CARBON IN FRESHWATER SYSTEMS 251 



Although tne quantity of DOC secreted, in addition to that released by 

 autolysis, is small in comparison to other sources (Tables 2 and 3), these organic 

 substrates are labile (e.g., Wetzel and Manny, 1972b) and are subject to rapid 

 heterotrophic utilization by bacteria (<48 hr) (Miller, 1972). In oligotrophic 

 waters, labile DOC can be highly limiting to heterotrophic carbon metabolism 

 and serves as a major factor among a number of simultaneous interactions 

 suppressing productivity of freshwater systems (Wetzel, 1968, 1971). This 

 labile-organic-substrate limitation, among an abundance of accumulated refrac- 

 tory organic substrates of very high residence times (turnover rates 1 to 

 >100 years), can result from either an absolute deficiency in the system or 

 abiotic loss mechanisms (such as adsorption to CaC0 3 as discussed previously or 

 by sorption to mineral particulate materials (e.g., Bader, Hood, and Smith, I960; 

 Meyers and Quinn, 1971b). Once eutrophication and synthesis of DOC of the 

 system proceeds to the point of continually saturating these inorganic sinks, 

 heterotrophic metabolism increases relatively rapidly and can contribute 

 significantly in accelerating nutrient (inorganic and organic) regeneration and the 

 cycling of carbon (Wetzel and Allen, 1972). 



The dissolved organic-carbon pool exhibits a relative constancy from year to 

 year (Table 3). Maximum detrital biomass of DOC generally occurs in late 

 summer and autumn, indicative of an accumulation of more refractory organic 

 compounds prior to autumnal circulation of the lake. Highest values of DOC 

 occur in the epilimnion, and concentrations consistently fluctuated more there 

 than in the hypolimnion (see Wetzel, Rich, Miller, and Allen, 1972). These 

 epilimnetic fluctuations are related to photosynthetic metabolism. 



Biochemical origins and sources of DOC in such an ecosystem as Lawrence 

 Lake are largely photosynthetic. In oligotrophic, moderately large bodies of 

 water, phytoplanktonic photosynthetic metabolism dominates (Fig. 3). How- 

 ever, among a majority of lakes of the temperate region, of which Lawrence 

 Lake is a typical example, the ratio of littoral to pelagic photosynthetic activity 

 typically increases greatly as the mean depth of the basin decreases. 

 Allochthonous inputs of DOC are quite constant for a given lake; their relative 

 contribution to the system decreases as autochthonous sources increase. The 

 ratio of photosynthetic fixation to dark C0 2 fixation and bacterial chemo- 

 synthesis is generally very large in oligotrophic waters. In Lawrence Lake, for 

 example, the 4-year average of dark fixation was 16.3% of light fixation of 

 phytoplankton (Table 4). The percentage is much less if the littoral photo- 

 synthesis is considered in addition to that of the phytoplankton. The ratio of 

 photosynthetic to dark C0 2 fixation decreases markedly in the transition to 

 planktonic eutrophy and hypereutrophy. With further transition of the system 

 to dominance by emergent macroflora and associated attached and eulittoral 

 microflora, the relative contribution of dark C0 2 fixation apparently decreases. 



A detailed analysis of the heterotrophic utilization of DOC originating 

 largely from phytoplanktonic secretion and autolysis in Lawrence Lake (Miller, 

 1972) indicates at least 44% of planktonic primary production was transferred 



