102 PLANKTON OF WISCONSIN LAKES 



were secured and, on the basis of these counts, the amount of organic 

 matter in the 0-5 meter and in the 0-10 meter strata have been esti- 

 mated. These estimates together with the results obtained in the ob- 

 servations are given in table 28; they are indicated in kilograms per 

 hectare and in pounds per acre. According to the estimates nearly 

 37.0 per cent of the organic matter in the total plankton came from 

 the upper 5 meters, while a little more than two-thirds of it was ob- 

 tained from the 0-10 meter stratum. The catches show that 79.5 per 

 cent of the total quantity of organic matter came from the 0-13 meter 

 stratum, leaving only 20.5 per cent for the 14-23 meter stratum. 



Some data were obtained on the horizontal distribution of the plank- 

 ton by taking one of the two catches of a week about four kilometers 

 (2.5 miles) west of the regular station. The total plankton obtained 

 at the regular station on July 10, 1916, yielded 1,968.7 milligrams of 

 organic matter per cubic meter of water, while the catch taken in the 

 western part of the lake on July 14, 1916, gave 1,972.9 milligrams. The 

 difference is only 4.2 milligrams per cubic meter which indicates a 

 pretty uniform horizontal distribution of the material. 



Plankton of Entire Lake Per Unit of Area 



The monthly averages for organic matter may now be considered on 

 the basis of the amount per unit area when the surface of the whole 

 lake is taken into account. In general, the amount of plankton under 

 a unit of surface is dependent upon the depth of the water since the 

 volume of water occupied by the organisms is proportional to the depth. 

 Unfavorable factors, such as the absence of dissolved oxygen, insuffi- 

 cient light, and low temperature, may exclude many organisms from 

 the deeper strata, still certain forms occupy the lower water even when 

 such conditions exist. The population of this region may be scant in 

 comparison with the upper strata, as shown in table 27, yet those forms 

 which occupy the lower strata contribute their quota to the total quan- 

 tity of plankton under a unit of surface. Thus, while the increase in 

 plankton would not be directly proportional to an increase in depth 

 during periods of unequal vertical distribution, still an increase in 

 depth at such times would show a well marked increase in some of the 

 organisms. 



It must be borne in mind, therefore, that, in stating the results in 

 averages for the entire lake, a larger quantity of plankton than is 

 actually present there, is attributed to the water which is shallower 

 than the mean depth of the lake, while, on the other hand, the amount 

 assigned to the deeper water is smaller than it should be. For ex- 

 ample, the mean depth of Lake Mendota is 12.14 meters, so that the 

 average quantities give results that are too high for areas with a 



