The influence of diatom abundance on the egg production rate of Calanus helgolandicus in the English Channel

Weekly measurements of Calanus helgolandicus egg production rates and phytoplankton species identification and abundance have been carried out at a station in the English Channel for a period of 7 yr. These data have been analyzed to investigate the effects of the microplankton composition on egg production rates with special emphasis on groups where nutritional value is controversial, such as diatoms and the Prymnesiophyte, Phaeocystis pouchetii. Diatoms did not show any negative effect on the egg production rate. Phaeocystis pouchetii also showed no negative effects, whereas in contrast, a dinoflagellate, Gyrodinium aureolum, clearly showed a negative effect. We conclude that diatoms do not have a negative effect on egg production rates at the concentrations found in the field and for these natural species assemblages, and that when such an effect exists, as is the case with G. aureolum, the effect can be clearly identified.