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

Abstract

The Bering Sea is a rich marine ecosystem with 1995 U.S. landings of finfish and shellfish in excess of 2 × 106 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 d13C 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 d13C 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.

Stable C and N isotopic composition of sinking particles and zooplankton over the southeastern Bering Sea shelf

“…* Seasonal and inter-annual variability of d 15 N at the middle shelf site M2 were pronounced. Interannual and seasonal changes in nutrient supply are probably responsible for much of the variability in d 15 N. * Our data are consistent with a relationship between d 13 C and primary productivity, and hence lend some support to the Schell (2000) inference that decreasing d 13 C of bowhead whale baleen indicates declining Bering Sea productivity over the past 30 years. However, we can not rule out other factors influencing d 13 C, such as phytoplankton species effects.…”

Twentieth century <i>δ</i><sup>13</sup>C variability in surface water dissolved inorganic carbon recorded by coralline algae in the northern North Pacific Ocean and the Bering Sea

“…5). The δ 13 C values in bowhead whale baleen and steller sea lions bone collagen (Schell, 2000(Schell, , 2001Hirons et al, 2001) also declined at an enhanced rate; therefore, an algal-specific effect or site-specific variability were unlikely to be the primary cause of the decline observed in our study. It would appear more likely that changes in the δ 13 C DIC pool to which Keeling et al (2001); http://scrippsco2.ucsd.edu/ data/flask co2 and isotopic/monthly iso/monthly ptb c13.csv), (B) the δ 13 C values from the baleen of bowhead whales (data from Schell et al (2001)), (C) the δ 13 C values from algal specimen AM-KR-80 and combined δ 13 C values from specimens Attu 11-4 (1937-1967) and AM4-1 andAttu 11-4 (1968-2003) with line of best fit calculated using SigmaPlot (Systat Software Inc., San Jose, CA), and (D) ALPI values (data from Beamish et al (1997); http://www.pac.dfo-mpo.gc.ca/science/species-especes/ climatology-ie/cori-irco/indices/alpi.txt).…”

“…Decreases in primary productivity would increase the amount of CO 2 in surface waters, thus reducing carbon fractionation during the calcification process and leading to lower δ 13 C values in the resulting skeleton. In fact, a decline in phytoplankton cell growth rates, and hence primary productivity, has been hypothesized to drive the declining trends in the whale baleen and sea lion bone collagen δ 13 C (Schell, 2000(Schell, , 2001Hirons et al, 2001). However, available evidence does not support a decrease in new primary production (Hunt Jr et al, 2002).…”

Regional and seasonal feeding by bowhead whales Balaena mysticetus as indicated by stable isotope ratios

“…Also, we were interested in comparing feeding strategies in recent vs. earlier years. Recent evidence that the productivity of the Bering and Chukchi Seas may have declined suggested that feeding in the eastern Beaufort Sea might be more important to bowhead whales now than in previous decades (Schell 2000(Schell , 2001. As in earlier related studies, our approach was based on the fact that samples of bowhead tissues can be obtained in northern Alaska during the spring and autumn migrations, when bowhead whales are harvested by Inupiat hunters (Stoker & Krupnik 1993).…”