THE EFFECT OF SUSPENDED CLAY ON CILIATE POPULATION-GROWTH RATES

1. Renewed batch cultures of three algivorous and one bacterivorous ciliate showed that ciliate reproductive rates increase with increasing food concentration up to an asymptote. Maximum r(m) values for Strobilidium gyrans, Bursaridium difficile and Euplotes eurystomus feeding on the cryptomonad alga Rhodomonas minuta were 0.87, 0.86 and 0.45 day-1, respectively, while Paramecium aurelia feeding on the bacterium Enterobactor aerogenes had an r(m) of 0.40 day-1. B. difficile had a food threshold (r(m) = 0) of 0.38 mug C ml-1 and P. aurelia had a threshold of 1.6 mug C ml-1. 2. The presence of suspended fine (particle size <1 mum) and coarse (particle size 1-2 mum) clay suppressed the half maximum reproductive rate of the ciliate Strobilidium gyrans by up to 69% but had no effect on B. difficile, E. eurystomus or P. aurelia. Populations of ciliates unaffected by clay may be released in situations where suspended particles negatively affect their metazooplankton predators and competitors.