CARBON IN FRESHWATER SYSTEMS 



253 



TABLE 5 



PARTICULATE ORGANIC CARBON, LAWRENCE LAKE, MICH. 



reduction in POC occurred under ice cover, a period during which primary 

 production and allochthonous inputs were reduced. The relationship of 

 approximately 10 times greater DOC than POC was very constant. The DOC to 

 POC ratio of about 10 : 1 also existed in all lake waters in the region of 

 Lawrence Lake, as well as in inlet streams and experimental and natural streams 

 of the area. 



Estimates of the replacement of algal-cell carbon in the pelagic POC can be 

 made from the biomass of algal-cell carbon and the rates of net primary 

 production. Compensating for respiratory loss of carbon (see Miller, 1972), the 

 daily net accumulation of POC can be estimated from net primary production. 

 Total suspended epilimnetic POC in Lawrence Lake had an average replacement 

 time (turnover) of 40.7 days (range 8.1 to 544 days). Of this POC pool, a mean 

 of 83.4 mg C/m 2 (range 30 to 241 mg C/m 2 ) was algal-cell carbon that was 

 replaced by primary productivity in 1.06 days (range 0.30 to 2.5 5 days) during 

 the ice-free seasons and with an annual mean of 3.6 days (Miller, 1972). 



Generalizing on the cycling of organic carbon among phytoplankton 

 populations is difficult because of the paucity of data, especially detailed annual 

 cycles, under natural conditions. A few generalizations are apparent, however. 

 POC concentrations of the pelagic zones are usually considerably larger (roughly 



