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Marine Ecology and Stock Assessment Program

Southeast Alaska: Oceanographic Habitats and Linkages

University of Alaska and ABL scientists conducted an overview of the physical oceanographic and geological processes that affect marine biological habitats and production in the marine waters throughout the archipelago and continental shelf of Southeast Alaska. Given the paucity of regional data, the overview summarizes work done in adjacent regions of the Gulf of Alaska shelf and basin and draws on research carried out in similar settings elsewhere.

The geological setting, which critically influences the regional meteorology and oceanography, includes a narrow continental shelf, deep channels that permeate the archipelago, fjords, glaciers, and a rugged, mountainous coast. The large-scale meteorology is influenced primarily by seasonal variations in the intensity and position of the Aleutian Low pressure center. Winds, freshwater runoff, tides, and cross-shelf exchange control the regional oceanography.

The large-scale flow field advects mass, heat, salt, nutrients and planktonic organisms northward from British Columbia (and even farther south) to the northern Gulf of Alaska along the slope, shelf, and within the channels of Southeast Alaska. The deep channels permeating the island archipelago and narrow continental shelf facilitate transfer between basin and interior waters.

Water properties and flow fields are subject to large annual variations in response to similarly large variations in winds and coastal freshwater discharge. The complex geological setting leads to large spatial heterogeneity in the physical processes controlling the local circulation fields and mixing, thereby creating numerous and diverse marine biological habitats. These various circulation and mixing processes substantially modify Southeast Alaska water masses and, thus, influence marine ecosystem processes downstream over the northern and western Gulf of Alaska shelf.

By Lisa Eisner

The Potential Role of Water-column Stability and Nutrients in Structuring the Zooplankton Community

The southeastern Bering Sea sustains one of the largest fisheries in the United States as well as wildlife resources that support valuable tourist and subsistence economies. The fish and wildlife populations in turn are sustained by a food web linking primary producers to apex predators through the zooplankton community. Recent shifts in climate toward warmer conditions may threaten these resources by altering productivity and trophic relationships in the ecosystem on the southeastern Bering Sea shelf.

AFSC researchers from ABL and the Center's Resource Assessment and Conservation Engineering (RACE) Division in collaboration with University of Alaska Fairbanks scientists examined the zooplankton community near the Pribilof Islands and on the middle shelf of the southeastern Bering Sea in summer 1999 and 2004 to document differences and similarities in species composition, abundance, and biomass by region and year.

Between August 1999 and August 2004, the summer zooplankton community of the middle shelf shifted from large to small species. Significant declines were observed in the biomass of large scyphozoans (Chrysaora melanaster), large copepods (Calanus marshallae), arrow worms (Sagitta elegans) and euphausiids (Thysanoessa raschii, T. inermis) between 1999 and 2004.

In contrast, significantly higher densities of the small copepods (Pseudocalanus spp., Oithona similis) and small hydromedusae (Euphysa flammea) were observed in 2004 relative to 1999.

Stomach analyses of young-of-the-year (age-0) walleye pollock (Theragra chalcogramma) from the middle shelf indicated a dietary shift from large to small copepods in 2004 relative to 1999.

The shift in the zooplankton community was accompanied by a 3-fold increase in water-column stability in 2004 relative to 1999, primarily due to warmer water above the thermocline, with a mean temperature of 7.3°C in 1999 and 12.6°C in 2004. The elevated water-column stability and warmer conditions may have influenced the zooplankton composition by lowering summer primary production and selecting for species more tolerant of a warm, oligotrophic environment.

A time series of temperature from the middle shelf indicates that the warmer conditions in 2004 are part of a trend rather than an expression of interannual variability. These results suggest that if climate on the Bering Sea shelf continues to warm, the zooplankton community may shift from large to small taxa which could strongly impact apex predators and the economies they support.

By Lisa Eisner

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