CHAPTER 4 



shore region, with no values lower than 0.5 ji.g-at/1. At 

 mid-shelf, values up to 8 |xg-at/l at 65 m depth were meas- 

 ured. The bottom nitrate concentrations were above 4 \i.g- 

 at/l on the outer shelf region, and some nitrate was prob- 

 ably introduced to the Bight Apex by Hudson-Raritan 

 river runoff. 



In spring 1976 off New Jersey (fig. 4-5), bottom nitrate 

 concentrations were slightly lower than in early spring 

 1975. Concentrations at the inshore stations dropped to 

 less than 0.5 (xg-at/l, probably as a result of the spring 

 phytoplankton bloom. Concentrations at the deeper off- 

 shore stations also declined. April 1976 bottom nitrate 

 concentrations were high at the shelf edge off Long Island 

 and corresponded closely to deep values observed in 

 March 1975 off New Jersey (fig. 4-5). 



The BNL transects taken in April 1975 (fig. 4-4) and 

 April-May 1976 show interesting differences between the 

 2 years (fig. 4—6). Inshore concentrations were markedly 

 lower in 1976; nitrate values integrated over the euphotic 

 zone were as low as 2.2 ixg-at/m- (Conway and Whitledge, 

 personal communication). The nitrate gradient at the shelf 

 break was smaller in 1976 than in 1975 and deeper source 

 waters off the shelf contained 3 (xg-at/l less nitrate. This 

 lower concentration in the deep water persisted over 2 

 weeks and could represent a deficiency in the 1976 source 

 water. If so, any cross-shelf movement of this water would 

 have brought less nitrate to the midsheif and inshore re- 

 gions in 1976. 



A late spring sampling off New Jersey in May 1975 

 showed little or no nitrate inshore of the 60-m isobath 

 (fig. 4-7). However, the deep water, which probably acted 

 as a nitrate source, had concentrations similar to those 

 observed in February-March 1975, indicating a general 

 nitrate depletion on the shelf — most pronounced off New 

 Jersey and less apparent off Long Island (area of transect 

 V on fig. 4-4). A similar sampling in May 1976 found 

 slightly higher nitrate near the bottom of the euphotic 

 zone off New Jersey, but no value was greater than 1 p.g- 

 at/l (fig. 4-7). The May 1976 bottom nitrate concentrations 

 were negligible over the entire shelf. Note that the shelf 

 break nitrate concentrations shown for May 1975 and May 

 1976 are similar; this weakens the evidence for depletion 

 of nitrate in the 1976 "source" water. 



A 1975 summer section off New Jersey (fig. 4-8) showed 

 nitrate values similar to those seen in May of that year. 

 Data for about the same place and time in 1976 are limited, 

 even though they were taken about the same time that the 

 initial anoxic conditions arose; however, data collected at 

 that time for that section are all below the detection limit 

 of 0.5 |xg-at/l and are not shown in the figure. For the 

 1976 cruise also, all bottom nitrate values except those off 

 the shelf were less than the detection limit. Apparently, 

 bottom nitrate concentrations inside the shelf break were 

 an order of magnitude less in 1976 than in 1975 — possibly 



26 MAR -9 APR 1975 

 STATIONS 



43 21 23 46 48 



18120 I 22 I 45 I 24 I 25 26 27 28 29 



.1.1 I I I L_j I I . .1 . 1 . . .1 I ..I . I 



180 - 



B 



DISTANCE OFFSHORE (km) 



20 APR -4 MAY 1976 



STATIONS 

 6 8 10 12 



20 40 60 80 100 120 140 

 DISTANCE OFFSHORE (km) 



FIGURE 4-6. — Nitrate concentratieins for spring 1^75 (A) and spring 

 1976 (B) BNL cross-shelf transect values in jjig-at 1 



a result of reduced transport of nitrate across the shelf 

 break or a result of denitrification. 



Late summer sections off New Jersey for 1975 and 1976 

 (fig. 4—9) both show the effects of bottom shoreward trans- 

 port of nitrate-rich water. At this time in 1976, oxygen- 

 deficient water was still located off some areas of the New 

 Jersey coast. The bottom nitrate concentration increased 

 8 or 9 |xg-at/l off Long Island during both years, but some 



101 



