RESPONSES OF MARINE-PHYTOPLANKTON IN IRON ENRICHMENT EXPERIMENTS IN THE NORTHERN NORTH-SEA AND NORTHEAST ATLANTIC-OCEAN

Shea-term iron enrichment experiments were carried out with samples co llected in areas with different phytoplankton activity in the northern North Sea and northeast Atlantic Ocean in the summer of 1993. The res earch area was dominated by high numbers of pico-phytoplankton, up to 70,000 ml(-1). Maximum chlorophyll a concentrations varied from about 1.0 mu g 1(-1) in a high-reflectance zone (caused by loose coccoliths, remnants from a bloom of Emiliania huxleyi) and about 3.5 mu g 1(-1) in a zone in which the phytoplankton were growing, to about 0.5 mu g 1 (-1) in the northeast Atlantic Ocean. From the high-reflectance zone t o the northeast Atlantic Ocean, nitrate concentrations increased from 0.5 mu M to 6.0 mu M. Concentrations of reactive iron in surface water showed an opposite trend and decreased from about 2.6 nM in the high- reflectance zone to < 1.0 nM in the northeast Atlantic Ocean. In the r esearch area, no signs of true iron deficiency were found, but iron en richments in the high-reflectance zone, numerically dominated by Synec hococcus sp., resulted in increased nitrate uptake. Ammonium uptake wa s hardly affected. Strong support for the effect of Fe on cell physiol ogy is given by the increase in the f-ratio. Net growth rates of the p hytoplankton (changes in cell numbers over 24 h) were almost unchanged . Phytoplankton collected from the northeast Atlantic Ocean, did not s how changes in the nitrogen metabolism upon addition of iron. Net grow th rates in these incubations were low or negative, with only slightly higher values with additional iron. (C) 1998 Elsevier Science B.V. Al l rights reserved.