Effects of Early Sea-Ice Reduction on Zooplankton and Copepod Population Structure in the Northern Bering Sea During the Summers of 2017 and 2018

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/marshallae and Metridia 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).

Zooplankton size composition and production just after drastic ice coverage changes in the northern Bering Sea assessed via ZooScan

“…In both years, there were similar variations in water temperature and salinity from east to west and with cold saline waters distributed on the western side (cf. Figure 2 and 3 in Kimura et al, 2022). On the other hand, water mass distributions were slightly different between the years (Figure 2).…”

“…Chlorophyll a fluorescence was higher in the Bering Strait during both years (cf. Figure 2 and 3 in Kimura et al, 2022). The MD occurred from March 23 to April 29 in 2018, which was earlier than in 2017 (April 4 to May 11) (cf.…”

“…The MD occurred from March 23 to April 29 in 2018, which was earlier than in 2017 (April 4 to May 11) (cf. Figure 2 and 3 in Kimura et al, 2022).…”

“…Zooplankton connect primary production and higher trophic levels. The abundance of small copepods increased during the summer of 2018 (Duffy-Anderson et al, 2019) because the delayed phytoplankton bloom resulted in delayed reproduction and large copepods did not reach their late developmental stage (Kimura et al, 2022). However, although the community structure of zooplankton and the population structure of copepods have been reported, changes in the size composition of zooplankton have not been reported.…”

Meiofauna in the southeastern Bering Sea: community composition and structuring environmental factors

“…In the polar region, a recent metabarcoding approach revealed that season is a significant predictor of the diversity of sea ice and sediment-associated meiofauna at approximately 3 km from the ice edge near Utqiagvik (Barrow), Alaska (Leasi et al, 2021). The interannual change in seasonal sea ice reduction events in the northern Bering Sea could also result in the production of planktonic copepods (Kimura et al, 2022). Thus, we need further studies on unfolding annual changes in meiofaunal communities in the area, based on time series data covering both non-bloom and bloom seasons.…”