Environmental and nutritional factors determining seasonal variability in the fecundity and egg viability of Calanus helgolandicus in coastal waters off Plymouth, UK

Fecundity of the copepod Calanus helgolandicus was monitored at a coastal station in the western English Channel during January to September 1994 and varied seasonally from 3 to 33 eggs female(-1) d(-1) (mean 17 eggs female(-1) d(-1)). Particulate fatty acid levels, particularly in the <20 mu m size fraction, were more highly correlated with fecundity than either chi a, particulate carbon or nitrogen, and this reflected the size and species composition of the microplankton. Diatoms, dinoflagellates and ciliates all sustained high fecundity. The mean volume of eggs produced by C. helgolandicus was inversely related to both food availability and fecundity, with increased numbers of smaller eggs produced during periods of high food availability. Female C. helgolandicus with the highest body nitrogen and carbon produced eggs with the highest carbon and nitrogen contents, although these eggs were also smaller. Hatching viability was consistently high with a seasonal mean of 83% and both viability and the incidence of deformed nauplii were negatively correlated with the carbon and nitrogen content of females and eggs. Hatching viability was not correlated with the abundance of diatoms, but was closely correlated with the levels of 18:2(n-6) and 20:4(n-6) fatty acids in both the environment and in the eggs. Particulate concentrations of the polyunsaturated fatty acids 20:5(n-3) and 22:6(n-3), although abundant in the environment and therefore good indicators of food availability, were not correlated with either egg viability or naupliar survival. The levels of saturated and monounsaturated fatty acids in the eggs were important factors determining naupliar survival under starvation conditions. Although food availability limited fecundity, female recruitment was possibly limited by predation pressure, particularly during the late summer. This study highlights the value of seasonal and nutritional based studies in understanding variability in zooplankton fecundity.