INTEODUCTION 3 



the bottom stratum in the deeper water ceases to take part in the gen- 

 eral circulation the chlorophyllaceous organisms are at a disadvantage 

 because the light is insufficient at these depths for photosynthesis. 

 When stratification is finally completed the entire hypolimnion becomes 

 unsuitable for these organisms owing to the scarcity of light in this 

 stratum ; these forms, therefore, are restricted to the epilimnion, or in 

 some instances they may extend into the mesolimnion also. 



The disappearance of the dissolved oxygen from the lower stratum 

 also restricts the volume of water that can be occupied by some of the 

 plankton organisms, such as the rotifers and the Crustacea. Substan- 

 tially all of the water of the hypolimnion is without free oxygen in the 

 month of August so that only those forms which are able to live under 

 anaerobic conditions can occupy this region of the lake at that time. 

 The hypolimnion, therefore, yields a relatively small amount of plank- 

 ton material during this month because the scarcity of light excludes 

 the chlorophyllaceous organisms and a lack of oxygen excludes the 

 rotifers and Crustacea; the hypolimnion is about twelve meters thick 

 at this time so that these organisms are absent from about half of the 

 maximum depth of the lake. Following the spring maximum there is 

 a gradual decrease in the total quantity of plankton which is correlated 

 in time with the thermal stratification and the subsequent disappear- 

 ance of the oxygen in the hypolimnion ; the minimum amount for this 

 season is usually found in August. 



The autumnal overturning and circulation of the water again re- 

 lease the decomposition products which have accumulated in the hypo- 

 limnion during the summer and the entire body of water becomes 

 aerated, thus making the conditions favorable for the various plank- 

 tonts at all depths. These changes are accompanied by an increase in 

 the quantity of plankton which culminates in an autumn maximum in 

 late September or in October. Following this maximum there is a 

 gradual decline to a winter minimum which is reached in February or 

 in March; the winter minimum is somewhat smaller than the summer 

 minimum. Among the various constituents of the plankton the dia- 

 toms seem to respond most vigorously to the favorable conditions that 

 obtain in the spring and in the autumn; consequently they are the 

 chief forms concerned in the production of the vernal and the autumnal 

 maxima. 



Computations based on the area and the total volume of Lake Men- 

 dota show that the largest crop of spring plankton yielded 404 kilo- 

 grams of dry organic matter per hectare of surface (360 pounds per 

 acre), while the largest autumn crop gave 363 kilograms per hectare 

 (324 pounds per acre). The smallest summer minimum amounted to 

 139 kilograms of organic matter per hectare (124 pounds per acre) and 



