Influence of river discharge on phytoplankton structure and nutrient concentrations in four tropical semiarid estuaries

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.

摘要 本研究针对巴西东北部4处河口，探究了径流量增加后浮游植物与营养盐的变化特征。研究分别在显著淡水匮乏的旱季（2010年11月）与雨季（2011年5月）采集样本。其中2处河口位于城区（塞阿拉河口Ceará - CE与科科河口Cocó - CO），其余2处属于典型淡水匮乏型河口（帕科蒂河口Pacoti - PAC与皮兰吉河口Pirangi - PIR）。研究结果显示，相较于受人为排放影响较小的PAC河口，径流量增加对受人为活动排放影响的CO、CE及PIR河口产生了截然相反的生态效应：在CO与CE河口，雨季的淡水径流可起到稀释营养盐、抑制浮游植物水华的作用；在PIR河口，雨季仅磷与无机氮浓度出现下降。与之相对，PAC河口的所有营养盐浓度均显著上升，PIR河口的总氮（Total Nitrogen, TN）浓度亦随之升高，随后浮游植物生物量出现增长。雨季时，所有河口均以富营养淡水物种占主导；旱季时，CE河口与CO河口分别观测到隐藻属/红胞藻属（Cryptomonas/Rhodomonas）与水生集胞藻（Synechocystis aquatilis）形成的水华。同样在旱季，PAC与PIR河口处于高盐度环境，浮游植物群落以海洋物种为主导。典范对应分析（Canonical Correspondence Analysis, CCA）结果表明，盐度、硅酸盐与总氮的变化是驱动浮游植物群落结构演变的核心影响因子。