98 PLANKTON OF WISCONSIN LAKES 



The average quantity of dry organic matter in the entire series of 

 nannoplankton samples from Lake Mendota is 1,630.5 milligrams per 

 cubic meter of water, or about four and three-quarters times as much 

 as that in the net plankton. The amount varied from a minimum of 

 about 795.0 milligrams to a maximum of 3,151.0 milligrams per cubic 

 meter of water. The average quantity contained 111.5 milligrams of 

 nitrogen (equivalent to 697.0 milligrams of crude protein), 106.5 milli- 

 grams of ether extract, 78.6 milligrams of pentosans, and 84.6 milli- 

 grams of crude fiber. The crude protein, ether extract, and crude 

 fiber together constituted 888.4 milligrams, or 54.5 per cent of the 

 average quantity of organic matter. The remainder, 45.5 per cent, 

 consisted of nitrogen free extract; the latter, therefore, was 15.5 per 

 cent larger in the nannoplankton than in the net plankton. 



The total plankton, that is, the net plankton plus the nannoplankton, 

 yielded an average of 1,974.0 milligrams of dry organic matter per 

 cubic meter of water from April, 1915, to June, 1917. In this material 

 there were 878.2 milligrams of crude protein, 148.7 milligrams of ether 

 extract, 90.2 milligrams of pentosans, and 105.0 milligrams of crude 

 fiber. The crude protein, the ether extract, and the crude fiber 

 amounted to 1,131.9 milligrams, or 57.3 per cent of the average quan- 

 tity of organic matter, leaving 42.7 per cent as nitrogen free extract. 



Total Plankton Per Unit Area 



The total plankton may now be considered from the standpoint of the 

 quantity per unit of area; the averages for the deep water are pre- 

 sented first and those for the entire lake later. Since the observations 

 were made in the deeper part of Lake Mendota and extended to a 

 depth of 20 meters, the results apply more particularly to this portion 

 of the lake. For this part of the discussion, then, the 20 meter con- 

 tour line may be taken as the boundary of the particular portion of the 

 lake under consideration. This comprises an area of 6,641,000 square 

 meters, or 16.8 per cent of the total area of the lake. The volume of 

 water in this area down to a depth of 20 meters is 132,820,000 cubic 

 meters and below this depth it is 13,449,000 cubic meters, giving a total 

 of 146,269,000 cubic meters witEin this area, or 30.6 per cent of the 

 total volume of the lake. The quantitative computations for the plank- 

 ton of the deep water are thus based on this area and on this volume. 



Total Plankton in Deep Water 



The monthly averages for the entire set of observations on the total 

 plankton have been ascertained from table 24 and these averages have 

 been used to compute the quantity of organic matter per unit of sur- 



