Effects of salinity, food level, and the presence of microcrustacean zooplankters on the population dynamics of rotifer Brachionus rotundiformis

We investigated the population dynamics of the rotifer Brachionus rotundiformis fed with the alga Isochrysis galbana at two food concentrations (3 × 10⁴ and 40 × 10⁴ cells ml⁻¹) and four salinity levels (5, 10, 20, and 30) in the presence and absence of two copepod species, Pseudodiaptomus annandalei and Apocyclops royi and one cladoceran, Diaphanosoma aspinosum. Both the density and population growth rate of B. rotundiformis increased at higher food concentration and at salinity levels of 10 and 20. Among the microcrustaceans, only P. annandalei had a significant negative effect on the growth rate of the rotifer population because of its efficient predation. In contrast, the presence of both A. royi and D. aspinosum did not affect the growth rates at any of the salinity and food levels. Brachionus rotundiformis had significantly larger size during the log-phase, particularly if P. annandalei was present. Thus, B. rotundiformis grows better at higher food level and medium salinity levels. Unlike the larger calanoid, P. annandalei, B. rotundiformis can definitely coexist with relatively small cyclopoid copepods (A. royi) and cladocerans (D. aspinosum), because of the absence of interference.