IRON STRESS IN THE PACIFIC REGION OF THE SOUTHERN-OCEAN - EVIDENCE FROM ENRICHMENT BIOASSAYS

The effect of iron on phytoplankton physiology in a scarcely investiga ted part of the Pacific region of the Southern Ocean was studied durin g an expedition aboard RV 'Polarstern'. Physiological effects of iron enrichments were studied during short-term bottle experiments (72 h) i n high-nutrient, low-chlorophyll (HNLC) water samples. Throughout the expedition (March to May 1995), surface water nitrate concentrations w ere high (21 to 27 mu M) and surface water silicate concentrations var ied considerably north (<3.5 mu M) and south (>13 mu M) of the Polar F ront, whereas low chlorophyll a concentrations (<0.2 Pg l(-1)) were me asured in the research area. Small-sized cells (<5 mu m) dominated the phytoplankton community; diatoms were observed only occasionally nort h of the Polar Front. Total phytoplankton numbers, as determined by no w 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, indicatio ns of iron stress in the indigenous phytoplankton assemblage were foun d. Whereas the species composition remained virtually unaltered upon a ddition of 2 nM iron, significant increases in nitrate uptake and enha nced nitrate reductase activities were observed within 32 h after the start of the experiments. Ammonium uptake, which prevailed over nitrat e 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 o f incubation. Irrespective of iron deficiency, silicate concentrations in the northern part of the research area were likely below half-satu ration values for growth for diatoms and thereby likely prevented diat om blooms. The results of this study indicate that the nano-phytoplank ton in the Pacific region of the Southern Ocean are iron stressed; upo n iron enrichment, a physiological stimulation is observed. Our result s support a lack of iron as one of the causes of HNLC conditions in th is region.