Chemistry 157 



carboxylic acids (Black and Christman 1963, Ghassemi and Christman 

 1968). They frequently polymerize reversibly and form colloids as a 

 function of pH, temperature, and divalent cation concentration. Most of 

 the humic compounds have a quinone-like nucleus, that is, an unsaturated 

 double bond coordinated with an aromatic ring, which absorbs ultraviolet 

 light strongly and fluoresces. 



For relative quantification of humic compounds dissolved in pond 

 waters, we used their absorbance of ultraviolet light at 250 nm in filtered 

 water (984 H glass fiber filters). Samples were read at 25°C at the usual 

 pond pH's. 



Ultraviolet absorption of pond water over the season tended to follow 

 the DOC (Figure 4-30a and 30b), ranging from an absorbance of less than 

 0.070 at 250 nm(l cm) in unthawed Pond B to 0.545 by 5 July 1971. Like 

 the total DOC concentration, the UV absorption increased rapidly in the 

 post-thaw period. The relationship between DOC and absorbance at 250 

 nm was significant in 1971 (r = 0.81 to 0.94) for Ponds B, C, D, E. In 1972 

 and 1973, this correlation was not significant. 



In general, trough ponds all have higher amounts of color than low- 

 centered polygon ponds and frequently have some of the highest 

 concentrations of DOC. Trough ponds drain the ice-wedge troughs which 

 act as a repository for leachable lemming-cut grass, winter nests of 

 lemmings, and lemming feces. In addition, the input of runoff and 



9 Aug 1973 



FIGURE 4-31. UV absorbance at 250 nm in the water of 

 some ponds near the IBP aquatic site, 9 August 1973. 



