Possibility of N or P limitation for planktonic cladocerans: An experimental test

Abstract

Threshold food N : C or P : C ratios, below which net production is N or P limited, were calculated for two cladocerans, Daphnia galeata and Bosmina longirostris, based on the N and P contents of body tissue and the C balance between ingestion and net production rates under experimental conditions with various food concentrations from 0.05 to 2.50 mg C liter‒1. With changing food concentration, the threshold N : C and P : C ratios for D. galeata changed in weight ratio from 0.039 to 0.077 and 0.0067 to 0.0120, respectively, with the lowest value at the high food concentration. Changes in threshold ratios for B. longirostris, however, were small and mean values were 0.085 and 0.0080 for the N : C and P : C ratios. As a result, the threshold N : C ratio for D. galeata was lower than for B. longirostris at high food concentrations, while the P : C ratio for B. longirostris was lower than for D. galeata at lower food concentrations. Although the threshold elemental ratios calculated here are theoretically minimum estimates, they were higher than the lower end of the sestonic elemental ratios in some lakes, suggesting that it is not unusual for these cladocerans to be N or P limited at certain periods of the year. Sestonic elemental ratios may have potential importance in determining the community structure of zooplankton.