Algal blooms and trophic state in a tropical estuary blocked by a dam (northeastern Brazil)

Abstract The Bacanga River Estuary is socioeconomically important due to artisanal fishing and aquaculture. It is blocked by a dam and is under human pressure along its drainage basin, intensifying the eutrophication process. This study reports on the occurrence of phytoplankton blooms and trophic state (TSI and TRIX) at six sampling sites during the annual cycle. The estuary was divided into downstream and upstream regions. Higher salinity, turbidity, depth, and lower dissolved oxygen levels were found downstream; whereas, high levels of chlorophyll a and nutrient concentrations were observed in both regions. There were blooms of Leptocylindrus danicus (1.45 × 106 cells L-1) and Skeletonema costatum (1.89 × 106 cells L-1) downstream; whereas phytoflagellate proliferation, such as those of Chlamydomonas sp. (13.17 × 106 cells L-1), Euglena gracilis (7.84 × 106 cells L-1), and Euglena proxima (1.03 × 106 cells L-1) were recorded upstream, with Chlamydomonas sp. as the discriminant species of this zone. Both trophic indices (TSI; TRIX) indicated elevated trophic conditions for the estuary, classifying it as hypereutrophic. Nevertheless, TSI only showed a significant relationship with some specific phytoplankton blooms. Thus, TSI seems to be the trophic index with a better response in the assessment of estuarine ecological functioning.