NO A A PROFESSIONAL PAPER 11 



a NMFS cruise of August 6-17 (fig. 2-27). surface cooling 

 is suggested. This effect could have resulted from passage 

 of hurricane Belle (discussed later). 



By September, the salinity below the pycnocline had 

 changed relative to June (fig. 2-12). It is difficult to com- 

 pare the September and June salinity distribution at the 

 bottom of the pycnocline because of the few observations 

 in June. However, the salinity sections show a better de- 

 veloped halocline in inshore waters in June, at which time 

 bottom water was more saline off New Jersey and in the 

 shelf valley than in September. Surface water off Long 

 Island (section E-F) was fresher in September than in 

 June. The source of this relatively fresh water appears to 

 be the Hudson-Raritan estuary (fig. 2-12). 



The densitv difference, Aa,, was slightly less in Septem- 

 ber than in June (figs. 2-13, 2-14, and 2-16). By Septem- 

 ber the signature of the Hudson-Raritan outflow had dis- 

 appeared, and, except for a relatively weak pycnocline 

 associated with the fresher surface water off southwestern 

 Long Island, there was no significant density difference 

 in the pvcnocline off New Jersey and Long Island. How- 

 ever, the pycnocline was weaker in these nearshore lo- 

 calities than offshore. 



September presented a different picture than earlier 

 months relative to the norm. Although surface tempera- 

 tures were tvpical of the average, bottom waters, which 

 had temperatures of 8° to 9° C (figs. 2-11 and 2-15), were 

 considerably colder than the 12° C described by Bowman 

 and Wunderlich ( 1977). Because salinity values continued 

 to be somewhat high, densities were still above normal. 



Seasonal development of the pycnocline was not re- 

 flected by significant changes in the depth or configuration 

 of the pycnocline bottom or the salinity at the bottom of 

 the pycnocline. With the exception of the Hudson-Raritan 

 outflow, the bottom of the pycnocline seemed to be more 

 closely related to the presence of offshore water. 



Because of the depletion of oxygen in the subpycnocline 

 bottom layer (ch. 6), the depth of the pycnocline bottom 

 was used to determine the thickness of the bottom layer 

 for the four XWCC cruises (figs. 2-17 to 2-20). The pat- 

 terns of bottom layer thickness reflect isobathic control. 

 In June, the pycnocline bottom generally was 3 m higher 

 in the water column off New Jersey than off Long Island 

 (fig. 2-19). However, the depth of the ocean floor is con- 

 siderably less off New Jersey so that the thickness of the 

 subpycnocline layer was about 3 to 3 m less than off Long 

 Island. By September the bottom of the pycnocline was 

 observed at greater depths off Long Island, which tended 

 to equalize the thickness of the subpycnocline layer off 

 the two coasts. 



OXYGEN DISTRIBUTION 



Dissolved oxygen (D.O.) was determined by Winkler 

 titration on all water samples collected at discrete depths 



with a Rosette multisampler attached to the conductivity- 

 salinity-temperature-depth (CSTD) sensor. On the May 

 and June cruises D.O. also was monitored with an oxygen 

 probe on the CSTD, which functioned on a majority of 

 the stations of these two cruises. Although slow in re- 

 sponse time, this electrode allows a more accurate deter- 

 mination of the depths of oxygen maxima and mmima 

 than the discrete Winkler analyses. 



The distribution of average D.O. concentration in the 

 subpycnocline layer for May and June is shown in figures 

 2-23 and 2-24. These distributions were derived by av- 

 eraging all the Winkler-determined oxygen values below 

 the pycnocline at each station for each of these months. 

 The decrease in available oxygen and increase in patchi- 

 ness of its distribution between May and June are evident. 



The vertical sections of D.O. for April, May, June, and 

 September were determined from the water samples (figs. 

 2-21 and 2-22). The April oxygen range of 6.0 to 7.5 ml/ 

 I was what would be expected for weakly stratified waters 

 at that time of year. In May oxygen levels began to reflect 

 the onset of stratification, with some depletion to under 

 5 ml/I in the bottom water and some increase to as much 

 as 9 ml/I near the top of the pycnocline. In June, a very 

 strong gradient in oxygen values developed through the 

 pycnocline (figs. 2-13 and 2-22). High values (over 9 ml/ 

 I) near the top of the pycnocline occurred offshore and 

 low values (as little as 0.5 ml/i) were present below the 

 pycnocline, particularly near the head of the Hudson Shelf 

 Valley. Over the shelf valley, the minimum oxygen values 

 were not observed at the bottom but in the 30- to 40-m 

 depth range. 



By September, the high oxygen values in the top of the 

 pycnocline disappeared so that the DO. in the mixed 

 surface layer had average values of about 5.25 ml/I. Below 

 this, seaward over the shelf, the oxygen gradient was very 

 strong, from 5 ml/1 to 2 ml/i between 20 and 30 m depth. 

 On the inner shelf floor (fig. 2-22). D.O. values were well 

 below 2 ml/I, and off New Jersey below I ml/1. Close to 

 shore, the oxygen content was below the detection limit 

 of the Winkler titration. Offshore, in continental slope 

 water, oxygen values ranged between 4 and 5 mI/1. As 

 observed earlier in the Hudson Shelf Valley, the minimum 

 generally was not at the bottom except near the head of 

 the valley in Christiaensen Basin. 



The D.O. probe values were used to determine the 

 depth of the main oxygen minimum above the bottom 

 (figs. 2-25 and 2-26). In both cases, the minimum 

 "grounded" in the vicinity of northern New Jersey and 

 was terminated by the relatively highly oxygenated dis- 

 charge from the Hudson-Raritan estuary, which in this 

 location averaged about 5 ml/I, and showed no significant 

 gradient across the pycnocline (figs. 2-21 and 2-22). The 

 oxygen minimum was always below the base of the pyc- 

 nocline but its distance from the bottom generally de- 

 creased from May to June except in the Hudson Shelf 



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