Declines in the quality of aquatic resources in the Hudson drainage are clearly linked to increases in population and associated destruction of habitat, changes in land use, over-harvesting of fisheries, and the discharge of municipal and industrial effluents (Suszkowski, 1990). As noted at the beginning of this chapter, impacts associated with the discharge of untreated sewage into the Hudson River Estuary have been recorded since the seventeenth century.
Estimating water quality improvements due to sewage treatment is primarily achieved by tracking two key indicators of sewage-related pollution: dissolved oxygen (DO) as an indicator of the quality of the habitat to sustain life, and fecal coliform bacte-riaasan indicator of sanitary quality. For this analysis, long-term water quality data for the Albany Pool were retrieved from the U.S. Environmental Protection Agency's STORET system in October 2000.
Data for the lower Hudson were provided by the New York City Department of Environmental Protection's Harbor Survey Program. Methods used by the NYCDEP are documented in Brosnan and O'Shea (1996a).
Trends in aquatic health. Oxygen dissolved in the water column is necessary for respiration by all aerobic forms of aquatic life, including fish, crabs, clams, and insects. Dissolved Oxygen (DO) levels between 4.8 mg l-1 and 3.5 mg l-1 are generally protective of all but the most sensitive aquatic species, while levels below 2.3 mg l-1 may cause severe lethal and sub-lethal effects (USEPA 2000). DO varies seasonally, typically being lowest in summer and highest in early winter and spring. Year to year variability can be affected by a variety of natural and anthropogenic factors including weather, runoff, temperature and salinity stratification, tidal and gravitational circulation, algae blooms, the quality of water entering an area, and especially flushing
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