Size-fractionated phytoplankton biomass, production and biogenic carbon flux in the eastern Atlantic sector of the Southern Ocean in late austral summer 1997–1998

Size-fractionated chlorophyll- a (Chl- a ), primary production and biogenic carbon flux in the vicinity of the Winter Ice-edge Zone (WIE), Spring Ice-edge Zone (SIE) and Antarctic Polar Front (APF) were investigated in the eastern Atlantic sector of the Southern Ocean during a collaborative Scandinavian/South African Antarctic cruise conducted in late austral summer (December 1997/January 1998). Microphytoplankton (>20 μm) dominated integrated Chl- a biomass in the SIE and at the APF, comprising >50% of the total at all stations. Within the WIE, nanophytoplankton (2–20 μm) dominated total integrated Chl -a biomass comprising on average 36% of the total. Total integrated primary production ranged between 316 and 729 mg C m −2 d −1 at stations occupied in the region of the SIE, and between 292 and 317 mg C m −2 d −1 within the WIE. At stations occupied in the region of the APF, total integrated production ranged between 708 and 926 mg C m −2 d −1 . The contribution of various size fractions to total productivity generally displayed the same pattern as integrated Chl -a biomass. Microphytoplankton formed the most important contributor to total production at all stations in the SIE and at two stations in the region of the APF. Within the WIE, nanophytoplankton dominated total daily production. The elevated primary production rates in the region of the SIE and APF appear to be related to favourable light environment and the availability of iron. In the SIE, the partitioning of carbon between the microbial loop and classical food web was similar. Grazing activity by metazoans resulted in 1.5% of total daily production being transported to depth. In the region of the WIE and APF, the classical food web represented the main sink for daily primary production. Within the SIE and APF, the grazer-mediated carbon flux corresponded to 8.4% and 15.4% of the total production, respectively. The low rates of biologically mediated carbon flux resulting from grazing by zooplankton in the SIE can be related to the size structure of the phytoplankton assemblages and the absence of larger macrozooplankton in the region. As a consequence of the low grazing activity, the sinking of dead/senescent phytoplankton cells appears to be the major route for the transfer of carbon from the surface waters to depth in the SIE. Results of the investigation suggest that the efficiency of the biological pump in the three regions of investigation was largely determined by the zooplankton community composition and the structure of the local phytoplankton community.