Interactive effect of temperature and food concentration on growth rate:: A test case using the small freshwater ciliate Urotricha farcta

The combined effect of temperature and food concentration on the growth rate, cell volume, and production of the freshwater ciliate Urotricha farcta was investigated in laboratory batch cultures. Experimental temperatures ranged from 9 to 25degreesC and food levels ranged from 0.1 to 4.4 mug C ml(-1). The ciliates were fed the small cryptophyte Cryptomonas sp. The combined effect of temperature and food on growth and cell volume resulted in negative production rates at high temperatures and low to moderate food supply. Three main changes were observed in the shape of the numerical response (growth vs. food concentration) of U. farcta with temperature: change in the threshold level, where population net growth rates are zero; change in the initial slope of the numerical response curve; and change in maximum growth rate (mu(max)). The threshold food concentration and mu(max) were shifted up at the highest temperatures. The threshold level was also higher at the lowest experimental temperature. The initial slope of the numerical response curve was several-fold lower at both high and low temperatures. The analysis suggests that temperature altered the numerical response so that the species shifted from being adapted to low food concentrations at moderate temperatures to requiring, and potentially thriving at, high food concentrations at the temperature extremes. These findings support and extend conclusions previously obtained for metazooplankton and indicate that changes as small as 3degreesC could alter the role of protozoa in planktonic food webs.