Iron and nitrogen stress controls summertime biogeochemistry in the high‐latitude North Atlantic

Abstract

High‐latitude North Atlantic currents (60°–70°N) exhibit contrasting nutrient stoichiometries, but the spatial extent of post‐spring bloom iron (Fe) and nitrogen (N) stress on phytoplankton around Iceland remains poorly constrained. Here we pair in situ biogeochemical characteristics with 72 h Fe and N addition experiments in the East Greenland Current (EGC), Irminger Current (IC), Atlantic Current (AC), and East Icelandic Current (EIC) regions. Dissolved iron (dFe) was elevated in the EGC region and depleted in Atlantic surface waters. Unlike dFe, surface nitrate in the EGC region was depleted and elevated in Atlantic waters. Chlorophyll a (Chl a ) and particulate organic carbon standing stocks in the diatom and chlorophyte enriched EGC were ~ 40% lower than in the haptophyte dominated Atlantic waters. Changes in photophysiology during the incubation experiments revealed widespread Fe stress in the IC and in most of the AC stations. Notably, 80% of the experiments with nutrient stoichiometry suggesting Fe limitation (dFe −1 , N > 0.6 μ mol L −1 ) revealed Fe stress. In contrast, 30% of the experiments with nutrient stoichiometry suggesting N limitation (N μ mol L −1 , dFe > 0.2 nmol L −1 ) showed N stress, mainly in the EGC region. The EIC showed depleted concentrations of nitrate and dFe at the surface. Here, one incubation experiment revealed responses of combined N and Fe stress. The strong summertime density differences between the EGC and Atlantic water masses most likely limits the ability of advected dFe from Greenland and the Arctic to fertilize the Fe‐depleted Atlantic waters.

Bibliographic Information