256 



WETZEL AND RICH 



O 

 O 



Q- 

 CJ 



o 



Q- 



3 

 — 



O 

 c_> 



LU 



INCREASING FERTILITY — ** 



Fig. 5 Generalized planktonic, littoral, and allochthonous sources of POC to 

 lakes of increasing fertility, with shifts in the domination of pelagic and 

 littoral productivity (see Fig. 2). 



transition from nutrient-limited conditions of oligotrophic lakes to light 

 limitations of biogenic origins and dominance by emergent flora community. 



In conclusion, the organic-carbon budgets for Lawrence Lake can serve in 

 exemplary fashion to illustrate the various inputs, losses, and utilization 

 relationships of organic carbon. The pelagic organic-carbon budget (Table 7) 

 indicates losses of DOC exceed considerably inputs, resulting in a net production 

 of DOC of — 17 g C nv 2 year -1 . In contrast, particulate-organic-carbon produc- 

 tion exceeds losses (about 20 g C va 1 year -1 ) and results in a total organic net 

 production of +3 g C m -2 year l for the pelagic zone. 



In need of reiteration is the importance of the littoral region, not only its 

 contribution to autochthonous production of the lake as a whole but also its 

 indirect effects on pelagic and benthic carbon metabolism (Table 8). These 

 estimates for the littoral organic-carbon budget of Lawrence Lake, based on a 

 lake-wide area rather than an arbitrarily defined littoral zone, emphasize the 

 magnitude of these components in the lake system. The characteristics of this 

 lake are such that in no way is the littoral zone exaggerated. In a majority of 

 lakes, the littoral contribution is a major component of organic-carbon flux. 



NET ECOSYSTEM PRODUCTION 



Woodwell and Whittaker (1968) originally defined net ecosystem production 

 (NEP) as 



NEP = GP - [Rs (A) +Rs (H )] 



