Macro- and micro-nutrient flux to a highly productive submarine bank in the Gulf of Alaska: A model-based analysis of daily and interannual variability

Although the Gulf of Alaska is subjected to intense downwelling through much of the year, during early spring and summer, upwelling due to local wind stress curl can occur over major portions of the shelf, resulting in high production. Satellite observations indicate that shallow banks may have substantially elevated chlorophyll concentrations relative to surrounding waters during much of the summer. We use the Regional Ocean Modeling System (ROMS) and ocean observation data to examine circulation and stratification around Portlock Bank in the Gulf of Alaska, and to explore mechanisms contributing to interannual variability in the supply of iron and nitrate onto the bank in spring and summer. ROMS at 3-km resolution is coupled to a lower trophic level biology model for the Gulf of Alaska; the coupled model is driven by tidal forcing, sub-daily atmospheric forcing, freshwater runoff, and boundary and initial conditions from Simple Ocean Data Assimilation (SODA) products. Hydrographic observations were made as part of six surveys undertaken by the GLOBEC/NEP (Global Ocean Ecosystem Dynamics/Northeast Pacific) program. Modeling results suggest that iron supply to the shallow layer around Portlock Bank is controlled by both advection and vertical diffusion processes, while nitrate supply is dominated by tidally-induced vertical diffusion. Overall, higher chlorophyll concentration in summer around Portlock Bank is attributed to strong vertical mixing, which pumps nutrients onto the bank from the flanks on either side, and from the top of the bank into the euphotic zone. Recirculation attributed to tidal effects increases residence time over the bank, further enhancing potential production, but intense mixing atop the bank in early spring can lead to light limitation of phytoplankton production. We used July 2004 cruise data from Portlock Bank to help verify model results.