When a relatively warm year (1981) in the southeastern Bering Sea is compared with a cooler year (1980), the upper layer of both the middle shelf and outer shelf warmed at a faster rate in the warmer year, but the spring bloom of phytoplankton took place at approximately the same time both years. The middle front near the 100 m isobath separated the two major communities of zooplankton both years. Offshore of the front, large calanoid copepods such as Neocalanus plumchrus, Neocalanus cristatus, Eucalanus bungii, and Metridia pacifica dominated, while inshore of that front Pseudocalanus spp., Acartia spp., and Calanus marshallae dominated. Over the outer shelf (Sta. 5) at the end of April, N. plumchrus, N. cristatus, E. bungii, and Pseudocalanus spp. were significantly more abundant in 1980 than in 1981, while over the middle shelf (Sta. 12) all stages of Pseudocalanus spp. and C. marshallae were more abundant in 1981 than in 1980. Gradients in abundance of N. plumchrus and N. cristatus across the outer shelf reversed after periods of wind favourable to subsurface onshore flow suggesting that the shelf population of large calanoids is derived from overwintering slope populations during spring and summer storms. N. plumchrus and C. marshallae, which were thought to reproduce once per year based on 1980 data, produced two cohorts at some stations in 1981, and both E. bungii and C. marshallae reproduced earlier in 1981 than in 1980. Survivorship of N. plumchrus and N. cristatus over the outer shelf (Sta. 5), where only one cohort was produced both years, was higher in 1981 than in 1980, suggesting that predation occurred later in 1981 than in 1980. The abundance of the chaetognath Sagitta elegans over the middle shelf was higher in 1980 than in 1981 suggesting that both increased temperature and reduced predation may account for the larger numbers of small copepods found over the middle shelf in the warmer year.
ASJC Scopus subject areas
- Aquatic Science