VARIATIONS IN CARBON AND NITROGEN-CONTENT WITH BODY LENGTH OF DAPHNIA-HYALINA-GALEATA S L FROM LABORATORY AND FIELD OBSERVATIONS

Length-carbon (L/C) and length-nitrogen (L/N) regression equations on In-transformed data are presented for Daphnia hyalina-galeata s.l. cul tured for one generation in the laboratory at food levels of 1.36 and 0.17 mg Cl-1, and for a field population at different periods of the y ear. The equations of the two sets of data did not differ significantl y: 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 rea ched; 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 cha nges in age, with higher values for newborn and a decrease of similar to 7% for specimens 20 days old (2 mm in length). The line of the equa tion obtained from the laboratory data set fitted well the data from a natural population in spring. Both carbon and nitrogen content were q uite lower in summer, suggesting a marked decrease in available food. Changes in the intercept of In-transformed regression equations obtain ed assuming a slope between 2.7 and 3 (from the laboratory equations) seem to suggest that food limitation was taking place in summer. Never theless, the possibility of using the intercept values to infer food a vailability in natural environments is discussed.