The vertical distribution and migration of planktonic rotifers in a hypereutrophic tarn
The planktonic rotifers of Priest Pot were studied from January 1983 to September 1984. Routine weekly samples from January 1983 to March 1984 provided a uniquely detailed data set giving information on the vertical distribution of the rotifer community in relation to the environmental conditions. Seasonal abundance of the rotifers was also noted. Diel studies in 1983 investigated the vertical migration of the community and experimental work in 1984 produced new information on the causes of migration in planktonic rotifers. Sixteen species were found but six of these were very rare. The remaining ten were divided into perennial species (Keratella cochlearis, K. quadrata, Polyarthra vulgaris, Synchaeta kitina and S. pectinata), springautumn species (Brachionus calyciflorus, B. angularis and Filinia terminalis) and summer-autumn species (Anuraeopsis fissa and F. brachiata) according to their periods of occurrence and abundance. When the tarn was isothermal most of the rotifer species were homogeneously distributed in the water column. This was only altered when algal blooms occurred in spring and autumn, causing some species to aggregate at depths with high chlorophyll concentrations, and when the rotifers were hatching from resting eggs, producing large populations in one section of the water column before dispersal. When the tarn was stratified, from May to September, all the rotifers were confined to the epilimnion by the anoxic water of the bottom two metres. F. brachiata, B. calyciflorus, B. angularis and P. vulgaris were concentrated nearer the surface than the other species. K. cochlearis and K. quadrata were aggregated just above the oxycline and A. fissa often occurred in very low oxygen concentrations and always had maxima at the oxicanoxic boundary. These stratified distributions are linked to food availability and show clearly the oxygen and temperature requirements of the different species. Diel studies in 1983 showed that most of the rotifer species can perform vertical migrations. These were more often a reverse migration (up during the day and down at night) than the expected normal migration (down during the day and up at night) commonly found in studies of crustaceans. The movements were not only linked to the photoperiod but also showed some response to changing chlorophyll levels and to oxygen concentration in the water column. One species, A. fissa, did not show a response to either light or chlorophyll stimuli but its distribution was linked to movements of the oxic-anoxic boundary. Experimental work on the vertical migration of the rotifers produced further evidence that reverse migrations are more common in rotifers than the classical "normal" migration. Manipulation of the community using altered photoperiods showed light to be a controlling stimulus for the migrations. It is suggested that the migration is a result of a circadian rhythm entrained to the exogenous 24 hour cycle by the changing light intensity.