Size-fractionated Primary Production in the South Atlantic and Atlantic Sectors of the Southern Ocean

Abstract

Results are presented from size fractionated chlorophyll a (Chl a) and primary production studies along a transect between Antarctica and southern Africa during the second South African Antarctic Marine Ecosystem Study (SAAMES II), conducted in late austral summer (January to February) 1993. Total integrated Chl a along the transect was highest in the vicinity of the Marginal Ice Zone (MIZ) and Antarctic Polar Front (APF). At these stations, integrated Chl a biomass was always >25 mg Chl a m–2 and was dominated by microphytoplankton. Although nominal increases inChl a biomass were also associated with the Subantarctic Front (SAF) and Subtropical Convergence (STC), total Chl a biomass in these regions was dominated by nanophytoplankton. Within the inter-frontal regions, total integrated Chl a biomass was lower, generally <25 mg Chl a m–2, and was always dominated by nanophytoplankton. An exception was found in the Agulhas Return Current (ARC) where picophytoplankton dominated. Total daily integrated production along the transect ranged between 60 and 436 mg C m–2 day–1. Elevated production rates were recorded at stations occupied in the vicinity of the MIZ and at all the major oceanic frontal systems. The contributions of the various size fractions to total daily production displayed the same spatial pattern as integrated biomass, with microphytoplankton being the most important contributor in areas characterized by elevated phytoplankton biomass. Outside these regions, nanophytoplankton dominated the total phytoplankton production. Again, an exception was found in the ARC north of the STC where picophytoplankton dominated total production. There, the lowest production along the entire transect was recorded, with total daily integrated production always <90 mg C m–2 day–1. The increased production rates recorded in the MIZ appeared to result from increased water column stability as indicated by a shallow mixed-layer depth. Within the inter-frontal regions, the existence of a deep mixed layer appeared to limit phytoplankton production. Low silicate concentrations in the waters north of the APF may also have limited the growth of large microphytoplankton.