Seasonal and event-driven changes in the phytoplankton communities in the Araçá Bay and adjacent waters

Abstract

Biomass and composition of phytoplankton are reported for the Araçá Bay (AB) and adjacent waters in the São Sebastião Channel (SSC) in relation to physical-chemical variables. The AB has unequivocal ecological and socio-economic importance but it is under current anthropic pressures that are expected to increase. Needed future management programs require establishing the natural scales of variability of the phytoplankton communities and main drivers. In this study, samplings were conducted every 5–6 weeks from September 2013 to August 2014 to evaluate the extent to which AB and SSC differ in time. The approach was: 1) to compare nutrient concentration and phytoplankton abundance and composition found at AB and adjacent SSC waters; 2) to associate significant changes in nutrient concentration and in phytoplankton composition with time and main meteo-oceanographic processes, such as changes in wind direction or intrusions of upwelled South Atlantic Central Water (SACW). Sampling design included two parallel sections of oceanographic stations (inside and outside AB) that were repeated in the morning and afternoon for 3 consecutive days. Temperature, salinity and chlorophyll-a fluorescence were measured vertically, and discrete water samples were collected for analyses of inorganic nutrients, chlorophyll (Chla) and enumeration of selected groups of phytoplankton. SSC water masses characteristics showed clear seasonality but also a significant hourly to daily variability driven by changes in wind speed and direction. Nutrients and Chla varied mainly on a scale of days, as did phytoplankton abundances and compositions and all variables compared were consistently higher at AB. Samplings were characterized in four groups: 1) warm and stratified waters, 2) cold and homogenous waters, 3) low salinity waters, and 4) an intermediary condition between groups 1 and 3. Diatoms were numerically important, including the potential bloom forming Pseudo-nitzschia spp, which were more frequent during warm and stratified conditions, when SACW intrusions were observed. Occurrences of Asterionellopsis spp and Trichodesmium spp were associated with changes in wind direction. Future monitoring programs in the region should combine short-time coastal observations and near-real-time regional hydrodynamic models. The results raise the awareness for the increasing urban expansion, inefficient sewage treatment and changes in the hydrodynamics of AB.