Description: The Southern Ocean is the world's largest high nutrient low chlorophyll (HNLC) region. However, satellite images highlight several areas associated with island chains and shallow topographic features which display high phytoplankton biomass. Here we present the first study of seasonal variations in phytoplankton biomass and iron availability in the Scotia Sea over both austral spring and summer seasons. Based on dissolved iron (dFe) and Chlorophyll a (Chl a) concentrations, the study area is be divided into three regions: North of South Georgia, south of South Georgia and the vicinity of South Orkney Islands. The Scotia Sea to the south of South Georgia exhibited low dFe concentrations (〈0.027-0.05 nM) in surface waters during both the spring and summer seasons. Nevertheless, nitrate concentrations were considerably lower in spring compared to summer (difference similar to 8 mu M). Summer Chl a concentrations were similar to 1.4 mg m(-3) and in situ phytoplankton populations displayed evidence of iron stress, suggesting the development of seasonal iron limitation. Surface water dFe concentrations in the South Georgia bloom waters (north of the islands) were elevated and slightly lower during spring than summer (0.20 nM compared to 0.31 nM, P 〉 0.05). Nitrate concentrations were 16 mu M lower in summer compared to spring, whilst Chl a standing stocks remained high. Enhanced dFe (similar to 0.25 nM) and Chl a concentrations were furthermore observed in the vicinity of the South Orkney Islands, located in the southern Scotia Sea. Iron addition experiments showed that in situ phytoplankton were iron replete spring and summer north of South Georgia and in the vicinity of South Orkney Islands during summer. We thus suggest that increased iron supply in high productivity areas including the area north of South Georgia and the South Orkney Islands, was sustained by a continuous benthic supply from their shelf systems, with a potential additional input from seasonally retreating sea ice in the South Orkney system

Description: 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.