Midsummer succession of rotifer plankton in a shallow eutrophic pond

Temporal changes in the density of rotifer plankton were examinedin a shallow eutrophic pond during July to September 1990, whenabiotic environmental variables were relatively stable. In earlyJuly when Daphnia similis was abundant, rotifer populationsremained at a low density. This is probably due to interferenceby the large cladoceran, because possible food was abundantand the rotifers showed high egg ratio. After late July, whenthe large cladoceran disappeared, a striking succession wasfound in the rotifer plankton. In late July, when Filinia longisetaand Conochilus dossuarius dominated, and in mid- to late Septemberwhen F.opoliensis and Brachionus falcatus dominated, there wasa causal relationship between the density and egg ratio, suggestingthat temporal changes in rotifer density were regulated mainlyby food abundance. However, in August, changes in the densityof most rotifers were not necessarily related with those inthe egg ratio. Brachionus angularis, B.forficula and Keratellaspp. decreased or remained at a low density regardless of theegg ratio, when the population of Asplanchna brightwelli wasdeveloped. However, Polyarthra vulgaris, B.calyciflorus andH.intermedia increased or maintained a high density at thistime. Stomach contents of A.brightwelli revealed that B.angularis,B.forficula and Keratella spp. were the preferred prey, whereasP.vulgaris, B.calyciflorus and H.intermedia were not. Theseresults provide strong evidence that the structure of rotiferplankton can change strikingly within a season due to species-specificdifferences not only in diet, but also in the ability to escapepredation, even if abiotic environmental variables are stable.