Top-down control of population dynamics of the dominant rotifers in two mesotrophic lakes in Hokkaido, Japan.

The effects of abiotic and biotic factors on the population dynamics of rotifers were investigated over two years in mesotrophic Lakes Ohnuma and Konuma (Hokkaido, Japan). Four rotifer species, Polyarthra vulgaris, Trichocerca rousseleti, Keratella cochlearis and Filinia longiseta, dominated rotifer assemblages of both lakes. Their population densities changed seasonally with species-specific trajectories, but these dynamics were not related to seasonal variations in water temperature and dissolved oxygen content. Analysis of the population dynamics revealed that the instantaneous death rate affected the rate of the density change more strongly than the instantaneous birth rate in all dominant rotifer species. The estimated daily ration of predatory rotifers and copepods was higher than the estimated finite death rate of the rotifers in most cases. Effects of food limitation and competition with herbivorous crustaceans seemed to be less important compared to that of the invertebrates predation. These results indicate that in Lakes Ohnuma and Konuma the population dynamics of rotifers were mainly determined by invertebrate predators through top-down control.