The Plankton Algae of the Palisades Interstate Park 123 



With these possibiHties of new algae being introduced into the 

 waters of the Park there is always the chance of new algae, and 

 sometimes algae that will grow luxuriantly, appearing in the plankton 

 flora. The extent to which these new algae will dominate the flora 

 of a lake is, however, a matter of conjecture. 



METHODS OF CONTROLLING ALGAL GROWTHS 



Proper Construction o£ Artificial Lakes. Since the method of 

 building the Park lakes has undoubtedly increased conditions favoring 

 the growth of algae, remedial measures should be considered. The 

 depth of a lake is of importance both in the way that it affects the 

 fish and in the way that it influences the growth of algae. In the 

 warm months the water of a deep lake is not of uniform tempera- 

 ture, but consists of an upper region where there is a vertical cir- 

 culation of the water (the epilimnion) and a bottom region of 

 stagnation (the hypolimnion). These two layers do not fuse 

 gradually, but are separated by a sharply defined transition belt (the 

 thermocline). The thickness of the epilimnion, or zone of circula- 

 tion, depends upon the size of the lake, being about ten feet thick 

 in small lakes and 30 to 40 in large ones (Birge and Juday, 'ii). 

 This thermal stratification of the lake is accompanied by changes 

 in the gaseous content of the different parts; Birge and Juday having 

 shown that carbon dioxid, the gas essential for food manufacture 

 by algae, tends to decrease in the epilimnion and to increase in the 

 hypolimnion. The accumulation of carbon dioxid in the lower 

 waters is of no particular advantage to the algae since the hypo- 

 limnion is generally at a depth below that where there is sufficient 

 light for the effective utilization of the carbon dioxid by the algae. 

 The increase in the carbon dioxid at the bottom of the lake comes 

 from the decay of algae that are sinking from the surface as well 

 as from the decay of other organic material at the bottom. In 

 shallow lakes without thermal stratification there is a continuous cir- 

 culation of the waters and their dissolved gases from top to bottom, 

 so that the products of algal decay are immediately available for 

 re-utilization. In addition, the carbon dioxid and other products from 

 the decay of organic material at the bottom are also carried to where 

 they can be utilized by the algae, and so shallow lakes in general 

 support a proportionally greater mass of algal vegetation. In the 

 construction of new lakes in the Park attention should be given to 

 the question of the proper depth of lakes, since the inauguration of 

 the policy of building shallow lakes with a depth of 8 to 12 feet 

 will certainly invite a repetition of the experiences of algae in large 

 numbers that have been encountered with some of the lakes already 

 constructed. Artificial lakes with a depth of 30 to 40 feet, and an 

 average depth of 15 to 20 feet, will undoubtedly be productive of 

 less trouble from algae. 



In addition to the problem of the proper depth of the lake atten- 

 tion should also be given to the preparation of the lake bed. Hazen 

 and Fuller Cgy) have made an exhaustive study of this problem in 



