Calcium balance in Daphnia grown on diets differing in food quantity, phosphorus and calcium

1. The influences of dietary phosphorus (P) and food concentration on the calcium (Ca) balance in Daphnia magna were examined in this study at two different ambient Ca concentrations (0.5 and 10 mg Ca L^?1). Daphnia were grown by feeding the young adults differentially under contrasting dietary P conditions [molar C : P ratio = c. 900 and c. 90 as low P (LP) and high P (HP), respectively], ambient Ca concentrations [0.5 mg and 10 mg Ca L^?1 as low Ca (LCa) or high Ca (HCa), respectively] and food levels [0.15 or 1.5 mg C L^?1 as low food (LF) or high food (HF), respectively] for 5 days. 2. The specific Ca contents of daphniids (1.9–6.5% of dry weight^?1) increased with increasing Ca concentration, food level and dietary P content, although the food level did not affect the Ca content in the HPHCa treatment. A radioactive tracer method showed that the food level did not affect the influx of Ca from the water under LP conditions, but the Ca influx under HP conditions doubled with a HF level. A LP condition also led to a decrease in Ca influx with a HF level. 3. During the 3 days of efflux, generally only a small proportion of Ca (2.6–3.3%) was retained by the daphniids, but this retention increased (14–23%) under low ambient Ca concentrations and under P‐limitation. Excretion was the most important pathway for Ca loss (accounting for 50–60% of body Ca), followed by moulting (20–47%), but the relative contribution of these two pathways (excretion and moulting) did not vary among all the different treatments. The absolute loss of Ca through excretion and moulting, on the contrary, differed with different ambient Ca concentrations and dietary P conditions. A HF level led to an increase in the loss rates in most cases. 4. Our study strongly suggested that there is an interaction between an essential metal (Ca) and macronutrients (C and P) in freshwater crustaceans with HCa and P contents. The results imply that variation in environmental nutrient conditions may change the Ca budget in crustaceans and may affect the dynamics of Ca in the epilimnion of freshwaters.