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.

O estuário do rio Bacanga apresenta um comportamento hidrodinâmico com fluxo de marés limitado por uma barragem. Ele é considerado como um ambiente hipereutrófico que recebe diariamente altas cargas de esgoto doméstico sem tratamento. Este trabalho teve como objetivo avaliar a variação espaço-sazonal da comunidade fitoplanctônica e suas relações com parâmetros ambientais. Amostragens bimestrais foram realizadas em seis pontos fixos entre 2012 e 2013, obtendo valores dos parâmetros físico-químicos e biológicos (clorofila a, composição e abundância do fitoplâncton) para realização das análises estatísticas. Os resultados indicam que a comunidade fitoplanctônica é representada por diatomáceas, sendo Skeletonema costatum a espécie dominante responsável por pulsos de florações em abril e junho de 2012. O predomínio dessa espécie está relacionado aos elevados teores de silicato, pH e turbidez da água. Outros eventos de florações como da Euglena gracilis e Chlamydomonas sp. foram registrados em fevereiro de 2013, quando os teores de fósforo total estiveram elevados e as taxas de oxigênio dissolvido foram superiores. Os dinoflagelados, cianobactérias e a diatomácea Thallassiosira sp. apresentaram ampla distribuição no período de estiagem e estão altamente correlacionados com a salinidade, transparência da água e nutrientes. Desta forma, a distribuição da comunidade fitoplanctônica é mais definida sazonalmente que espacialmente

Abstract The Bacanga River Estuary has a hydrodynamic behavior and its tidal flow is limited by a dam. It is considered as a hypertrophic environment that receives daily high loads of domestic sewage without treatment. This study aimed to evaluate the spatial and temporal variation of phytoplankton community and its relationship with environmental parameters. Bi-monthly sampling campaigns were carried out at six fixed sites between 2012 and 2013. Physical-chemical and biological parameters were collected (chlorophyll a, phytoplankton composition and abundance) to perform the statistical correlations. The results indicate that phytoplankton community is mostly represented by diatoms, with Skeletonema costatum being the dominant species responsible for bloom in April and June of 2012. The dominance of this species is related to the high silicate concentrations, pH and turbidity. Other blooms events as well as the Euglena gracilis and Chlamydomonas sp. were recorded in February 2013, when the total phosphorus concentrations were high and the dissolved oxygen concentrations were higher. Dinoflagellates, cyanobacteria and diatom Thallassiosira sp. were widely distributed in the dry period and highly correlated with salinity, water transparency and nutrients. Hence, the distribution of phytoplankton community is more defined seasonally, rather than spatially.