BIOMASS, GROWTH, AND DEVELOPMENT OF POPULATIONS OF HERBIVOROUS ZOOPLANKTON IN THE SOUTHEASTERN BERING SEA DURING SPRING

Two distinct communities of herbivorous zooplankton, separated by an oceanographic front, inhabit the continental shelf and slope of the southeastern Bering Sea during spring. The community over the outer shelf and slope is dominated by populations of large-sized oceanic copepods (mainly Neocalanus spp.) that developed early in spring and attain maximum biomass and growth rates by mid- to late spring. Total biomass and growth rates of herbivores follow the spring outburst of phytoplankton; during April and May biomass increases from .ltoreq. 1 to .apprx. 14 g C m-2 on the slope and to .apprx. g C m-2 on the outer shelf, and maximum growth rates > 500 and .apprx. 300 mg C m-2 day-1 occur on the slope and outer shelf, respectively, in May. The dominant species, N. plumchrus, grows from copepodid I to V between late March and early May, and after attaining maximum body weight in late May and early June it begins its downward migration. The inshore community on the middle shelf is dominated by the euphausiid Thysanoessa raschi in April and May and by the copepod Calanus marshallae in late May and early June. Total biomass (.ltoreq. 1 g C m-2) and growth rates (.ltoreq. 50 mg C m-2) of the inshore community are substantial lower than those of the offshore community, and show a delayed response to the spring bloom of phytoplankton; both biomass and growth rates increase about one month after the bloom. Small herbivorous copepods contribute little to the total biomass and growth rates of either community and the cumulative community growth rates during April and May decrease from 18.3 g C m-2 on the slope to 2.5 g C m-2 on the middle shelf.