84 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



bonate goes into solution again. McMullen (1941), when eradicating 

 snails in a hard-water Michigan lake, found that there were 75.6 mg of 

 copper per square foot of bottom 4 hours after treatment; 30 mg at the 

 end of 24 hours, and 4.8 mg at the end of 54 hours. Whipple cites a con- 

 clusion of Hale (without exact reference) that at least during the winter 

 months all copper fed into the New York water supply came through the 

 distributing system. Conversely, he notes another case where .3% copper 

 was reported in the bottom mud of a reservoir that had been treated. 



The most conclusive evidence on the accumulation of copper in the 

 bottom muds of treated lakes is that of Nichols et al. (1946) who analyzed 

 muds from three Wisconsin lakes with a long history of copper sulphate 

 treatment. In the treated lakes a range of 18 to 1093 mg of copper per 

 kilogram of bottom mud was found in Lake Menona; 18 to 595 mg in 

 Lake Waubesa and 18 to 595 mg in Lake Kegonsa. In contrast, untreated 

 Lake Mendota showed a range of 32 to 135 mg of copper per kilogram of 

 bottom mud. These workers conclude that ". . .it appears that by far 

 the greatest amount of that copper applied remains as a deposit in the 

 mud of the lake." 



The amount of siltation is also important. In lakes, such as those 

 of the Fairmont chain, which are continually receiving silt from adjacent 

 land, any copper precipitated to the bottom would be buried and its possi- 

 ble toxic effect on bottom fauna thereby greatly lessened. 



Algal Control and the Growth of Larger Aquatic Plants 



Larger aquatic plants are quite tolerant to copper sulphate and there 

 is no record of injury to them from algal control measures in Minnesota. 

 The higher algae are more susceptible than the aquatic seed plants and 

 the growth of such forms as Chara and Hydrodictyon has been controlled 

 with copper sulphate in fish ponds. Greater concentrations are necessary 

 than those ordinarily used for plankton algae (Surber 1943; O'Donnell 

 1945). In reviewing the literature on the effect of copper on terrestrial 

 seed plants, Miller (1931 p. 284) states that although copper is usually 

 toxic, low concentrations of .02 to .2 ppm ''not only increased the length 

 of life of various plants but also their dry weight." 



Generally, there is an inverse relationship between plankton produc- 

 tion and that of larger aquatic plants. Minnesota lakes with little plank- 

 ton may have aquatic weeds to a depth of 25 feet. In those with moderate 

 plankton production, the littoral zone usually extends to about 15 feet 

 and in those with a heavy algal bloom there are often few weeds beyond a 

 depth of 4 feet. The production of a heavy plankton crop by fertilization 

 has been found to be an effective way to control weed growth in fish ponds 

 (Smith and Swingle 1941). Conversely, plankton production may be 

 limited by robust growth of the larger aquatic plants. Observations along 

 this line have been made by Kofoid (1903) and more recently by Bennett 

 (1943) who recommends the elimination of larger aquatic plants from fish 



