Grazing by a dominant rotifer Conochilus unicornis Rousselet in a mountainlake: in situ measurements with synthetic microspheres

Grazing rates of zooplankton were analysed in the summer of 1999 in Yellow Belly Lake, an oligotrophic system in the Sawtooth Mountains of Idaho (U.S. A.). The colonial rotifer Conochilus unicornis was a dominant species in th e epilimnion, with densities reaching 20 colonies l(-1) (ca. 400 ind. l(-1) ). Clearance rates were measured with an in situ Haney Grazing chamber and synthetic microspheres 5, 9 and 23 mum in diameter. At epilimnetic temperat ures of around 14 degreesC, mean clearance rates for 5 mum particles ranged from 30 to 65 mul ind.(-1) h (-1). Clearance rates were 2-9 times higher o n the 5 mum spheres than on the 9 mum spheres, and C. unicornis almost neve r fed on the 23 mum spheres. Grazing rates did not change over the diel cyc le. Clearance rates declined more than 10-fold as temperatures declined fro m 14 degreesC in the epilimnion to 7 degreesC in the metalimnion. In the ep ilimnion, grazing by C. unicornis was more important than grazing by crusta ceans in the community, at least on particles less than or equal to9 mum. T he results show the importance of grazing by rotifers in lakes, and the sig nificance of spatial variations that influence grazing rates.