| Summary: | Populations of Steller sea lions (Eumetopias jubatus) and harbor seals (Phoca vitulina) have declined in the past decade to 10-50% of initial populations in the western Gulf of Alaska and Bering Sea. Several studies have sought to determine if these declines arise from "top down" or "bottom up" controls. We have used stable carbon isotope ratios in whale baleen as a means of testing the hypothesis that climate change has forced a decrease in ecosystem carrying capacity. Recent findings in laboratory and natural environments indicate that carbon isotope ratios of phytoplankton are closely linked to cell growth rates where other floristic and environmental conditions are similar. Once incorporated into phytoplankton, the isotope ratios are conservatively transferred into the food webs supporting consumer organisms. Bowhead whale (Balaena mysticetus) baleen, grown while the whales fed in the Bering-Chukchi seas, provides a multiyear temporal record of isotope ratios in their zooplankton prey and, by proxy, the phytoplankton supporting the consumer food webs. By using baleen plates from 26 whales archived at the Los Angeles County Museum and recently taken by Native hunters, an isotopic record was constructed extending from 1947 to 1995. From this, we infer that seasonal primary productivity in the Bering Sea was at a higher rate over the period 1947-1966, then underwent a general decline continuing to the most recent samples (1995). Assuming a close similarity to the published relationships established between primary productivity and carbon isotope ratios, the decline in the Bering Sea carbon isotope ratios suggests a loss of 35-40% of the carrying capacity of 30 years ago. This drastic decline is evident in recent zooplankton biomass estimates and is very likely implicated in the continuing decline of marine mammal populations in the western Gulf of Alaska and Bering Sea. Seeking the environmental physical and chemical causes is the focus for future work.
|
|---|