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Rates of body growth, development and egg production of Pseudocalanus elongatus were measured in the laboratory, in relation to the taxonomy and physiology of algal food. Four types of experiments were performed to measure the copepod's response to (1) 7 algal species of similar size and shape, but different taxonomic groups, (2) inferior food species that were offered with good food as a check of toxicity, (3) nitrogen limitation and the growth rate of food species, and (4) highly unsaturated fatty acids that were supplied with inferior food to test for lack of essential fatty acids. Grazing was measured to check that the offered food was really ingested. The best food species proved to be Rhodomonas sp., whlch induced a fast rate of development, good somatic growth and egg production and low mortality. The development rate was almost equally fast 1~1th Thalassiosira weissflogi~, Gyrnnodinium simplex and Tetraselmjs suedca, but the rates of somatic growth or egg production were lower and mortahty generally higher than with Rhodornonas sp. Three algal species, Dunaljella sp , Amphidinium sp, and Chrysochrornulina polylepjs, were poor food; copepod development was not completed, the rates of somatic growth and egg production were low and mortality was high. Ingestion was equally high with most of the species; only C. polylepis was not eaten No clear toxic effects were found when the 3 poor-food species were offered in mixtures w~t h Rhodomonas sp. N-limited Rhodomonas sp. did not reduce the rate of copepod development in comparison to a N-replete culture; however, N-limited T weissflogii reduced the development rate to the low level of poor-food species. No effect of different growth rates of Dunaliella sp. was found. Lipids rich in highly unsaturated fatty acids supplied with DunalieUa sp. did not substantially improve the slow development and low egg production observed with this species. The weight-specific somatic growth rate was always higher than the weight-specific egg production rate, especially with less optimal food, which seems to hamper the estimation of the secondary production of copepods based on egg production alone. It is concluded that large differences in the food quality of different algal species are due to differences in digestibility or in mineral and biochemical composition.

The development of 2 copepod species was studied with nutrient-limited algae as the sole food source. Thalassiosira weissflogii was grown under different levels of nitrogen and phosphorus limitation. Young copepodite stages of Temora longicornis (Müller) and Pseudocalanus elongatus (Boeck) developed at significantly reduced rates, both when fed with the nitrogen-and the phosphorus-limited diatom. At high levels of nutrient limitation, the copepods often did not reach maturity. The lipid composition of the diatom was strongly affected by nutrient limitation. Both the proportion and the content of long-chain polyunsaturated fatty acids (PUFAs) were reduced, particularly under phosphorus limitation. The dominant sterol, Δ 5,24(28) C28:2, was reduced by about a factor of 2 both under nitrogen and phosphorus limitation. The results suggest that the different growth rates of the copepods observed can be explained by the different lipid composition of the algae due to nutrient limitation.

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