Trophic state of an urban coastal lagoon (northern Brazil), seasonal variation of the phytoplankton community and environmental variables

Abstract Jansen Lagoon is an urban coastal area formed by damming the Ana Jansen Creek, which is located in the northwest of Sao Luis Island (northern Brazil) and is influenced by semidiurnal macro-tides. This study aimed to characterize the seasonal variation of the phytoplankton communities and its relationship with environmental variables to determine the trophic conditions in the Jansen Lagoon. Samples were collected bimonthly in five fixed sites in 2011 and 2012 to analyze biological and physicochemical data. A total of 55 taxa were identified including Bacillariophyta (25), Cyanobacteria (17), Chlorophyta (5), Charophyta (2), Euglenophyta (4) and Miozoa (2). The Closterium setaceum , Pleurosigma elongatum e Oscillatoria sp 3 occurrence had a positive correlation with the higher rainfall period, increase in dissolved oxygen concentrations and nutrients (DIP and NO 2 − ) and elevated trophic levels (TRIX). Cyclotella stelligera and Microcystis aeruginosa were strongly correlated with chlorophyll a concentration (microphytoplankton) under turbidity and wind speed influence. The trophic state of Jansen Lagoon ranged from eutrophic to hypereutrophic according to the TRIX index, and this poor water quality is explained by an intense eutrophication process in the rainy season due to elevated nutrient availability. Hence, to change the scenario presented in this study the reduction of point and non-point sources is necessary to slow down the eutrophication and growth of algal blooms in Jansen Lagoon.