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

Observations of natural feedlng and egg-production rates of planktonic copepods have revealed distinct responses, Independent of phytoplankton blomass, to oceanographic processes that ferhlize the photic layer Are such responses caused by changes ln phytoplankton growth rate, influencing feeding behaviour, and/or by changes In the chemical composition of the phytoplankton, lnfluenclng fecundlty? The dlatom Thalassioslra welssflogu, grown in continuous cultures at different d ~l u h o n rates and different nltrogen concentrations In the growth medlum, was offered to the copepod Acarha tonsa at a concentrahon of 1 5 p p m 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 remalned constant whereas ingested nltrogen as well as rate of egg production increased by a factor of ca 6 Vanahon in mgested cell volume was interpreted as a behavioural feedlng response to vanations In algal growth condltlons, vanation in ingested mtrogen and egg production, as being caused malnly by vanatlon In algal nltrogen content It 1s concluded that vanable fecundlty may potentially be explained by vanable chemical composition of the algae while the observed feeding response is probably msufflclent to account for the vanabllitv observed In the sea