Grazing by a dominant rotifer Conochilus unicornis Rousselet in a mountain lake: 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 the 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 23Mum in diameter. At epilimnetic temperatures of around 14 deg. C, mean clearance rates for 5Mum particles ranged from 30 to65 Mul ind.^-1 h^-1. Clearance rates were 2-9 times higher on the 5Mum spheres than on the 9 Mum spheres, and C. unicornis almost never fed on the 23 Mum spheres. Grazing rates did not change over the diel cycle. Clearance rates declined more than 10-foldas temperatures declined from 14 deg. C in the epilimnion to 7 deg.C in the metalimnion. In the epilimnion, grazing by C. unicornis wasmore important than grazing by crustaceans in the community, at least on particles ???9Mum. The results show the importance of grazing byrotifers in lakes, and the significance of spatial variations that influence grazing rates. [ABSTRACT FROM AUTHOR]