CHAPTER 6 



suming that stratification began 2 months earher than nor- 

 mal in 1976 (February rather than April), the annual max- 

 imum oxygen concentrations would have occurred in mid- 

 January (ranging from 5.9 to 7.1 ml/1 and averaging 6.5 

 ml/1). If these January values of oxygen were then allowed 

 to decrease at rates obtained from the relationship estab- 

 lished in figure 6-4 (average curve), then the cycle of D.O. 

 for 1976 can be estimated from stratification, along with 

 the range of predicted conditions (fig. 6-5). Based on this 

 model, minimum average concentrations in August would 

 have been 1.75 ml/1, ranging from 0.15 to 3.30 ml/1, which 

 is well below the normal annual minimum (2.9 ml/1). 



For comparison with the 1976 D.O. cycle as predicted 

 from stratification, the cruise averages and ranges of D.O. 

 concentrations are also shown in figure 6-5. It can be seen 

 that stratification may account for the April and May D.O. 

 concentrations, but, by the end of June and continuing 

 through summer, actual conditions were much more se- 

 vere than could have been anticipated from strength of 

 stratification alone. Particularly during June, other factors 

 must have affected the utilization-replenishment bal- 

 ance — for example, the flow reversals in the bottom waters 

 (ch. 7) and the increase in biological oxygen demand that 

 resulted from the congregation, respiration, and decom- 

 position of Ceratium in the subpycnocline waters. (See 

 chapter 9, part 2.) 



REGIONAL ASPECTS 



Anoxic conditions and the resulting mortalities in 1976 

 were apparently limited to the shelf waters off the New 

 Jersey — Cape May area and did not develop in waters of 

 adjacent shelf regions to the northeast or south. To con- 

 struct hypotheses for the cause of the geographic extent 

 of anoxic conditions, data archived at NODC and obser- 

 vations by the Woods Hole Oceanographic Institution 

 (1961) were examined for annual cycles of water-column 

 stratification and bottom-water D.O. concentrations in 

 adjacent shelf areas. Using the methods employed in de- 

 veloping figure 6-3 for the New Jersey — Cape May shelf 

 waters, historical values were compiled into cruise aver- 

 ages and ranges for the Long Island shelf region (areas 

 LI and L2 in fig. 1-9 of chapter 1 ) . The values were plotted 

 on the average date of station occupations, regardless of 

 year, and annual trend curves and smoothed ranges of 

 values were drawn (fig. 6-6). The compilation involved 

 40 cruises (123 stations) made in 1932-75, with observa- 

 tions available in all months. 



Off Long Island, bottom D.O. concentrations normally 

 are maximum in March (averaging about 7.0 ml/1), de- 

 crease in spring and summer to the annual average min- 

 imum in August of about 3.9 ml/1, and begin to increase 

 in September. The annual cycle of D.O. parallels the cycle 

 of density stratification. Although the annual cycles of 



bottom D.O. concentrations in the two areas were quite 

 similar, oxygen decline off Long Island proceeded slightly 

 more rapidly in April and May than off New Jersey, but 

 less rapidly during summer, resulting in an average annual 

 minimum about 1.0 ml/1 greater than off New Jersey. 



AOML cruises in December 1975 and in April, May, 

 June, and September 1976 surveyed the waters off Long 

 Island. A NMFS Sandy Hook Laboratory cruise in March 

 1976 provided data for computing stratification, but D.O. 

 measurements were not made. AOML data, processed 

 with the same depth limitations as used with the historical 

 data, are shown for comparison as cruise averages and 

 ranges (fig. 6-6). As off New Jersey, oxygen values were 

 near normal in December, somewhat below normal in 

 April, and distinctly below historical conditions in May, 

 June, and September. Values in May through September 

 were not as low as off New Jersey, but were almost as 

 anomalous. Averages for the stations occupied in each 

 area indicated that, at the end of June 1976, bottom D.O. 

 concentrations were 2.6 ml/1 below the normal annual 

 trend curve off Long Island and 3.8 ml/1 below normal off 

 New Jersey for that time of year. 



Comparison of historical data and stratification values 

 from the cruise data of 1975-76 shows the same general 

 pattern off Long Island (fig. 6-6) as off New Jersey. In- 

 dications of early stratification are apparent, with a return 

 to normal for May through September. 



From the historical annual trend and range curves of 

 figure 6-6, monthly rates of decrease in bottom D.O. 

 concentration and monthly mean stratification values were 

 determined. Following the same methods used for figure 

 6-4 for the New Jersey waters, figure 6-7 was developed 

 to show the correspondence between stratification and 

 rate of oxygen decline for bottom waters on the conti- 

 nental shelf off Long Island. Figures 6-4 and 6-7 show the 

 same tendencies. For stratification values greater than 

 about one a, unit, however, the rates of oxygen decline 

 per month (for comparable strength of stratification) are 

 about one-third greater for New Jersey waters than for 

 Long Island waters. 



Using the stratification data from observations made in 

 the cruises of December 1975 through September 1976, 

 monthly mean averages and ranges were computed, as- 

 suming that 1 ) all stratification was destroyed by the strong 

 cooling of January, and 2) stratification became estab- 

 lished over most of the area in February 1976. From these 

 monthly means of stratification, rates of decrease in bot- 

 tom D.O. were determined from the appropriate curves 

 in figure 6-7. In developing an estimate of the oxygen 

 cycle as a function of stratification for 1976, it was assumed 

 that the observed values in December 1975 increased at 

 the normal (historical) rate until mid-January, yielding at 

 that time an average concentration of 6.4 ml/1 (range 5.9 

 to 6.8 ml/I). To develop the curves in figure 6-8, a coun- 

 terpart to figure 6-5, these values were then allowed to 



143 



