Primary production and bacterial carbon metabolism around South Shetland Islands in the Southern Ocean

Phytoplankton and bacterioplankton dynamics were studied around South Shetland Islands (Antarctica) with special emphasis on the Drake Passage region, during austral summer, in order to expand our knowledge on the coupling between the autotrophic and heterotrophic microbial plankton compartments in polar ecosystems. In addition, we directly estimated bacterial growth efficiency in the Drake Passage with the aim of better constraining total bacterial carbon utilization in this important polar ecosystem. Integrated chlorophyll-a concentration (21–86 mg m−2), primary production rates (0.7–19.3 mg C m−3 d−1) and mean water-column photochemical efficiency (0.24–0.60) were significantly correlated with Si⁎ tracer (r2=0.55, 0.46 and 0.64, respectively), which indirectly points to iron as the major limiting factor for phytoplankton growth in the area. Bacterial production was considerably low (0.002–0.3 mg C m−3 d−1) and was best explained by chlorophyll-a concentration, protein-like fluorescence of dissolved organic matter and temperature (r2=0.53, p<0.001). Water temperature appeared to influence bacterial activity when organic substrate availability is high. Bacterial production accounted on average for only 3.9% of co-occurring primary production, which has been frequently interpreted as an indicator of the marked uncoupling between bacteria and phytoplankton in cold waters. However, using the experimentally derived mean bacterial growth efficiency for the photic zone (6.1±1.3%) the bacterial carbon demand represented on average 63±18% of concomitant primary production, similar to what is found in warmer productive waters. Thus, our study suggests that bacterioplankton and phytoplankton appear to be connected in this polar area.