CHAPTER lb 



sessing long-term effects. Actual payoffs in terms of de- 

 tecting changes in the ecosystem will take many years. If 

 future monitoring shows that there is a statistically sig- 

 nificant decreasing trend, it might be presumed to be re- 

 lated to the continuous and probably increasing anthro- 

 pogenic inputs of nutrients and carbon as pointed out by 

 O'Connor (ch. 15). 



Evidence to date suggests that human waste inputs to 

 the New York Bight did not cause the 1976 oxygen-de- 

 pletion event. Nevertheless, concern continues that hu- 

 man waste inputs could produce certain long-term effects 

 and deplete the D.O. in waters off the New Jersey coast. 

 Thus, in a sensitive system, even a slight imbalance in the 

 normal cycle of environmental conditions (brought about 

 by natural or human causes) might cause severe depletion 

 of D.O. in any year. This leads to the question: Is it 

 possible to predict the development of such conditions and 

 the consequences? 



PREDICTING FUTURE EVENTS 



After the publicity given the 1976 oxygen-depletion 

 event and mass mortalities in the New York Bight, a fre- 

 quent question was: Is it possible to predict oxygen de- 

 pletion in waters of the New York Bight? Complexities 

 of the marine ecosystem and many interacting environ- 

 mental conditions limit development of a reliable predic- 

 tive model. However, considerable attention is now given 

 to carbon, oxygen, and nitrogen (C/O/N) modeling of 

 Bight waters, in order to make it possible to detect trends 

 in D.O. concentrations and identify the need to observe 

 certain indicators more closely. 



Before developing elaborate techniques for predicting 

 D.O. depletion, consideration must be given to how the 

 information could be used and who would use it benefi- 

 cially. For example: 



• If severe D.O. depletion in a region can be pre- 

 dicted several months in advance, who can use this 

 information and how can it be used? 



• Could alternative waste management strategies be 

 put into effect quickly enough to alleviate any pre- 

 dicted oxygen depletion, if further studies indicate 

 any bearing on such conditions? 



• Could the fishing industry increase its efforts suf- 

 ficiently to harvest the resource before mass mor- 

 talities occurred? 



• Could onshore fish processing facilities handle the 

 increased load if fishing efforts were accelerated, 

 and how would the additional supply affect the 

 market? 



The answers to these questions are unclear at this time. 

 It is apparent that techniques and strategies must be de- 

 veloped in industry and resource management areas to 



properly take advantage of the progress being made in 

 predictive techniques. 



Despite our present difficulties in reliably predicting or 

 using information relative to D.O. depletion in the Bight, 

 monitoring some of the contributing factors listed in table 

 16-1 is proposed to provide advanced warning of a severe 

 problem. It seems likely that severe D.O. depletion might 

 be predicted up to a month in advance. 



The positive occurrence of several contributing factors 

 or indicators during spring and early summer is a warning 

 of possible severe D.O. depletion along the New Jersey 

 coast. Among these signs are early persistent southerly 

 winds, early large river discharge, and massive plankton 

 blooms, which tend to concentrate below the pycnocline 

 in summer. An important sign would appear to be the 

 early formation of a pycnocline below the photic zone. 

 This would provide favorable conditions for accumulations 

 of C. tripos, which function efficiently at low light levels. 

 A bloom at that depth will consume the available D.O. 

 near the bottom because of the organisms' net respiration. 



The normal seasonal progression of D.O. levels in bot- 

 tom waters of the Bight is reasonably well known. Mon- 

 itoring during the period of critical annual decline can 

 indicate a possible abnormal year. Off the New Jersey 

 coast the critical period is between mid-May and mid- 

 August (fig. 16-3). If there are usual D.O. concentrations 

 of 6 ml/1 in May, and a typical rate of decrease of about 

 0.025 ml/l/d, the depletion of D.O. should not be severe, 

 because the minimum values reached would only be about 

 3 ml/1 before reoxygenation at the autumn deterioration 

 of the pycnocline. However, a later than normal autumn 

 breakdown of the pycnocline by prolonging the period 

 during which D.O. declines might also cause severe D.O. 

 depletion. There also can be localized patches of water 

 where oxygen is severely depleted, as observed in June 

 1977, August 1978, and July 1979. These can recover 

 quickly and do not necessarily indicate the severe deple- 

 tion of D.O. over an extensive area. 



If the rate of decrease in D.O. is observed to be greater 

 than 0.025 ml/l/d, close scrutiny is required. In 1976 off 

 the New Jersey coast (segment Jl) the rate of decrease 

 was found to exceed 0.05 ml/l/d (ch. 4). For short periods 

 a rate of decrease in D.O. approaching 0.1 ml/l/d has 

 been observed. 



Stress in surf clams has been observed when D.O. ap- 

 proaches 2 ml/1 (Thurberg and Goodlet, ch. 11, pt. 2). 

 Thus, the critical level of D.O. for this species is between 

 the typical minimum of 3 mI/1 and the critical level of 2 

 ml/1. 



Figure 16-4 shows the days required for the D.O. con- 

 centration to drop from the typical annual minimum of 3 

 ml/1 to the critical level of 2 ml/1 for different rates of 

 decrease. When the D.O. level decreases to the typical 

 annual minimum of 3 mI/1, the number of days to further 



343 



