298 PRINCIPLES OF ANIMAL BIOLOGY 



is poor in carbon dioxide (a condition like^vise accentuated by any 

 photosynthesis going on there). The temperature of the shallow water 

 along the shore is likely to fluctuate greatly between day and night, 

 especially on the side of the lake toward the prevailing wind and among 

 vegetation, where there is little agitation; but out in the open water in 

 the middle of the lake temperature is much more nearly constant. Shore 

 regions exposed to the wind are subject to vigorous wave action which 

 usually prevents vegetation from gaining a foothold (Fig. 263). 



Below the thermocline the water is always cold, often varying only 

 3 or 4° throughout the year. It is always dark. There is no wave 

 action, and almost the only mechanical disturbance is that occasioned by 

 the complete circulation of the water in spring and fall. A solid sub- 

 stratum is available for attachment. There is very little oxygen, some- 

 times none at all, for whatever oxygen is brought in by the spring and 

 fall overturn of the water is consumed by decay of dead organisms that 

 fall to the bottom, and there can be no photosynthesis in this dark region. 

 Carbon dioxide is always abundant, likewise because of the decomposition 

 of organic matter, except temporarily at the times of the spring and fall 

 overturn. 



The Organisms of Lakes. — It is obvious that the conditions described 

 above have much to do with the types of organisms inhabiting lakes, 

 and that different parts of a lake will have very different kinds. Plants 

 can as a rule occupy only about 3 meters of the depth of a lake, owing 

 to deficiency of light below that level. For plants springing from the 

 soil, this means that they are limited to a narrow strip along the shore 

 (Fig. 258). While a pond may have vegetation throughout, most lakes 

 have plants over only 10 to 30 per cent of their area. This difference 

 between ponds and lakes is indicated in Fig. 264. Since many animals 

 depend on these plants, the abundance of the latter is important in the 

 general ecology. Of the many animals found in such situations it is 

 possible to mention only a few. In the shore region with the plants 

 are usually snails and immature caddis flies, midges, IVIay flies, and 

 dragon flies. Where there is little or no vegetation because of waves, 

 there are often mussels and young insects \vith flattened bodies and 

 clinging habits (certain May flies). In the open water of the middle of 

 the lake are sometimes floating plants, chiefly algae, so abundant as to 

 reduce very materially the amount of light that enters the water. With 

 the algae, and often feeding upon them, are many small animals, chiefly 

 Crustacea, protozoa, rotifers, and mites, abundant in numl)ers but not 

 usually of many kinds in any one lake. 



On the bottom of a lake, below the thermocline, are found those 

 organisms requiring no light and little or no oxygen. Characteristic 

 examples are the minute plants known as diatoms, some of the annelid 



