Trace metal distributions within a Sitka eddy in the northern Gulf of Alaska

Matthew T. Brown, Sherry M. Lippiatt, Maeve C. Lohan and Kenneth W. Bruland

Limnol. Oceanogr., 57(2), 2012, 503-518 | DOI: 10.4319/lo.2012.57.2.0503

ABSTRACT: In the northern Gulf of Alaska, mesoscale, anticyclonic eddies have been implicated as a mechanism of the cross-shelf exchange of iron (Fe)-replete coastal waters with Fe-deplete subarctic Alaskan gyre waters. Based on existing hydrography and macronutrient distributions, a Sitka eddy sampled during August 2007 is divided into a surface eddy core, a shallow subsurface eddy core, and a deeper subsurface eddy core. The distributions of aluminum (Al), manganese (Mn), and Fe in the eddy are examined and compared to distributions at shelf stations similar to where the eddy formed as well as basin stations representative of the Fe-limited subarctic Alaskan gyre. Relative to basin stations, dissolved and particulate Al and dissolved Mn were elevated in eddy core waters. Reactive Fe concentrations within the shallow subsurface eddy core were nearly seven times greater than reactive Fe concentrations at similar densities at the basin stations. This shallow subsurface eddy core is likely mixed into the euphotic zone during storm-induced mixing as well as mixing during isopycnal relaxation as an eddy dies out. The flux of reactive Fe to surface waters from the shallow subsurface core of eddies in the Gulf of Alaska is a significant source compared to both remote and local atmospheric dust deposition. It is hypothesized that Fe supply from eddies in the eastern Gulf of Alaska shifts the core of the high-nutrient, lower-than-expected chlorophyll water farther west in conjunction with an “eddy desert” in that same region.

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