Variation in hatching success and egg production of Eurytemora affinis (Calanoida, Copepoda) from the Gulf of Bothnia, Baltic Sea, in relation to abundance and clonal differences of diatoms

We monitored the hatching frequencies and clutch sizes of Eurytemoraaffinis, and environmental variables in the Gulf of Bothnia,between May and October 2003. We tested the hypothesis thatthe hatching frequency of the copepod would be negatively affectedduring time periods with high diatom concentrations. Resultsfrom two stations showed significant differences between thedifferent sampling occasions, with the lowest observed hatchingfrequency during the spring diatom bloom. The reverse was truefor clutch size, with the highest average egg number duringthe diatom bloom. These results were not correlated to ambienttemperature, salinity or chlorophyll a (Chl a). In a separatelaboratory experiment, nine different local clones of the diatomSkeletonema costatum were used as food for adult E. affinisfemales, in order to screen for possible differences in toxicitywithin the Skeletonema community. The resulting average copepodhatching frequency varied between 5 and 75% for the differentclones, indicating that there can be large within-species variationin the toxic properties of diatoms. The significance of suchvariations in natural communities remains to be tested in futurestudies.