504 



THE COMMUNITY 



temperate lakes, at least, these six groups 

 give an over-all picture of population den- 

 sity similar to that of the marine com- 

 munity, that is, a relatively high peak in 

 April-May-early-June and a second, smaller 

 peak, in August-September-early-October. 

 Small lakes (Pennak, 1946) may or may 

 not follow this pattern. 



This seasonal parallel is clear when a 

 single group is used— for example, the dia- 

 toms. Year-around, quantitative studies on 

 Lake Erie (Chandler, 1942a; Gottschall 

 and Jennings, 1933) and on Lake Michigan 

 (Daily, 1938; Damann, 1940) demonstrate 

 a clearly defined vernal and an autumnal 



obtained since they must take into account 

 the reproductive potential of each species 

 under annual environmental conditions, 

 average hfe span of each species, and aver- 

 age weight of each species (Welch, 1935). 

 One of the few reliable estimates of an- 

 nual production is the early figure of Birge 

 and Juday (1922) for Lake Mendota, Wis- 

 consin. These authors estimated 12,000 kg. 

 of dry total plankton per hectare of lake 

 surface. This works out at about 10,700 

 pounds of plankton per acre per year, 

 which roughly equals the annual crop of 

 diatoms only, in the English Channel (p. 

 502). In other words, the diatom annual 



Table 42. Total Crop of Rooted Hydrophytes in Wisconsin Lakes 



Southern Wisconsin 

 (After Rickett 1922, 1924) 



Lake Mendota. 

 Green Lake . . . 



Total Crop in 



Kilograms (Dry 



Weight) 



2,100,000 

 1,527,900 



Average Crop in 



Grams per Square 



Meter 



202 . 00 

 178.00 



Northern Wisconsin 

 (After Wilson 1935, 1937, 1939) 



Little John Lake . . 

 Muskellunge Lake 

 Silver Lake 



000.52 

 000.45 

 000.08 



pulse, dominated by six genera of diatoms 

 {Asterionella, Cyclotella, Fragilaria, Melo- 

 sira, Synedra, and Tabellaria). 



Chandler's data for Lake Erie may be 

 summarized for the general picture in large 

 temperate lake communities. The vernal 

 pulse of nonbacterial phytoplankton reach- 

 ed a maximum of 374,000 organisms per 

 liter between March 14 and May 28; of 

 this pulse, the diatoms composed 98 per 

 cent. The observed autumnal pulse occur- 

 red in two parts and never exceeded a max- 

 imum of 34,000 organisms per liter be- 

 tween September 13 and November 29; of 

 this pulse, the diatoms composed 60 per 

 cent. 



These data refer to the standing crop, 

 viz., the total amount of phytoplankton in 

 the water at a given time. They do not rep- 

 resent the annual crop, viz., the total 

 quantity of phytoplankton produced in a 

 given year. Annual crop data are not easily 



production of the marine community about 

 equals the total phytoplankton, plus total 

 zooplankton annual production of the fresh- 

 water community, per unit of water sur- 

 face, in about the same range of latitude. 



The rooted aquatic vegetation of the 

 fresh-water community makes up the bal- 

 ance of the photosynthetic industry, exclu- 

 sive of the work of autotrophic photosyn- 

 thetic bacteria about which relatively little 

 is known. In the United States five Wis- 

 consin lakes have been studied with refer- 

 ence to total crop. These data are sum- 

 marized in Table 42. 



Using data in the right hand column, 

 it will be seen that there is a great range 

 in dry bulk of rooted plants in lakes rela- 

 tively close to each other. Wilson (1939) 

 correlates this disparity in anabolic poten- 

 tial with several factors, the most obvious 

 of which is type of bottom. Thus the lakes 

 studied in southern Wisconsin had bottoms 



