Zooplankton are small animals that live suspended in water. This water column life-style requires coping with a number of distinct abiotic and biotic factors. Zooplankton movement is largely the consequence of currents and not directed swimming activities. The flow of the Hudson River pushes plankton continuously downstream. Tidal currents in the estuary retard this downstream movement, but populations must still contend with constant net displacement toward the sea with associated increases in salinity. Plankton in the Hudson are adapted to live in freshwater or moderate salinity conditions, and as a consequence there are two fairly distinct communities - freshwater plankton and those tolerant of salinity that we will call true estuarine plankton. In addition to adapting to the abiotic features of the Hudson, zooplankton must acquire food to support growth and reproduction. Appropriate food resources like algae are embedded in a mixture of organic and inorganic particles from which zooplankton must acquire energy. Hudson River zooplankton are also preyed on by many fishes and invertebrates that may be sufficiently abundant to regulate their prey populations.

There are many ways to organize a discussion of a community of organisms. Here, we describe the composition of the freshwater and estuarine zooplankton communities including their distribution and abundance at several temporal and spatial scales. Measurements of abundance allow us to assess the importance of zooplankton relative to other groups (for example, benthos) in the Hudson. Our main contention is that zooplankton are not particularly significant in the context of the overall flow of energy in the Hudson ecosystem. Zooplankton do not account for a large component of system biomass, respiration, or productivity, nor do they consume a large fraction of primary production. Nutrient recycling by zooplankton is also not important, because dissolved nutrients are in plentiful supply. Zooplankton, however, are key intermediaries in the food web. They are a major diet item for many fishes, particularly critical early life history stages. Thus, zooplankton consume phyto-plankton, bacteria, detritus, and other small animals, and provide suitably-sized food for larger invertebrates and fish.

A second focus of the chapter concerns the changes of zooplankton communities in the Hudson as a consequence of human impacts. Major alterations to the ecosystem include the introduction of sewage treatment, changes in pollutant loadings, alterations of fisheries and fish populations, andinvasions of exotic species. Ideally we could assess how each of these impacts affected zooplankton and their interactions with other components of the ecosystem. However, both general understanding and specific data are insufficient. Instead, we focus on one documented case - the invasion and spread of zebra mussels (see chapter on invasive species). These animals invaded the freshwater Hudson estuary in the early 1990s and by late 1992 established a large population (Strayer et al., 1999; Strayer, Chapter 21 this volume). Since 1993 the zebra mussel population has sustained high filtration rates with consequent impacts onphytoplank-ton (Caraco et al., 1997) and other components of the Hudson ecosystem (Strayer et al., 1999; Strayer and Smith, 2001). Zebra mussels have also had significant differential impacts on zooplankton (Pace, Findlay, and Fischer, 1998) that are important in assessing how changes imposed by the invasion may propagate to fish populations.

Our strategy for this chapter is not an exhaustive treatment of the ecology of the Hudson zooplankton or even a partial review of the ecological interactions affecting zooplankton in a riverine-estuarine system. For a more detailed treatment of life history, physiological ecology, evolutionary biology, predator-prey dynamics, and population processes, the reader should consult more comprehensive texts and reviews. We recommend Day et al. (1989) for a basic introduction to estuarine zooplankton, and for more advanced treatments of zooplankton, Kerfoot (1980) andMauchline (1998).

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