IRON ENRICHMENT EXPERIMENTS IN THE SOUTHERN-OCEAN - PHYSIOLOGICAL-RESPONSES OF PLANKTON COMMUNITIES

The physiological responses of plankton to iron enrichment were invest igated in experiments performed in 20-1 culture vessels. Natural phyto plankton communities in sea water, with mean ambient Fe concentrations ranging from 0.3-0.4 nM in the Antarctic Circumpolar Current to 1.2-1 .9 nM in the Polar Frontal region, were incubated for several days. Up on addition of 2 nM of iron, synthesis of chlorophyll a and nutrient u ptake was stimulated. The specific nitrate-uptake rates as determined by N-15-uptake experiments consistently increased, as well. as the rat ios of chlorophyll a to particulate carbon. Growth rates in iron-enric hed bottles were consequently enhanced relative to control bottles. Th e biochemical composition of the plankton community, indicated by carb on to nitrogen ratios and fatty acid composition, remained unaffected by iron addition. On the basis of C-14 incorporation into the major bi ochemical pools, no changes were observed in the allocation of carbon into proteins, polysaccharides, lipids and low-molecular-weight metabo lites in the particulate fraction. Antarctic phytoplankton endures the low ambient iron concentrations by maintaining physiological processe s at lower activity rates, whereas the biochemical composition of the plankton remains virtually unaffected. (C) 1997 Elsevier Science Ltd.