NO A A PROFESSIONAL PAPER 11 



nocline, in circulation that could affect horizontal advec- 

 tion of oxygen, or in the use of oxygen brought about by 

 variations in biological oxygen demand. (See chapter 9, 

 part 1.) 



STRATIFICATION AND 

 DISSOLVED OXYGEN IN 1976 



A comparison of D.O. observations made off New Jer- 

 sey during cruises in December 1975 through September 

 1976 and historical data (fig. 6-3) shows that bottom oxy- 

 gen concentrations were typical in December. However, 

 D.O. concentrations in near-bottom waters were below 

 normal by April, and progressively declined until late in 

 the summer. The sharp drop in air and surface water tem- 

 peratures in December-January probably destroyed the 

 slight stratification that was present in December 1975 

 (fig. 6-3). In March, April, and May, the early increase 

 in river discharge, and reduced storm activity, probably 

 led to greater than normal stratification. Stratification was 

 typical in June and into August, before the passage of 

 hurricane Belle. Stations occupied by the NMFS Sandy 

 Hook Laboratory immediately after the hurricane indi- 

 cated a distinct decrease in stratification, particularly at 

 shallower stations, but with less impact in deeper water. 



as discussed in chapter 2. In September 1976, stratification 

 had weakened and values were typical for that time of 

 year, except at one station with particularly weak strati- 

 fication. 



Using the values of stratification observed in 1976 (fig. 

 6-3), the curves of figure 6-A can be used to model the 

 cycle of bottom D.O. decline that could have been ex- 

 pected from the strength of stratification alone during that 

 year. For the modeling, the intense cooling of surface 

 waters in January 1976 was assumed to have destroyed 

 the stratification remaining from December, leaving the 

 waters unstratified by mid-January. Beginning with no 

 stratification in mid-January, the cruise-averaged values 

 of stratification strength observed in 1976 were connected 

 to define the average cycle during the season of stratifi- 

 cation. Similar cycles were drawn connecting each of the 

 minimum and maximum points, all beginning with the 

 value of zero in mid-January. From these three cycles, 

 monthly mean values were derived for each monthly in- 

 terval from mid-January to mid-August. Monthly rates of 

 decrease in D.O. were determined from the appropriate 

 curves of figure 6-4. 



D.O. concentrations in the bottom waters off New Jer- 

 sey were assumed to increase from December 1975 con- 

 ditions (ranging from 5.4 to 6.6 ml/1, 6.0 average) at the 

 normal rate for that time of year until mid-January. As- 



Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep 



FIGURE 6-5. — Dissolved oxygen content of bottom waters (>20 m) of New Jersey-Cape May shelf as predicted from stratification model and as 



observed in 1976. 



142 



