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Abstract:
We investigated the changes in life histories imposed on the water flea, Daphnia magna, due to biochemical and mineral limitations. Phosphorus-deficient Scenedesmus obliquus were incubated with or without a single essential fatty acid, eicosapentaenoic acid (EPA). Additionally, the algae were spiked with dissolved phosphorus to create a range of C : P ratios from 600 to 200. This procedure created the possibility to study the importance of different essential resources. We found that somatic growth is retarded until a C : P ratio (molar) similar to350 is reached. Adding more phosphorus did not further increase growth. At the same time, at high C : P ratios the addition of EPA did not make a difference in growth, whereas below the nutrient threshold (C: P = 350), the fatty acid had a strong positive impact on growth. In a second experiment we studied how the food conditions (with regard to EPA) affected the growth and investment in reproduction and whether this effect was passed on to the next generation. We found that animals fed EPA made an earlier and larger investment in reproduction. Also, the EPA-enriched animals had a higher mortality. The juveniles from mothers fed EPA-enriched algae had a higher growth rate than neonates from control mothers.
