Variations in carbon and nitrogen content with body length of <i>Daphnia hyalina-galeata s.l</i>. from laboratory and field observations

Abstract

Length-carbon (L/C) and length-nitrogen (L/N) regression equations on In-transformed data are presented for Daphnia hyalina-galeata s.l . cultured for one generation in the laboratory at food levels of 1.36 and 0.17 mg C l −1 1, and for a field population at different periods of the year. The equations of the two sets of data did not differ significantly: animals were selected during the intermolting phase, thus excluding the effect of egg production. The length-standardized carbon content (C/L) changed exponentially with length until size at maturity was reached; in large-sized mature females it remained fairly constant, thus suggesting that above threshold food concentrations, variations should be mainly associated with egg production. The C:N ratio reflected changes in age, with higher values for newborn and a decrease of ˜7% for specimens 20 days old (2 mm in length). The line of the equation obtained from the laboratory data set fitted well the data from a natural population in spring. Both carbon and nitrogen content were quite lower in summer, suggesting a marked decrease in available food. Changes in the intercept of In-transformed regression equations obtained assuming a slope between 2.7 and 3 (from the laboratory equations) seem to suggest that food limitation was taking place in summer. Nevertheless, the possibility of using the intercept values to infer food availability in natural environments is discussed.