In order to apply the primary production modeling tools described above to the lower Hudson River Estuary, it is necessary to understand some of the physical characteristics of the system. Geographically, the lower Hudson River Estuary includes the saltwater portion of the Hudson River. Although the Hudson River is tidal as far north as the Federal Dam at Troy, New York, approximately 250 km (150 miles) north of the Battery (at the southern tip of Manhattan Island in New York City), salinity generally propagates up river only as far north as Poughkeepsie, New York, approximately eighty miles north of the Battery. Thus, for our purposes, the lower Hudson River Estuary refers to the eighty miles of the Hudson River north of the southern tip of Manhattan Island in New York City.
The lower Hudson River Estuary is the central water body in the larger urban estuary commonly referred to as New York/New Jersey Harbor Estuary. Water quality in the New York/New Jersey Harbor Estuary is influenced by numerous point (including more than 30 freshwater tributary inputs, 100 wastewater treatment plants (WWTPs) and 700 combined sewer outfalls (CSO) and nonpoint source discharges.
Consistent with the concentration of large point source discharges in the region, nutrient levels are high, in excess of what is required by the phy-toplankton populations which the available light regime and residence time in the photic zone can support. Water quality in the lower Hudson River Estuary also includes a depression in dissolved oxygen of several mg/L below saturation, which is an integrated response to both nutrient and carbon loadings from all loading sources.
In addition to the loadings described above, water quality in the lower Hudson River Estuary is influenced by the ocean. Tidal exchange with the ocean through the Sandy Hook-Rockaway transect is an important sink of nutrients and organic carbon in New York/New Jersey Harbor and the lower Hudson River Estuary. In addition, the influence of the ocean is an important consideration for modeling water quality. An important requirement for numerical models of natural water systems which are to be used for predictive or diagnostic purposes, that is, to assign causality, is open boundary conditions which are not affected by internal loads. In estuarine systems this requirement for indepen-dentboundaries necessitates an openboundary located far out into the ocean. Conditions in the lower Hudson River Estuary are also influenced by conditions in Long Island Sound by exchanges through the East and Harlem Rivers. Therefore, the boundaries for a proper evaluation of the Hudson River Estuary must include the New York Bight and Long Island Sound.
The exchanges between the lower Hudson River, New York/New Jersey Harbor, the New York Bight and the Long Island Sound have been extensively studied. Jay and Bowman (1975) present a comprehensive literature review of regional hydrodynamics and water quality from 1848 to the early 1970s. It is appropriate to consider New York/New Jersey Harbor, New York Bight and the Long Island Sound as an integrated system. This integrated approach has been undertaken in the development of the System-wide Eutrophication Model (SWEM).
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