Predation by ciliates on a metalimnetic Cryptomonas population: feeding rates, impact and effects of vertical migration

Abstract

Deep algal maxima are frequently overlayed by dense populations of ciliates, rotifers and crustaceans. This has been interpreted as evidence of heavy predation on the algae, although the impact of this predation has never been determined experimentally. We determined the vertical and seasonal distribution of the alga Cryptomonas phaseolus and its most relevant predators, the ciliates Coleps sp. and Prorodon sp., forming metalimnetic maxima in Lake Cisó. On several dates, in situ feeding rates of ciliates were determined by three independent methods: (i) epifluorescence counts of ingested algal cells together with estimates of the food turnover time of the ciliates; (ii) in situ incubations with radioactively labeled algae: (iii) HPLC determination of alloxanthine content in the predator size fraction. Feeding rates varied between 0.07 and 0.64 Cryptomonas ciliate⁻¹ h⁻¹. We then calculated integrated predation on the algae. using the functional response of the ciliates and the vertical distribution of each population. We found that even though the ciliates were always food saturated, their predation impact on Cryptomonas was not very large: as an average, 5–25% of the biomass of Cryptomonas was removed daily by the ciliates. Finally, we studied the effects of the diel vertical movements of these populations on predation impact. By migrating into the sulfide-rich hypolimnion during the night, Cryptomonas could reduce its predation losses by 38%. Thus, the algae were protected from predation during several hours of each diel cycle and maintained a very large biomass throughout stratification, although this resulted in a very slow growth. Slow growth, coupled to large biomass, seems to be a general feature of metalimnetic accumulations of organisms.