Dissolved and particulate primary production and bacterial production in offshore Antarctic waters during austral summer: Coupled or uncoupled?

The dependence of heterotrophic bacteria on dissolved organic products released by phytoplankton was estimated in the Weddell and Scotia Seas during austral summer 1998. We used time-course experiments of ¹⁴C-bicarbonate assimilation and compartmental analysis to estimate primary production of total (TOC), particulate (POC) and dissolved (DOC) organic carbon. Three in situ incubations were also performed in Bransfield Strait. Phytoplanktonic biomass and production were within the ranges previously reported for offshore Antarctic waters, with mean values of 0.45 mg chlorophyll a m^-3 and 0.51 mg C m^-3 h^-1 for total primary production (0.45 and 0.06 mg C m^-3 h^-1 of POC and DOC, respectively). Percent extracellular release (PER = DOC/[POC+DOC]) averaged 13 % (range 5 to 33 %), a value comparable with those reported for lower latitudes. The production rate of DOC correlated positively with TOC and POC production rates, but not with chlorophyll a concentration, suggesting that availability of recently fixed photosynthate was a key factor regulating phytoplanktonic DOC release. As much as 82 % of the variance in bacterial heterotrophic production (BHP), estimated by ³H-leucine incorporation, was accounted for by dissolved primary production. BHP bore no relationship with other phytoplankton-related variables. Assuming a conservative bacterial growth efficiency of 14 %, as reported by recent work in Antarctic waters, our experiments indicate that phytoplanktonically produced DOC would suffice to meet bacterial carbon demand. These results suggest a strong coupling between phytoplankton and bacterioplankton through DOC release and uptake under non-bloom conditions in the Southern Ocean.