260 WETZEL AND RICH 



the system by the rather constantly maintained equilibrium concentration of 

 DOC. Thus export in this system represents the superimposition of a rather 

 stochastic hydrology and homeostatic factors regulating both production and 

 DOC concentration. The arbitrary exclusion of "export" from NEP in this 

 situation is a gross oversimplification that obscures the operation of important 

 regulatory functions of the ecosystem. 



Detrital Electron Flux 



The value of the original NEP formula is also seen in the case of a second 

 phenomenon where the application of NEP to noncarbon substrates may be 

 conceptualized. This is the special case of a detrital food chain wherein detrital 

 energy is subsequently transferred by noncarbon substrates in an anaerobic 

 environment. Because of the low solubility of oxygen in water, oxygen is 

 frequently exhausted as an electron acceptor during hypolimnetic and benthic 

 heterotrophy. The concentration of electrons then increases in the environment, 

 and the redox potential becomes negative. Alternate electron acceptors, e.g., 

 SO4 and NO3, are reduced by continued bacterial activity and are converted, in 

 general, to more soluble products. Ferric iron may also be reduced to ferrous 

 iron with a resulting decrease in the binding capacity of the sediments. The 

 diffusion interface between the reduced products and dissolved oxygen then 

 becomes the site of chemosynthesis where bacterial metabolism, actually 

 noncarbon heterotrophy, reoxidizes the substrate. The term "detrital electron 

 flux" reminds us that electrons, not carbon, represent the actual continuity in 

 this case and are the ultimate trophic medium of exchange following 

 photosynthesis. 



In the case of anaerobic respiration, the NEP equation is misleading in terms 

 of carbon because, although the carbon reduced in photosynthesis is oxidized, 

 the oxygen oxidized in photosynthesis is not the substrate reduced: 



PS 

 C0 2 + H 2 O CH 2 O + 2 -> respiration 



Aerobic ^ C0 2 + H 2 

 Anaerobic -> C0 2 + H 2 S + 2 



{ 



NEP = CH 2 O - C0 2 = (carbon) 

 NEP = 2 - H 2 = (oxygen) 



NEP = CH 2 O - C0 2 = (carbon) 

 NEP = 2 - H 2 S =* (oxygen) 



It may be argued that H 2 S is ultimately equivalent to H 2 0; however, 

 energetically there is a discrepancy equivalent to the distance between oxygen 

 and sulfur in the electromotive series. This discrepancy is manifested in the 

 inequality of the NEP equation for oxygen. 



The NEP concept is a powerful insight, open to much elaboration. In the 

 final analysis its unique usefulness derives from its compatibility with both 

 material and energy units and with both trophic and compartmental models of 

 ecosystems. Although the role of detritus in ecosystems is complex and poorly 

 understood (Wetzel et al., 1972) and the current definition of NEP (Woodwell 



