ecological indices describe well the spatio-temporal variation of phytoplankton in these reservoirs and are a good approach in monitoring environmental quality.