COMMUNITY ORGANIZATION: STRATIFICATION 



447 



Nipissing, although this factor was quali- 

 fied by many others, such as food, tem- 

 perature, chemical constituents, wind, 

 gravity, and age and sex of zooplankter. 

 As noted by Tressler (1939), gravity af- 

 fects all plankters heavier than water, and 

 most of these must actively exert them- 

 selves to maintain their position in the 

 gradient. Food is undoubtedly important. 

 Naturally it should be pointed out that sea- 

 sonal and twenty-four hour migrations, dis- 

 cussed later, influence the vertical position 

 of these organisms. 



The specific nature of such vertical gra- 

 dients must be realized. Tressler (1939, p. 

 82) states that in each lake "or perhaps 



bulk of the concentration zones within the 

 hypolimnion." 



From an extensive study of thirty-six 

 New York lakes by Tressler, Bere, Wagner, 

 and others, still in progress, some valuable 

 generalizations have appeared (Tressler, 

 1939 ) : The average depth of these thirty- 

 six lakes was 22.6 meters, and the range in 

 depth from 5 to 50 meters. The average 

 maximum abundance of the chief zoo- 

 plankters is shown in Table 31. 



The variation of maximal population den- 

 sity with depth is caused by (a) differences 

 in species composition of a given plankton 

 group, (b) time of day, and (c) season. 

 Differentiation within the several zones is 



Table 31 . Vertical Distribution of Chief Groups of Fresh-Water Zooplankton 

 in New York Lakes (Modified from Tressler, 1939) 



Range of Depths for 



Maximal Popula- Maximal p.d. for 



Zooplankton tion Density at: Thirty-six Lakes 



Protozoa 5.3 meters to 15 meters 



Cladocera 6.5 meters to 25 meters 



Copepoda 7.1 meters to 30 meters 



Rotifera 7.5 meters 5 to 40 meters 



Nauplii 9.9 meters to 35 meters 



in each type of lake, every organism has its brought about either directly by differences 

 own preferred level." In Wisconsin lakes in the physical environment, or indirectly 

 with thermal stratification it has been through the effects organisms produce on 

 demonstrated (Woltereck, 1932) that each the environment, or also indirectly by the 

 stratum has its own peculiar group of reaction of residents to each other. 

 Daphnia and allied genera. This popula- The vertical distribution of marine 

 tion-domination of the gradient gives indi- plankton exhibits distinctive features (Rus- 

 viduality to each community. We will re- sell, 1927; Pavillard, 1935). The marine 

 turn to this subprinciple later in the dis- plankton gradient varies with season, with 

 cussion of the vertical organismal gradient the twenty-four hour cycle, with latitude, 

 in terrestrial communities. and with turbidity and local weather. Its 

 Whenever a particular taxonomic group chief characteristics are determined pri- 

 is investigated for vertical stratification, its marily by the physical environmental gra- 

 more uniform physiological requirements dients, as in the fresh-water communities, 

 permit attention to be focussed upon the The plankton distribution is apparently cor- 

 lower taxonomic units, and the distribu- related with both the intensity and com- 

 tional gradient becomes more apparent, as position of the penetrating sunlight. Sea- 

 in the studies of Woltereck just cited. This sonal and day-night migrations are dis- 

 is clearly evident in the careful analysis of cussed later, and attention is focussed here 

 Campbell (1941) of the plankton Rotifera on the stratification of the community, 

 of Douglas Lake, Michigan. Concerning From accumulated oceanographic re- 

 distribution of rotifers, he says (p. 15) : search it is clear that, notwithstanding the 

 "Certain characteristics of the distribution relative uniformity in the proportion of the 

 patterns of rotifers are in part due to the more abundant mineral salts in solution 

 distinct distributional patterns of the more (Coker, 1938), and despite the reality of 

 abundant species. That is, certain species stratification, there is little taxonomic imi- 

 are, in the main, responsible for the sur- formity in horizontal distribution of plank- 

 face or near-surface concentration zones, ton in the ocean from locality to locality, 

 and certain deep-water species form the The areas are too vast, subject to too much 



