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 rele ased by phytoplankton was estimated in the Weddell and Scotia Seas during a ustral summer 1998. We used time-course experiments of C-14-bicarbonate ass imilation and compartmental analysis to estimate primary production of tota l (TOC), particulate (POC) and dissolved (DOC) organic carbon. Three in sit u incubations were also performed in Bransfield Strait. Phytoplanktonic bio mass 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 mgC 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 rep orted for lower latitudes. The production rate of DOC correlated positively with TOC and POC production rates, but not with chlorophyll a concentratio n, suggesting that availability of recently fixed photosynthate was a key f actor regulating phytoplanktonic DOC release, As much as 82% of the varianc e in bacterial heterotrophic production (BHP), estimated by IH-leucine inco rporation, was accounted for by dissolved primary production. BHP bore no r elationship with other phytoplankton-related variables. Assuming a conserva tive bacterial growth efficiency of 14%, as reported by recent work in Anta rctic waters, our experiments indicate that phytoplanktonically produced DO C would suffice to meet bacterial carbon demand. These results suggest a st rong coupling between phytoplankton and bacterioplankton through DOC releas e and uptake under non-bloom conditions in the Southern Ocean.