Daphnia Effects on Microzooplankton: Comparisons of Enclosure and Whole- Lake Responses

Abstract

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.

The relative importance of fish predation and excretion effects on planktonic communities

“…Indeed, our study suggests that fish predation on large cladocerans (Ͼ500 m) was the most important mechanism accounting for the observed increase in protozoans and rotifers in the presence of fish (Table 3). A plausible explanation is that the selective predation of perch on large cladocerans may have released the microzooplankton from the negative effects of these large herbivores, such as mechanical interference and exploitative competition for food (Sarnelle 1997;Pace et al 1998). In spite of their strong positive effect on protozoans and rotifers after 14 d, planktivorous fish had no effect on bacteria biomass (Table 3).…”

Fading out of the trophic cascade at the base of the microbial food web caused by changes in the grazing community in mesocosm experiments

“…The experiment carried out in large enclosures for over two weeks met the requirements for 'mesocosms', a large-scale experiment (Bloesch 1988). According to Sarnelle (1997), in terms of duration and enclosure-size, such large-scale experiments imitate more precisely the natural dynamics of a lake compare with small-scale (microcosm) experiments, and thus provide reliable results allowing a prediction of whole-lake responses to changes within top consumers of the food webs. Our mesocosm experiment allowed to track changes within communities of varying body size, as well as, with short and long generation time, such as bacteria and eu-APP, ciliates and NF, as well as rotifers and phytoplankton.…”

Short‐term impacts of nutrients,Daphnia, and copepods on microbial food‐webs of an oligotrophic and eutrophic lake

“…Most experimental studies of the effects of cladocerans on microbial food-webs have focused on Daphnia (e.g., Pace & Funke 1991;Wickham & Gilbert 1993;Jiirgens et al 1994b;Massana et al 1994;Pace & Vaque 1994;Marchessault & Mazumder 1997;Sarnelle 1997;Callieri et al 1999); there have been fewer studies of the impact of copepods, particularly calanoid copepods, and there have been few comparisons of the consumer effects (top-down) of cladocerans and copepods on microbial food-webs. In studies of the effects of Daphnia and calanoid copepods (Boeckella, Eudiaptomus) on microbial food-web organisms in mesotrophic lakes in New Zealand and Germany, the copepods had a much stronger negative effect on ciliates than Daphnia (Burns & Schallenberg 1996;Adrian & Schneider-Olt 1999).…”