Daphnia effects on microzooplankton: Comparisons of enclosure and whole-lake responses

The effects of the herbivorous zooplankter, Daphnia, on a natural community of microzooplankton (rotifers, ciliates, and nanoflagellates) were compared in enclosures of two sizes (15 and 10 000 L) and in a eutrophic lake before and after a fish kill. The magnitude of Daphnia's effect on microzooplankton taxa varied quantitatively and qualitatively between experiments. The rotifer Synchaeta pectinata and the ciliate, Didinium sp., showed extreme differences between experiments, exhibiting opposite responses in small vs. large enclosures. The relative rankings of responses for two rotifers (Polyarthra dolichoptera and Synchaeta pectinata) and two ciliates (Coleps sp. and Didinium sp.) were reversed in small vs. large enclosures. Polyarthra was depressed by Daphnia more than Synchaeta in small enclosures, while Synchaeta was depressed more than Polyarthra in the large enclosures. Didinium was depressed more than Coleps in the small enclosures, while the reverse occurred in large enclosures. Microzooplankton responses also changed over time in the large-enclosure experiment. Daphnia significantly suppressed total ciliate dry biomass, but not total rotifer biomass after 6 d, but suppressed dry biomass of both groups by the end of the 23-d experiment. This result may reflect changes in the relative importance of Daphnia's predatory and competitive effects over time. Daphnia's effects on the rotifers, Polyarthra and Hexarthra mira, took 3 wk to be expressed but were very strong by the end of the large-enclosure experiment, suggesting that short-term experiments may underestimate competitive effects. Polyarthra's delayed response was associated with a delayed response of cryptophytes to the Daphnia manipulation. Comparisons of enclosure results with temporal correlations in lake data and with microzooplankton abundances before and after a fish kill indicated better congruence between lake data and the large-enclosure results. More importantly, large-enclosure results agreed more closely with whole-lake dynamics than with small-enclosure results. Consequently, large-scale enclosure experiments may provide reliable predictions of whole-lake phenomena, but small-scale (microcosm) experiments may not.