Abstract
A remarkable early sea-ice reduction event was observed in the northern Bering Sea during 2018. In turn, this unusual hydrographic phenomenon affected several marine trophic levels, resulting in delayed phytoplankton blooms, phytoplankton community changes, and a northward shift of fish stocks. However, the response of the zooplankton community remains uncharacterized. Therefore, our study sought to investigate the zooplankton community shifts in the northern Bering Sea during the summers of 2017 and 2018 and evaluate the effects of early sea-ice melt events on the zooplankton community, population structure of large copepods, and copepod production. Five zooplankton communities were identified based on cluster analysis. Further, annual changes in the zooplankton community were identified in the Chirikov Basin. In 2017, the zooplankton community included abundant Pacific copepods transported by the Anadyr water. In 2018, however, the zooplankton community was dominated by small copepods and younger stages of large copepods (Calanus glacialis/marshallaeandMetridia pacifica), which was likely caused by reproduction delays resulting from the early sea-ice reduction event. These environmental abnormalities increased copepod production; however, this higher zooplankton productivity did not efficiently reach the higher trophic levels. Taken together, our findings demonstrated that zooplankton community structure and production are highly sensitive to the environmental changes associated with early sea-ice reduction (e.g., warm temperatures and food availability).
Highlights
The northern Bering Sea is a shallow shelf that connects the Bering Sea and the Arctic Ocean
The melt day (MD) in 2018 was much earlier than that in 2017; MD occurred from April 4 to May 11 in 2017 and from March 23 to April 29 in 2018 (Supplementary Table 1 and Figures 2, 3)
The highest abundance (581,378 ind. m−2) of the mitraria larvae was recorded at Stn. 19 in 2018, where the integrated water-column salinity was 29.4, which was lower than the previous value (29.9–30.8) defined as Alaskan Coastal Water (ACW) in Hopcroft et al (2010)
Summary
The northern Bering Sea is a shallow shelf (approximately 50 m in depth) that connects the Bering Sea and the Arctic Ocean. In the winter of 2017 and 2018, the seaice coverage reached a historical minimum based on satellite observations dating back to 1978 (Cornwall, 2019) This early sea ice retreat has been largely attributed to relatively warm winds from the south, accompanied by a westward shift of the Aleutian low from its typical position (Stabeno et al, 2019; Basyuk and Zuenko, 2020). This drastic change in sea ice coverage may weaken the water column stratification of the Chirikov Basin (Ueno et al, 2020), decreasing the cold pool in the bottom layer of the Bering shelf (Stabeno and Bell, 2019)
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