Trophic interactions between zooplankton and the microbial community in contrasting food webs: the epilimnion and deep chlorophyll maximum of a mesotrophic lake

Abstract

Trophic interactions between the mesozooplankton and the microbial web (bacteria, protozoa) were studied in 2 in situ grazing experiments in the epilimnion and the deep chlorophyll maximum (DCM) of a mesotrophic lake. We combined the Landry-Hassett dilution technique, whereby growth and grazing rates can be determined simultaneously, with the presence or absence of mesozooplankton and additional nutrients. The epilimnion was cladoceran-dominated, and had relatively high ciliate, and low heterotrophic nanoflagellate (HNF) abundance, while the DCM was rotifer-dominated, with fewer ciliates, but higher HNF abundance. Temperature differed by 13°C (22°C in the epilimnion; 9°C in the DCM). The community differences between the epilimnion and DCM and the experimental manipulations had their greatest effects on ciliates, lesser effects on HNF, and essentially no net effect on bacteria. Bacteria growth rates and grazing losses did not differ between the epilimnion and DCM, despite the large differences in chlorophyll a and abiotic parameters, and were independent of nutrient addition or mesozooplankton removal. In contrast, the experimental manipulations produced relatively moderate changes in HNF growth rates and grazing losses in both the epilimnion and DCM. Although HNF densities were 3 times higher in the DCM than the epilimnion, epilimnetic HNF growth rates were approximately twice the rate found in the DCM. This was consistent with both higher temperature and a release from predation through indirect predatory effects on alternative HNF predators like ciliates. In the DCM, crustacean zooplankton were less abundant and therefore had smaller direct and indirect effects on the HNF, while the metazoan microzooplankton (mainly rotifers) were more important. Ciliate growth rates were lower in the epilimnion than in the DCM, but only in the epilimnion were ciliate growth rates enhanced by the addition of nutrients. The cladoceran-dominated community of the epilimnion was able to reduce ciliate growth rates to negative values, while in the rotifer-dominated DCM, ciliate growth rates were always positive along a predator density gradient. These results demonstrate that while ciliates, and to a lesser extent heterotrophic flagellates, respond quickly to changes in predator community composition, there are so many other direct and indirect pressures on bacteria that changes in the mesozooplankton community composition have no net effect on their adundance. KEY WORDS: Trophic interactions · Microbial web · Deep chlorophyll maximum · MesozooplanktonResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 24: 83-97, 2001 is not a good predictor of bacterial abundance (Pace 1993). Predatory effects of zooplankton on the microbial community are species-specific , Burns & Schallenberg 1996, Wickham 1998, Adrian & Schneider-Olt 1999, Mohr & Adrian 2000. Thus, the trophic links should differ in the course of the seasonal planktonic succession (Sanders et al. 1994) or in different c...

Hidden diversity among aquatic heterotrophic flagellates: ecological potentials of zoosporic fungi

“…can also consume other HF . Among metazoa, several studies have provided evidence that cladocerans are the main grazers of HF (Köthe & Benndorf, 1994;Jürgens & Stolpe, 1995;Jürgens & Jeppesen, 2000;Adrian et al, 2001;Auer and Arndt, 2001), as well as of small ciliates and bacteria (Hartmann et al, 1993;Jürgens & Jeppesen, 2000). Several studies have highlighted that rotifers and copepods are also able to consume small ciliates and HF in the size range of 20-50 lm (Dolan & Gallegos, 1991;Arndt, 1993;Sanders et al, 1994;Adrian & Schneider-Olt, 1999;Jürgens & Jeppesen, 2000;Zöllner et al, 2003;Bundy et al, 2005).…”

Importance of zooplankton for the persistence of a deep chlorophyll layer: A limnocorral experiment

“…Others have found that phytoplankton migrate between strata (Tilzer 1973;Gasol et al 1992), and our limnocorrals contained many flagellated species capable of movement (chrysophytes, cryptophytes, and dinophytes). Removal of macrozooplankton from the limnocorrals may also have allowed microzooplankton to proliferate and graze heavily on the picoplankton that were most abundant in the DCL (Adrian et al 2001).…”

Predicting grazing mortality of an estuarine dinoflagellate, Pfiesteria piscicida

“…Copepod grazing may indirectly have a negative effect on the total grazing pressure on zoospores because copepods have higher clearance rates for ciliates than they do for small flagellates such as P. piscicida zoospores (Stoecker & Sanders 1985, Gifford & Dagg 1988, Hansen et al 1994. Microzooplankton populations, particularly planktonic ciliates, respond quickly to changes in mesozooplankton abundance, and, in turn, microzooplankton strongly control <15 µm flagellates (Miller et al 1995, Adrian et al 2001.…”