152 Annals of the Carnegie Museum. 



far as the bottom is concerned, conditions in Presque Isle Bay (44), 

 the principal source of material used in this investigation, represent 

 the extreme of finest sediment. 



The friction of wind on the surface of the lake produces very decided 

 mo\-ements in the waters. On the Laurentian Lakes, waves fifteen 

 to eighteen feet in amplitude have been observed during long, con- 

 tinued storms. As Lake Erie is the shallowest of the Great Lakes 

 and as its axis lies almost directly in the normal path of the cyclonic 

 storms (25), the wave action thus produced is particularly strong at 

 times. The efTect of the prevailing westerly winds on the surface 

 movement of the water in the lake is indicated by the trend of the 

 principal currents. It has been found that the currents of the Lauren- 

 tian Lakes have in general a speed of from four to twelve miles a day, 

 but in certain observed instances this is increased to two and one-half 

 to four miles per hour. When the currents follow the shore, important 

 results in a physiographic sense may follow. When the wind blows 

 obliquely to the shore strong currents are frequently produced which 

 follow the general trend of the coast, but sweep across bays and inlets. 

 These currents with the assistance of the waves sweep along sand 

 and gravel and produce important changes in the bottom particularly 

 where the water is shallow. The prevailing littoral current unaided 

 is however, not strong enough to transport any considerable amount 

 of coarser material and in a general way it may be said that the condi- 

 tion of the water is not as disturbed as in the LTpper Ohio Drainage 

 where a considerable amount of such material is carried along. 



As a rule, the temperature of the water in Lake Erie is much cooler 

 than that in the Upper Ohio Drainage. The shallow lakes of the 

 Northern states have been found to have a nearly uniform temperature 

 during the summer months of 75° F. (42). In the winter the tempera- 

 ture is generally 32° F. This condition has an important bearing 

 upon the growth of Naiades. It is a well-known physiological fact 

 that the rapidity of nutritive processes in "cold-blooded" animals 

 depends largely upon the temperature to which they are subjected. 

 Again the food of the Naiades consists largely of plankton. 



I quote from a letter from Mr. A. F. Shira, Director of the Biological 

 Station at Fairport, Iowa, "It may be said that an increase in the 

 temperature of a lake favors an increase in plankton, and there is an 

 increase in plankton during the spring and in early summer followed 

 generally by a decrease in autumn. Temperature affects the character 



