From Strayer and Smith (2000, 2001).

Gammarus tigrinus is common throughout the freshwater tidal Hudson River. Like other benthic animals, its local density varies from place to place by more than 1,000-fold. Of this variation, 11 percent canbe attributed to sampling error, 14 percent can be explained by environmental variables like bottom type, and 75 percent remains unexplained (Strayer and Smith, 2001). This unexplained variation could be due to biological factors (e.g., sediment bacteria, amphipod behavior, fishpreda-tion), disturbance history, unmeasured characteristics of the environment (e.g., local current regime, sediment stability), and so on. Understanding the causes and consequences of spatial variation in large-river benthic communities represents a major research challenge.

Temporal Variation in the Hudson Zoobenthos

Benthic communities vary over time in response to season, disturbances, species invasions, human alteration of riverine habitats, long-term climate change, and so on. Unfortunately, very little is known about how the Hudson's benthos varies over time. Based on work done in other rivers and estuaries, we can assume that there are significant seasonal changes in the community (for example, Wolff, 1983; Beckett, 1992). The Hudson's zoobenthos must have changed greatly over the past one hundred to two hundredyears in response to pollution and habitat alteration by humans. Further, it seems likely that there has beennaturallong-term variation in the benthic community. However, we know almost nothing about the nature of these changes. The only temporal change in the zoobenthos that has been well studied in the Hudson is its response to the zebra mussel invasion.

Zebra mussels first appeared in the river in May 1991 and by the end of 1992 constituted over half of heterotrophic biomass in the freshwater tidal Hudson (Strayer, Chapter 21, this volume). They reduced the biomass of phytoplankton and small zooplankton by 80-90 percent (Caraco et al., 1997; Pace, Findlay, and Fischer, 1998; Chapter 9, this volume; Chapter 16, this volume), changed the species composition of the remaining phytoplank-ton (Smith et al., 1998), increased water clarity by 45 percent (Caraco et al., 1997), changed concentrations of dissolved oxygen and plant nutrients (Caraco etal., 2000), and increased numbers ofbac-terioplankton (Findlay, Pace, and Fischer, 1998).

The zoobenthos showed three kinds of response to the zebra mussel invasion. First, there was an overall depletion of the zoobenthos other than zebra mussels (Fig. 19.4, upper). Riverwide, we estimated a loss of about 4,000 animals/m2 (Strayer and Smith, 2001), or roughly three benthic animals lost for every zebra mussel that appeared. Taken together with losses in the zooplankton (Pace et al., 1998), we estimated that about half of the biomass of invertebrates useful for fish forage was lost from the Hudson with the zebra mussel invasion (Strayer and Smith, 2001).

Second, the response of benthic species to the zebra mussel invasion depended on their trophic group. Species that feed on plankton (that is, suspension-feeders plus the phantom midge Chaoborus punctipennis, which eats small zooplankton) declined much more severely than species that feed on benthic food (that is, predators and deposit-feeders) (Fig. 19.4, middle). Since the zebra mussel invasion, benthic planktivores have declined by 46-100 percent, and several formerly common species appear to be on the verge of disappearing from the Hudson. Because these benthic planktivores constituted more than half of heterotrophic biomass in the freshwater tidal Hudson River before the zebra mussel invasion,

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