Effect of ammonia toxicity on the competition among three species of cladocerans (Crustacea: Cladocera)

Among the natural abiotic variables affecting the species density and composition of cladocerans, ammonia is important. Using population growth data as tool, we studied the competitive outcome of three cladoceran species (Ceriodaphnia dubia, Daphnia pulex, and Moina macrocopa) grown alone and together at three different concentrations (0, 25, and 100 mg L−1) of ammonia at one density (1×106 cells ml−1) of the microalga Chlorella vulgaris. Regardless of the presence or absence of competing species, C. dubia showed little population growth at 100 mg L−1 of ammonia. At 25 mg L−1 of ammonia, the population density of C. dubia increased but the value was much lower than that in control. D. pulex was able to grow at the ammonia concentration of 25 mg L−1, comparable to controls. However, at 100 mg L−1 of ammonia, the population declined and more rapidly when competed with other cladocerans. M. macrocopa showed decreased population density with increasing ammonia concentration in the medium. This trend was accelerated by the presence of competing cladocerans. M. macrocopa was benefited by the presence of D. pulex at 0 and 25 mg L−1 of ammonia in the medium. The peak population density of C. dubia varied from 0.5 to 9 ind. ml−1, whereas for D. pulex and M. macrocopa, these values varied from 1 to 3 ind. ml−1 and 7 to 18 ind. ml−1, respectively, depending on the ammonia concentration and the presence of competing species. The rates of population increase per day of C. dubia, D. pulex, and M. macrocopa ranged from −0.12±0.001 to 0.30±0.01, 0.04±0.001 to 0.22±0.002, and 0.201±0.028 to 0.235±0.019, respectively, depending on the ammonia concentration and the presence of competitors. Our study showed that while competition within the chosen cladoceran species caused suppression of one or two competing species, the presence of ammonia expedited this process based on the relative sensitivities of the tested zooplankton.