CHAPTER 8 



they are capable of vertical motility and choose to remain 

 below the pycnocline. Organisms which are transported 

 into the convergent areas do not move through the pyc- 

 nocline into the divergent area above. This leads to a 

 concentration of organisms in the lower layer of the con- 

 vergent areas off New Jersey and in the Apex. 



CONCLUSION 



The first conclusion to be drawn from the use of the 

 diagnostic model is that even during May and June 1976, 

 the replacement time of water in New York Bight was 

 relatively short compared to the general seasonal cycle of 

 property changes for the Bight as a whole and for the 

 inshore segments of principal interest. A second conclu- 

 sion is that oxygen depletion in the critical region during 

 the May-June period was due to oxygen utilization about 

 3 times greater than in other regions of the inner shelf and 

 nearly 10 times greater than that occurring over the outer 

 shelf, rather than simply being due to the length of strat- 

 ified season, stagnation, or advection of low-oxygen 

 water. The most critically affected regions, in fact, had a 

 relative advective oxygen input substantially greater than 

 other areas. 



The key problem in examining the anoxia episode is 

 explaining the high consumption rates that occurred off 

 New Jersey during spring and summer 1976. Numerous 

 causes have been suggested, including ocean dumping, 

 estuarine discharge, primary production (Segar and Ber- 

 berian 1976; ch. 10), and an anomalous bloom of dinofla- 

 gellate organisms (ch. 9, pt. 2). All these suggestions do 

 not adequately address the questions of why 1976 and why 

 the inner shelf off New Jersey. Although observations 

 obtained and techniques used are deficient for explicit 

 quantitative modeling of carbon/oxygen relationships, or 

 even the distribution of particulate material or Ceratium 

 tripos in the Bight, a strong qualitative case can be made 

 from results of the diagnostic circulation model of the 

 Bight, and the model study of particulate material trans- 

 port by Festa and Hansen (1976), that the 1976 anoxia 

 episode off New Jersey came about in the following way. 



For reasons not well understood but probably related 

 to the pattern of surface wind, and quite possibly to the 

 heavy discharge from the Hudson River during early 1976, 

 the circulation in and near the Apex was kinematically 

 equivalent to that commonly occurring in coastal plain 

 estuaries for the entire period from May 18 to June 29. 

 A similar situation existed in the average along the entire 

 inner New Jersey shelf over the last 26 days of the period. 



Such circulations tend to trap and concentrate suspended 

 particulate matter having a small sinking velocity. This 

 concentration process functions irrespective of where such 

 particulate materials are introduced into the circulation 

 pattern. Hence, particulates from the Hudson River, the 

 dumping activities, and plankton productivity would have 

 concentrated in the Apex and over the inner shelf off New 

 Jersey. This concentration of oxidizable material had an 

 exorbitant biochemical oxygen demand that depleted the 

 oxygen supply in spite of reasonably active ventilation of 

 waters below the thermocline. 



If Ceratium tripos, which are capable of relatively rapid 

 movement (see chapter 9, part 2), seek a position below 

 the thermocline, this behavior will couple to the circula- 

 tion just as do inanimate particle distributions. In the case 

 of Ceratium tripos the concentration mechanism can be 

 expected to function with particular efficiency because the 

 organism can optimize its vertical movement relative to 

 that of the water. A principal outstanding question is 

 whether Ceratium tripos did in fact contribute in a dom- 

 inant way to the oxygen demand off New Jersey, or 

 whether the concentration of other oxidizable particulates 

 would have led to anoxia even in the absence of Ceratium 

 tripos. 



Conditions in May and June 1975 have not been diag- 

 nosed. But the comparison of current meter data between 

 1975 and 1976 made in chapter 7 shows that the convergent 

 circulation pattern which fostered inshore concentration 

 of particulates did not exist during spring 1975. 



Though we can calculate what occurred in 1976, the 

 comparison of oxygen utilization rates to average rates 

 suffers because we have a poor idea of what the average 

 really is. Changes in both terms of the oxygen mass bal- 

 ance, CLQC + QyC) and SS, contributed to the large de- 

 crease in concentrations with time preceding the anoxic 

 episode. A really illuminating result is that an order of 

 magnitude change in these terms is not necessary to pro- 

 duce the observed conditions. Relatively small changes, 

 smaller than a factor of two, can considerably influence 

 the mass balance. The simplicity of the oxygen mass bal- 

 ance equation conceals many feedback loops between the 

 water transport and oxygen production. Decreased trans- 

 port will, for example, prevent nutrients from reaching an 

 area, thus decreasing plankton growth rates. In the so- 

 lution to these problems and others, such as nutrient sup- 

 ply, organic loadings, and planktonic growth rates, and 

 the oxygen consumption rates of all these constituents, lie 

 the answers to why the anoxia developed off New Jersey 

 in summer 1976. The oxygen transport is only a piece of 

 the puzzle. 



191 



