Climate-related differences in zooplankton density and growth on the inner shelf of the southeastern Bering Sea

Zooplankton abundance and biomass were measured during spring and late summer on the inner shelf of the southeastern Bering Sea in years of climate extremes. Samples were taken during late spring and late summer of three years: 1997, a year of moderate spring ice cover and unusually warm, calm summer conditions; 1998, a year of warm, but stormy summer conditions with very little spring ice cover; and 1999, a year of extensive spring ice cover, cold spring conditions and storms during spring and summer. Mean water column temperature was significantly lower during June 1999 than June 1998 and 1997. Copepod abundance and biomass during June were correlated with mean water column temperature and mean temperature below the thermocline. Mean calanoid abundance during June 1999 was 8-52% of the mean abundance during 1998 and 1997. Significantly lower abundances during June 1999 were observed for Calanus marshallae, Acartia spp., Pseudocalanus spp. and calanoid nauplii. Significant interannual differences in mean water column temperature and calanoid abundance during late summer (late July-early September) were not detected. The Hirst-Lampitt equations were used to estimate the mean daily copepod production during the warm and cold years. The mean production estimate during warm conditions was 13 mg C m(-2) d(-1), with a range of 3-37 mg C m(-2) d(-1), similar to previous estimates. Production estimates during the cold spring, 1999, were 3-4% of the production during warm periods. Assuming a 35% gross growth efficiency, calanoids could consume an average of 37 mg C m(-2) d(-1) during warm periods, but only about 2.6 mg C m(-2) d(-1) during a cold spring. Comparison of the above estimates to concurrent measures of primary production indicate that during warm, calm seasons, calanoids could remove most or all of the available water column primary production, thus indicating that calanoids may have been food limited. During cool conditions, the calanoids could remove only about 3% of the estimated daily primary production. Lower post-bloom primary production rates and higher calanoid grazing rates may result in substantially lower annual carbon flux to benthic communities on the inner shelf during warm calm years relative to cold years with extensive spring ice cover. (C) 2002 Elsevier Science Ltd. All rights reserved.