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.

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We evaluated the food quantity and quality over a seasonal cycle for the development and egg production of the common North Sea copepods Temora longicornis and Pseudocalanus elongatus, using a bioassay approach. Seston was sampled from December to October from a wellmixed water column of the Marsdiep (Dutch Wadden Sea) and fed to cultured copepods at a constant temperature of 15°C, thus excluding seasonal effects of temperature, body size, age, and maternal nutrition. Copepod response was evaluated by measuring egg production and juvenile development, while the seston quantity and quality were measured as the concentrations of chl a, specific phytoplankton pigments, particulate organic carbon (POC), particulate organic nitrogen (PON), fatty acids, and sterols. The egg production of both copepods was low when feeding on seston collected in winter, but increased to peak values with the seston from the spring bloom in March-April. The juveniles of both species were able to complete their development only in spring experiments. A multiple regression analyses and comparison to a good-quality standard food of the same concentration suggested that, in an annual scale, the egg production and development of T. longicornis mainly depended on phytoplankton concentration, while the egg production and development of P. elongatus appeared also to benefit from detritus or heterotrophic food sources. The present study did not detect an influence of a specific food quality variable; however, an unexplained high juvenile mortality in summer suggests that all factors are not understood yet. KEY WORDS: Copepod · Food quantity · Food quality · Seasonal dynamics · Bioassay Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 399: [141][142][143][144][145][146][147][148][149][150][151][152][153][154][155] 2010 other variables, such as the amount of nitrogen (Kiør-boe 1989), polyunsaturated fatty acids (PUFAs) (Jónas-dóttir 1994, Jónasdóttir et al. 1995, Pond et al. 1996, Hazzard & Kleppel 2003, amino acids (Kleppel et al. 1998), sterols (Klein Breteler et al. 2005), or the concentration of heterotrophic food (Ohman & Runge 1994, Dutz & Peters 2008. Theoretical arguments suggest that, since the potential food items are likely to differ in their biochemical components, copepods should be able to overcome limitation due to PUFAs, amino acids, or sterols by selective feeding (Anderson & Pond 2000). In contrast, a frequent limitation by mineral nutrients (e.g. nitrogen) or food quantity could be expected. However, in the field, effects of food quantity and quality are difficult to distinguish (see e.g. Müller-Navarra & Lampert 1996), since many of the quality indicators are related to the quantity. For instance, many of the PUFAs co-vary with the phytoplankton concentration (see e.g. Jónasdóttir et al. 1995), and a correlation between, for example, copepod egg production and concentration of PUFAs thus does not necessarily indicate limitation by food quality, but can simply be due to ...