Nutrients, seston and phytoplankton biomass in the São Sebastião inner continental shelf - summer of 1994 and spring of 1997

Abstract

The distribution of inorganic nutrients, seston and chlorophyll-a (Chl-a) in the São Sebastião inner continental shelf was studied through a mesoscale stations grid in the summer of 1994 and spring of 1997. In the summer, the South Atlantic Central Water (SACW) intrusion was weak, but the availability of inorganic nitrogen and seston in the Coastal Water (CW) was higher than during the spring due to the high runoff contribution caused by intense rain before the samplings. Phytoplankton biomass was low and the primary limiting factor for phytoplankton biomass development was probably the phosphorous availability, as indicated by the high N:P and 480/665 ratios in CW domain. In the spring, Chl-a concentrations were up to threefold higher than the summer ones, possibly favored by the higher light penetration into the water column and by the more intensive SACW presence over the inner shelf that promoted a better nutritional balance in the euphotic zone, increasing the active Chl-a percentual. The effect of the presence of the São Sebastião Island (SSI) was reflected in the seston distribution and composition (organic matter richer), which accumulated in the North side of the area due to the local dominant currents circulation pattern. The main axis of São Sebastião Channel is deeper and shows similar characteristics to the neighboring shelf, in terms of nutrients and Chl-a concentrations.