EFFECT OF FOOD CONCENTRATION ON THE ASSIMILATION AND PRODUCTION EFFICIENCIES OF DAPHNIA-GALEATA GO SARS (CRUSTACEA, CLADOCERA)

Measurements of growth, respiration and ingestion were used to examine the influence of food concentration on the assimilation and individual production efficiencies of Daphnia galeata G. O. Sars at food concentrations of 2.50, 0.25 and 0.05 mg C l-1. A plot of assimilation rate against body size showed an S-shaped curve. In the first adult instar, the rates at 2.50 mg C l-1 were two and five times higher than those at 0.25 and 0.05 mg C l-1, respectively. Assimilation efficiency, gross growth efficiency (K1) and net growth efficiency (K2) showed binomial curves with maxima at the last juvenile or early adult instars. Assimilation efficiency increased with decreasing food concentration; an opposite trend was found for K2. Except for early juvenile instars, K1 was highest at the intermediate food concentration, suggesting that there is an optimal resource concentration for energy transfer efficiency. The lower value of K1 at the higher food concentration was due to lower assimilation efficiency, while that at the lower food concentration was due to high respiration rate relative to the amount of food assimilated. These results demonstrate that food concentration affects the carbon balance not only quantitatively (rates), but also qualitatively (efficiencies). The change in carbon balance with changing food concentration is interpreted as a strategy for D. galeata to maximize its assimilation rate at given food concentrations.