Phytoplankton growth rate and nitrogen content: implications for feeding and fecundity in a herbivorous copepod

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Abstract
Observations of natural feeding and egg-production rates of planktonic copepods have revealed distinct responses, independent of phytoplankton biomass, to oceanographic processes that fertilize the photic layer. Are such responses caused by changes in phytoplankton growth rate, influencing feeding behaviour, and/or by changes in the chemical composition of the phytoplankton, influencing fecundity? The diatom Thalassiosira weissflogii, grown in continuous cultures at different dilution rates and different nitrogen concentrations in the growth medium, was offered to the copepod Acartia tonsa at a concentration of 1.5 ppm. Over more than 2 orders of magnitude increase in algal growth rate, the ingested cell volume increased by less than a factor of 2, ingested carbon remained constant whereas ingested nitrogen as well as rate of egg production increased by a factor of ca 6. Variation in ingested cell volume was interpreted as a behavioural feeding response to variations in algal growth conditions; variation in ingested nitrogen and egg production, as being caused mainly by variation in algal nitrogen content. It is concluded that variable fecundity may potentially be explained by variable chemical composition of the algae, while the observed feeding response is probably insufficient to account for the variability observed in the sea.