A study of the planktonic rotifers (Rotatoria) of Grasmere

The planktonic rofiters of Grasmere, a small lake in the English Lake District, were studied from August 1969 to December 1972. Twenty-four species were recorded but five were very rare. The remaining species were divided into three groups according to their seasonal occurrence: spring-autumn species (Keratella quadrata. K. cochlearis. Gastropus stvlifer. Asplanchna priodonta. Kellicottia longispina. Conochilus hippocrepis). spriqg-early summer species (Polvarthra dolichoptera, Synchaeta tranula. S. pectinata. S. stylata, S. oblonga. C. unicornis), summer- autumn species (Polvarthra vulgaris, P. major, Filinia terminalis. S. grandis. Ploesoma hudsoni, Trichocerca capucina. T. similis). The months in which each species were abundant are given.Keratella quadrata and Filinia terminalis were most abundant - in the deepest stratum, Kellicottia longispina and Conochilus spp. were most abundant in the upper and middle strata, Trichocerca spp. showed no pronounced vertical distribution, and all the remaining species were most abundant in the upper stratum. Keratella cochlearis, Kellicottia longispina and Synchaeta spp. (temperature only) attained their highest densities over a wide range of temperature and oxygen concentration, and Filinia terminalis, Conochilus spp., Asplanchna priodonta, K. quadrata, Polyarthra spp., Synchaeta spp. (oxygen only) attained their highest densities within a narrow range of temperature and oxygen concentration. Optimum ranges are given for each species. Major changes in abundance occurred between 1971 and 1972 when Asnianchna priodonta, Kellicottia longispina. Conochilus unicornis and Filinia terminalis increased in abundance, and Keratella quadrata. Gastropus stylifer, C. hippocrepis. Polyarthra spp., Synchaeta spp. and Ploesoma hudsoni decreased in abundance. These changes are discussed in relation to the temperature and oxygen requirements of each species and also to the probable enrichment of the lake after the opening of a new sewage works in June 1971.