CHAPTER 16 



10 ■* ^.l/cell/h. The average C. tripos cell counts below the 

 pycnocline were 1.1 x 10'* cells/m'' off New Jersey (seg- 

 ment Jl) and 0.64 X 10**cells/m'off Long Island (segment 

 LI). Respiration rates for these cell concentrations are 

 0.37 ml/l/d and 0.22 ml/l/d, respectively. If the mean 

 benthic respiration rate of 17 ml/m-/h (Thomas et al. 1976) 

 is distributed over the measured thickness of the lower 

 layer — 10 m for segment Jl and 16 m for segment LI — 

 then the benthic respiration rates are 0.04 ml/l/d for seg- 

 ment J 1 and 0.03 ml/l/d for segment LI. Thomas (personal 

 communication) believes that 17 ml/m'/h is a better esti- 

 mate than the 11 ml/m'/h used by Malone (ch. 9, pt. 1); 

 however, the net effect of this assumption on the final 

 result is small. 



The total respiration of the C. tripos and benthic com- 

 munities is then 0.41 ml/l/d for segment Jl and 0.25 ml/ 

 1/d for segment LI. The net utilization rates calculated 

 from the observed oxygen concentrations using the di- 

 agnostic model (ch. 8) were 0.17 ml/l/d for segment Jl, 

 0.05 ml/l/d for segment LI. and 0.15 ml/l/d for the Apex 

 (segment A). Table 16-2 gives values computed for these 

 segments. 



In mid-May D.O. concentrations near the bottom were 

 nearly the same in all segments. The time rates of change 

 in concentration in the Apex and along the New Jersey 

 coast were comparable, as were the rates of net utilization. 

 The local rate of change and the net utilization were much 

 less in bottom waters off the south shore of Long Island. 

 The major difference was between the estimated rates of 

 C. tripos respiration off the coast of New Jersey and the 

 shelf waters off Long Island during this period of time. 

 If there were no inputs of D.O. into the system, respiration 

 alone could have caused anoxic conditions along the New 

 Jersey coast in about 10 days. Thus, simply the respiration 

 of the large population of C. tripos was sufficient to ac- 

 count for the observed oxygen decline through June. It 

 is still unclear from these calculations why the anoxic con- 

 ditions initially occurred in the southern Apex and ex- 

 tended to waters off Atlantic City about 3 weeks later. 

 Diagnostic model computations and direct current obser- 



vations indicated that southward advective processes did 

 not transport low D.O. water from the Apex. 



The average carbon loading to the Apex, including in- 

 puts from human activities, is insufficient to cause gen- 

 eralized anoxic conditions (Garside and Malone 1978). 

 However, the normally large carbon load, in addition to 

 the respiration of C. tripos (which to some degree may 

 have been concentrated more in the southern Apex by 

 flow up the Hudson Shelf Valley), likely created a greater 

 total load in the southern Apex. Thus, the available D.O. 

 in the Apex may have been utilized more rapidly than 

 elsewhere in the Bight from mid-May until the end of 

 June. The oxidation rates of the dissolved and particulate 

 carbon are not known. The real impact of human inputs 

 may have been to accelerate oxygen depletion locally in 

 the Apex, but not to have caused the widespread oxygen 

 depletion in 1976. Thus, while human inputs into the Apex 

 are not considered important in causing the widespread 

 anoxia observed in 1976, these inputs coupled with the 

 convergent flow field there may explain the apparent 

 anomaly in the spread of the anoxic area from north to 

 south. 



Thomas et al. (ch. 10) found that C. tripos were not 

 present in the water column in August and September, 

 though Mahoney (ch. 9, pt. 2) found a decaying floe of 

 C. tripos near the bottom in July. Malone et al. (ch. 9, pt. 

 1) estimated the biological oxygen demand (BOD) from 

 the decay of the C. tripos. Assuming that the average 

 depth below the pycnocline was 10 m off New Jersey and 

 that the decay took place over 60 days (July and August), 

 we find that the oxidation rate is 0.16 ml/l/d compared 

 to a C. tripos respiration rate of 0.37 ml/l/d. Thus, the 

 BOD from the decay of C. tripos alone was much smaller 

 than the respiration of C. tripos cells but it was probably 

 sufficient to maintain anoxic conditions throughout the 

 rest of the summer. 



Thomas et al. (ch. 10) observed anaerobic conditions 

 and sulfide generation in the anoxic area along with high 

 rates of seabed D.O. consumption at the periphery of the 

 area. The oxygen deficiency and hydrogen sulfide result- 



Table 16-2. — Comparison of mean bottom dissolved oxygen (DO.) concentrations, local rates of change, and net utilization with C. tripos plus 

 benthic respiration. May 18 to June 29, 1976, segments A, Jl, and LI, New York Bight 



Segment' 



Mean bottom DO. 

 concentration- 



May 18 



June 29 



A 

 Jl 



LI 



ml/1 



5.5 

 5.2 



5.4 



ml/1 



1.9 

 1.3 



4.5 



' Segments of New York Bight are shown in figure 16-2. 

 ' Data from Han et al., chapter 8. 



341 



