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Planktonic copepods are primary consumers in the ocean and are perhaps the most numerous metazoans on earth. Secondary production by these zooplankton supports most food webs of the open sea, directly affecting pelagic fish populations and the biological pump of carbon into the deep ocean. Models of marine ecosystems are quite sensitive to the formulation of the term for zooplankton mortality, although there are few data available to constrain mortality rates in such models. Here we present the first evidence for nonlinear, density-dependent mortality rates of open-ocean zooplankton. A high-frequency time series reveals that per capita mortality rates of eggs of Calanus finmarchicus Gunnerus are a function of the abundance of adult females and juveniles. The temporal dynamics of zooplankton populations can be influenced as much by time-dependent mortality rates as by variations in 'bottom up' forcing. The functional form and rates chosen for zooplankton mortality in ecosystem models can alter the balance of pelagic ecosystems, modify elemental fluxes into the ocean's interior, and modulate interannual variability in pelagic ecosystems.

Overwintering of Calanus finmarchicus and C. helgolandicus was studied in the field and in laboratory experiments in a shallow (120 m) Swedish fjord and in a deep (650 m) Norwegian fjord. In the Norwegian fjord 2 populations were found in late summer and autumn: in the surface layer the copepods were smaller and more active with high respiratory and digestive enzyme activities. The deep population, consisting of Copepodid stage V (CV) and a few females, was torpid, had large oil sacs and empty guts. Their respiratory and digestive enzyme activities were very low. In the Swedish fjord CV in deep layers weighed much less than those in the Norwegian fjord. Weight-specific respiration was intermediate between deep and surface population in the Norwegian fjord. It is concluded that overwintering copepodids do not feed. Metabolic rates allowed successful overwintering only in the Norwegian fjord. Experiments performed on several occasions during overwintering witnessed -in contrast to the situation in field populations -increased rates of respiration and moulting. Mortality during moulting and time span between capture and onset of moulting decreased during autumn and winter. These observations suggest internal development during overwintering.

The reproductive biology of Calanus finmarchjcus was investigated at a permanent station in the Norwegian Sea (Weathership Stn M, 66" N, 2" E) during a time series between March and June 1997 The temporal development of female abundance, egg production rate and gonad development stage in relation to the phytoplankton production cycle is described Abundance of females, copepodite stage 5 and males as well as female gonad morphology were examlned from MrP2 net samples taken daily from the upper 100 m. Daily egg production rate and number of spawn.ing females we]-e determined from 50 individual females placed in multiwells or beakers. Once a week a multinet haul from 1000-500-250-100-50-0 m was performed to study the depth d~stribution of females and gonad development stages. Results show that the reproductive period of C. finmarchicusin the Atlantic rcqion of the Norwegian Sea can be subdivided in 3 periods in relation to phytoplankton devrlopmt,nt.(1) During the prebloom over a period of 49 d mean egg production rate was 8 eggs female-' d ' and an average of 47 of the females were mature. (2) Coincident with the bloom in mid May the e g g production rate increased up to a maximum of 44 eggs female-' d-' while more than 80% of the females were mature. (3) After the bloom at the beginning of June, egg production decreased, and mature females were only rarely found. Feedlng expenments indicate that food quantity limited egg production prior to the bloom, while presumably food quality was not sufficient dunng postbloom. However, due to high female abundance the total population egg production prior to the bloom was the same as during the bloom. This implies that the reproduction of C. finmarchicus in the Norwegian Sea is to some extent decoupled from the phytoplankton bloom.