Chemistry 155 



0.6 g C m~^. During the third year, some 54% of the dry weight of the 

 plant matter is lost as small particulate material (less than 20 ^m) whose 

 leaching rate is probably much like that of the surface sediments (Table 4- 

 23). The leaching of old leaves and the re-solution from the sediments add 

 humic compounds which are resistant to further decomposition and which 

 accumulate in the pond. From all these calculations, we estimate that 2.4 g 

 C m "^ of refractory DOC enters the ponds from grasses and sedges [this is 

 87% of the 2.8 g DOC m '^ found in Pond B in early July (20 cm average 

 depth)], and another 1.4 g C comes from re-solution from the sediments. 

 In view of the relatively small amount of labile DOC that is estimated to 

 be present at any one time (maybe 10%), it is surprising that leaf leaching 

 contributed 5.6 g C m '" of labile DOC each year. Most of this comes from 

 the early season leaching of the grasses and sedges and almost all of it is 

 used by planktonic and benthic bacteria (see Chapter 8). Macrophytes 

 were also an important source of DOC in Lake Wingra where they cover 

 31% of the lake. Prentki et al. (1979) measured a movement of 15 g C m '^ 

 of labile DOC from the submersed weedbeds to the open water. 



In August of all 3 years, there was sufficient rain to raise the pond 

 water level; yet, contrary to our expectation, the DOC concentration 

 increased. Sometimes the rainfall appears to dilute the DOC, and at other 

 times it appears to increase DOC by leaching vegetation. One explanation 

 may be that the August rains frequently occur after early frosts have 

 damaged terrestrial vegetation so that cell contents are easily leached. 

 Another possibility is that overland runoff, which quickly reaches 

 equilibrium with the soil solution, is entering the ponds. This runoff was 

 observed on 19 to 26 August 1971, for example. The rain can not add 

 appreciable DOC, for even if it contained 1 mg DOC liter', 5 cm of rain 

 during the summer would add only an insignificant 0.05 g DOC m~^. 

 (Table 4-23). 



In 25 ponds contained in the same drained lake basin as the IBP 

 ponds, total DOC concentrations ranged from 10.2 to 34.5 mg C liter ^ ' in 

 1972 and 1973. Some of the highest concentrations were found in the 

 ponds formed in polygon troughs. 



Humic Compounds 



The ponds contain very brown water, equivalent to the color of water 

 in bogs. The color can reach 370 ppm on the Pt-Co scale (Kalff 1965). 

 Although we can only speculate, it is likely that the origin of the color is 

 similar in these ponds and in bogs — the leachate from dead emergent 

 vegetation and organic deposits in the littoral and benthic zones. 



These colored materials are soluble humic compounds and tannins, 

 mainly of terrestrial origin, which are resistant to bacterial breakdown. 

 They have been classed generally as aromatic polyhydroxy methoxy 



