Kr. Timmermans et al., IRON STRESS IN THE PACIFIC REGION OF THE SOUTHERN-OCEAN - EVIDENCE FROM ENRICHMENT BIOASSAYS, Marine ecology. Progress series, 166, 1998, pp. 27-41
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.