Northern Bering Sea: Early sea ice melt reduces productivity — Revealed by Hokkaido University zooplankton size survey

A research group comprising Graduate Student Ms. Shino Kumagai, Assistant Professor Kohei Matsuno, and Associate Professor Atsushi Yamaguchi of the Graduate School of Fisheries Sciences at Hokkaido University announced their findings that small zooplankton in the northern Bering Sea had increased in 2018, a year when sea ice melting occurred early, and there were associated decreases in productivity and energy transfer efficiency to a higher number of organisms in the ecosystem. This was found by examining the size composition of zooplankton communities in the northern Bering Sea during the summer months (June-July) of 2017 and 2018, when sea ice melt was normal and early, respectively. The results are expected to lead to improved accuracy in predicting the future of marine ecosystems due to global warming and were published in the November 29 issue of the international academic journal Frontiers in Marine Science.

The northern Bering Sea, which is a shallow shelf area that connects the Arctic Ocean to the Pacific Ocean, is known for its high primary productivity. In this respect, zooplankton communities play an important role in ecosystems as prey for fish and other higher-order predators. The area has recently experienced earlier sea ice melt, speculated to be caused by global warming, and this change has reportedly altered the zooplankton communities and reduced the population of higher-order predators. The year 2018 had the fastest sea ice melting of any period known for this region, but the causal relationship between sea ice melting and the marine ecosystem was not well understood.

To clarify how the zooplankton size composition and production across higher trophic levels are affected by sea ice fluctuations, the research group used the ZooScan image analyzer for size and community structure analysis. Based on the size composition, the research group examined the energy amount across higher trophic levels.

In 2018, a distinctive community dominated by small copepods and Bivalvia larvae began to appear throughout the northern Bering Sea. According to the size range of possible prey for pollack larvae and juveniles, small copepods and Bivalvia larvae were predominant. The low biomass and predominance of the small, nutrient-poor copepods and Bivalvia larvae in 2018 probably resulted in an unsuitable feeding environment for the fish larvae and juveniles.

With regard to energy transfer efficiency, the slope of the regression line was found to be steeper in 2018 and energy transfer was less efficient (i.e., increased frequency of predation within the ecosystem). Changes in the zooplankton community composition may have reduced the amount of energy available to higher predators, which may have influenced the decline of fish and other populations.

As the northern Bering Sea is expected to experience significant sea ice fluctuations in the future owing to global warming, a better understanding of the response of marine organisms to environmental changes is expected to lead to the conservation of marine ecosystems and the sustainable use of marine resources in the future.

Matsuno said, "It is not easy to unravel how marine ecosystems are affected by reduced sea ice. This is due to the limitations of the study and the diversity of the species. However, understanding the characteristics of the organisms and gathering as much evidence as possible can help us understand what is happening in the ocean. We believe that properly documenting the current state of the ocean during this transitional period of change will be an asset for the future."

Journal Information
Publication: Frontiers in Marine Science
Title: Zooplankton size composition and production just after drastic ice coverage changes in the northern Bering Sea assessed via ZooScan
DOI: 10.3389/fmars.2023.1233492