Temperature- and cholesterol-induced changes in eicosapentaenoic acid limitation of Daphnia magna determined by a promising method to estimate growth saturation thresholds
Limnol. Oceanogr., 56(4), 2011, 1273-1284 | DOI: 10.4319/lo.2011.56.4.1273
ABSTRACT: We present data on eicosapentaenoic acid (EPA)-limited growth responses of Daphnia magna under different temperatures and different dietary cholesterol availabilities to assess how EPA growth saturation thresholds depend on changing environmental conditions. D. magna was raised on gradients of dietary EPA at 15°C and 20°C with high cholesterol supply and at 20°C with low and high cholesterol supply in laboratory experiments. A new method was applied to estimate EPA growth saturation thresholds on the basis of fitted saturation curves using bootstrapped data. The EPA threshold at which 75% and 90% of maximum growth was reached ranged from 0.7 to 1.6 µg EPA (mg dietary C)−1 and 2.0 to 4.9 µg EPA (mg dietary C)−1, respectively. Previously reported EPA concentrations in natural seston of many different lakes suggest that the thresholds measured here indicate a frequent potential for at least moderate EPA limitation in nature. Furthermore, the calculated EPA thresholds were higher in treatments of low compared with high temperature and higher in treatments of low compared with high cholesterol availability. The EPA-dependent growth responses were more strongly affected by temperature than by cholesterol availability. Our results suggest that EPA growth saturation thresholds for a particular Daphnia species probably vary in nature under different environmental conditions.