Flagellates versus diatoms: Phytoplankton trends in tropical and subtropical Estuarine-coastal ecosystems

Abstract

Attempts to provide general patterns of phytoplankton and their regulating factors benefit from ecosystem comparisons, but these are strongly biased toward high-latitude environments of the northern hemisphere (> 20°N). In the present study, we compare the phytoplankton biomass and composition variability in two coastal environments in the southern hemisphere, the tropical Guanabara Bay, GB (23°S), and the subtropical Patos Lagoon Estuary, PLE (32°S), located on the South American southeast coast at the state of Rio de Janeiro and Rio Grande do Sul, respectively. These environments present contrasting features regarding the magnitude of anthropic impacts, the watershed size, geomorphology, and hydrology. Our goal was to identify the main factors that regulate the phytoplankton biomass and composition comparing data obtained at monthly intervals between the years 2011 and 2012 at a single station located in an area of significant water exchange in each environment. Surface water temperature, salinity, inorganic dissolved nutrients, chlorophyll a, phytoplankton biomass (carbon) and composition were analyzed. Phytoplankton biomass in the GB and PLE was dominated, respectively, by flagellates and diatoms, whereas cyanobacteria were more important in the former. Salinity was about twofold higher in the GB (mean 32.6 ± 1.5) than PLE (mean 15.4 ± 9.1) and, together with nutrient concentrations and their proportions, largely explained the observed different communities and much higher biomass in GB. GB presented strong eutrophication signals, with high ammonium and phosphate and lower, closer to limitation, silicate concentration. In contrast, high silicate concentration favored the predominance of diatoms in the PLE. Despite large environmental differences between both environments, the chlorophyll a presented a rather similar seasonal pattern, with maxima in austral summer/autumn and spring in both ecosystems. We suggest the seasonal pattern was associated to the incident light variation, but this hypothesis should be further explored.