Model and field observations of effects of circulation on the timing and magnitude of nitrate utilization and production on the northern Gulf of Alaska shelf

The GLOBEC program was tasked with understanding the mechanistic links between climate forcing and the ocean-ecosystem response on the northern Gulf of Alaska (GOA) shelf. To address this task, samples were collected five to six times 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 water masses. The ROMS circulation model with an embedded ecosystem model was used to extend GLOBEC observations in space and time on the GOA shelf. The timing of the spring bloom in simulations was related to shallowing of the pycnocline. The spring bloom began in late March-April on the inner shelf and in May on the mid and outer shelf. The simulations suggest that the magnitude of shelf production is a balance between the amount of iron from freshwater runoff and nitrate, with iron limitation on the outer shelf and adjacent ocean and nitrate limitation on the inner shelf. Simulated shelf-break eddies form near Yakutat, have elevated iron concentrations relative to surrounding waters, and propagate westward, influencing production and nitrate concentrations on the outer shelf and in the adjacent ocean during spring and summer. Simulated primary production in the Seward Line region was about 100-130g cm(-2) y(-1), but production of up to 300 g cm(-2) y(-1) predicted for regions in Lower Cook Inlet and around Kodiak. (C) 2012 Elsevier Ltd. All rights reserved.