Combined Effects of Algal (Chlorella vulgaris) Density and Ammonia Concentration on the Population Dynamics of Ceriodaphnia dubia and Moina macrocopa (Cladocera)

Ammonia is a natural variable in ponds and lakes. Although an important source of nitrogen for microalgae, at high concentrations ammonia can affect the density and diversity of cladocerans. Using the cladocerans Ceriodaphnia dubia, and Moina macrocopa, the effect of nominal concentrations of ammonium chloride under acute and chronic exposures at different levels of algal food was tested. Regardless of food level, C. dubia was more sensitive than M. macrocopa to ammonia. In the absence of food, the median lethal concentration of ammonia (LC50 24 h) for C. dubia was (112 mg L-1) less than half that of M. macrocopa (232 mg L−1). When algal food (0.5×106 and 1.5×106 cells ml−1 of Chlorella) was used, the LC50 values were much higher for both cladoceran species. Based on the population growth studies, it was found that in controls of M. macrocopa an increase in the availability of Chlorella from 0.5 to 1.5×106 cells mL−1 led to an increase in the maximum density from 4.7±0.2 to 16.4±1.2 ind•mL−1, while in C. dubia the peak population density decreased from 7.9±0.6 to 5.0±l.0 ind•mL−1. An increase in ammonia concentration (10 to 40 mg L−1 for C. dubia and 20 to 120 mg L−1 for M. macrocopa) resulted in a corresponding decrease in peak population densities of the tested cladocerans. The rate of population increase (r) values for M. macrocopa in the controls ranged from 0.21±0.001 and 0.25±0.02 at 0.5 and 1.5×106 cells mL−1 of Chlorella, respectively. The corresponding values of C. dubia in controls were 0.21±0.004 and 0.18±0.01. At 0.5×106 cells mL−1 of algal food, the r values became negative under 40 and 120 mg L−1 of ammonia for C. dubia and M. macrocopa, respectively. The role of algal food in ammonia toxicity to cladocerans was discussed.