Declining carrying capacity in the Bering Sea: Isotopic evidence from whale baleen

The Bering Sea is a rich marine ecosystem with 1995 U.S. landings of finfis h and shellfish in excess of 2 X 10(6) tons. Recently, major declines of ma rine mammal and bird populations have occurred, with the question arising a s to whether the causes were top down, i.e., from fishing pressure, or bott om-up, i.e., driven by climate-change effects. We assessed the latter scena rio by using the inverse relationship between phytoplankton cell-growth rat es and carbon isotope fractionation to compare seasonal rates of primary pr oductivity. The isotope ratios in the long baleen plates from bowhead whale s (Balaena mysticetus) laid down while the whales fed in the Bering and Chu kchi seas were used as a proxy for the average annual isotope ratios in the ir zooplankton prey and, by extension, phytoplankton. Plates from 37 whales produced a continuous isotopic record from 1947-1997 and indicate that sea sonal productivity was higher in 1947-1976, peaking in 1966. Since that tim e, average delta(13)C values have declined by over 2.7 parts per thousand u ntil 1997, inferring a drop in seasonal carbon fixation of similar to 30-40 %. Alternatively, lowered isotope ratios could arise if phytoplankton stock s significantly increased over time while cell-growth rates declined. Howev er, long-term literature estimates for phytoplankton in the Western and Eas tern Pacific Ocean and Eastern Bering Sea show either no trend or a decreas e over time. Archaeological samples of baleen from ca. 100 and 2,200 yr. B. P. showed average delta(13)C values higher than any in the last five decade s, implying still higher productivity in the past. The lowered carrying cap acity is a likely contributor to the decline of top consumers in the region .