Chemistry 163 



day ~ ' . Since the plant beds cover 40% of the pond area, this would amount 

 to 9 to 15 mg C m^^day^ over the pond area or about 900 mg Cm' 

 yr~' for a 75-day growing season (Table 4-23). This 45 to 75 Mg DOC 

 liter"' day ' (assuming a 20-cm-deep pond) is a large input to the DOC 

 pool. 



The excretion from the zooplankton may be the only significant 

 animal input into the open water (Table 4-23). We have no direct measure 

 of this, but if the annual net production of zooplankton is 1 mg C liter ', if 

 the zooplankton ingest 20 times their net production, and if release of 

 DOC from its metabolism or from mechanical destruction of cells passing 

 through the gut is 40% of ingestion, then the zooplankton may add 1.0 g 

 DOCm 'yr \ 



We were not able to measure the total input of DOC from the 

 sediment. Obviously the maximum microbial activity occurs here but we 

 could measure only the production of refractory DOC. A large quantity of 

 labile DOC could have been produced and rapidly used. This amount, not 

 entered in Table 4-23, is likely 2 or 3 mg C liter '\ 



The input of total DOC is 12.4 g m "' yr "' or 4 times the maximum 

 standing crop of total DOC (Table 4-23). This input is made up of an 

 estimated 3.8 g C of refractory DOC, primarily humic materials from 

 sediment re-solution and from sedge and grass leaching, and 8.5 g m"^ 

 yr~' of labile DOC, primarily from the leaching of plant matter in the 

 littoral weed beds and from leakage of autotrophs. Obviously bacterial 

 utilization of the labile DOC must be very large (see Chapter 8). Other 

 sources of loss include washout during snowmelt in the first 2 weeks after 

 the thaw and photo-decomposition by sunlight. Based on June 

 measurements on the concentration of humic material in Pyrex bottles left 

 in the sunlight, we calculate that all of the dissolved humic material is 

 decomposed every 28 days. 



In conclusion, because of the importance of the refractory humic pool 

 in the total DOC budget, the observed concentrations of DOC appear to 

 be largely controlled by physical and hydrologic processes. In general, the 

 concentration of DOC in shallow lakes with large drainage basins is 

 determined more by allochthonous inputs than by autochthonous ones 

 (Birge and Juday 1934). The tundra ponds do not follow this rule as they 

 are very shallow, low-volume aquatic systems whose DOC pool is largely 

 dominated by vegetation decay within the pond itself and by sediment 

 solution of surficial bottom deposits. This pool, however, is mainly the 

 refractory DOC remaining after the bacteria have removed the labile 

 material. 



Particulate Organic Carbon 



Most of the material in suspension in the water is particulate organic 

 carbon (POC). This is separated from the dissolved material by filtration. 



