Abstract
Fatty acids contribute to the nutritional quality of the phytoplankton and, thus, play an important role in Daphnia nutrition. One of the polyunsaturated fatty acids (PUFAs)––eicosapentaenoic acid (EPA)––has been shown to predict carbon transfer between primary producers and consumers in lakes, suggesting that EPA limitation of Daphnia in nature is widespread. Although the demand for EPA must be covered by the diet, the demand of EPA in Daphnia that differ in body size has not been addressed yet. Here, we hypothesize that the demand for EPA in Daphnia is size-dependent and that bigger species have a higher EPA demand. To elucidate this, a growth experiment was conducted in which at 20 °C three Daphnia taxa (small-sized D. longispina complex, medium-sized D. pulicaria, and large-bodied D. magna) were fed Synechococcus elongatus supplemented with cholesterol and increasing concentrations of EPA. In addition, fatty acid analyses of Daphnia were performed. Our results show that the saturation threshold for EPA-dependent growth increased with increasing body size. This increase in thresholds with body size may provide another mechanism contributing to the prevalence of small-bodied cladocera in warm habitats and to the midsummer decline of large cladocera in eutrophic water bodies.
Highlights
Allometric relationships for many of the essential features of organisms are widely observed (Woodward et al 2005)
S. elongatus was chosen for the experiments, because it is non-toxic and lacks sterols and polyunsaturated fatty acids (PUFAs)
With increasing concentrations of eicosapentaenoic acid (EPA) in the food suspensions, juvenile growth rates increased for all the daphnids tested (Fig. 1a–c)
Summary
Allometric relationships for many of the essential features of organisms are widely observed (Woodward et al 2005). The competitive ability of zooplankton is strongly correlated with zooplankton body size, and large-bodied species are competitively superior (Brooks and Dodson 1965). Zooplankton species differ with respect to their elemental composition (C:P ratio). The phosphorus demand is higher in large-bodied species due to their higher growth rate than in small-bodied species that grow slower (Sterner and Schulz 1998). Similar considerations among aquatic consumer taxa have led to the concept of threshold elemental ratios of carbon and phosphorus. These threshold ratios varied considerably among 41 investigated taxa, which could be attributed to interspecific differences in body C:P ratios and gross growth efficiencies of carbon (Frost et al 2006)
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