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across the pycnocline, and mixing is in fact rapid compared with gas exchange with the atmosphere (Clark et al., 1995; Swaney et al., 1999). Even in completely mixed water columns, high levels of GPP can lead to hypoxia, as was demonstrated experimentally in the Marine Ecosystems Research Laboratory (MERL) facility at nitrogen loadings comparable to those that occur in the Hudson estuary (Frithsen, Keller, and Pilson, 1985). Eutrophica-tion leads to anoxic and hypoxic events in estuaries as a result of spatial and/or temporal separation of the production of oxygen associated with GPP and its consumption in respiration.

Freshwater discharge can dramatically affect oxygen concentrations in the saline Hudson estuary, with concentrations lower at times of lower discharge (Clark et al., 1995; Brosnan and O'Shea, 1996). This may result from the slower flushing that accompanies reduced discharges (Fig. 10.2B; Brosnan and O'Shea, 1996). The mesohaline Hudson estuary also becomes more stratified at times of lower freshwater discharge, in contrast to the general expectation that stratification lessens in estuaries as discharge decreases (Howarth and Swaney et al., 2000). This greater stratification may also contribute to lowered oxygen concentrations. Thus, the lower discharge that increases GPP and so, in situ oxygen production, also makes the Hudson more sensitive to the effects of this organic loading on oxygen levels.

Further Improvements to Water Quality in the Hudson River Estuary

The upgrade of sewage treatment in the New York City metropolitan area to secondary treatment has resulted in marked improvement in water quality (Brosnan and O'Shea, 1996; DEP, 2001) and was highlighted by a 1993 report from the National Research Council as one of the greatest success stories in water quality management in estuaries over the past several decades (NRC, 1993). However, while oxygen concentrations in the Hudson estuary have improved and usually meet the New York State standard for secondary contact recreation (4 mg O2 l-1), they still do not reliably meet the standard for primary contact recreation (5 mg O2 l-1), as noted above. Our analysis shows that the Hudson estuary is often hypereutrophic, and despite fairly rapid flushing, is more sensitive to nutrient pollution than has been previously assumed. Further, water quality management in estuaries is moving beyond consideration just of dissolved oxygen levels, and must now consider other adverse effects of eutrophication, such as reduced biodiversity, increased incidences and duration of harmful algal blooms, and alteration in food web structure (NRC, 1993,2000;Howarth andAnderson et al., 2000; EPA, 2001). Nutrient pollution from the Hudson estuary also contributes to eutrophication in downstream ecosystems, including the plume of the Hudson River on the continental shelf, where hypoxia is a regular event.

The nitrogen and phosphorus loads to the Hudson River Estuary and to the downstream ecosystems could be significantly reduced through improved sewage treatment. While the nitrogen in effluent from an average secondary sewage treatment plant in the United States contains 19 g N m-3, plants designed for nutrient removal on average discharge only 3gN m-3; for phosphorus, nutrient removal technology results in an average effluent concentration of 1.5 g P m-3, as compared to 3 g P m-3 for secondary treatment (Table 10.4; NRC, 1993). If all the municipal wastewater plants that discharge into the saline Hudson estuary were to upgrade to this level of treatment, nitrogen loading from the sewage plants would be reduced from a current estimated 23 x 103 tons N y-1 to 3.7 x 103 tons N y-1. Assuming no change in discharges from CSOs and from storm sewers, and no change in the nitrogen coming down the Hudson River from upstream sources, total nitrogen loading to the estuary would be reduced to 24 x 103 tons y-1, or 150 g N m-2 y-1 per area of estuary. Similarly, phosphorus loading from sewage plants upgraded to nutrient removal technology would be reduced from a current estimate of 3.7 x 103 tons y-1 to 1.9 x 103 tons y 1, resulting in a total P loading to the saline estuary of 3 x 103 tons y-1 or 20 g P m-2 y-1.

The cost of building and maintaining sewage treatment plants that include nutrient-reduction technology in the United States is on average $0.37 per m3 treated, compared to a cost of $0.28 per m3 for only secondary sewage treatment (Table 10.4; NRC, 1993). Thus, if the New York metropolitan region had upgraded to nutrient removal technology rather than just to secondary over the past few decades, the incremental cost would have been an estimated $0.09 per m3 of effluent, or $112 million per year for the plants that discharge into the Hudson estuary. To build new nutrient reduction plants in the future would cost more, and if there were no capital savings from converting sec-ondaryplants to nutrient reduction plants, the capital cost would be an estimated $0.22 per m3 and the increased operating costs over that for secondary plants would be $0.01 per m3 of effluent (Table 10.4), or a total cost of $277 million per year. There probably is some saving of capital costs when converting secondary treatment to nutrient reduction treatment, so the actual cost of nutrient reduction technology for the Hudson estuary is probably between$112and$277millionperyear,orbetween $0.08 and$0.17per personin the watershedper day, if national average costs apply. Note that these estimates are based on 1990 dollars and do not include land costs, but they are otherwise conservative as they are based on 8 percent interest rates and 20-year depreciation of plants (NRC, 1993).

The CSO and storm sewer discharges could in theory be eliminated as nitrogen sources to the Hudson estuary, and although the cost would be high, this would be desirable for other water quality reasons as well, such as reducing the pathogen load to the estuary. Most pathogens enter the Hudson estuary from CSOs (DEP, 2001). Ending CSO discharges should perhaps be a priority of rebuilding the urban infrastructure of the New York metropolitan area. Reducing the nitrogen from upstream tributaries would also be difficult, but a reduction of 50 percent or more seems possible through a combination of improved sewage treatment upstream, reduction in nitrogen deposition from fossil fuel pollution, improved farming practices, and other measures such as wetland creation (NRC, 2000). With this effort, it seems possible to reduce nitrogen loading to the Hudson estuary to 13 x 103 tons y-1,or87gNm-2y-1. The regression illustrated in Figure 10.5 indicates a maximum potential rate of primary production at this loading rate of 480 g C m-2 y-1. We conclude that, given sufficient public will and effort, the Hudson estuary can be restored to an ecosystem that is only moderately eutrophic rather than hypereutrophic, and where the risk of hypoxic events is greatly lessened (Table 10.5).

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