Survivals of D. galeata in sub-tropical reservoirs: harmful effects of toxic cyanobacteria in food source

In the present study, we tested the hypothesis that massive occurrence of cyanobacteria as food source is one of the factors limiting the distribution of the zooplankton Daphnia galeata in the warm water reservoirs of South China. D. galeata was fed with Chlorella pyrenoidosa, 4 strains of cyanobacteria (three strains of Microcystis aeruginosa: MC1, MC2 and MC3, and one strain of Pseudoanabaena sp. :PA), a mixture of C. pyrenoidosa and cyanobacterial strains, and natural particles from the reservoir where the animal was initially collected. On medium and high food levels (1 and 1.5 mg C L-1) of C. pyrenoidosa, D. galeata showed a typical Type III of survivorship, and had a high intrinsic rate of population increase (0.37-0.41). On low food levels, the intrinsic rate of population increase declined but lifespan largely increased (84 days). When feeding with natural particles, the intrinsic rate of increase was evidently reduced (0.19), but the maximal body length (2 mm) was much longer than that of individuals observed in the fields implying that D. galeata was strongly preyed upon by size-selective predators. Applying a mixture of Chlorella and cyanobacteria, the strain and proportion of cyanobacteria significantly affected life history variables of the animal and its somatic growth. On the sole diet of each stain of cyanobacteria, D. galeata failed to reproduce. Microcystin and colonial morphology of Microcystis strains reduced clutch size and somatic growth rate. In comparison, the less negative effect of the strain MC3 indicates that the morphology was critical for actual ingestion of toxic cyanobacteria by the zooplanktons. Pseudoanabaena sp. had a short filament (15 mu m), can be easily used as a food by D. galeata. Composition of phytoplankton community plays a significant role in survival and population sizes of D. galeata and massive occurrence of cyanobacterial blooms limits the distribution of D. galeata in the reservoirs of South China.