Predation on rotifers by the small suspension-feeding Calanoid copepod Diaptomus pallidus was examined in order to quantify the effects of prey density, prey type, and the presence of algal food resources on ingestion rates, and to determine whether ingested rotifer biomass could be utilized to enhance the survival and reproduction of the copepods. Clearance and ingestion rates of D. pallidus on rotifers were 5.5-6.2 times greater than on algae presented at the same concentration and similar to the maximum rates previously reported for more carnivorous cyclopoid copepods preying on rotifers. The survival and reproduction of D. palhdus were substantially enhanced by the addition of rotifers to a threshold algal diet. When presented with a natural assemblage of plankton, D. pallidus preferentially ingested certain rotifer species over others. Predation on rotifers by such diaptomids may form an important trophic link in freshwater planktonic food webs.

Grazing rates of a freshwater copepod (Diaptomus kenai) and a freshwater cladoceran (Daphnia rosea) on the green alga Selenastrum minutum were determined to be dependent on the nutritional status of individual cells. Cells that were less nitrogen limited were ingested at a greater rate than cells reared under more nitrogen limited conditions. Diaptomus displayed active discrimination, possibly via chemodetection, while Daphnia expressed passive selection, probably via differential retention on filters. These results suggest that the impact of grazing zooplankton varies with the physiological state of components of the phytoplankton community.

No 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...

The relationships between survivorship and fecundity of Diaptomus pakdus and algal food concentrations (Cryptomonas reflexa) were examined with a flow-through system that prevents the depletion and settling of algal cells. Egg production in adult females increased with increasing food concentration up to a maximum at 600 pg dry wt liter-*. Under starvation conditions, egg production stopped and survival decreased, whereas at 60 pg dry wt liter-l production was halted and survival was high. Adults survived longer under conditions of starvation than did nauplii. Adult survival was high at all food concentrations; naupliar survival was highest at 60 pg dry wt liter-*, slightly lower at 600 pg dry wt liter-l, and much lower at 6,000 pg dry wt liter-l. Naupliar development was faster at 600 and 6,000 pg dry wt liter-l than at 60 pg dry wt liter-l.