104 



PROBLEMS OF LAKE BIOLOGY 



TABLE V 

 Distribution of Copepoda in Three Series of 10 cc Sand Samples. Specimens Per Sample. 



Copepoda in the various beaches, we believe 

 that the most important element is the 

 amount of interstitial water. In general, 

 the greatest numbers of Copepoda were 

 found in the middle portions of the beaches 

 where the sand is neither saturated nor too 

 dry. This response to an optimum amount 

 of water is shown in Fig. 8. In beaches 

 having gradual slopes the largest numbers 

 of copepods were found at some distance 

 from the edge of the water (200 to 300 cm), 

 but in beaches having steep slopes the 

 larger numbers were found from 100 to 200 

 cm from the edge of the water. No cope- 

 pods, except a few planktonic Cyclops, were 

 found in the submerged sand near the 

 shore. 



Calculations of the mean vertical distri- 

 bution of the copepods (Table VI) showed 

 a variety of conditions. At the narrow 



Boulder and W. Crystal beaches, for ex- 

 ample, few copepods were found and nearly 

 all of these were in the top 4 cm of sand. 

 S. Trout, E. Crystal, and Starrett beaches 

 showed well-defined gradients with a com- 

 parativeh" large population in the top centi- 

 meter of sand and less than one specimen 

 per sample in the eighth centimeter from 

 the top. At N. Trout the mean population 

 in the surface samples was 20.7 specimens 

 per 10 cc sand sample while the bottom 

 centimeter contained 7.4. At White Sand 

 (a comparatively flat beach) the usual type 

 of distribution was reversed, with less than 

 one in the surface sample and 8.1 in the 

 bottom centimeter. This condition was due 

 to the fact that many copepods were found 

 in the deeper layers of sand at 300 and 350 

 cm from the edge of the water where the 

 surface of the sand was quite dry. Con- 



