National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2010

Quantifying cross-shelf and vertical nutrient flux in the Coastal Gulf of Alaska with a spatially nested, coupled biophysical model

Hermann, A.J., S. Hinckley, E.L. Dobbins, D.B. Haidvogel, N.A. Bond, C. Mordy, N. Kachel, and P.J. Stabeno

Deep-Sea Res. II, 56(24), 2474–2486, doi: 10.1016/j.dsr2.2009.02.008 (2009)

The Coastal Gulf of Alaska (CGOA) is productive, with large populations of fish, seabirds, and marine mammals; yet it is subject to downwelling-favorable coastal winds. Downwelling regions in other parts of the world are typically much less productive than their upwelling counterparts. Alternate sources of nutrients to feed primary production in the topographically complex CGOA are poorly known and difficult to quantify. Here we diagnose the output from a spatially nested, coupled hydrodynamic and lower trophic level model of the CGOA, to quantify both horizontal and vertical nutrient fluxes into the euphotic zone. Our nested model includes both nitrogen and iron limitation of phytoplankton production, and is driven by a fine-scale atmospheric model that resolves the effects of local orography on the coastal winds. Results indicate significant “rivers” of cross-shelf nitrogen flux due to horizontal advection, as well as “fountains” of vertical transport over shallow banks due to tidal mixing. Using these results, we constructed a provisional budget of nutrient transport among subregions of the CGOA. Contrary to expectations, this budget reveals substantial upwelling of nutrients over major portions of the shelf, driven by local wind-stress curl. These effects are large enough to overwhelm the smaller downwelling flux at the coast throughout the growing season. Vertical mixing by winds and tides, and horizontal flux from the deep basin, are other substantial contributors to nutrients above the 15-m horizon. These findings help to explain the productivity of this coastal ecosystem.

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