Abstract We evaluate how the concentrations of inorganic nutrients and chlorophyll a vary in a heterogeneous area (Equatorial SW Atlantic), covering a gradient from stations closer to the coast to others more distant associated or not with turbid-zone reefs. Vertical temperature (27.9 ± 0.10 °C; mean ± standard deviation) and salinity (36.2 ± 0.14) profles showed that the water column is well mixed (0-30 m depth). The oligotrophic condition was marked by low concentrations of phosphate (0.30 ± 0.22 µM) and dissolved inorganic nitrogen (0.64 ± 0.74 µM). Moreover, dissolved reactive silicon (DSi) was low in most samples (< 2.0 µM), but higher (>10 µM) in nearshore stations, probably related to continental runoff and/or resuspension of the bottom sediments. The pelagic phytoplankton biomass indicated that chlorophyll a (0.25 ± 0.08 µg L-1) was low, positively correlated with light and negatively correlated with nutrients, indicating possible phytoplankton uptake. Chlorophyll a concentrations were lower in stations closer to the coast and higher in some stations near the reefs, indicating that the latter could be more prone to phytoplankton development and depletion of nutrients, especially DSi. Therefore, although oligotrophy is present along this coast, we found some unexpected heterogeneity of nutrient and chlorophyll a distributions, which were probably infuenced by benthic-pelagic coupling due to the presence of extensive reefs, sponge gardens (18-30 m depth), and the proximity to the coast. These results highlight the importance of understanding the heterogeneity of ocean productivity, especially in lesser known low-latitude areas, which showed distinct nutrient and chlorophyll a levels related to the occurrence of tropical reefs that are capable of supporting important fsh stocks and unique biological communities.

Resumo Neste estudo foram avaliados os efeitos do aumento da vazão fluvial sobre o fitoplâncton e os nutrientes em quatro estuários do Nordeste do Brasil. Para isso, foram coletadas amostras durante um período de forte déficit hidrológico (Nov/2010, período seco), e durante um período chuvoso (maio/2011). Dois estuários estão localizados em áreas urbanas (Ceará - CE e Cocó - CO) e dois têm forte déficit de água doce (Pacoti - PAC e Priangi - PIR). Os efeitos do aumento da descarga fluvial foram diferentes entre os que recebem descargas antrópicas (CO, CE e PIR) e aquele menos impactado (PAC). No CO e CE, o fluxo fluvial no período chuvoso diluiu os nutrientes e controlou as florações fitoplanctônicas. No PIR, o fósforo e o nitrogênio inorgânico decresceram neste período. Foi observado um aumento de todos os nutrientes no PAC, e de NT no PIR, seguidos de um aumento do fitoplâncton. No período chuvoso, espécies dulcícolas de ambientes eutróficos dominaram nos estuários. No período seco, observaram-se florações de Cryptomonas/Rhodomonas (CE) e Synechocystis aquatilis (CO). No período seco, sob a condição de hipersalinidade, predominaram espécies marinhas no PAC e PIR. A Análise de Correspondência Canônica sugeriu a salinidade, a sílica e o NT como os principais responsáveis pela variação do fitoplâncton.

Abstract This study evaluated changes in phytoplankton and nutrients after an increase in river discharge in four estuaries of northeastern Brazil. Samples were taken during a markedly freshwater-deprived period (Nov/2010, dry season), and during a rainy season (May/2011). While two estuaries are located in urban areas (Ceará - CE and Cocó - CO), the others are markedly freshwater-deprived (Pacoti - PAC and Pirangi - PIR). Our findings indicate that increased freshwater had opposing effects on estuaries impacted by anthropogenic discharges (CO, CE and PIR) compared with the other, less impacted (PAC). In CO and CE, the freshwater flow during the rainy period had a nutrient diluting effect and controlled phytoplankton blooms. In PIR, only phosphorus and inorganic nitrogen decreased in the rainy season. In contrast, an increase of all nutrients was observed in PAC, and of TN in PIR, followed by an increase in phytoplankton biomass. In the rainy season, eutrophic freshwater species dominated in all the estuaries. In the dry season, blooms of Cryptomonas/Rhodomonas and Synechocystis aquatilis were, respectively, observed in CE and CO. Also in the dry season, PAC and PIR had a predominance of marine species under a condition of hypersalinity. Canonical correspondence analysis suggested the variation observed for salinity, silica, and TN are the main driver of phytoplankton structure.