Iron stress in the Pacific region of the Southern Ocean: evidence from enrichment bioassays

The effect of iron on phytoplankton physiology in a scarcely investigated part of the Pacific region of the Southern Ocean was studied during an expedition aboard RV 'Polarstern'. Physiological effects of iron enrichments were studied during short-term bottle experiments (72 h) in high-nutrient, low-chlorophyll (HNLC) water samples. Throughout the expedition (March to May 1995), surface water nitrate concentrations were high (21 to 27 mu M) and surface water silicate concentrations varied considerably north (<3.5 mu M) and south (>13 mu M) of the Polar Front, whereas low chlorophyll a concentrations (<0.2 Pg l(-1)) were measured in the research area. Small-sized cells (<5 mu m) dominated the phytoplankton community; diatoms were observed only occasionally north of the Polar Front. Total phytoplankton numbers, as determined by now cytometry, ranged from 1500 to 8000 cells ml(-1). Ambient dissolved iron concentrations in the experimental area were low and ranged from 0.1 to 0.4 nM in surface waters. In the experimental series, indications of iron stress in the indigenous phytoplankton assemblage were found. Whereas the species composition remained virtually unaltered upon addition of 2 nM iron, significant increases in nitrate uptake and enhanced nitrate reductase activities were observed within 32 h after the start of the experiments. Ammonium uptake, which prevailed over nitrate uptake, remained unaltered by the iron additions. In addition to the physiological effects, a molecular marker of iron stress, flavodoxin, was detected in phytoplankton protein extracts collected after 72 h of incubation. Irrespective of iron deficiency, silicate concentrations in the northern part of the research area were likely below half-saturation values for growth for diatoms and thereby likely prevented diatom blooms. The results of this study indicate that the nano-phytoplankton in the Pacific region of the Southern Ocean are iron stressed; upon iron enrichment, a physiological stimulation is observed. Our results support a lack of iron as one of the causes of HNLC conditions in this region.