Interactions of Bacterio- and Phyto-Plankton in Nutrient Cycling Within Eutrophic Heywood Lake, Signy Island

Summary

Recent estimates of whole lake carbon budgets within eutrophic Heywood Lake and oligotrophic Sombre Lake, have revealed a significant disparity between production (as measured by ¹⁴C) and turnover values in the eutrophie system. Whereas the turnover of photosynthetically fixed C in Sombre Lake could be adequately explained in terms of bacterial decomposition, grazing, sedimentation and loss to the outflow, up to 70% of Heywood Lake production remained unexplained by the above processes. In Heywood Lake, carbon dioxide can become limiting to phytoplankton populations at certain periods of the production season. Bacterial breakdown of algal debris and extra-cellular products of photosynthesis (detected by ¹⁴C method) is too slow to generate the necessary levels of carbon dioxide, so a second extra-cellular DOC pool comprising small readily assimilable substrates was postulated. Using a variety of techniques, evidence was found to support this proposal. The possibility that N and P are also rapidly recycled was investigated using axenic algal cultures. Preliminary experiments indicated that bacteria were having a beneficial effect on algal production in both N and P-depleted environments. It is suggested that such mechanisms could explain the high production values recorded from enriched Antarctic lakes.