Oxygen Depletion and Associated Benthic Mortalities 

 in New York Bight, 1976 



Chapter 4. Chemical Factors 



Donald K. Atwood,^ Terry E. Whitledger and Jonathan H. Sharp^ 



Adriana Y. Cantillo, George A. Berberian, Joan M. Parker, and Philip G. Hanson' 



James P. Thomas and Jay E. O'Reilly^ 



' Atlantic Oceanographic and Meteorological Labo- 

 ratories, Environmental Research Laboratories, NOAA, 

 Miami, FL 33149 



' Oceanographic Sciences, Brookhaven National Lab- 

 oratory, Upton, NY 11973 



^ College of Marine Studies, University of Delaware, 

 Lewes, DE 19958 



* Northeast Fisheries Center, National Marine Fish- 

 eries Service, NOAA, Highlands, NJ 07732 



INTRODUCTION 



In this chapter we discuss the chemistry associated with 

 what has been termed the 1976 anoxic event in New York 

 Bight in hght of what is tcnown about anoxic conditions 

 in the ocean. We also look at how the chemistry of Bight 

 waters in 1976 differed from other years for which data 

 exist. 



When an ocean system like New York Bight moves 

 toward anoxia, chemical events can be described in this 

 general way (Richards 1965; Deuser 1975; Dugdale et al. 

 1977): 



1. Once the system is isolated or overloaded with an 

 oxygen demand, respiration and oxidation of organic mat- 

 ter deplete the dissolved oxygen (D.O.) available. 



2. When all or nearly all the D.O. has been utilized, 

 the next source of energy for oxidation of organic matter 

 is nitrate (NO,^ ). As the nitrate is utilized, and chemically 

 reduced, concentrations of nitrite (NO,^ ) and ammonium 

 (NHj*) increase. There is some evidence that this reduc- 

 tion should continue to nitrous oxide (N^O) and to ele- 

 mental nitrogen (N.). 



3. When oxygen, nitrate, and nitrite have been con- 

 sumed, the system uses dissolved sulfate (SOj') as its 

 source of oxidative energy, and sulfate is reduced to sul- 

 fide (S°), which occurs in the system as hydrogen sulfide 

 (HoS). Since hydrogen sulfide is very toxic, a good deal 

 of sudden mortalities can be expected at this stage. 



4. As oxygen, nitrate, nitrite, and sulfate are depleted, 

 the system's oxidizing capability drops, and the environ- 

 ment becomes a reducing one; that is, instead of being an 

 electron sink, the system becomes an electron source. As 

 a result, the oxidation states of numerous dissolved spe- 

 cies, notably metals, are changed. This, combined with 



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