Seasonal and inter-annual variability in phytoplankton over a 22-year period in a tropical coastal region in the southwestern Atlantic Ocean

Abstract Plankton time-series studies are necessary to improve our understanding of changes in aquatic ecosystems worldwide. Nevertheless, coastal tropical and sub-tropical regions long-term data series are scarce, especially in the Southern Hemisphere. Thus, the present study aims at identifying the factors controlling plankton variability at different temporal scales in a tropical coastal ecosystem in Brazil between 1987 and 2009. Three sampling stations were visited monthly at Ilha Grande Bay, a meso-oligotrophic coastal ecosystem in Rio de Janeiro, Brazil, to define the seasonal and inter-annual variability in the chlorophyll a (Chl a ) and nanoplankton and microphytoplankton abundances, and the main controlling factors. Salinity, water temperature, Secchi disk depth and inorganic nutrient concentrations were also measured. The dataset from the three sampling stations did not show significant differences for most of the measured variables; the stations were considered replicates. The mean annual Chl a values were −3 , and the mean phytoplankton densities were 5 cells L −1 . Both variables exhibited maximum values during the warmer and rainy season (November to March), with decrease in February, and minimum values during the colder and dry season (May to October). The microphytoplankton abundance, mainly diatoms, was high especially after the strong 1997/1998 El Nino event, whereas higher nanoplankton abundances were associated with La Nina events. In conclusion, over the short-term (seasonal), seasonal rainfall regime influences the phytoplankton, probably via the greater dissolved inorganic nutrients availability, especially nitrate and silicate. However, over the long-term (inter-annual), El Nino/La Nina events are responsible for the large-scale phytoplankton variability.