NO A A PROFESSIONAL PAPER 11 



ment of bottom DO. Continued cooling and overturning 

 through autumn and winter typically cause a steady in- 

 crease in oxygen values to the annual maximum in March. 

 The influence of stratification on rates of oxygen decline 

 may be greater off New Jersey than in adjacent regions 

 because of bathymetric differences. However, this assess- 

 ment does not take into account any effects on D.O. con- 

 centrations that might result from unusual advective and 

 biologic processes. Comparison with the normal cycle of 

 bottom D.O. indicates that concentrations were already 

 below normal by April in 1976 throughout the New York 

 Bight. 



In 1976, the early occurrence of increased river dis- 

 charge accompanied by early warming led to the early 

 development of stratification; above-normal stratification 

 persisted through May. The early onset of stratification 

 in 1976 would have contributed to the occurrence of ab- 

 normally low bottom D.O. in the New York Bight in three 

 different ways: 



1. If D.O. concentrations increased as per the normal 

 trend into January 1976 (as indicated from observations 

 made in December 1975), then with a 2-month earlier 

 than normal onset of stratification, maximum concentra- 

 tions for the year, would have been in January at about 

 6.5 ml/1, which is 0.5 ml/1 less than the usual March max- 

 imum of 7.0 ml/1. Given normal conditions through the 

 rest of the year, the D.O. values in 1976 could have been 

 somewhat below normal each month until autumn. 



2. Normally, the season of strengthening stratification 

 and declining D.O. lasts about 5 months (April-August). 

 In 1976, this season was apparently lengthened as much 

 as 2 months, because of the early onset of stratification. 

 This means that 1976 had 7 months in which utilization 

 of oxygen would have exceeded replenishment. 



3. From March through May 1976, and probably be- 

 ginning as early as February, stratification was stronger 

 than normal and about typical for June through August. 

 As a result, the decline in D.O. would have been greater 

 than normal during as many as 4 months in 1976 and at 

 typical rates during summer. Thus the cycle of bottom 

 D.O. concentration would have proceeded at values below 

 normal for as long as February through August. 



Historical observations, stratification values, and bot- 

 tom D.O. decline rates were used to develop a graphic 

 model. The model was then used to estimate the cycle of 

 bottom D.O. concentration for 1976. These estimates 

 were compared with 1976 observations and indicated that 

 stratification, with the early arrival of spring conditions, 

 accounted for the below-normal D.O. values in April and 

 May. The model-derived estimate further implied that 

 D.O. would have become almost totally depleted over the 

 New Jersey shelf in August. But DO. measurements in 

 the area during June indicate that concentrations fell much 

 more rapidly than estimated, because of stratification 

 alone. Similarly, model-derived estimates for Long Island 



shelf waters indicated that below-normal concentrations 

 should have developed there in 1976, but not at such low 

 values as off New Jersey. Also, in June, Long Island D.O. 

 concentrations fell more rapidly than predicted from the 

 model. 



Because stratification alone failed to explain the full 

 magnitude of the oxygen depletion, there must be other 

 contributory factors. Possibly the dramatic decline in bot- 

 tom D.O. during June resulted from excessive demand 

 from the presence of the large mass of Ceratium and, 

 perhaps, from factors associated with the circulation. 



Since instances of anoxia-related mortalities have been 

 reported during previous years in the New Jersey shelf 

 waters, historical records of sea-surface temperature and 

 of river discharge were examined for indications that the 

 season of stratified conditions may have been longer than 

 normal in these years. For the 30 years of records ex- 

 amined conditions that could have caused a lengthening 

 of the time the waters were stratified occurred 12 times, 

 or 40 percent of the time. Of these potential cases, five 

 resulted from the early arrival of spring and seven from 

 the late arrival of autumn. Anoxic conditions and mor- 

 talities were reported for the four most recent occurrences. 

 This history of fairly frequent instances when stratifica- 

 tion-related, depressed D.O. conditions may have devel- 

 oped implies that future recurrences of anoxia should be 

 expected. 



ACKNOWLEDGMENTS 



Steven K. Cook helped compile data, and J. Lockwood 

 Chamberlin and Merton C. Ingham gave advice. The fol- 

 lowing individuals provided data: E. L. Burke, William 

 Embree, and Sheila Perry of the U.S. Geological Survey; 

 Ellsworth C. Smith of NOAA NODC; Henry Diaz of 

 NOAA NCC; James Hubbard and Charles Muirhead of 

 NOAA National Ocean Survey; Thomas Azarovitz of 

 NOAA NMFS, Sandy Hook; and George Berberian, Ad- 

 riana Cantillo, and John Hazelworth of NOAA AOML. 



REFERENCES 



Bowman, M. J., and Wunderlich. L.D., 1976. Distribution of hydro- 

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Bumpus, D. F., 1973. A description of the circulation on the Bcontinental 

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Stearns, Franklin, 1969. Bathymetric maps and geomorphology of the 

 Middle Atlantic Continental Shelf, Fish. Bull. 68:37-66. 



Uchupi, Elazar, 1968. Atlantic continental shelf and slope of the United 

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Woods Hole Oceanographic Institution, 1961. Biological, chemical, and 

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