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 finfish and shellfish in excess of 2 × 10 6 tons. Recently, major declines of marine mammal and bird populations have occurred, with the question arising as to whether the causes were top down, i.e., from fishing pressure, or bottom‐up, i.e., driven by climate‐change effects. We assessed the latter scenario by using the inverse relationship between phytoplankton cell‐growth rates and carbon isotope fractionation to compare seasonal rates of primary productivity. The isotope ratios in the long baleen plates from bowhead whales (Balaena mysticetus) laid down while the whales fed in the Bering and Chukchi seas were used as a proxy for the average annual isotope ratios in their zooplankton prey and, by extension, phytoplankton. Plates from 37 whales produced a continuous isotopic record from 1947‐1997 and indicate that seasonal productivity was higher in 1947‐1976, peaking in 1966. Since that time, average d 13 C values have declined by over 2.7‰ until 1997, inferring a drop in seasonal carbon fixation of ∼‐40%. Alternatively, lowered isotope ratios could arise if phytoplankton stocks significantly increased over time while cell‐growth rates declined. However, long‐term literature estimates for phytoplankton in the Western and Eastern Pacific Ocean and Eastern Bering Sea show either no trend or a decrease over time. Archaeological samples of baleen from ca. 100 and 2,200 yr. B.P. showed average d 13 C values higher than any in the last five decades, implying still higher productivity in the past. The lowered carrying capacity is a likely contributor to the decline of top consumers in the region.