Ecology and biogeochemistry in the Atlantic sector of the Southern Ocean during austral spring: the first JGOFS expedition aboard RV `Polarsternʹ

Investigations of phytoplankton spring bloom development and its biogeochemical impacts in different water masses of the Atlantic sector of the Antarctic Circumpolar Current were carried out during RV `Polarsternʹ cruise ANT X/6 as part of the international Southern Ocean JGOFS programme. The regions investigated were the Polar Front region, the southern branch of the Antarctic Circumpolar Current (sACC) and the Marginal Ice Zone (MIZ), between 48°S and 60°S along the 6°W meridian from 29 September to 30 November 1992. This paper summarises the major findings of the cruise and complements the paper of Smetacek et al. [Smetacek, V., Bathmann, U.V., de Baar, H.J.W., Lochte K., Rutgers van der Loeff, M.M., 1997a. Ecology and biochemistry of the Antarctic Circumpolar Current during austral spring: results of the JGOFS expedition ANT X/6 aboard RV `Polarsternʹ. Deep-Sea Res. II, 44, 1–21; Smetacek, V., de Baar, H.J.W., Bathmann, U.V., Lochte, K., Rutgers van der Loeff, M.M. (Eds.), 1997b. Ecology and Biogeochemistry of the Antarctic Circumpolar Current During Austral Spring: Southern Ocean JGOFS Cruise ANT X/6 of R.V. `Polarsternʹ. Deep-Sea Res. II, 44, 519 pp.]. In the surface waters of all regions a recycling community with low biomass but high turnover rates was present. This had little impact on large scale biogeochemical cycles. Superimposed on this community were diatom blooms formed by large species in the region of the Polar Front. It was these massive diatom blooms that caused biogeochemical export to the deep ocean. Physical stability of the upper water layers was recognized as the most important factor in determining initiation and development of these blooms. Their growth was favoured by relatively high natural iron availability. Intrinsic factors in diatom life cycle and reproduction may have triggered the demise of blooms accompanied by massive sedimentation events. Feeding by larger zooplankton in general appeared minimal, except for the occurrence of salps in the low productivity zone of the sACC. Seasonal warming and ice-retreat in the sACC and MIZ caused outgassing of carbon dioxide, but there was clear CO2 draw-down by the diatom blooms in the iron-rich Polar Frontal region. Top predators had negligible direct impact on the CO2 budget but would have affected population dynamics of the plankton community. Thus, it was not the community with the highest turnover rates (microbial network) but the species which yielded most biomass (diatoms) which had the greatest impact on export of C, N and Si in this part of the Southern Ocean, and is therefore most important on biogeochemical cycling on global scales.