Distribution of Nutrients and Changes in Phytoplankton Composition in a Tropical Mesotidal Estuary, Northeastern Brazil

Abiotic parameters and phytoplankton were collected during 2010 and 2011 with the purpose of describing the phytoplankton distribution and the environmental characteristics. The diatoms were the most representative group in terms of species richness; in terms of density, the Cyanobacteria were more representative. Diversity and equitability were generally low in the estuary due to the dominance of Microcystis aeruginosa, an opportunistic and potentially toxic species of Cyanobacteria. The estuarine region is strongly impacted by high pollutant loads, especially nitrogen and phosphate compounds. Historical series of Apparent Oxygen Utilization (AOU) showed negative trends associated with changes in the estuarine system. The main biological components changed through 1999-2011 period. The dominance of the species changed from the Bacillariophyta in previous studies to the Cyanobacteria in our study. The species Microcystis aeruginosa, through its high density, dominance and frequency of occurrence, was the key species in the area.