Bering Sea | Lower Trophic Levels
Changes in copepod size in response to warm and cold conditions in the eastern Bering Sea: NPRB Project number 2008 Presenter: Deana Crouser , deana.crouser@noaa.gov, Lynker technologies in support of NOAA’s Alaska Fisheries Science Center Jan Ohlberger , janohl@uw.edu, School of Aquatic & Fishery Sciences, University of Washington & WA Department of Fish and Wildlife Grant Woodard , gwooda@uw.edu, School of Aquatic and Fishery Sciences, University of Washington David Kimmel , david.kimmel@noaa.gov, Alaska Fisheries Science Center, NOAA Fisheries Large marine ecosystems of Alaska are experiencing significant warming and one response to warming predicted by ecological theory is a decline in organism body size. Here we report preliminary results for NPRB Project 2008: The effect of global warming on long-term changes in copepod size in the Gulf of Alaska and Bering Sea. We used an archive of preserved specimens to measure body size (prosome length) of copepods across warm and cold periods in the eastern Bering Sea. Average prosome length for large copepods ( Calanus spp . and Neocalanus spp .) was lower during the warm years, whereas prosome length for smaller copepods ( Acartia longiremis , Oithona similis , Pseudocalanus spp , and Metridia pacifca ) did not show differences. The result confirms observed size variability in larger copepods in response to shifts in temperature. The differences in body size translate to differences in individual mass and lipid content. For example, the average size of Calanus spp . C5 during warm years was 2.76 ± 0.29 (SD) mm and 3.03 ± 0.12 (SD) mm during the cold years. Based on length-weight regression, this results in an average individual body mass of 237.4 μg during warm years and 288.7 μg during cold years, i.e. C5 Calanus spp . are ~20% heavier during cold years. Lipid sac volume for Calanus spp . C5 during warm years was 1621 mm3 compared to 1780 mm3 during cold years, i.e. 10% more lipid. This suggests that the fewer Calanus spp . observed during warm conditions are smaller and less lipid rich compared to cold years. This has implications for the eastern Bering Sea food web as many species rely on Calanus spp . as an important forage item, including Walleye pollock ( Gadus chalcogrammus ), seabirds, and the critically endangered north Pacific right whale ( Eubalaena japonica ). We will expand this initial comparison to determine if copepod size has changed over the past two decades in both the Bering Sea and Gulf of Alaska. This will allow us to test the hypothesis that warming in Alaska ecosystems has caused copepod body size to decline over time. Should such a decline be detected, it would suggest a shifting relationship with higher trophic levels with ecosystem wide consequences. In addition to the statistical analyses of preserved specimens, we plan to develop a size-based population model parameterized to Calanus spp . to better understand the underlying mechanism of copepod responses to a warming ocean and to improve predictions of future changes in copepod body size.
Alaska Marine Science Symposium 2023 143
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