Note: the input from the ocean and downstream aquatic ecosystems are not included; see text for derivation of estimates.
parts of New York Harbor and to the coastal waters of the New York Bight. Primary production in lower NewYork Bay and in the plume of the Hudson River in the 1970s and 1980s was reported to be in the range of 600 to800gCm-2y-1 (O'Reilly etal., 1976; Malone and Conley, 1996), indicating that these systems are quite eutrophic (Nixon, 1995; NRC, 2000). Chlorophyll concentrations in the plume of the Hudson River on the continental shelf range up to 20 /xgl-1 (Malone and Conley, 1996), levels which also indicate a high degree of eutrophication (NRC, 1993). The apex of the NewYork Bight (an area of 1,250 km2) becomes hypoxic every year, and a large region of the Bight became anoxic in 1976 (Mearns etal., 1982).
Nutrients enter the saline Hudson estuary from wastewater in the New York City metropolitan area, from combined sewer overflows and storm runoff in the metropolitan area, from the freshwater portion of the Hudson River (above river km 66), and from the salt water entering the estuary from the ocean. In this chapter, we only estimate the inputs from sources within the watershed, including the tributary sources coming down the Hudson River. While first-order estimates of nutrient exchange with the sea are possible under the assumption of steady state
(Gordon et al., 1996), this term is difficult to assess without detailed hydrody-namic modeling, and generally has not been estimated in nutrient budgets for other estuaries (Nixon et al., 1996; NRC, 2000). In the case of the Hudson, it may be large, as the salt water entering the estuary first passes through RaritanBay and lower NewYork harbor, and these systems receive substantial nutrient inputs themselves.
The saline Hudson estuary receives a daily input of wastewater of approximately 3.4 x 106 m3 d-1 (calculated from data in Clark et al., 1992), or one third of the total wastewater flux for the entire New York City metropolitan area, an estimated 10 x 106 m3 d-1 (Clark et al., 1992; Brosnan and O'Shea, 1996). By the early 1990s, all of the dry-weather sewage discharges in the metropolitan area received secondary treatment (Brosnan and O'Shea, 1996; Hetling al., 2003; Chapter 23, this volume). The effluent from the average secondary sewage treatment plant in the United States has a total nitrogen concentration of 19 g N m-3 and a total phosphorus content of 3 g P m-3 (NRC, 1993). Assuming that these values apply to the treatment plants in the New York City metropolitan area, we estimate nitrogen and phosphorus loads to the saline Hudson estuary in the 1990s as 24 x 103 metric tons N y-1 and3.7 x 103 metric tons Py-1 (Table 10.2). Other estimates of nutrient inputs from wastewater to this portion of the estuary can be made by scaling the estimates of Brosnan and O'Shea (1996) for the entire metropolitan waste flow to the percentage of the wastewater flow that enters the saline Hudson estuary (34 percent) and by similarly scaling the estimates of Hetling et al. (2003) and Brosnan et al. (Chapter 23, this volume), both of whom used the Verrazano Narrows as the lower limit of the Hudson estuary and therefore included a significantly greater urban watershed and population. The estimates for nitrogen input to the estuary calculated this way are all very similar to that estimated using the NRC, concentration data and wastewater flows (within 15 percent); however, the estimates for phosphorus input are only
2.1 x 103 to 2.5 x 103 tons P y-1, values that are 30 percent to 40 percent lower.
For the entire metropolitan New York City area, Brosnan and O'Shea estimate that nitrogen and phosphorus inputs in combined sewer overflows (CSOs) and in storm water runoff are 4.1 x 103 tons N y-1 and 0.67 x 103 tons P y-1. If we assume that the percentage of these inputs that go directly into the Hudson estuary is one third, as is true for wastewater effluent, then we can estimate these other urban inputs to the estuary as 1.4 x 103 tonsNy-1 and 0.22 x 103 tonsPy-1 (Table 10.2).
Lampman, Caraco, and Cole (1999) have estimated the flows of total nitrogen and phosphorus down the Hudson River as 18 x 103 tons N y-1 and 0.9 x 103 tons P y-1 at a point 125 km north of the Battery, or 60 km north of the beginning of the oligohaline estuary in Haverstraw Bay. This is a conservative estimate of the input of these nutrients to the saline estuary, as additional nitrogen and phosphorus enter the freshwater Hudson from tributaries over this 60 km stretch. Nonetheless, the estimates of Lampman et al. (1999) are the best available for total nutrient loading to the estuary from the upstream Hudson and its tributaries (Table 10.2).
These non-point source estimates ofN and P input from the freshwater Hudson, combined with the total urban inputs (wastewater, CSOs andstorm water runoff) result in an estimate of the total nutrient load to the saline Hudson estuary as of the mid 1990s of 43 x 103 tons N y-1 and 4.8 x 103 tons P y-1 (Table 10.2). For nitrogen, a similar but slightly smaller estimate (38 x 103 tons N y-1) is obtained from the population density in the watershed and a regression model that relates population density to total nitrogen flux for large regions in the temperate zone (Howarth, 1998). Wastewater effluent plus CSOs and storm water runoff contribute 58 percent of the nitrogen and 81 percent of the phosphorus, while the upstream tributary sources contribute 42 percent of the nitrogen and 19 percent of the phosphorus (Table 10.2).
Nitrogen and phosphorus loading rates expressed per area of the saline Hudson are 290 g N m-2 y-1 and 32 g P m-2 y-1. These are far higher than reported for any other large estuary in the United States (NRC, 1993). For comparison, total nitrogen and phosphorus loadings to Chesapeake
Bay are estimated to be twenty to thirty-fold less than the Hudson (13 g N m-2 y-1 and 1 g P m-2 y-1; Boynton et al., 1995), yet the Chesapeake is considered an ecosystem that is highly degraded from nutrient pollution (Bricker et al., 1999; NRC, 2000). Nitrogen loadings to Delaware Bay, Narra-gansett Bay, and Boston Harbor are also all substantially lower than to the saline Hudson, at 27,27, 130 g N m-2 y-1, respectively (Nixon et al., 1996). Total phosphorus loadings to these three systems are 5, 2.6, and 21 g P m-2 y-1, respectively (Nixon et al., 1996). Even by European standards, the nutrient loading to the Hudson estuary is high. The highly polluted Scheldt estuary in Belgium has a nitrogen loading of 190 g N m-2 y-1 and a phosphorus loading of 33 g P m-2 y-1 (Billen et al., 1985).
Of the nutrients entering the estuary from upstream in the Hudson River, a substantial portion likely comes from non-point sources. Boyer et al. (2002) compared the nitrogen cycle of sixteen major watersheds in the northeastern United States, including the Mohawk River valley and the basin of the upper Hudson River (above river km 260 where theMohawkjoins the Hudson). Together, these two tributaries comprise 60 percent of the area of the entire Hudson River basin (Boyer et al., 2002) and contribute 65 percent of the total freshwater discharge of the Hudson River (Howarth, Schneider, and Swaney, 1996a). For the upper Hudson and Mohawk River valleys combined, nitrogen deposition from the atmosphere contributes 41 percent of the flux of nitrogen from the landscape into the rivers. Agriculture contributes another 39 percent of the flux, with 28 percent of the nitrogen in the rivers originating from nitrogen fixation by agricultural crops and 11 percent from nitrogen fertilizer (Table 10.3; Boyer et al., 2002). The total nitrogen flux from these two watersheds is 13 x 103 tons y-1 (Table 10.3; Boyer et al., 2002), as compared to an estimated flux of 18 x 103 tons y-1 for nitrogen further down the Hudson River at river km 125 (Lampman et al., 1999). We would expect the nutrient flux between river km 125 and 66 to be even greater than this, since the watersheds of the tributaries there are more disturbed. The watersheds of the Mohawk River and upper Hudson are 73 percent forested on average (Table 10.3; Boyer et al., 2002), while the lower Hudson is 57 percent forested (Swaney et al., 1996). Also, atmospheric
Table 10.3. Characteristics of the Mohawk and Upper Hudson River basins (data from Boyer et al.,
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