Seward Line LTOP stations in the Northern Gulf of Alaska: Red circles denote CTD-nutrient-phytoplankton-zooplankton stations; blue circles denote CTD-nutrient-phytoplankton stations. Click here for a map with the station names incorporated. ADCP and sea surface data are collected continuously underway and while on station during GLOBEC. Acoustic data for zooplankton biomass determinations were collected at all zooplankton stations and between stations on the Seward Line during the GLOBEC period. Since 2005 sampling has been reduced to the Seward Line and those stations inside western Prince William Sound.

Over twenty years of historical CTD data exist for stations on the Seward and Cape Fairfield Lines. The GAK1 station CTD time series extends back to 1970. Stations along the Cape Cleare Southeast Line were occupied on the April, May and July cruises in years 2001 and 2003 to provide along-shore coverage for comparisons to the Seward Line transects.

The Gulf of Alaska (GOA) shelf supports a diverse ecosystem that includes several commercially important fisheries such as crab, shrimp, pollock, salmon, and halibut. Together these stocks indicate that the gulf is amongst the world's largest fisheries with annual catches exceeding 300 grams per thousand cubic meters. The mechanisms that underlie this high productivity were not known prior to the Seward Line studies, and were somewhat enigmatic because the GOA shelf is a coastal "downwelling" shelf. By contrast, the rich fisheries along the eastern boundaries of the Pacific Ocean to the south are supported by vigorous, wind-driven coastal upwelling, whereby the euphotic zone is regularly replenished with nutrients pulled upward from depth.

Intriguingly, the relative dominance of the commercially important fish species changed in the mid-1970s; crab and shrimp declined while salmon and groundfish populations increased. These population shifts coincided with the beginning of a decadal North Pacific change in the atmosphere and ocean. From the human perspective these alterations required the commercial fishing industry to invest substantially in modifying gear, ships and processing facilities to remain economically viable. Subsequent changes in the ecosystem followed in the 1980s with substantial declines in populations of sea lions and puffins. Dramatic though this "regime shift" was, there was evidence that the abundance of halibut and other commercially important species varies on decadal time scales in conjunction with northern North Pacific Ocean temperatures.

These correlations and observations suggested that the GOA ecosystem is sensitive to climate variations on time scales ranging from the interannual to the interdecadal; however, the specific mechanisms linking climate to ecosystem alterations are still unknown. Determining and understanding these mechanisms requires an understanding of the seasonal cycle of the principle physical, chemical and biological variables.

The Seward Line is obtaining the multi-year data set that will lead to a better understanding of the seasonal cycle and the interannual variability in the physical-chemical structures and biological productivity of the Gulf of Alaska shelf.