70 M.C. Miller etal. 



on mean water depths taken on several transects and they assume that the 

 area of the ponds does not change during the summer. 



LIGHT 



Water 



The light extinction in the ponds was measured in situ with an 

 underwater selenium photocell (Schueler, Waltham, Mass.). The spectral 

 response of this instrument resembles that of the human eye. The data are 

 summarized as an extinction coefficient {n, per meter) calculated from 



I^=IoG"" 



where /^ is the light intensity at depth z (in meters) and h is the light 

 intensity immediately beneath the water surface. Unfortunately, it is 

 difficult to be precise with this instrument, designed for oceanic work, 

 when measuring a water column of only 30 cm. There are also other 

 problems, such as the rapid extinction of the infrared and ultraviolet light 

 in the top few centimeters, that make it very difficult to obtain a good 

 extinction coefficient for the total light over these short distances. 



The extinction coefficient is high in the ponds (Table 3-12), reflecting 

 a high concentration of colored organic compounds. In similar ponds and 

 lakes, the organic compounds absorb strongly in the short wavelengths 

 (UV and blue) so that, unlike clear lakes, the maximum transmission 

 occurs in the red wavelengths. One measure of the amount of dissolved 

 humic matter in the water is the absorption of light at 250 nm (Miller 

 1972) (Table 3-12). There is good agreement between the n and the optical 

 density (OD) except that Pond E, the oil experiment pond, has more humic 

 compounds than expected from the extinction coefficient. 



TABLE 3-12 Mean Extinction Coefficient and Mean Optical 

 Density (OD) at 250 nm and a 1-cm Path 

 Length, for 4 Ponds during 1971 



Pond Mean extinction coefficient Mean OD 



B 3.84 0.446 



C 3.73 0.444 



D 4.40 0.611 



E 3.04 0.482 



