NOAA PROFESSIONAL PAPER II 



ing from the decaying floe increased stress on benthic 

 organisms (ch. 11, pt. 2), and the continuous yet typical 

 anthropogenic loadings and oxidation of dissolved and 

 particulate matter and carbon could only intensify and 

 maintain the already depressed conditions. 



Anoxic conditions were not relieved until breakdown 

 of the pycnocline in autumn when surface coohng caused 

 mixing of the water column. Even the effects of hurricane 

 Belle on mixing across the pycnocline were only transi- 

 tory. During autumn bottom oxygen values continued to 

 increase and reached saturation during winter. 



New Jersey had three other open coastal mortality ep- 

 isodes since 1965, although none were as severe as that 

 which occurred in 1976. We cannot define quantitatively 

 the probability of a recurrence of a major mortality. How- 

 ever, based upon limited data over the last decade, a 

 similar major occurrence can be expected in the future. 



If both the interpretation of events causing the 1976 

 mass mortality and the assumption is correct that low bot- 

 tom D.O. caused the earlier localized mortalities in the 

 late 1960s and early 1970s, then the long-term trend of 

 bottom D.O. should be considered. In order to do this it 

 is necessary to examine several available data sets. Figure 

 16-3 shows the August history of Apex bottom D.O. con- 

 centrations from the MESA data base (1974 to present). 

 August data are used, because generally the annual D.O. 

 minimum occurs then. Data are sparse before 1974. but 

 O'Connor et al. (1977) report a mean D.O. concentration 

 and standard deviation in waters of the bottom 5 m of the 

 Apex of 4.3 ± 0.4 mi/1 in 1948 and 3.4 ± 0.3 ml/1 in 

 1969. 



A least-squares regression of the mean values shown in 

 figure 16-3 was calculated for the MESA data. These 

 values represent observations of D.O. within 6 m of the 

 bottom and below the pycnocline. The hypothesis that the 

 slope of the regression over the 1974-79 period was dif- 

 ferent than zero was tested by using the F statistic. The 

 test leads to rejection of the hypothesis that the slope is 

 not zero, based on this sample, and that there is no de- 

 tectable change in the average value for D.O. of bottom 

 waters in the Apex during August over the 6-year period. 

 This conclusion appears to be consistent with the 1948 and 

 1969 values already mentioned. Thus, 1976, with all its 

 unusual circumstances (table 16-1), appears to be an ex- 

 ception. 



Changes of bottom D.O. occur rapidly during the period 

 of the average D.O. minimum. Such changes occur as a 

 result of short-term fluctuations in meteorological forcing 

 and advective processes. The mean of four Apex stations 

 near the mouth of the estuary and west of the Christiaen- 

 sen Basin changed from 4.0 ± 1.2 ml/1 during the flrst 

 week of August 1978 to 3.7 ± 0.2 ml/1 a week later. Note 

 that while the change in the means are small the large 

 change in the standard deviation indicates a considerable 



1974 1975 



1976 



1977 



1978 1979 



YEARS 



FIGURE 16-3. — Dissolved oxygen concentrations in bottom waters of 

 New York Bight Apex during August. 1974-79. Source: National 

 Oceanic and Atmospheric Administration, Environmental Data and 

 Information Service. MESA data base. 



fluctuation. A fifth station, not included in the mean be- 

 cause it did not meet the criteria stated above, changed 

 from 5.5 ml/1 to 3.0 ml/1 during the same period. The 

 change at the fifth station apparently resulted from the 

 pycnocline being at the bottom during the flrst period of 

 observations and considerably above the bottom during 

 the second. 



In 1979, the means and standard deviations of three 

 Apex stations in and west of the Christiaensen Basin var- 

 ied in the sequence of 3.0 ± 1.0, 5.3 ± 0.3, and 3.6 ± 

 0.9 ml/1 over the period August 13-23. Five other stations 

 were excluded from the samples, because of large changes 

 in D.O. related to the changes in depth of the pycnocline. 



The documentation of these changes raises the question 

 of why hurricane Belle did not have a more long-lasting 

 effect on replenishment of bottom D.O. in 1976. Perhaps 

 the reason is that this weak, fast-moving storm did not 

 result in a breakdown of the pycnocline structure and re- 

 plenishment of D.O. throughout the water column, but 

 rather brought about advective oscillations due to the im- 

 pulse nature of the storm. 



The year-to-year fluctuations of bottom D.O. reflect a 

 distribution with a large variance which, at this point, is 

 still undersampled if we wish to predict confidently the 

 long-term trend of the distribution. This is important to 

 consider in terms of establishing monitoring programs as- 



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