Iron limitation on the northern Gulf of Alaska shelf: model and field observations of affects of circulation on the timing and magnitude of production (Invited)

The GLOBEC program was tasked with understanding the mechanistic links between climate forcing and the ocean-ecosystem response on the northern Gulf of Alaska shelf. To address this task, samples were collected 5-6 times annually along the Seward Line between 1998 and 2004. However, interpreting Seward-Line field observations in space and time is complicated by the complex circulation on the GOA shelf. The Alaska Current/Alaskan Stream and Alaska Coastal Current produce eddies and meanders which mix the iron-limited small-cell oceanic community with the iron-rich large-cell coastal community. Thus observations at any point in space and time are the result of the degree of mixing of the oceanic and coastal communities. The ROMS circulation model with embedded ecosystem component was used to extend GLOBEC observations in space and time on the GOA shelf and to provide quantitative implications for our conceptual models of system functions. Results indicate that the timing and magnitude of the spring bloom on the outer shelf is influenced by the degree of mixing of oceanic and coastal water, while the timing of the bloom on the inner shelf is related to thermal and salinity stratification. Outer shelf circulation is influenced by shelf-break eddies propagating westward. Simulations must have sufficient resolution to resolve eddies and meanders for model results to approximate field observations on the GOA shelf. Simulations indicate primary production in the Seward Line region is about 130 g C m-2 y-1, but production of up to 300 g C m-2 y-1 is predicted for Lower Cook Inlet and regions around Kodiak.