258 WETZEL AND RICH 



TABLE 8 



LITTORAL ORGANIC-CARBON BUDGET, 

 LAWRENCE LAKE, MICH.* 



* Estimates are on a lake-wide-area basis rather 

 than on an arbitrarily defined "littoral zone." 



where GP = gross production 



Rs(A) = respiration of autotrophs 

 Rs(H) = respiration of heterotrophs 



More recently Woodwell, Rich, and Hall (1972) modified the original expression 

 to account for import and export of organic material: 



NEP = (GP + NPj n ) - [Rs( A ) + Rs( H ) + NP out ] 



where NP^ = net production from another ecosystem 

 NPout = net production exported 



Both formulations interpret NEP as the net positive or negative increment of 

 organic matter, either living biota or dead organic storage, after total respiration 

 within the ecosystem. The latter definition of NEP, which specifically subtracts 

 organic exports, limits the application of the term to only that material which 

 remains inside ecosystem boundaries. Our experience with the Lawrence Lake 

 study leads us to conclude that the original definition of NEP (Woodwell and 

 Whittaker, 1968) is the more generally useful when considering aquatic 

 situations (Table 9). 



Aquatic systems appear to have much more dynamic equilibria than forest 

 ecosystems, the context in which the term NEP was originally defined. In 

 aquatic systems the biotic (living) structure is small compared to annual 



