CHAPTER 16 



FACTORS LEADING TO OXYGEN 

 DEPLETION 



The cause of mass mortalities in portions of the New 

 York Bight in 1976 has been clearly established as ab- 

 normally low D.O. concentrations in bottom waters and 

 poisoning by sulfide generation at the low D.O. levels. 

 Possible causes of previous oxygen depletion in the Bight 

 Apex have been discussed by Segar and Berberian (1976) 

 and Garside and Malone (1978). It is less clear why there 

 was severe oxygen depletion in bottom waters throughout 

 a larger area of the Bight in 1976. 



Historical data on D.O. levels off the New Jersey coast 

 are sparse, but Armstrong (ch. 6) gathered scattered ob- 

 servations for 30 years and constructed an annual cycle 

 from the data. The cycle shows the D.O. in bottom waters 

 normally declines during spring and summer to a minimum 

 concentration in August. The usual minimum is about 3 

 ml/1. The minimum in 1976 was 1 ml/1 or lower. Because 

 D.O. concentrations probably approach critical levels 

 (<2 ml/1) in localized areas during many years, a relatively 

 small imbalance between rates of oxygen supply and uti- 

 lization has the potential to change the normal coastal 

 productivity of the region to one of mortality and decay. 



Normally, D.O. concentrations in the bottom waters off 

 both New Jersey and Long Island decline through spring 

 and summer, paralleling the development of stratification. 

 An unusual feature of the 1976 oxygen-depletion event 

 was that bottom D.O. concentration values began (in 

 April) to decline earlier than usual, and remained below 

 normal until fall. During June 1976 bottom D.O. concen- 

 tration values declined abruptly off both New Jersey and 

 off Long Island (ch. 6), achieving values comparable to 

 the normal minimum more than a month early. Values 

 continued to decline throughout the summer. 



Environmental factors that can contribute to oxygen 

 depletion in bottom waters of the Bight are considered in 

 the preceding chapters. In addition to these factors, high 

 river discharge may influence biological and chemical 

 processes in the Bight and therefore may be important. 

 For some factors there is a substantial historical record of 

 observations. Records for wind velocity, air temperature, 

 sea-surface temperature, river discharge, and fish catches 

 permit quantitative comparisons between conditions in 

 previous years and conditions in 1976. Table 16-1 sum- 

 marizes the occurrence of potentially important environ- 

 mental conditions during the years 1966-76. Before 1976, 

 at least two contributing environmental conditions oc- 

 curred in each year when mortalities caused by oxygen 

 depletion were observed. In 1976 six such conditions were 

 observed: high Hudson River discharge, early water-col- 

 umn stratification, persistent southerly winds, larger than 

 usual dumping of wastes, reversals of summer bottom cur- 

 rents off New Jersey, and an excessive phytoplankton 



bloom. Some information needed to make the table com- 

 plete for the 1960s is not available, which lessens the value 

 of this analysis. However, it is apparent that annual mon- 

 itoring of these environmental factors and their seasonal 

 development can be useful in anticipating the possible 

 occurrence of mass benthic mortalities. Such a projection 

 would be limited to about 1 month, although some indi- 

 cators may be apparent earlier. 



SEQUENCE OF 1976 CAUSAL EVENTS 



No single factor has been identified as causing the oxy- 

 gen depletion and resulting mass mortalities of benthic 

 organisms during the summer of 1976. The observed con- 

 ditions seemed to develop in response to atmospheric and 

 oceanic processes that departed from normal (average) in 

 both intensity and time of occurrence — possibly aggra- 

 vated by unusually large inputs of nutrients and organic 

 carbon from human activities and wastes in the area sur- 

 rounding the Bight. 



The conditions causing depressed D.O. began early in 

 the year. As early as January 1976 the dinoflagellate Cer- 

 atium tripos had a large bloom throughout New York 

 Bight and over much of the northeastern continental shelf, 

 peaking between April and June (ch. 9, pt. 1). Though 

 the cause is not clear, the bloom was so large in geographic 

 extent that nutrient inputs to the Bight from human 

 sources cannot reasonably be regarded as the cause. 



An early and warm spring also coincided with the 

 bloom. This occurred when atmospheric circulation pat- 

 terns inhibited transport of arctic air into the Northeast 

 region (ch. 3). The unusual weather conditions contrib- 

 uted to greater river discharge and runoff throughout the 

 region, and to earlier-than-normal warming of sea-surface 

 waters. These conditions produced a lens of relatively 

 warm, fresh surface water, contributed to early density 

 stratification of coastal waters, and isolated bottom waters 

 from replenishment of D.O. at the sea surface (chs. 2 and 

 6). Although stratification developed nearly 6 weeks ear- 

 lier than usual, analysis has shown that this condition is 

 not sufficient to have caused the extensive depletion. 



Perhaps the most significant indirect factor influencing 

 the development and duration of the 1976 oxygen-deple- 

 tion event was the wind field generated by the anomalous 

 atmospheric pressure patterns from late winter to mid- 

 summer 1976. Winds consistently had a southerly com- 

 ponent during this period (ch. 3). Generally, the wind 

 does not shift to the south until April. Also during May 

 and June the wind was more persistently from the south. 

 The resulting wind field established a potential for up- 

 welling of bottom waters along the New Jersey coast. The 

 bottom current-meter data (ch. 7) give evidence of up- 

 welling off the New Jersey coast. 



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