1000 2000 3000 4000 5000 6000 Net anthropogenic N inputs (kg N km-2 yr-1)

Figure 10.6. Export of nitrogen per area of watershed from large watersheds in the northeastern United States as a function of the net anthropogenic nitrogen inputs to the watersheds. Inputs include fertilizer use, nitrogen fixation in agricultural systems, deposition of NOy from the atmosphere, and the net import or export of nitrogen in food and animal feeds. "HUD" refers to the upper Hudson River basin, and "MOH" refers to the Mohawk River basin. Data are from Boyer et al. (2002).

Figure 10.7. Mean oxygen concentrations in bottom waters of the saline Hudson River estuary during the summer, from 1985 to 2000. Bars represent 95% CI; dashed lines indicate the State of New York standards for secondary (4 mg l-1) and primary contact recreation (5 mg l-1). Reprinted from DEP (2001).

Figure 10.7. Mean oxygen concentrations in bottom waters of the saline Hudson River estuary during the summer, from 1985 to 2000. Bars represent 95% CI; dashed lines indicate the State of New York standards for secondary (4 mg l-1) and primary contact recreation (5 mg l-1). Reprinted from DEP (2001).

1996). Assuming that the phosphorus content of this eroded sediment has not changed over time, then the pre-European input of phosphorus to the estuary can be estimated as 10 percent of the current input from present upstream tributary sources (Table 10.2), or0.09 x 103 tons Py-1. The actual flux was likely less than this, both because the current input from upstream tributary sources includes some wastewater inputs from the tributaries and because the phosphorus content of the soils in the Hudson basin have probably increased from fertilization with phosphorus. In comparison to the estimated load for the 1990s (Table 10.2), human activity in the Hudson basin appears to have increased phosphorus loading to the estuary by at least fifty-fold. Note that phosphorus fluxes in the Hudson at the peak extent of deforestation in the ninteenth century may well have been greater than at present, due to very high rates of erosion (Swaney et al., 1996).

A pristine nitrogen loading of 23 g N m-2 y-1 would predict a rate of primary production of 270 g C m-2 y-1 (Fig. 10.5; Nixon et al., 1996) if the nitrogen loading were as inorganic nitrogen and if other factors such as short water residence times were not constraining GPP. As discussed in previous sections, GPP in the 1990s in the mesohaline estuary was roughly equal to the potential rate of14C production estimated from nitrogen loading (Fig. 10.5). Assuming this was true before European settlement, GPP can be estimated as 270 g C m-2 y-1. The actual value was probably lower, because in fact, most of the nitrogen export from the pristine landscape was probably as organic nitrogen (Howarth et al., 1996b;Lewis, 2002), of which the refractory component would be unavailable to phytoplankton, and short water residence times caused by spring tide mixing would have resulted in lower rates of primary production even during periods of low freshwater discharge. However, this exercise suggests that human activity has increased GPP in the mesohaline Hudson estuary by three-fold or more (Table 10.5).

Dissolved Oxygen: Historical Trends and Controls

The Hudson estuary is somewhat protected against low dissolved oxygen events both by the rapid flushing that removes organic wastes and by a rapid mixing over depth that can quickly replenish oxygen as it diffuses in from the atmosphere (Clark et al., 1995; Swaney et al., 1999). Nonetheless, the estuary has historically had problems with low oxygen (See Ch. 23). Much of this can be ascribed to organic loading from sewage effluents (BOD), and oxygen concentrations in the saline Hudson estuary have increased steadily in response to improved sewage treatment (Fig. 10.7; Suszkowski, 1990;Clarket al., 1995;BrosnanandO'Shea, 1996). The estuary is classified by the State of New York as Class I for water quality goals (secondary contact recreation, not primary contact), which sets a limit of 4 mg l-1 for minimum dissolved oxygen concentrations. As a result of reduced BOD loadings from upgrades to secondary sewage treatment throughout the New York City metropolitan area, this goal has been met most of the time since 1990

according to the data collected by the City of New York (Fig. 10.7; DEP, 2001). However, even in recent years the Hudson estuary would have difficulty meeting the state standard of 5 mg l-1 for primary contact recreation. In our cruises in 1998, 1999, and 2000, we found dissolved oxygen concentrations in the bottom waters of the estuary in early morning (when concentrations are lowest; Swaney etal., 1999) tobebelow5mgl-1 on roughly half the cruises and below 4 mg l-1 on two cruises (out of a total of twenty cruises), in August 1999 and July 2000 (our unpublished data).

In the saline Hudson estuary, theprimarysources of organic matter that fuel respiration leading to oxygen depletion are BOD inputs from sewage and phytoplankton primary production. The estuary also received substantial inputs of organic matter from upriver, estimated as 50 x 103 tons C y-1 in the late 1980s (Howarth et al., 1996a). Much of the labile organic carbon that enters the freshwater Hudson is respired in situ (Howarth et al., 1996a), and as a result most of the organic C that is exported downstream is likely fairly refractory and has little influence on oxygen dynamics in the saline estuary. At the peak of agricultural activity in the Hudson River basin a century ago, the inputs of organic matter may have been 80 percent greater than at present due to higher erosion, and prior to European settlement in the basin, the flux may have been 40 percent of the current rate (Swaney et al., 1996).

By the early 1970s when the new environmental movement focused attention on water quality resulting in the Clean Water Act of 1972, the organic carbon inputs to the Hudson estuary were dominated by sewage. As discussed above, in the early 1970s, 38 percent of the wastewater discharge into the Hudson estuary was raw sewage, 15 percent received primary treatment, and 47 percent received secondary treatment (calculated from data in Clark et al., 1992). Using average values for the United States for the BOD load from treatment plants receiving various levels oftreatments (Table 10.4), we estimate BOD loadings to the saline Hudson estuary in the early 1970s as 49 x 103 tons C y-1. By the 1990s, virtually 100 percent of the wastewater inputs to the Hudson estuary during dry-weather conditions receivedsecondarytreatment(Brosnan and O'Shea, 1996). Using the same approach as for our 1970s estimate, we calculate that the complete conversion to secondary level would have reduced the input of labile organic carbon from wastewater treatment plants to 7.5 x 103 tons C y-1 in the 1990s. Scaling the estimates of Brosnan and O'Shea (1996) for discharges from CSOs and storm water discharge for the entire metropolitan area to only the area of the saline Hudson estuary, as we did for nutrients above, suggests a further BOD loading of 6 x103 tons C y-1. If we assume that the CSO and storm runoff remained constant over the past several decades, then we estimate that the total BOD from wastewater effluent and other urban sources decreased by 75 percent between the early 1970s and the mid 1990s, from 55 x 103 tonsCy-1 to 14 x 103 tons Cy-1.

At the same time as BOD loadings from waste-water treatment plants decreased, rates of GPP increased in the Hudson estuary, probably due to the longer water residence times resulting from the decrease in freshwater discharge between the 1970s and the 1990s. Given a rate of GPP of 200 to 250 g C m-2 y-1 in the early 1970s (O'Reilly et al., 1976;Malone, 1977;Sirois and Fredrick, 1978), phytoplankton production would have provided an input of 33 x 103 tons C y-1 of labile organic matter to the estuary. If we assume that in the 1990s, GPP was on average 850 g C m-2 y-1 in the mesohaline estuary and 450 g C m-2 y-1 in the oligohaline estuary, the total input of organic carbon from GPP to the saline estuary would be approximately 90 x 103 tons C y-1. Despite the uncertainty in these estimates, the relative importance of sewage effluent and GPP clearly shifted between the early 1970s and the mid 1990s (Table 10.5; Fig. 10.8). BOD from sewage sources contributed over 60 percent of the labile carbon to the Hudson estuary in the early 1970s but only 10 percent in the 1990s. Surprisingly, the total input of labile organic matter to the estuary actually increased over those two decades due to the large increase in GPP (Table 10.5).

GPP by phytoplankton produces oxygen as well as labile carbon, and so given a comparable input of organic matter from GPP and BOD, the BOD loading will have a much greater negative impact on dissolved oxygen concentrations. However, excess GPP can lead to hypoxia and anoxia in estuaries, particularly when the water column is stratified (NRC, 1993, 2000). The saline Hudson estuary is generally stratified, yet significant mixing occurs

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