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 Kenneth O. Coyle a,⇑ , Wei Cheng c , Sarah L. Hinckley b , Evelyn J. Lessard d , Terry Whitledge a , Albert J. Hermann c , Kate Hedstrom e a Institute of Marine Science, University of Alaska, Fairbanks, AK 99775-7220, USA b Alaska Fisheries Science Center, 7600 Sand Point Way NE, Seattle, WA 98115, USA c Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Box 357941, Seattle, WA 98195, USA d Dept. Oceanography, University of Washington, Seattle, WA 98195, USA e Arctic Region Supercomputing Center, University of Alaska, Fairbanks, AK 99775, USA article info Article history: Received 6 June 2011 Received in revised form 1 February 2012 Accepted 4 March 2012 Available online 20 March 2012 abstract 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 100–130 g cm 2 y 1 , but production of up to 300 g cm 2 y 1 is predicted for regions in Lower Cook Inlet and around Kodiak. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Potential impacts of climate change have become central polit- ical, social and economic issues. As the growing human population places increasing demands on renewable resources, concern over the effects of climate on renewable resources has become the focus of many environmental studies. In the ocean, climate change is ex- pected to change temperature and precipitation patterns, poten- tially altering primary production in marine ecosystems, affecting species composition, abundance and biomass of the constituent marine organisms, especially those in polar and sub-polar regions. In response to these concerns, the United States Global Ocean Eco- system Dynamics program (GLOBEC) was undertaken to collect field observations and generate models to improve our under- standing of the mechanistic links between climate and the ecosys- tem response in four regions including the northern Gulf of Alaska shelf (Fogarty and Powell, 2002). This work focuses on the GLOBEC observational and modeling results from the Gulf of Alaska (GOA) shelf. The GOA climate is characterized by energetic storm systems associated with the Aleutian Low (Weingartner et al., 2005); these storms are constrained by coastal mountains, causing uplift of the moist air and elevated precipitation. Much of the precipitation is sequestered on the continent as snow and ice in winter but dis- charged into the coastal ocean in summer and fall as the snow pack melts and summer rains add to the runoff. The amount of runoff can be massive, averaging about 24,000 m 3 s 1 (Royer, 1982). Freshwater discharge into the coastal ocean is apparently a critical source of iron to the shelf ecosystem (Wu et al., 2009). Concentra- tions of dissolved iron in coastal surface waters range from 0.5 to 4.1 nM, but dissolution of leachable particulate iron, present at 0079-6611/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pocean.2012.03.002 ⇑ Corresponding author. E-mail address: coyle@ims.uaf.edu (K.O. Coyle). Progress in Oceanography 103 (2012) 16–41 Contents lists available at SciVerse ScienceDirect Progress in Oceanography journal homepage: www.elsevier.com/locate/pocean