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treatment plants. The estuary receives runoff from a watershed having an area of 34,600 km2 (USGS, 2002). The freshwater discharge from the watershed is similar to that in several other large estuaries, such as Delaware Bay, San Francisco Bay, and Long Island Sound (Limburg, Moran, and McDowell, 1986; Howarth and Swaney et al., 2000; http://data.ecology.su.se/MNODE/wmap.htm; http://cads.nos.noaa.gov). However, when compared to other well-known estuaries with large drainage basins which are part of the Land-Ocean Interactions in the Coastal Zone (LOICZ) project data set, the surface area of the Hudson River Estuary is fairly small, and thus the freshwater discharge per area of estuary is quite high (Table 10.1). This contributes to a rapid flushing of the estuary, with water residence times on the scale of 0.1 to 4 days (Howarth and Swaney et al., 2000). By comparison, the water residence times in Delaware Bay, Chesapeake Bay, and Long Island Sound are on the order of 60, 250, and 1,100 days, respectively (Nixon et al., 1996; http://data.ecology.su.se/ MNODE/wmap.htm; http://cads.nos.noaa.gov). Rapid flushing, particularly when water residence times are less than one to two days, makes the Hudson less sensitive to nutrient pollution than other estuaries with longer water residence times (Howarth and Swaney et al., 2000).

Some 4.4 million people live in the watershed of the Hudson River upstream of the Battery in Manhattan (U.S. Census Bureau, 2002), giving an average population density of 128 individuals km-2. The majority live in the New York City metropolitan area, with fewer living in the mostly forested basin upriver (Howarth, Fruci, and Sherman, 1991; Swaney, Sherman, and Howarth, 1996; Boyer et al., 2002). Note that the population for the entire New York City metropolitan area is substantially higher than 4.4 million, but many of these live in watersheds and sewersheds that do not flow into the Hudson, but rather into Long Island Sound, the Raritan, lower New York Harbor, Jamaica Bay, and other water bodies. The Hudson River Estuary receives a substantial amount of sewage from the New York City metropolitan area, and as a result, the nutrient loading per area or volume of estuary is the highest for any large estuary in the United States (Nixon and Pilson, 1983; NRC, 1993). BOD loading is also high. The first sewers in New York City were constructed in 1696, and the sewer system grew steadily in collection capacity until the middle of the twentieth century (Brosnan and O'Shea, 1996). The first primary sewage treatment plants in the New York City metropolitan area were only built in the 1930s, and secondary treatment plants to reduce BOD loadings were not constructed until after such treatment was mandated by the Clean Water Act in 1972. Full secondary treatment of the waste stream was only achieved in the past decade (Brosnan and O'Shea, 1996). The result has been a dramatic increase in water quality, as measured by dissolved oxygen levels in the estuary (see chapter 23).

In this chapter, we discuss how freshwater discharge and nutrients affect primary productivity in the saline Hudson River estuary, how human activity has altered the inputs of nutrients and labile organic matter to the estuary over time, what regulates oxygen concentrations in the estuary, and what actions might be taken to further improve water quality in the Hudson in the future. Our focus is on the portion of the Hudson River which begins at the Battery at the southern tip of Manhattan and extends northward 66 km to the northern end of Haverstraw Bay (Fig. 10.1). So defined, the saline Hudson estuary has an area of 149 km2 and a volume of 1.11 km3 (estimated from NOAA navigational charts). The estuary can be divided into the saltier, mesohaline region from the Battery north 36 km and the less salty, oligohaline region from river km 36 north to river km 66, an area that includes the Tappan Zee and Haverstraw Bay.

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