Midsummer succession of rotifer plankton in a shallow eutrophic pond

Temporal changes in the density of rotifer plankton were examined in a shallow eutrophic pond during July to September 1990, when abiotic environmental variables were relatively stable. In early July when Daphnia similis was abundant, rotifer populations remained at a low density. This is probably due to interference by the large cladoceran, because possible food was abundant and the rotifers showed high egg ratio. After late July, when the large cladoceran disappeared, a striking succession was found in the rotifer plankton. In late July, when Filinia longiseta and Conochilus dossuarius dominated, and in mid- to late September when F.opoliensis and Brachionus falcatus dominated, there was a causal relationship between the density and egg ratio, suggesting that temporal changes in rotifer density were regulated mainly by food abundance. However, in August, changes in the density of most rotifers were not necessarily related with those in the egg ratio. Brachionus angularis, B.forficula and Keratella spp. decreased or remained at a low density regardless of the egg ratio, when the population of Asplanchna brightwelli was developed. However, Polyarthra vulgaris, B.calyciflorus and H.intermedia increased or maintained a high density at this time. Stomach contents of A.brightwelli revealed that B.angularis, B.forficula and Keratella spp. were the preferred prey, whereas P.vulgaris, B.calyciflorus and H.intermedia were not. These results provide strong evidence that the structure of rotifer plankton can change strikingly within a season due to species-specific differences not only in diet, but also in the ability to escape predation, even if abiotic environmental variables are stable.