CHAPTER 6 



Belle were excluded because of uncertainties about the 

 storm's effects on the normal cycle of D.O. 



Parallel patterns in the normal annual cycles of strati- 

 fication and D.O. in the bottom waters off New Jersey 

 (fig. 6-3) imply a direct relation between the two. During 

 autumn, surface cooling in shelf waters causes overturning 

 and vertical mixing. This diminishes the stratification and 

 replenishes oxygen in the bottom waters. By December 

 and continuing through March, shelf waters are essentially 

 unstratified and bottom waters are saturated with D.O. 

 Further increases in bottom D.O. during winter result 

 from increased oxygen solubility of the water brought 

 about by decreasing water temperature. Bottom D.O. in- 

 creases to the annual average maximum of about 7.0 ml/ 

 1 in March. 



Initiation of stratification normally begins during April 

 when surface water density declines, because of warming 

 (fig. 6-1), and river discharge increases (fig. 6-2). With 

 the onset of stratification, vertical replenishment of oxy- 

 gen into the bottom water becomes limited. Within the 

 normal ranges and cycles of water circulation and biolog- 

 ical activity, utilization exceeds replenishment, resulting 

 in a decrease in oxygen concentration. Based on the his- 

 torical data of figure 6-3, the annual minimum D.O. value 

 occurs in August, averaging about 2.9 ml/1. In September, 

 when surface cooling begins to destroy stratification, re- 

 plenishment exceeds utilization and oxygen concentra- 

 tions increase. 



To depict the correspondence between increase in 

 strength of stratification and D.O. decline, monthly values 

 of both were derived from the annual trend curves of 

 historical data in figure 6-3. For each 1-month period from 

 mid-March through mid-August (spanning the interval of 

 developing stratification and of declining D.O.), monthly 

 means of the strength of stratification were computed. 

 Against these were plotted the amount that D.O. declined 

 during the corresponding 1-month periods; the data points 

 were connected by a hand-smoothed curve to give monthly 

 rates of oxygen decline as a function of stratification (fig. 

 6-4). To provide some estimate of the range of conditions 

 around the average trend curves, similar monthly com- 

 putations were made from the smoothed ranges of values 

 in figure 6-3, assuming that the high D.O. values corre- 

 spond to weakest stratification and low oxygen to strongest 

 stratification in a given month. Again, smooth curves were 

 drawn through the data points. 



As stratification increases the rate of oxygen decline 

 increases (fig. 6-4) as would be expected, since enhanced 

 stratification acts to further isolate bottom waters from 

 replenishment. Basically, the relationships in figure 6-4 

 represent the imbalance in replenishment and use of D.O. 

 which, on the replenishment side, is strongly affected by 

 stratification. The reason for the change in slope of these 

 curves at greater stratification values (at about 3.5 u, units 

 for the average curve) is unclear, but probably is associ- 

 ated with seasonal changes in the structure of the pyc- 



Stratification ((^ Units) 

 12 3 4 



Max. Stratification, 

 Mi n . Oxygen 



Min. Stratification 

 Max. Oxygen 



Average 



New Jersey Shelf 



FIGURE &A -Relation between strength of stratification and decline in dissolved oxygen ,n bottom waters (>20 m) of New Jersey-Cape May 



shelf. 



141 



