Environmental variables and phytoplankton community composition of Água Doce River, Maranhão, Brazil

Abstract

Estuaries provide food resources and habitat for several species, recreational opportunities, scientific and educational experiences and other important ecosystem services. We aimed to evaluate, seasonally and spatially, the physical, chemical and biological variables of the Água Doce River in the municipality of Água Doce, state of Maranhão. The samples were collected monthly, from July 2017 to June 2018, at six points, during high and low tide. In situ data of temperature, pH, salinity, turbidity, electrical conductivity and transparency of the water were recorded, and in the laboratory the values of N-NH4, N-NO2, N-NO3, total nitrogen, total phosphorus, total solids, dissolved oxygen, chlorophyll a, rainfall and phytoplankton community composition were recorded. Statistical analyses of variance and Spearman correlation were applied. Temporally, a dry period, July to December 2017, and a rainy period, January to June 2018, were evidenced. The Freshwater River showed warm, saline waters with low nutrient concentration. The phytoplankton community was represented by 22 generic taxa and 32 species distributed in the divisions Bacillariophyta (22), Dinophyta (eight) and Cyanophyta (two). The rainy season was the main factor of interference in the physicochemical dynamics and composition of the phytoplankton community, despite tidal fluctuations and anthropic actions.