NO A A PROFESSIONAL PAPER 11 



decrease until mid-August at the rates defined by the ob- 

 served strength of stratification. Also shown in figure 6-8 

 are the cruise averages and ranges of values observed 

 during 1976. The estimated average minimum, in August, 

 was 2.30 ml/1 (range 1.40 to 3.95 ml/1). 



The comparison in figure 6-8 of predicted bottom D.O. 

 concentrations for the waters off Long Island with con- 

 ditions observed in 1976 indicates that the stratification- 

 dependent oxygen model accounts fairly well for the ob- 

 served conditions in April and May. By the end of June, 

 bottom D.O. concentrations were below the model values, 

 but not as much as in the waters of the New Jersey — Cape 

 May area (fig. 6-5). As with the waters off New Jersey, 

 one or more additional circumstances must have devel- 

 oped in the Long Island waters, particularly in June, to 

 affect the replenishment-utilization ratio in such a way 

 that the oxygen decline was more rapid. But the impact 

 on the D.O. concentrations was not as intense for waters 

 off Long Island as off New Jersey. 



South of New Jersey, over the continental shelf of the 

 Delmarva Peninsula, no anoxia was reported in 1976. Pre- 

 sumably the early warming and early occurrence of spring 

 river discharge would also have influenced these waters. 

 Historical observations in the NODC archives for that 

 area were too sparse to demonstrate an annual cycle of 

 bottom oxygen, and data were not available for describing 

 developing conditions during 1976. Presumably, bottom 

 D.O. concentrations off the Delmarva Peninsula normally 

 follow an annual cycle similar to those off New Jersey and 

 Long Island, but do not normally decrease to concentra- 

 tions as low as off New Jersey. 



Possibly the differences in the annual cycle of bottom 

 D.O. concentration between the New Jersey — Cape May 

 areas and Long Island, and presumably the Delmarva 

 Peninsula, are the result of bathymetric differences. 

 Stearns (1969) noted that on the New Jersey side of the 

 Hudson Shelf Valley, gravel deposits have caused the con- 

 tinental shelf waters off northern New Jersey to be about 

 5 fm (9 m) shoaler than on the Long Island side of the 

 valley. He pointed out that the effect of the gravel deposits 

 is apparent in bathymetric charts: the 20- to 30-fm (37 to 

 55 m) isobaths are farther off New Jersey than off Long 

 Island. 



To depict bathymetric differences of the continental 

 shelf off Long Island, New Jersey, and the Delmarva 

 Peninsula, average depth profiles were made for the three 

 regions (fig. 6-9). The profiles were constructed from the 

 bottom topography maps of Uchupi (1968) by computing 

 the average distance offshore to selected isobaths con- 

 toured on the maps (20, 40, 60, 80, 100, 140, and 200 m). 

 In general, the continental shelf off New Jersey is shoaler 

 than the shelf off Long Island and wider than the shelf off 

 the Delmarva Peninsula. Figure 6-9 shows that the depth 

 of the shelf is about 20 m less off New Jersey than off 

 Long Island. 



Distance Offshore (km) 

 50 100 



150 



New Jersey. 

 Cape May 



200 



FIGURE 6-9. — Average bottom depth profiles across selected areas 

 of continental shelf in Middle Atlantic Bight. 



To explore how the difference in water depth in the 

 Long Island and New Jersey areas might affect seasonal 

 oxygen declines, the data displayed in figure 2-18 of chap- 

 ter 2 were examined for differences in thickness of bottom 

 water (pycnocline bottom to ocean bottom) for the two 

 areas. The May cruise was chosen because it was made 

 during stratified conditions and provided more extensive 

 coverage than either the April or June surveys. Area av- 

 erages of the thickness of subpycnocline or bottom waters 

 from the coasts to the 60-m isobath were calculated for 

 each of the two shelf regions. Average thicknesses were 

 13 m for the Long Island shelf and 9 m for the New Jersey 

 shelf, reflecting the greater depth to the bottom off Long 

 Island. 



With the greater volume of water that would tend to be 

 isolated in the subpycnocline layer during the months of 

 stratification off Long Island, and realizing that maximum 

 oxygen concentrations are typically about the same in both 

 regions, then there would be a volume of about 44 percent 

 more oxygen available in the bottom waters off Long Is- 

 land than off New Jersey. In the most simplistic case 

 (where there is no advection and presuming that the bi- 

 ological oxygen demand per unit area is the same in both 

 shelf regions) the lesser volume of oxygen available in the 

 thin layer of bottom water off New Jersey would be di- 

 minished to lower concentrations. From the historical, 

 annual trends of figures 6-3 and 6-6, D.O. values in the 

 bottom waters off Long Island normally decrease a total 

 of 3. 1 ml/1 (from the annual maximum in March of 7.0 ml/ 

 1 to the August minimum of 3.9 ml/1), and in bottom waters 

 off New Jersey decrease about 4.1 ml/1 (from a maximum 

 in March of 7.0 ml/1 to a minimum in August of 2.9 ml/1). 

 That is, bottom D.O. concentrations normally decrease 

 about one-third more off New Jersey than off Long Island. 



In a comparison of the averaged bathymetric profiles 

 of the Delmarva Peninsula and the continental shelf off 



146 



