Chemistry 153 



DOC (measurements of 0.4 and 3.3 mg C liter"' in Pond B, 1971) but 

 once the sediments begin to thaw the DOC concentrations increase 

 rapidly. In Pond B, the maximum was reached on 5 July 1971 (15.6 mg C 

 liter '), 30 June 1972 (14.6 mg C liter"') and 16 July 1973 (11.9 mg C 

 liter '). Through much of the summer the concentration was fairly 

 constant at 10 to 14 mg C liter"'. However, in late August of 1971 and 

 1972, the concentration of DOC was changed by rainfall and its runoff 

 (Figure 4-30a). We have no data for concentrations during freezing, but 

 Kalff (1965) has shown that in the autumn under the ice cover, the color 

 (measured in Pt units, a relative measure of DOC) increased to seasonal 

 maxima just before the ponds froze solid. 



The sources of organic carbon are: (1) leachates from the sediments 

 including re-solution of the previous year's DOC, (2) leachates from dead 

 emergent sedges in the flooded peripheral areas around the ponds, 

 (3) leachates of soil, sedge and lemming feces washed into the ponds by 

 overland snowmelt and rainfall runoff, (4) leakage of photosynthate from 

 aquatic sedges and grasses {Carex aquatilis and Arctophila fulva), (5) 

 leakage by planktonic and epipelic algae during photosynthesis, and (6) 

 excretions and autolysis of living and dead chironomids, oligochaetes, 

 flatworms, zooplankton, tadpole shrimp, and other fauna and flora. 



Much of the increase in DOC early in the thaw season comes from the 

 re-solution of DOC which was frozen out in the previous autumn. 

 Evidence for this comes from a series of 1 .4-m^ subponds, set up in 1970 in 

 ponds B and C. These excluded runoff water but included sediments. 

 Hence all the total DOC in these subponds in the early summer of 1971 

 had to be redissolved from the sediments as there could be no new DOC 

 from new grass detritus and runoff water. By 21 June 1971, nine days after 

 the ponds thawed to the bottom, the subponds contained only 50% and 

 64% of the DOC in the open-pond waters. By 7 July 1972, they contained 

 55% and 38% of the open-pond DOC. In another type of measurement, 

 surface sediments (Pond J) were incubated in distilled water for six days 

 (25 to 28°C) and the DOC determined. This leachable DOC was 1% of the 

 total sediment carbon in June but was only 0.1% in August. Thus, the 

 leachable DOC in the sediments decreased by an order of magnitude over 

 the summer. 



A second major source of DOC appears to be the leachates from dead 

 emergent grasses and sedges. Each fall, the leaves of emergent aquatic 

 plants die but most remain as standing dead. The next spring, there are 

 three types of leaves present: green (grown during the previous summer but 

 frozen in ice over the winter), yellow (grown during the previous summer 

 but overwintered beneath snow), and brown (grown during the summer 

 before the previous summer). When the green leaves of Carex aquatilis 

 were placed in 20-Mm mesh litterbags and put into the pond, the leaves lost 

 47% of their initial weight in 33 days. By means of tests described in 

 Chapter 8, it was determined that 48% of the loss was caused by bacterial 

 activity and 52% by leaching. 



