Zooplankton are typically viewed as grazers on phytoplankton, but consumption of heterotrophic organisms, especially bacteria, flagellates, and ciliates, can be significant both in terms of the mortality of heterotrophs and as a resource for zooplankton. Additionally, in many aquatic systems zooplankton constitute a significant biomass pool and are important in the cycling of carbon, nitrogen, and phosphorus as well as other elements.
Relative zooplankton biomass, however, is not large in the Hudson ecosystem. For example, in the freshwater Hudson the biomass of benthic animals including zebra mussels averages 12gC m-2 d-1 (Strayer and Smith, 2001) while zooplankton biomass is less than 0.1 g C m-2 d-1. Calculations of the contribution of zooplankton to system respiration and secondary production indicate similar low relative contributions from zooplankton (Lints, Findlay, and Pace, 1992; Caraco et al., 2000). Nutrient recycling of nitrogen and phosphorus by zooplankton is also not significant as dissolved nutrients (PO4, NH4, and NO3) are available at high concentrations (Clark et al., 1992; Lampman, Caraco, and Cole, 1999), and phytoplankton are limited primarily by light (Cole, Caraco, and Peierls, 1992).
Grazing by larger zooplankton (i.e., the mi-crometazoa and mesozooplankton) also has little effect on phytoplankton. For example, Lonsdale et al. (1996) found that grazing was less than 10 percent and often less than 1 percent of primary production per day during spring and summer in the lower estuary. Grazing impacts were higher (60 percent) infallbutprimaryproductionwas also low at this time of the year. In the freshwater estuary, grazing by zooplankton (>73 |m) was also low, ranging from 1-13 percent d-1 (Pace and Findlay, unpublished data). Therefore, in both freshwater and saline sections of the estuary, direct measurements as well as extrapolations based on biomass indicate that grazing is not a major fate of phyto-plankton. However, the grazing impact of smaller zooplankton, including heterotrophic flagellates and ciliates, has yet to be evaluated in the Hudson. These organisms can consume a significant fraction of phytoplankton biomass.
Flagellates and ciliates can contribute significantly to the diets of larger zooplankton such as copepods (Stoecker and Capuzzo, 1990). Ciliate concentrations in the Hudson estuary are similar to other coastal waters and preliminary research suggests that ciliates, includingbothnonloricate forms and tintinnids, contribute to the diets of larger zooplankton (Lonsdale, unpublished data). Copepod addition experiments, conducted in March and June using seawater collected from the Verrazano
Narrows, resulted in mean clearance rates of 2.35.8 ml copepod-1 h-1 for Acartia adults feeding on ciliates >20 |im. These clearance rates are within the range reported previously for Acartia (Stoecker and Capuzzo, 1990). Copepod nauplii also ingested some ciliates in both seasons. Cladocera (for example, Bosmina) and rotifers also prey on ciliates and heterotrophic flagellates (Sanders and Wickham, 1993).
Zooplankton also consume bacteria. Rates of consumption by heterotrophic flagellates, ciliates, and cladocerans are similar to those observed in other systems (Vaque etal., 1992). Copepods probably do not feed directly on bacteria, at least the unattached forms that dominate numerically in the Hudson. However, detritus and the associated microbial populations do contribute to cope-pod diets in the estuary (Chervin, 1978; Chervin, Malone, and Neale, 1981), and observations from the Columbia River estuary suggest most consumption by copepods in the turbidity maximum is on bacteria associated with particles (Simenstad, Small, and McIntire, 1990). Copepods also are often selective for flagellates and ciliates that, in turn, are significant consumers of bacteria (Sherr and Sherr, 1987; Stoecker and Capuzzo, 1990; Merrell and Stoecker, 1998).
Zooplankton grazing does not appear to be sufficient to balance bacterial production (Vaque et al., 1992). Nevertheless, grazing is an important fate of bacteria with an average of 10 to 20 percent; of bacteria consumed daily. Further, estimates of carbon requirements of zooplankton suggest that ingestion of bacteria can satisfy much of their demand. Bacterial production in the Hudson is largely uncoupled from primary production (Findlay et al., 1991), hence much of the carbon that ultimately fuels zooplankton production may arise from the watershed and move through into the food web via bacteria and bacterial predators. This conjecture, however, is based on limited evidence and more direct analysis using appropriate tracers (for example, stable isotopes, fatty acids) is needed to substantiate the hypothesized linkage.
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