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Figure 10.3. Average freshwater discharge to the Hudson at the USGS gauging station at Green Island, NY. Upper curve shows annual average flows, lower curve shows average summertime flows. Horizontal lines indicate mean values for annual and summertime discharge. Reprinted from Howarth et al. (2000a).

Figure 10.3. Average freshwater discharge to the Hudson at the USGS gauging station at Green Island, NY. Upper curve shows annual average flows, lower curve shows average summertime flows. Horizontal lines indicate mean values for annual and summertime discharge. Reprinted from Howarth et al. (2000a).

chlorophyll appear to be similar in both sets of studies (Howarth and Swaney et al., 2000). A conceptual model of how freshwater discharge down the Hudson River affects primary productivity in the Hudson estuary is presented in Figure 10.4.

Most of our data were collected between May and October, and we have not previously estimated an annual rate of production. However, for the six-month period from May through October over several years, we found GPP to range between 300 and 370 g C m-2 in the oligohaline estuary and 500 to 750 g C m-2 in the mesohaline estuary (Swaney et al., 1999, and our unpublished data). By comparison, for the May to October periodfor various studies in 1972-4, production was roughly 150gC m-2 in the mesohaline estuary and 180 g C m-2 for the oligohaline estuary (Swaney et al., 1999, using data from Sirois and Fredrick, 1978 and Malone, 1977). Overall, it would appear that GPP in the 1990s was perhaps twice the rate of production reported for the 1970s in the oligohaline Hudson estuary and four times the rate reported for the mesohaline estuary. Annual productivity reported for the saline Hudson estuary in the 1970s was approximately

Figure 10.4. Conceptual model of how the freshwater flow down the Hudson River interacts with the bimonthly tidal cycle to regulate water residence times and rates of primary productivity in the saline Hudson River estuary. The estuary receives extremely high inputs of nutrients, but a rapid flushing of water from the estuary prevents phytoplankton blooms from occurring except when both freshwater flow and tidal mixing are low

Figure 10.4. Conceptual model of how the freshwater flow down the Hudson River interacts with the bimonthly tidal cycle to regulate water residence times and rates of primary productivity in the saline Hudson River estuary. The estuary receives extremely high inputs of nutrients, but a rapid flushing of water from the estuary prevents phytoplankton blooms from occurring except when both freshwater flow and tidal mixing are low

Figure 10.5. Primary productivity as function of the input of inorganic nitrogen per area for a variety of marine ecosystems. The open circles are from experimental mesocosm studies at the MERL facility. Dark circles represent natural ecosystems. Reprinted from Nixon et al. (1996).

Figure 10.5. Primary productivity as function of the input of inorganic nitrogen per area for a variety of marine ecosystems. The open circles are from experimental mesocosm studies at the MERL facility. Dark circles represent natural ecosystems. Reprinted from Nixon et al. (1996).

1.3 times the rate measured over the May to June period in the 1970s (O'Reilly, Thomas, and Evans, 1976; Malone, 1977; Sirois and Fredrick, 1978; Limburg et al., 1986; Malone and Conley, 1996; Swaney et al., 1999). Assuming that this relationship holds for our data, mean annual rates of GPP in the Hudson during the 1990s can be estimated as 850 g C m-2 y-1 in the mesohaline estuary and 450 g C m-2 y-1 in the oligohaline estuary. The freshwater discharge during the 1990s is much more characteristic of the situation for the Hudson over the past six decades than is that of the 1970s (Fig. 10.3), so our estimates are a better reflection of long-term average rates of GPP in the saline Hudson. Future climate change may well lessen freshwater discharge from the Hudson during the summer, continuing a trend of high production (Howarth and Swaney et al., 2000; Scavia et al., 2002).

For many marine and estuarine ecosystems, the log-transformed rate of primary production (measured by the 14 C method) is a linear function of the log-transformed inorganic nitrogen loading rate (Fig. 10.5; Nixon et al., 1996). While primary productivity in many estuaries is less than predicted by the regression of Nixon et al. (1996) due to a variety of factors including rapid flushing and light limitation (NRC, 2000; Cloern, 2001), the regression sets a reasonable upper bound for the relationship between nutrient loading and production in estuaries if these physical factors were not limiting. Nitrogen (N) loading to the Hudson estuary is estimated as

43 x 103 tons N y-1 (Table 10.2, and discussion in section to follow in "Nutrient Loading in the Past 30 Years"), most of which is as inorganic nitrogen (Malone and Conley, 1996). This corresponds to an average loading per area of the estuary of 290 g N m-2 y-1, although in fact the loading in the oligohaline estuary would be somewhat less, as most of the nutrient input enters directly into the meso-haline estuary near Manhattan and some of this N is exported from the estuary rather than being mixed into the oligohaline estuary. This level of nitrogen loading corresponds to a predicted value for 14C primary production in the Hudson estuary of 820 g C m-2 y-1 (Fig. 10.5), which is remarkably close to our roughly estimated annual value of GPP in the mesohaline estuary (850 g C m-2 y-1). While we might expect the maximum value of productivity predicted from nutrient loading to be higher than that measured in situ, 14C productivity underestimates GPP. We tentatively conclude that the rates of GPP measured during the 1990s reflect the maximum rate that can be obtained in the Hudson under its current nutrient loading regime because of the constraint imposed by short water residence times from advection and tidal mixing.

Nutrient Loading over the Past Thirty Years

Nutrient inputs to the Hudson estuary are of interest both because they set an upper limit on rates of GPP there and because much of the nitrogen and phosphorus is exported from the Hudson to other

Table 10.2. Loadings of total nitrogen and phosphorus to the saline Hudson River estuary

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