a mean for long-term monitoring station near Kingston. b mean for the freshwater estuary.

From Strayer and Smith (1996, 2001), Caraco et al. (1997, 2000), Strayer et al. (1999), Pace, Findlay, and Fischer (1998), and Findlay, Pace, and Fischer (1998).

a mean for long-term monitoring station near Kingston. b mean for the freshwater estuary.

From Strayer and Smith (1996, 2001), Caraco et al. (1997, 2000), Strayer et al. (1999), Pace, Findlay, and Fischer (1998), and Findlay, Pace, and Fischer (1998).

bacteria increased substantially. Native planktonfeeders declined sharply, probably because their phytoplankton food was so depleted. Growth, abundance, and distribution of young-of-year fish changed substantially after the zebra mussel invasion. Physical and chemical characteristics of the Hudson changed as well. Water clarity and dissolved nutrients rose in response to the loss of phytoplankton. The increase in water clarity probably led to an increase in the size and thickness of submersedvegetation, although goodpre-invasion data are lacking. Even dissolved oxygen concentrations in the Hudson fell because of respiration by the enormous zebra mussel population. Thus, the zebra mussel invasion led to a series of large, ecologically important changes in the Hudson ecosystem that probably are long-lasting (decades) to permanent.

In addition to these ecological changes, zebra mussels cause economic damage in the Hudson. They attach to water intakes, boat hulls, and other submerged objects, increasing costs for inspecting and maintaining submerged equipment. Power plants and drinking water intakes have increased the frequency of intake inspections, and most now add anti-fouling chemicals such as chlorine, potassium permanganate, or polyquaternary ammonium compounds to prevent fouling by zebra mussels. Annual costs probably are in the range of $100,000 to 1,000,000/peryear.

ii. black bass (Micropterusspp) Most of the freshwater sport fish in eastern New York (including rainbow and brown trout, northern pike, largemouth and smallmouth bass, rock bass, black and white crappie, bluegill, and walleye) are not native to this region, but were deliberately introduced in the late ninteenth and early twentieth century (Mills et al., 1996,1997). Of these, the black bass (the largemouth bass, Micropterus salmoides, and the smallmouth bass, M. dolomieu) are the most important in the Hudson River.

Until the late nineteenth century, black bass were widespreadin the Great Lakes and Mississippi River drainages, but absent from the Northeast (Robbins and MacCrimmon, 1974). They moved eastward into the Hudson basin along the Erie Canal when it was opened in 1825, and were stocked into hundreds of lakes and rivers in the Northeast in the late nineteenth century (Cheney, 1895). Both species of black bass are now among the most common freshwater fish in the region. Largemouth bass typically occur in quiet, weedy waters such as ponds, lakes, and slow-moving rivers, while smallmouth basspreferrunningwatersorrockylakeshores(e.g., Robbins and MacCrimmon, 1974; Smith, 1985).

Both species occur throughout the freshwater and oligohaline Hudson River. The largemouth is the more common species (10,000-30,000 fish >280 mm long; Carlson, 1992; Green et al., 1993); the smallmouth is less common (5,000-10,000 fish >280 mm long) and found chiefly in the upper and middle parts of the estuary.

Black bass are important in the Hudson because of their value to the sport fishery and their impacts on prey populations. Black bass are among the most popular sport fish in the freshwater estuary. Between 1986 and 1991, 50-60 bass tournaments were held annually in the Hudson estuary, averaging 1,500 angler-days of effort per year (Green et al., 1993). The economic benefit of these tournaments was considerable. Of course, people fish for black bass in the Hudson outside of the organized tournaments, resulting inmany hours of recreation and many dollars spent in the local economy. The ecological impacts of blackbass in the Hudsonhave not been studied, but can be roughly assessed. Black bass are large, omnivorous predators which have important effects that cascade through food webs (e.g., Carpenter and Kitchell, 1993;Fuller, Nico, and Williams, 1999). Based on their biomass and typical physiological efficiencies, the black bass populations in the Hudson estuary probably consume very roughly 107 gdrymass of prey each year. While this is less than 1 percent of the annual production of forage fish and invertebrates in the freshwater estuary (Lints, Findlay, andPace, 1992), blackbass are highly localized (Carlson, 1992; Nack et al., 1993) and have strong preferences for specific prey, so it is likely that the black bass invasion has affected at least the abundance of preferred prey in local areas in the Hudson.

iii. water chestnut (Trapa nutans) Water chestnut is a striking aquatic plant (Fig. 21.4, color plate 5) native to Eurasia. Its biology was well summarized by Kiviat (1993), from whom much of the present account was drawn. The plant consists of a rosette of floating leaves, buoyed by air bladders in the petioles, which is attached to the sediments by a long, tough stem. This annual plant produces hard, spiny nuts (Fig. 21.4) that are viable for a decade or more. Although the seed encased in this nut is edible, Trapa is not the familiar water chestnut of Chinese cuisine, which is a sedge

(Eleocharis). Water chestnut lives in quiet waters up to 5 m deep, and may form dense, nearly inpenetrable stands of >1 kg dry matter/m2.

Water chestnut was introduced into North America in the late nineteenth century by well-meaning botanists, one of whom wrote: "but that so fine a plant as this, with its handsome leafy rosettes, and edible nuts, which would, if common, be as attractive to boys as hickory nuts now are, can ever become a nuisance, I can scarcely believe" (Kiviat, 1993). It arrived in the Hudsonbasinwhenit was deliberately introduced into Collins Lake near Schenectady in 1884. It escaped into the Mohawk River by the 1920s, and then into the Hudson estuary in the 1930s. By the 1950s, water chestnut was a nuisance in the Hudson. It is now widespread in quiet bays and backwaters of the Hudson estuary from Troy to Iona Island, with larger beds reaching 10-100 hectares in extent.

Water chestnut is a nuisance because its thick beds impede boating and other recreational activities, and because its spiny nuts can injure swimmers. In addition, water chestnut has been accused of having undesirable ecological impacts, including outcompeting native plants, increasing sedimentation rates, and lowering dissolved oxygen, thereby reducing the value of shallow-water habitats to fish and waterfowl. Because water chestnut has floating leaves that release photosynthetic oxygen into the air while shading out and preventing photosynthesis in the underlying water, dissolved oxygen concentrations can fall to zero in large, dense beds (Fig. 21.5; Caraco and Cole, 2002). Nevertheless, water chestnut supports dense communities of invertebrates and fish, although not necessarily the same species that live in native vegetation (Findlay, Schoeberl, and Wagner, 1989; Pelczarski and Schmidt, 1991;HankinandSchmidt, 1992; Gilcrest and Schmidt, 1998; Strayer et al., 2003).

Because of its negative impacts, people have tried to eradicate water chestnut from the Hudson. In the 1960s and early 1970s, the Department of Environmental Conservation used 2,4-D as a control. Since high doses of 2,4-D have been banned, hand-pulling or cutting in local areas (for example, around marinas and beaches) is the only control that has been practiced. There has been considerable interest in using herbivorous insects as a cc E < —

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