The Northeast Pacific GLOBEC program seeks to quantify the biological and physical processes governing the growth and survival of juvenile salmon in the Coastal Gulf of Alaska (CGOA). Although this broad (100-200 km in the north and northwest), deep (200 m) shelf experiences downwelling-favorable winds and copious, nutrient-poor freshwater runoff, it supports a rich, productive ecosystem. The region serves as a nursery for several commercially important fish species (OCSEAP staff, 1986; Rogers, 1986) that spend the early portions of their lives on the shelf. In particular, juvenile salmon occupy the shelf during their first year of life, a period thought to be critical to recruitment success (Francis and Hare, 1994). The mechanisms that govern spatial and temporal (seasonal and interannual) variability in nearshore circulation, vertical stratification and heat, salt and nutrient content bear directly on the distribution and health of both young fish and the zooplankton populations on which they depend. Achieving the program's ultimate goal of enhancing predictability and management of the region's rich marine resources and informing the design of regional monitoring systems requires knowledge of the dynamics underlying biological and physical variability at a range of timescales. Understanding the processes that maintain high productivity over the shelf contributes towards answering the larger question of how the Gulf of Alaska's marine ecosystem responds to long-timescale climate change. We propose a continuous, five-year program of physical and bio-optical measurements which complements existing Long Term Observation Program (LTOP) efforts by providing: (1) three-dimensional spatial coverage, (2) increased temporal resolution and (3) observations spanning the sparsely observed winter period. This study will focus on the processes governing circulation and stratification over the Alaskan shelf and their roles in driving onshore nutrient flux and modulating primary productivity.
Wind forcing and freshwater flux govern circulation in the Coastal Gulf of Alaska. In winter, frequent storms pass through the Gulf, producing strong, cyclonic winds and heavy precipitation. The mountains ringing the Gulf constrain onshore propagation of these systems, often resulting in intensified along-shore winds and enhanced precipitation in the coastal regions. Heavy precipitation accumulates as snow, which later melts to produce extremely large freshwater fluxes along the entire coast from May-October (Royer, 1982). Freshwater runoff peaks in October, has a secondary peak in May, and is at a low in mid-winter (Royer, 1982). Downwelling favorable winds dominate during winter, driving onshore Ekman transport, trapping the freshwater plume near the coast and maintaining the cross-front pressure gradient which supports the fast, narrow Alaska Coastal Current. During summer, a high pressure system occupies the Gulf, producing a relaxation, and even a reversal of alongshore winds (Royer, 1975; Wilson and Overland, 1986) accompanied by weaker downwelling or even occasional upwelling.
Two major currents dominate circulation in the CGOA (Fig. 1). The northern edge of the Pacific subarctic gyre forms the Alaska Stream, a ~100 km wide, swift (peak speeds ~1 m/s) current that flows westward near the shelfbreak, approximately 150 km from the coast (Reed, 1984; Reed and Schumacher, 1986; Musgrave et al., 1992). A combination of downwelling winds and large buoyancy flux from rivers and streams along the Alaskan coast supports the narrow (< 30 km wide), fresh, Alaska Coastal Current (ACC). The ACC typically remains within 30 km of the (provided by Danielson and Weingartner) Schematic of circulation in the Northeast Pacific and Gulf of Alaska. coast (Schumacher and Reed, 1980; Royer, 1981), flowing westward past Prince Williams Sound, through Shelikof Strait (Stabeno et al., 1995) between the coast and Kodiak Island, and possibly exiting to the Bering Sea at Unimak Pass. Observations reveal mesoscale eddies (Schumacher et al., 1993) within Shelikof Strait, likely generated by baroclinic instability of the mean flow through the region (Mysak et al., 1981). These eddies translate at speeds slower than the mean flow within the Strait (Schumacher et al., 1993) and may play a significant role in retaining larvae on the shelf. In contrast, large, slow-moving eddies generated in the eastern Gulf of Alaska (Musgrave et al., 1992) propagate westward and could impinge on the shelf, removing water in a fashion similar to that of Gulf Stream rings interacting with Georges Bank (Joyce et al., 1992), acting as a loss mechanism for target species and possibly effecting an onshore flux of nutrients. Strong seasonal and interannual variability in winds and freshwater discharge are reflected in the ACC (Royer, 1981; Johnson et al., 1988). Modeling studies (Hermann and Stabeno, 1996) suggest that the barotropic transport of the ACC responds strongly to changes in local winds and more weakly to alterations in buoyancy flux. However, strong buoyancy flux produces enhanced eddy generation within Shelikof Strait and decouples the surface intensified ACC from the underlying topography, permitting it to flow more easily across isobaths.
This study focuses on the Alaska Coastal Current due to the important, but poorly understood, role it plays in the early life history of juvenile salmon. Young fish migrate onto the shelf in late summer/early fall, though whether this is primarily an advective or a swimming process remains an active area of study (P. Rand and A. Hermann, personal communication). Given maximum swimming speeds of ~0.2 m/s, the fish cannot move directly upstream against the ACC, and advection likely plays a role in determining the path of their migration. Likewise, episodic circulation features (e.g. eddies and meanders) may represent significant mechanisms for transporting fish, zooplankton, larvae and nutrients across the front formed by the ACC. Freshwater discharge, lateral advection and the shift from strong downwelling-favorable winds to weaker, less directional patterns drive springtime restratification over the shelf, which plays a pivotal role in producing the spring bloom. Spatial and temporal variations in spring restratification and the resulting bloom are likely propagated through the food chain to influence prey abundance experienced by salmon migrating onto the shelf. The vertical structure established in spring serves as a starting point for mixed layer evolution through summer and will exert a strong influence on the mixed layer depths and temperatures that develop. Finally, processes associated with the ACC may play important roles in transporting nutrients onto the shelf. Weingartner (personal communication) hypothesizes that onshore advection of deep, nutrient-rich waters in spring combined with subsequent wintertime vertical mixing may act to replenish surface layer nutrients over the shelf, perhaps explaining the high productivity of this downwelling region.
The processes described above depend on the ACC and its response to forcing by winds and freshwater discharge, leading us to define the following scientific objectives:
• Quantify seasonal and interannual variability in ACC freshwater content and transport.
• Investigate the role ACC variability plays in governing springtime restratification, mixed layer depth and temperature over the shelf.
• Document interannual variability in the spring bloom. How does it progress across the shelf? What controls spatial and temporal variability in bloom development?
• Examine processes that may produce onshore nutrient flux, focusing on the mechanism proposed by Weingartner (personal communication, detailed below)
• Investigate the role played by episodic mechanisms (e.g. eddies, meanders) and their response to changes in freshwater discharge and winds.
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