river flow to determine the residence time for these freshwater aquatic environments. 

 Using the total volume in an estuary gives an erroneously long residence time, but it 

 is sometimes done and produces a meaningless number. Another error that occurs in 

 the evaluation of estuaries for the disposal of pollutants is the assumption that the 

 entire volume of the tidal prism is available for dilution on each tidal cycle. This 

 error has led to gross overestimates of the receiving capacity of estuaries and 

 accounts for some highly polluted conditions. If this paper does nothing but prevent 

 the perpetuation of these two errors in evaluating the receiving capacity of estuaries, 

 it will have achieved a useful purpose. 



Nitrogen in the Hudson Estuary 



The excellent studies by M alone ( 1976, 1977) of the phytoplankton productivity in 

 the Hudson Estuary and the New York Bight* provide information to illustrate the 

 application of these basic principles of estuarine circulation. A large amount of sew- 

 age (8 X 10 6 m /day -1 [10 X I0 6 yd /day']) is added to the lower reaches of the 

 Hudson Estuary, but the phytoplankton productivity within the estuary is low in spite 

 of the rich nutrient content. Because of high turbidity, the euphotic zone is limited to 

 the upper 3 to 5 m ( 10 to 16 ft) of the water column. Garside et al. (1976) estimated 

 that only about 10 percent of the sewage-derived inorganic nitrogen is assimilated 

 within the estuary; the remaining 90 percent of the nitrogen is discharged through the 

 mouth of the estuary to the New York Bight. Essentially, therefore, the dissolved in- 

 organic nitrogen within the estuary can be considered as a conservative pollutant, 

 and its distribution should be determined primarily by circulation and mixing. 



The average river flow of the Hudson has been estimated by Ketchum et al. 

 (195 lb) to be nearly 100X 10 6 m 3 /day ' (130X I0 6 yd 3 /day" , )(Table7).f Thus, there 

 are 12.4 volumes of river water to dilute each volume of sewage added. The resulting 

 concentration of dissolved inorganic nitrogen would equal 1 2 1 mg at m~ 3 as a result 

 of dilution by river water alone. This is more than three times the inorganic nitrogen 

 concentration of "average" seawater (Table 1 ) and is greatly in excess of the usual 

 concentration in surface estuarine or coastal water. 



Seawater further dilutes the sewage nutrients added to the Hudson Estuary. The 

 expected concentration of dissolved inorganic nitrogen at any location within the 

 estuary can be estimated from the available information on sewage input, river flow, 

 and the observed salinity in the water. This has been done for one location within the 

 lower Hudson Estuary and for another location offshore using an average of 12 obser- 

 vations throughout the year as presented by Malone( 1976). The station chosen with- 

 in the estuary for this analysis is south of Manhattan Island in the main channel of 

 the Hudson Estuary. All of the sewage pollution is added upstream of this location. 

 The annual mean fraction of freshwater there was 36 percent, and the calculated ni- 

 trogen content was 47.56 mg at m~ 3 . This calculated value is 94.2 percent of the 

 observed annual average nitrogen content of this water. A similar computation was 

 made for the surface water at the station just outside of the entrance to the harbor. 

 The annual mean freshwater content there was 10 percent. Here, the calculated nitro- 

 gen content was 17.78 mg at m~ , 96.8 percent of the average annual mean nitrogen 

 content of the same water. J The nitrogen content of the water is thus reduced to 

 about half of that found in "average" seawater given in Table .1 at the harbor 

 entrance. 



•The sewage has an N:P ratio of 10 14 by weight (22.46 by atoms) so that within the estuary, phosphorus is the probable 

 limiting nutrient In the offshore waters of the New York Bight, the N:P ratio was generally 10 or less (by atoms) so that 

 nitrogen is the probable limiting nutrient offshore 



♦ Malone (1977) gives 12 daily estimates of river flow at the times of his observations. The average of these daily flows is 66 

 percent ol the mean annual given by Ketchum et al. (1951), which was derived from more complete records. 



JThe calculated nitrogen content ignores the nitrogen in the source Hudson River since Malone (1976) reported no ob- 

 servations in the freshwater river above the estuary An estimate can be made assuming that the difference between the 

 observed and calculated content is all derived from the Hudson River Dividing the nitrogen difference by the fraction of 

 freshwater gives for station A-3. 8. 1 1; for station P- 1. 5.80 mg at m--\ These values do not seem unreasonable for the Hudson 

 River, which receives some pollution above the estuary. 



79 



