Discharge (m0 sec-1)
Figure 10.2. Relationship between freshwater discharge and water residence time (A), GPP (B), and light penetration (C) in the estuary during 25 cruises conducted during the spring, summer, and fall of 1994, 1995, and 1997. Open symbols represent times when tidal amplitude was < 1.15 m;dark symbols represents tides greater than 1.15 m. The dashed line in (B) represents the approximate value for GPP above which an estuary is considered to be hypereutrophic. Discharge data are from the USGS monitoring station at Green Island and represent approximately two-thirds of the total discharge into the estuary. Reprinted from Howarth etal. (2000a).
bottom sediments. Estuaries have been classified as eutrophic when annual production ranges between 300 and 500 g C m-2 y-1, and as hypereu-trophic when annual production exceeds 500 g C m-2y-1 (Nixon, 1995;NRC, 2000). This corresponds to a daily production rate of 2 to 3 g C m-2 d-1 on average during the active growing season (NRC, 2000; Howarth and Swaney et al., 2000). Thus, the saline Hudson estuary would be considered hyper-eutrophic during neap tide periods when freshwater discharge is low, but not when discharge is high (Fig. 10.2B).
Prior to our measurements in the 1990s, the only published data on rates of primary productivity in the saline Hudson estuary were from studies in the early1970s. Then rates were generally less than 1 g C m-2 d-1 and were never higher than 2 g C m-2 d-1 (Malone, 1977; Sirois and Fredrick, 1978). These lower primary production values are consistent with our measurements during the 1990s for periods of high discharge, and in fact the decade of the 1970s was a wetter decade with generally higher rates of freshwater discharge (Fig. 10.3). Average summer discharge rates at Green Island exceeded 200 m3 sec-1 in most of the years of the early to mid 1970s and exceeded 300 m3 sec-1 in every year except 1972. The resulting high flushing and corresponding low rates of productivity during that period gave rise to the thought that the Hudson estuary is relatively insensitive to eutrophica-tion despite the very high nutrient concentrations present (Garside et al., 1976; Malone, 1977; Bricker etal., 1999).
Different methods were used to measure productivity in the 1970s than in the 1990s, which can complicate direct comparison of the data sets. The measurements in the 1990s were made from in situ changes in dissolved oxygen concentrations over a diel cycle (Swaney et al., 1999), whereas the measurements in the 1970s were made by the 14C method (O'Reilly et al., 1976; Malone, 1977) and by the light-dark bottle method (Sirois and Fredrick 1978). The in-situ oxygen technique would be expected to give a more reliable and most likely higher estimate of gross primary production (GPP) than the other two methods (Swaney, Howarth, and Butler, 1999; Howarth and Michaels, 2000). The 14C method in particular would be expected to give a lower estimate, as it measures a rate of carbon fixation somewhere between net and gross primary production (Howarth and Michaels, 2000). Nonetheless, the apparent increase in production between the 1970s and the 1990s is probably real at least in part, as chlorophyll levels were also higher in the 1990s, while rates of production per
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