Size-fractionated primary production in the south Atlantic and Atlantic sectors of the Southern Ocean

Results are presented from size fractionated chlorophyll a (Chl a) and prim ary production studies along a transect between Antarctica and southern Afr ica during the second South African Antarctic Marine Ecosystem Study (SAAME S II), conducted in late austral summer (January to February) 1993. Total i ntegrated Chl a along the transect was highest in the vicinity of the Margi nal Ice Zone (MIZ) and Antarctic Polar Front (APF). At these stations, inte grated Chl a biomass was always > 25 mg Chl a m(-2) and was dominated by mi crophytoplankton. Although nominal increases in Chl a biomass were also ass ociated with the Subantarctic Front (SAF) and Subtropical Convergence (STC) , total Chl a biomass in these regions was dominated by nanophytoplankton. within the interfrontal regions, total integrated Chl a biomass was lower, generally < 25 mg Chl a m(-2), and was always dominated by nanophytoplankto n. An exception was found in the Agulhas Return Current (ARC) where picophy toplankton dominated. Total daily integrated production along the transect ranged between 60 and 436 mg C m(-2) day(-1). Elevated production rates wer e recorded at stations occupied in the vicinity, of the MIZ and at all the major oceanic frontal systems. The contributions of the various size fracti ons to total daily production displayed the same spatial pattern as integra ted biomass, with microphytoplankton being the most important contributor i n areas characterized by elevated phytoplankton biomass. Outside these regi ons, 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 m g C m(-2) day(-1). The increased production rates recorded in the MIZ appea red to result from increased water column stability, as indicated by a shal low 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 gr owth of large microphytoplankton.