Iron limitation of the postbloom phytoplankton communities in the Iceland Basin

Measurements performed on a cruise within the central Iceland Basin in the high-latitude (>55 degrees N) North Atlantic Ocean during late July to early September 2007 indicated that the concentration of dissolved iron (dFe) in surface waters was very low, with an average of 0.093 (<0.010-0.218, n = 43) nM, while nitrate concentrations ranged from 2 to 5 mu M and in situ chlorophyll concentrations ranged from 0.2 to 0.4 mg m(-3). In vitro iron addition experiments demonstrated increased photosynthetic efficiencies (F-v/F-m) and enhanced chlorophyll accumulation in treatments amended with iron when compared to controls. Enhanced net growth rates for a number of phytoplankton taxa including the coccolithophore Emiliania huxleyi were also observed following iron addition. These results provide strong evidence that iron limitation within the postspring bloom phytoplankton community contributes to the observed residual macronutrient pool during summer. Low atmospheric iron supply and suboptimal Fe:N ratios in winter overturned deep water are suggested to result in the formation of this seasonal high-nutrient, low-chlorophyll (HNLC) condition, representing an inefficiency of the biological (soft tissue) carbon pump in the region.