The Effect of Cold Pool Variability on Zooplankton Dynamics of the Eastern Bering Sea Shelf

Jennifer Johnson
Thesis Defense

Master of Science

Friday, Jul. 31, 2020, 9:30am

Interannual variability of ocean temperatures and sea ice extent has been observed on the eastern Bering Sea (EBS) shelf, where annual conditions have resulted in regional Cold or Warm year discretization. Consecutive years of Cold or Warm year characterization has resulted in regime states within the past two decades. A characteristic feature of the EBS is a subsurface layer linked to seasonal sea ice (SSI) and defined by bottom temperatures less than 2°C, termed the Cold Pool. Cold Pool variability is tied to the dynamics of fish distribution in the Arctic and subarctic ecotones. Water column backscatter was collected remotely using upward looking echosounders along the EBS shelf from 2008-2018. Acoustic data were coupled with additional bottom temperature, regional SSI, and local SSI data from the Cold regime between 2006-2013 and the Warm regime from 2014-2018 to assess the relationship between zooplankton communities and Cold Pool variation. Water column averaged area backscatter was two orders of magnitude greater in the Cold vs. Warm regime, with decreased spring and fall blooms of Warm year acoustic abundance coupled with early ice edge receding. Acoustic multifrequency analysis indicated a shift in the Warm regime zooplankton communities from larger to smaller bodied species on the EBS shelf resulting in an alternative equilibrium in the average acoustic abundance. Cold Pool proxy regional SSI was a better predictor variable for zooplankton abundance than bottom temperature in the Cold regime, while Warm regime bottom temperature and regional SSI were equal in predictive power and resulted in improved model performance. Although the models did not capture the dynamics of the regime shift in 2013, the Cold regime exhibited increased stochasticity in bottom temperature, SSI, and acoustic backscatter prior to the ecosystem tipping point and ultimate shift. Regime shift early warning signals from further mining of acoustic and environmental data warrant exploration for comprehensive management practices in the Bering Sea and neighboring Arctic ecosystems.


Jennifer Johnson first gained experience as a NOAA contractor at the Atlantic Hydrographic Branch in Norfolk, VA. She was a NOAA Hollings Scholar which led to internships at the Southeast Fisheries Science Center (SEFSC) with the National Ocean Service (NOS) and the National Center for Coastal Ocean Science (NCCOS) in Silver Spring, MD. Following graduation at Virginia Tech with a Bachelor of Science in Biology, Jennifer was a contractor at the National Marine Fisheries Service (NMFS) at the Northeast Fisheries Science Center (NEFSC) in Woods Hole, MA where she used unmanned aerial systems to gather optical imagery and furthered her experience with active fishery acoustic data collection and processing on research vessels. While in Woods Hole, she participated in a Woods Hole Oceanographic Institution (WHOI) collaborative project with fishery acoustic and trawl operations in the Arctic Ocean.