primarily on benthic animals, or on benthivorous fish (Table 19.1). Thus, benthic animals form the main link between phytoplankton, macrophytes, and allochthonous inputs at the base of the Hudson's food chain, and fish at its top. Because the zebra mussel invasion radically reduced the biomass of invertebrates that serve as fish food in the Hudson (Pace et al., 1998; Strayer and Smith, 2001), we might expect to see consequent changes in the Hudson's fish communities.

Finally, the feeding, burrowing, and movement of benthic animals mix sediments. Such mixing activities may alter exchanges of materials between sediment and overlying water (e.g., McCall and Tevesz, 1982; Robbins, 1982; Van de Bund et al., 1994). Although sediment mixing by benthic animals has not been investigated in the Hudson, its benthos is dominated by animals that are known to be effective sediment mixers (i.e., tubi-ficid oligochaetes, chironomids, amphipods, and unionid mussels - Robbins, 1982; Van de Bund, Goedkoop, and Johnson, 1994; McCall etal., 1995), and many important substances in the river (notably PCBs) are associated with the sediments. Thus, it seems likely that benthic animals play important roles as sediment mixers in the Hudson.

The role of benthic animals in the Hudson ecosystem is thus larger and more complex than would be suggested from a conventional assessment of biomass or metabolism. The overall importance of the zoobenthos in the ecosystem differs across specific roles, as does the importance of different members of the zoobenthos. Thus, bivalves are important suspension-feeders, amphipods are especially important as fish food, and oligochaetes probably are important in mixing sediments. Even this brief consideration of a few specific roles of benthic animals shows that they form a vital part of the Hudson River ecosystem.

The relative importance of the two major groups of invertebrates - zooplankton and zoobenthos -differs across types of aquatic ecosystems. Pace et al. (1992) pointed out that zooplankton densities are lower in advective habitats such as estuaries and rivers than in still-water habitats such as lakes. In contrast, because benthic animals are not carried en masse downriver by water flow, we would expect that benthic animal densities could be just as high in rivers and estuaries as in lakes. Available data support this idea, and further show

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