158 Theories and Techniques of Management 



reduction in water volume and surface area afiFects all parts of an aquatic 

 habitat and all components of the animal and plant communities that 

 inhabit the water. 



Ejfects Upon the Exposed Lake Bottom. According to Neess ^° the bot- 

 tom of a lake or pond is divided into regions, "an upper, loose well 

 aerated, and often highly colloidal layer of decomposed organic material, 

 plant debris . . . and a lower anaerobic zone, differing widely in composi- 

 tion from place to place and often containing a large proportion of 

 mineral matter." These soil layers have the ability to direct certain 

 processes in the pond because the mineral composition of water is largely 

 a reflection of the mineral composition of the soils of the pond bottom and 

 the surrounding basin; also the colloidal fraction of the bottom materials 

 consisting of humic substances, ferric gels, and clay is capable of absorbing 

 certain soluble nutrient elements and governing their later distribution. 



In a pond or lake where there is a shortage of oxygen near the bottom, 

 decomposition of organic matter is slow and the products are reduced to 

 incompletely oxidized compounds such as hydrogen sulfide, methane, and 

 short-chain fatty acids. When the water is drawn off of a lake bottom and 

 the bottom is allowed to dry out and crack open, an abundance of oxygen 

 becomes available, the processes of decomposition are stepped up, and 

 the pH of the bottom soils is raised. Under these conditions there may be 

 a release of certain fertilizing substances from organic colloidal systems, 

 making available greater quantities of potassium and phosphate. In Euro- 

 pean pond culture it was once considered important to grow a crop plant 

 or a legume on the exposed pond bottom. Later the need for this practice 

 was questioned -' although the crop furnished income to the pond owner 

 when the pond was not producing fish. 



Whether a lake or pond bottom exposed by a drawdown will develop 

 a vegetative cover depends upon the length of time the bottom lies ex- 

 posed and the season of the year when the drawdown is made. A winter 

 or early spring drawdown, which is prolonged by drought or purposely 

 extended throughout the following plant growing season, will insure a 

 luxuriant growth of terrestrial weeds on the exposed lake bottom. These 

 weeds will reflect the fertility of the exposed lake bottom by their height 

 and the density of the stand. Drawdowns made in July and August will 

 be followed by some germination of seeds and growth of terrestrial plants, 

 but drawdowns made as late as early September in the north are not 

 followed by growth of terrestrial vegetation in the basin. 



Whether or not plants grow upon the exposed bottom seems to be 

 unimportant; of primary significance is the exposure of the bottom to 

 rapid and complete oxidation. 



Effects Upon Rooted Aquatic Vegetation. Most forms of submersed 

 rooted aquatic plants are not greatly affected by a drawdown, e.g., ex- 



