Implications for Management

To control eutrophication, reducing nutrient load is a key goal of ecosystem management. In systems in which the phytoplankton are strongly nutrient limited, the loading of nutrients (typically N and P) lead to large and often undesirable blooms of phytoplankton (Howarth, 1988). The sinking and decomposition of these blooms can lead to oxygen depletion in the sediments and bottom waters in systems in which the water column is physically stratified. The tidal-freshwater Hudson is neither stratified nor nutrient limited. Nutrient concentrations are high, but phytoplankton are limited by the other factors we have discussed here: light; deep mixing; and grazing. Thus lowering the input of these nutrients will not greatly affect phytoplankton or oxygen in this part of the river. On the other hand, if one were to try to manage the Hudson for water clarity by reducing the input of silts and clays, dramatic increases in phytoplankton would be expected (Caraco et al., 1997). If the

Hudson River lacked the suspended matter that now absorbs light, phytoplankton wouldbe able to use much of the pools of inorganic N and P that are now exported downstream. In a simulation model of this effect, Caraco et al. (1997) suggested that summertime chlorophyll-« would be as high as 70 to 80 |g liter-1 (about 20 times the present values) before self shading or P-limitation further limited bloom formation in the mid-Hudson region. Thus while the abatement of turbidity might seem desirable to improve the river aesthetically and for swimming, it is likely to also lead to dramatic increases in eutrophication.

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10 Wastewater and Watershed Influences on Primary Productivity and Oxygen Dynamics in the Lower Hudson River Estuary

Robert W. Howarth, Roxanne Marino, Dennis P. Swaney, and Elizabeth W. Boyer

The Hudson estuary suffered from low dissolved oxygen concentrations over much of the twentieth century, but by the mid 1990s, all dry-weather discharges from sewage treatment plants in the New York City region received secondary treatment. This greatly reduced labile organic carbon (BOD) loadings, and resulted in dissolved oxygen concentrations that in most recent years have met New York State standards. Between the1970s and the 1990s, the estuary switched from sewage as the primary input of labile organic matter to phytoplankton primary production as the major input. Further improvement in water quality is desirable, with the goal of reducing the tropic status of the estuary from hypereutrophic to moderately eu-trophic. This will require upgrading of sewage treatment plants to nutrient reduction technology, at an estimated cost of $112 to $277 million per year, and a substantial reduction in nitrogen loading from combined sewer overflows and from nonpoint sources.

abstract Primary productivity in the saline Hudson River estuary is strongly regulated by water residence times in the estuary. Nutrient loads and concentrations are very high, and when residence times are more than two days, production is extremely high. When water residence times are less than two days, production rates are low to moderate. Residence times are controlled both by freshwater discharge into the estuary and by tidal mixing, so residence times are longest and production is highest during neap tides when freshwater discharge is low. Freshwater discharge was generally high in the 1970s, which kept primary production low. In contrast, freshwater discharge rates were lower in the 1990s, and the estuary became hy-pereutrophic.

Nutrient loading per area of estuary to the saline portion of the Hudson is probably the highest for any major estuary in North America. As of the 1990s, approximately 58 percent of the nitrogen and 81 percent of the phosphorus came from wastewater effluent and other urban discharges in the New York City metropolitan area. Some 42 percent of the nitrogen and 19 percent of the phosphorus came from upriver tributary sources. For nitrogen, these tributary inputs are dominated by nonpoint sources, with atmospheric deposition from fossil fuel combustion and agricultural sources contributing equally. Human activity has probably increased nitrogen loading to the Hudson estuary twelve-fold and phosphorus loading fifty-fold or more since European settlement. Nitrogen and phosphorus loadings to the estuary have decreased somewhat since 1970 due to universal secondary treatment of dry-weather wastewater effluents and a ban on phosphates in detergents.

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