CHAPTER 8 



Table 8-2. — Water and oxygen transport characteristics in New York Bight. May IS to June 29. 1976 



Segment of Bight' 



LI 



L2 



L3 



H 



Jl 



J2 



Net advective input of water, 



(2,„ (lO'mVs) 

 Water flushing time, 



V/&„ (d) 

 Change in oxygen concentration 



with time. 



AC/A/ (10' ml/l/d) 

 Divergence of oxygen flux, 



(IQC + Q.C)/V{\0 ' ml/l/d) 

 Net oxygen utilization rate. 



S5/V(10-' ml/l/d) 

 Oxygen ventilation time. 



CWrS.{QC),„d-' 



' Segment numbers refer to figure 8-1. 



putations are based on simple Ekman divergency ("Ek- 

 man suction") upwelling appropriate to the open ocean 

 rather than on coastal Ekman divergency appropriate to 

 the coastal ocean. 



An objective estimate of the advective flushing time of 

 the several segments of the Bight emerges from these 

 calculations; that is, the ratio of the volume of each seg- 

 ment to the flux into or out of the segment as determined 

 by the diagnostic model for the conditions observed below 

 the thermocline during spring and early summer 1976, V/ 

 Q,„. As seen in table 8-2, these flushing times within the 

 bottom layer varied from about 2 weeks to 2 months. The 

 most rapid flushing occurred in the Apex; the slowest 

 occurred off the New Jersey coast and in the outer seg- 

 ments off Long Island. A comparison with table 8-1 shows 

 a general correlation between flushing time and individual 

 segment size; this is to be expected because current speeds 

 are generally similar throughout the Bight. Also, the ratio 

 of an appropriate dimension of a segment to the speed of 

 the mean flow provides a first-order estimate of the ad- 

 vective flushing time. The corresponding flushing time for 

 the entire Bight was of the order of 75 days. Such calcu- 

 lations are limited in that there is no assurance that water 

 "flushed" from the Bight, or any part of it, does not return 

 later. On the other hand, it also does not take into account 

 the flushing effect of motions that occur on time scales 

 much shorter than 10 days. Even in this year of extreme 

 oxygen depletion, there were flows between the various 

 segments in the Bight, and between the Bight and sur- 

 rounding waters, which exchange these waters on time 

 scales that are short compared to the seasonal cycle of 

 thermal stratification. 



The oxygen flux calculations indicate a net advection 

 of oxygen into the Bight as a whole, and into each segment 

 defined for this analysis, except for the Hudson Shelf Val- 

 ley. All segments characterized by upwelling had a net 

 input of oxygen by horizontal advection. Those charac- 



terized by downwelling had a net loss of D.O. by hori- 

 zontal advection, which was more than compensated by 

 the downwelling of water with high D.O. 



The results of the primary objectives of these calcula- 

 tions are summarized in table 8-2. Interpretation of these 

 results requires an appreciation of how the values were 

 obtained. We began with observations of the rate of de- 

 crease in time of oxygen in the various segments. If there 

 was no net transport of oxygen into or out of the segment, 

 then the net utilization of oxygen would be the observed 

 time rate of change of concentration. However, if there 

 was some net advective flux of oxygen, say into a segment, 

 then the net utilization rate, SS, in the segment is equal 

 to the sum of the change of oxygen content in the segment, 

 VAC/Ar, and that advected into the segment, IQC + 

 QvC. To put the utilization rates into perspective, we 

 calculated the utilization rate per unit volume, SS/V, for 

 easy comparison with the observed time rate of change 

 of concentration, AC/ A/, and for comparison of effective 

 ratios between sections. 



It is readily seen in table 8-2 that in the inner and mid- 

 shelf segments the actual utilization rate is approximately 

 twice as large as would be inferred from the observed 

 change of concentration alone. Furthermore, the utiliza- 

 tion rate found for the seriously impacted areas in the 

 Apex and the inner shelf off New Jersey is about 3 times 

 greater than for other regions of the inner and middle 

 shelf and more than 10 times higher than that found over 

 the outer shelf. Segar and Berberian (1976) estimated the 

 oxygen consumption rate during summer 1974 as 10' kg 

 OJd for water below 10 m depth in the Bight Apex. By 

 dividing the appropriate volume (26 km'), this is equal to 

 a utilization rate of 0.27 x 10"' ml/l/d. Their estimate is 

 some 55 percent greater than our values for the Apex, 

 and 2 to 17 times greater than the other values from over 

 the shelf. Although Segar and Berberian's rate is not 

 greatly more than our maximum value, the unusual cir- 



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