CHAPTER 6 



New Jersey (fig. 6-9), the most striking feature is that the 

 New Jersey shelf is about 40 km, or 44 percent, wider than 

 the Delmarva shelf, as measured from the coast to the 

 100-m isobath. Although no comparative evidence ap- 

 parently exists, it would seem as though the narrower shelf 

 off the Delmarva Peninsula would promote greater across- 

 shelf exchange, which may tend to diminish any isolating 

 effect of stratification development. The results from re- 

 covered seabed drifters (Bumpus 1973) indicate slightly 

 higher drift speeds and distinctly lower recovery rates for 

 the bottom waters off Delmarva as compared to the New 

 Jersey shelf. One interpretation of these results would be 

 that the flushing rate of the bottom waters off the Del- 

 marva Peninsula is greater, which might tend to maintain 

 a higher rate of oxygen replenishment. 



PREVIOUS BENTHIC MORTALITIES 



Three previous episodes of benthic mortalities have 

 been reported in the same area as the 1976 anoxia: Sep- 

 tember through early October 1968, October 1971, and 

 August 1974 (ch. 1). None of these earlier mortalities was 

 as extensive or enduring as in 1976. Low D.O. concen- 

 trations in bottom waters off New Jersey were reported 

 with each occurrence. 



In comparing conditions during earlier mortalities and 

 what occurred in 1976, indexes associated with unusual 

 stratification were the only factors for which sufficient 

 historical records exist. To determine whether conditions 

 similar to 1976 occurred in the earlier cases of anoxia, and 

 at other times, climatological records of sea-surface tem- 

 perature from shore stations and discharge rates for the 

 Hudson River at Green Island were examined for the 30- 

 year period 1947-76. During this period, high discharge 

 (arbitrarily defined as greater than 150 percent of the 

 monthly mean) occurred five times in January (1949, 1950, 

 1952, 1973, and 1974) and four times in February (1949, 

 1951, 1954, and 1976). Shore station temperature records 

 for Sandy Hook and Atlantic City, N.J., indicated early 

 warming of the water (monthly mean for February warmer 

 than for January) 12 times at Sandy Hook and 9 times at 

 Atlantic City. Early warming was reported at Sandy Hook 

 in 1970 and 1971, but at Atlantic City observations were 

 not made those years. Early warming and high discharge 

 coincided in 1949, 1952, 1954, 1974, and 1976. Therefore, 

 these 5 years are considered potential times when anoxic 

 conditions might have developed during summer in the 

 shelf waters off New Jersey as the result of early onset of 

 stratification. For the 30-year record, the greatest warming 

 and discharge recorded in February were in 1976. Included 

 in the list of potential years of early onset of stratification 

 is 1974, one of the times of reported mortahties. The 1968 

 and 1971 instances are not included. 



The 1974 and 1976 mortalities were during summer, but 



the 1968 and 1971 mortalities were during autumn. The 

 implication is that very low D.O. can result from an early 

 spring or a late autumn; either would tend to lengthen the 

 stratification period. 



The effect of a prolonged summer (late onset of cooling) 

 can be derived from the relation between strength of strat- 

 ification and monthly rate of D.O. decline as shown by 

 the annual cycles of bottom D.O. and stratification (fig. 

 6-3). During summer, stratification usually is about 4.0 

 CT, units, which corresponds to an average, monthly rate 

 of oxygen decline of about 2.0 ml/1. Typically, the average 

 annual minimum oxygen is about 2.9 ml/1 in August. If 

 the usual breakdown of stratification by surface cooling 

 is delayed a month (October rather than September) then 

 minimum bottom D.O. concentrations off New Jersey 

 would be expected in September at an average concen- 

 tration of about 0.9 ml/1. 



In examining conditions that might indicate the late 

 arrival of autumn, surface temperatures were considered 

 the only significant factor, because river discharge in sum- 

 mer and autumn is typically small. In the 30-year record 

 for the Hudson River (1947-76) the discharge for August 

 is less than the monthly means for December, January, 

 and February, and the September discharge was as great 

 or greater than the monthly means for December through 

 February only once — 1975. Sea-surface temperature rec- 

 ords for Atlantic City (1947-76) show that August was 

 typically the warmest month and September was warmer 

 than August only seven times — in 1948, 1957, 1959, 1965, 

 1966, 1968, and 1971. Of these years the highest rate of 

 September warming was in 1968 and the second highest 

 in 1971. These are the years of autumn mortalities. There 

 were no instances of early-spring and late-autumn con- 

 ditions occurring in the same year. 



Included in the NODC historical data for bottom D.O. 

 concentration (fig. 6-3) were some values in February and 

 June 1968 and March 1971. At these times, bottom D.O. 

 values were above or equal to the average trend values, 

 implying that the low D.O. reported with the mortalities 

 did not result from early onset of stratification. 



SUMMARY 



Based on historical oceanographic data, the DO. con- 

 tent of bottom waters over the New Jersey-Cape May and 

 Long Island continental shelves typically declines during 

 spring and summer, reaching minimum values in August. 

 The seasonal decline in D.O. closely parallels the devel- 

 opment of density stratification, and the rate at which 

 D.O. declines seems to correspond with the strength of 

 stratification. Stratification tends to isolate bottom waters 

 from vertical replenishment until September, at which 

 time cooling of the surface and mixing begins to destroy 

 the vertical structure. This results in increased replenish- 



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