CARBON IN FRESHWATER SYSTEMS 245 



inadequate for metabolism or indirectly inadequate as a result of chemical losses 

 from the system. Although it is highly doubtful that inorganic carbon per se is 

 seriously limiting to photosynthetic metabolism under most natural situations, 

 physiological shifts to assimilation of bicarbonate at metabolic expense appear 

 to occur rapidly. Possession of an affinity for bicarbonate is an adaptive 

 advantage in a significant percentage of freshwaters, particularly for larger 

 submersed angiosperms. 



ORGANIC CARBON 



Nearly all the organic carbon of lake water consists of dissolved organic 

 carbon (DOC) and dead particulate organic carbon (POC). Almost universally 

 the ratio of DOC to POC approximates 10 : 1 in both lacustrine and stream 

 systems. DOC is defined rather arbitrarily in most studies by the practical 

 necessity of fractionation of POC from DOC by filtration at the 0.5-fl 

 level; hence DOC concentrations can include a significant colloidal fraction in 

 addition to truly dissolved organic carbon. Although the living POC of the biota 

 constitutes a very small fraction of the total POC, the metabolism of this biota 

 mediates a series of reversible fluxes between the dissolved and particulate 

 phases of detrital carbon. 



The composition and sources of organic carbon are diverse and poorly 

 understood. Production of dissolved and particulate carbon is, in part, a result of 

 autotrophic or heterotrophic metabolism. Instantaneous measurements of the 

 chemical biomass of DOC and POC, however, are highly biased toward the 

 refractory components of detrital organic carbon that accumulate in freshwaters. 

 The labile components cycle rapidly at low equilibrium concentrations but 

 represent major carbon pathways and energy fluxes. Furthermore, in streams 

 and most lakes, much of the detrital organic carbon, mostly dissolved, is of 

 terrestrial origin. A majority of lakes are small with a high proportion of their 

 surface area as littoral zone. Allochthonous and littoral sources of dissolved and 

 particulate detrital organic carbon form major inputs to the lacustrine system 

 and can markedly modify pelagic metabolism. In streams, major metabolic 

 carbon pathways for processing dissolved organic carbon are by planktonic 

 bacteria rather than benthic microflora (Wetzel and Manny, 1972a; Cummins 

 et al., 1972). In the relatively static waters of lakes, greater rates of 

 sedimentation displace a major portion of carbon metabolism to the benthos 

 (Wetzel, Rich, Miller, and Allen, 1972). To approach the complex carbon cycle 

 that dominates both the structure and function of lakes, one must have a 

 complete representation of the productivity of all components of the ecosystem 

 as well as an understanding of the origins and metabolism of detrital organic 

 carbon. 



DOC and detrital POC exceed the amount of organic carbon of the fauna, 

 plankton, and bacteria manifold (e.g., Birge and Juday, 1926, 1934; Saunders, 



