A high frequency time series at Weathership M, Norwegian Sea, during the 1997 spring bloom:: the reproductive biology of Calanus finmarchicus

The reproductive biology of Calanus finmarchicus was investigated at a permanent station in the Norwegian Sea (Weathership Stn M, 66 degrees N, 2 degrees 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 examined from WP2 net samples taken daily from the upper 100 m. Daily egg production rate and number of spawning females were 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 distribution of females and gonad development stages. Results show that the reproductive period of C. finmarchicus in the Atlantic region of the Norwegian Sea can be subdivided in 3 periods in relation to phytoplankton development: (1) During the prebloom over a period of 49 d mean egg production rate was 8 eggs female(-1) d(-1) and an average of 47 % of the females were mature. (2) Coincident with the bloom in mid May the egg production rate increased up to a maximum of 44 eggs female(-1) d(-1) 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. Feeding experiments indicate that food quantity limited egg production prior to the bloom, while presumably food quality was not sufficient during 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.