5° 



FRESH-WATER BIOLOGY 



breeding places. The oxygen content decreases, particularly on 

 the bottom. The distribution of the fish present in these ponds, 

 in so far as breeding habits were known, was found to be corre- 

 lated with the distribution of the bottom upon which they breed. 

 This becomes less and less in amount as the ponds grow older. 



TABLE X 



On the whole the carbonates, CO2, and bacteria are greater in 

 quantity according to age. Oxygen on the whole is less. 



The increase in quantity of animals with increase of soil fer- 

 tihty supports Knauthe's contention that with fishes productivity 

 of water is directly correlated with the richness of the soil. The 

 weak place in Knauthe's ideas Hes in the fact that as quantity in- 

 creases quahty decreases. The game basses and sunfishes give way 

 to the more inferior types and these are gradually succeeded by 

 bullheads, mud-minnows and dogfish. This is due to the destruc- 

 tion of breeding bottom for the desirable fishes by putrescible 

 organic matter which results in much carbon dioxide, hydrogen 

 sulphide, ammonia, and lack of oxygen. The German carp comes 

 into such a series rather late and thus productivity in carp is no 

 doubt correlated with a fertile substratum. 



The amount and kind of rooted vegetation are very important to 

 animals. Of all the aquatic situations which present themselves 

 the largest lakes have fewest attached plants, and these are all 

 algae. Cladophora, Chara and filamentous algas are the most com- 

 mon. These do not appear to have been recorded below about 

 25 meters; some of them require solid bodies for attachment and 

 are probably most abundant on the rock outcrops of shallow water. 



The vegetation of young streams consists largely of holdfast 



