CARBON IN FRESHWATER SYSTEMS 255 



TABLE 6 



ANNUAL BENTHIC PARTICULATE-CARBON BUDGET, 

 LAWRENCE LAKE, MICH. 



double) in eutrophic lakes (Fig. 4). Differences in DOC are less marked in the 

 transition from oligotrophic to highly eutrophic waters. Algal-cell carbon 

 commonly increases nonlinearly with increasing fertility, with a slight tendency 

 to algal cells of greater size. Replacement times of algal-cell carbon by net 

 primary production are usually greater (slower turnover times) in eutrophic than 

 oligotrophic waters but are not proportional to increases in cell carbon. Hence 

 the algal cells are photosynthesizing more per unit cell carbon (have a greater 

 carbon flux) in less fertile waters. 



While planktonic metabolism of detrital organic carbon is very significant, 

 especially in oligotrophic lakes, benthic metabolism of POC is of major 

 importance. The annual estimate of benthic respiration for Lawrence Lake was 

 117.5 g C m year . Maximal values were found in the spring, and seasonal 

 variations corresponded closely with littoral and pelagic productivity. Variations 

 in rates of benthic respiration from the shoreline to the deepest depression of 

 the lake correlated directly with biomass production of submersed macrophytes 

 in all cases during the ice-free period (March to November). Benthic respiration 

 was highest in the deepest sediments during the spring maximum of phyto- 

 planktonic productivity (Rich and Wetzel, 1972). 



The high rate of carbon flux as C0 2 from the sediments of Lawrence Lake 

 was over twice the rate of carbon fixation by the phytoplankton (Table 6) and 

 represents a good estimate of overall lake productivity since benthic metabolism 

 tends to integrate all sources of carbon production. The sources of POC 

 obviously shift their relative contributions to the POC pool as the freshwater 

 systems frequently progress through stages of increasing fertility (Fig. 5). Again, 

 it is difficult to generalize because of the high degree of lake individuality, but 

 the importance of littoral components manifests itself within the overall 



