Monitoring Changes in Oceanographic Forcing Using the Carbon and Nitrogen Isotopic Composition of Prince William Sound Pelagic Biota

Monitoring Changes in Oceanographic Forcing Using the Carbon and Nitrogen Isotopic Composition of Prince William Sound Pelagic Biota

Thomas C. Kline Jr.

Monitoring Changes in Oceanographic Forcing Using the Carbon and Nitrogen Isotopic Composition of Prince William Sound Pelagic BiotaThis is part of Ecosystem Approaches for Fisheries Management
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Description

Changes in the physical environment known to affect phytoplankton and zooplankton production can be linked to fish production through carbon and nitrogen stable isotope natural abundance measurements. Stable isotopic analyses of herbivorous copepods and juvenile fishes from Prince William Sound (PWS) and the northern Gulf of Alaska (GOA) were conducted as part of the Sound Ecosystem Assessment (SEA) project, a comprehensive multidisciplinary ecosystem study. The advective regime connecting the GOA with PWS was postulated by SEA to affect recruitment and nutritional processes in juvenile fishes. Herbivorous zooplankton, an indicator for pelagic production sources, had distinctive carbon isotope signatures when sampled in the GOA compared to those from PWS. PWS carbon had consistent spring bloom carbon isotopic signatures during 1994-1996 while GOA carbon differed in 1996.Nevertheless, PWS carbon was always distinctive from GOA carbon. This variation suggested that interannual fluctuations
occurring at the food chain base are driven by processes in the
gulf.

Analyses of nitrogen isotope ratios and C/N ratios of juvenile fishes were used to normalize their carbon isotope ratios, enabling determination of their relative affinity for GOA or PWS carbon. The data suggest a large affinity range, changing on annual time scales, consistent with observed oceanographic phenomena. For example, there was a shift to a greater dependency on GOA carbon in 1995 compared with 1994 and 1996. A parallel shift to increased GOA-originating copepods undergoing diapause (resting phase) in 1995 suggesting an influx of GOA zooplankton, provided a second line of evidence. Thus herring and other fishes partially dependent on GOA carbon are subject to vagaries of carbon flow that fall under the domain of physical oceanographic processes connecting the GOA with PWS as well as processes occurring on the GOA continental shelf adjacent to PWS.

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