82 R. T. Prentki et al. 



240 



200 - 



E 



" 160 

 o 



E 



^ 120 



I 80 



o 



o 



40 



FIGURE 4-4. Conductivity of five ponds, 1970. 



The seasonal cycles of the major ions that are not nutrients, such as 

 sodium, magnesium, and calcium, are usually similar from year to year. 

 For example, the concentration of sodium (Figure 4-5a) is almost the same 

 in 1970 and 1971 for June, July, and part of August. The divergence of the 

 two graphs in the middle and end of August is attributable to the 

 difference in amount of rainfall. Thus, the peaks and valleys in the 

 concentration graph during August of 1971 are the exact inverse of the 

 pond water levels (Figure 3-8). 



Potassium, iron and silica have more complicated seasonal cycles and 

 their concentrations do not follow the conductivity. Potassium 

 concentrations (Figure 4-3) are highest during runoff as a result of 

 leaching from vascular plants and from lemming feces, but they decrease 

 through the rest of the summer because of some unknown abiotic reaction 

 (perhaps with clays?). The iron concentrations (Figure 4-5b) are low early 

 in the season, reach a maximum in July, and then decline during August. 

 The concentration of dissolved iron is correlated with humic color 

 (r = 0.72, n=40) and, therefore, follows the dissolved organic carbon rather 

 than the conductivity. There is an undoubted cause-and-effect here but it 

 can not be determined if the dissolved iron is chelated by organic material 

 or if iron or some iron compound (ferric hydroxide?) is complexed with a 

 colloid. There is also an excellent correlation of the small peaks in the 

 concentration of iron with rainfall. Thus, the peaks on 10 and 28 July 1970 

 and on 21 July 1971 were on rainy days. Overall, the iron concentrations 

 were 25% higher on rainy days than on non-rainy days; it is likely that the 

 iron is being leached from the soil or from bottom sediments. The silica 



