Conclusions Drawn from SWEM

From SWEM projection work, two important generalizations can be drawn. First, primary production, as controlled by either the availability of nutrients, light, or residence time is location specific. For the Lower Hudson River Estuary and contiguous waterways, the areas for which primary production is controlled by the availability of nutrients include western Long Island Sound and New York Bight

Figure 11.6. SWEM 1994-95 temporal calibration for dissolved oxygen.

and potentially portions of Raritan Bay. This conclusion is supported by measured data. Dissolved inorganic nitrogenmeasurementscollectedinNew York/New Jersey Harbor during August 1995 have been separated into three groups: greater than 0.25 mg/l, between 0.25 mg/l and 0.05 mg/l, and less than 0.05 mg/l. Dissolved inorganic nitrogen measurements less than 0.05 mg/l, as observed in western Long Island Sound and New York Bight, indicate that nitrogen is limiting algal growth. Dissolved inorganic nitrogen measurements between 0.05 and 0.25, as observed in Raritan Bay, indicate that while nitrogen levels are low enough to contribute to less thanmaximum algal growth, the algal growth is being controlled more by light availability and residence time in the photic zone than by nitrogen availability. Dissolved inorganic nitrogen measurements greater than 0.25 mg/l, as observed throughout most of New York/New Jersey Harbor including the Hudson River Estuary, indicate that algal growth is not being limited by nitrogen, but almost entirely by these other factors. Our results imply that:

1. increasing or decreasing the amount of nitrogen reaching western Long Island Sound and New York Bight will affect algal growth directly,

2. increasing or decreasing nitrogen will have a more limited effect on algal growth in Raritan Bay, and

3. increasing or decreasing nitrogen will have even less effect on algal growth in most areas of New York/New Jersey Harbor, including the Hudson River Estuary.

The second observation from SWEM projection work concerns the location-specific relative significance that nitrogen and carbon have on the resulting dissolved oxygen deficit. Model projection results show that the relative contributions of nitrogen and carbon to dissolved oxygen deficit vary significantly by location. For example, on a summer average basis, carbon accounts for 68 percent of the dissolved oxygen deficit andnitrogen accounts for 32 percent of the dissolved oxygen deficit at a location in the lower Hudson River Estuary along Manhattan Island. This is consistent with the fact that oxidation of allochthonous (that is, from point and nonpoint sources rather than from primary production) carbon is a large component of the dissolved oxygen budget in the lower Hudson River Estuary. Conversely, nitrogen accounts for 62 percent of the dissolved oxygen deficit and carbon accounts for 38 percent of the dissolved oxygen deficit at a given location in western Long Island Sound. This is consistent with western Long Island Sound being nutrient limited and algal processing of nitrogen (that is, primary production) dominating the dissolved oxygen balance.

The comprehensive and regional nature of models such as SWEM are useful to both the New York/New Jersey Harbor Estuary Program (HEP) and the Long Island Sound Study (LISS), as well as the City of New York. Since 1994, SWEM has undergone extensive technical review by panels of experts convened by both HEP and the LISS. A future applicationofSWEMmayincludethe development of total maximum daily loads (TMDLs)/wasteload allocations (WLAs) for the Harbor, Bight, and Sound to the extent necessary to attain appropriate water uses and standards in the Lower Hudson River Estuary, the New York/New Jersey Harbor, and adjacent water bodies.

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