June, 1919] Fauna of Rock Bottom Ponds 457 



expected the plankton conditions were uniform throughout. 

 I have no actual data on the fish distribution but I assume that 

 the chief limiting factor would be extreme variation in the depth 

 of the water. Apparently, then, the pond offered an almost 

 equally suitable environment wherever a given species was 

 found. 



These counts give us data not only on the uniformity of 

 distribution, but they also furnish some conception of the actual 

 numbers which a pond of this comparatively small size can 

 maintain. Assuming that the entire bottom was as thickly 

 populated as the sample counts indicate the total number of 

 the one species of chironomid larvae would have been 2,444,400 

 and of the Heptagenia nymphs, 814,800. It is true that the 

 muck covering much of the bottom offered an unsuitable hab- 

 itat and was therefore, not as thickly populated but the above 

 calculation is based on a surface in a single dimension only and 

 when we consider that several thousand stones are equally 

 populated on all sides the calculation is probably not far from 

 the actual condition. 



FIFTEEN YEAR POND. 



Pond IV. 



The fifteen-year pond is on Kelley's Island, Lake Erie, five 

 miles off the Marblehead Peninsula. The entire island is a 

 solid mass of rock covered by a thin layer of soil. The pond 

 fills a shallow crescentic excavation in the rock, 100 by 150 feet 

 in surface area. Along one side there is a public road, on 

 another a lawn, and elsewhere a pasture. A portion of the 

 pond is shown on page 458. 



The depth of the water varied from eighteen inches to three 

 feet with an indicated fluctuation of six inches. At each end 

 of the pond, soil had slipped down to the water's edge and on this 

 some willow bushes had become established. Here there was 

 a foot or more of muck near shore which rapidly became less 

 deep farther out. Mixed in with it was a miscellaneous collec- 

 tion of sticks, leaves and other debris. The surface of the water 

 at one end was covered for several feet from shore with a thick 

 mat of filamentous algae. Narrow strips of this mat extended 

 some distance down each side. The shore was chiefly bare 



