19 



Usinger: Introduction 



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BENTHIC ZONE 



LIMNETIC ZONE 



LIGHT COMPENSATION UEVCL 



PROFUNDAL ZONE 



Intro, fig. 21. The major zones of a lake (Odum, 1953). 



open water the whirligig beetles (Gyrinidae) and 

 water striders (Gerridae, Veliidae, etc.) live on the 

 surface; nearly all aquatic insects inhabit or traverse 

 the limnetic zone at some time in their life history, 

 and only the Chaoborus "phantom larvae" regularly 

 inhabit open water in the profundal zone. 



As mentioned earlier, insects are only a part of 

 the aquatic communities in which they live. To under- 

 stand the dynamics of a lake or pond all aspects of 

 the community must be studied. This can be done 

 only on a large scale, for a lake or pond is in reality a 

 unit. However, within each body of standing water 

 local communities are recognized. These are con- 

 venient for study purposes but are always inter- 

 dependent and should not be permitted to obscure 

 the essential unity of the whole. 



The communities associated with the substrate are: 

 the littoral or shore community, the benthos or bottom 

 community, and the periphyton (epilithon) or organisms 

 attached to stems and leaves of rooted plants or other 

 surfaces projecting above the bottom (Odum, 1953). 



Of theso, tho littoral is divided into the region above 

 high water and wave action, opilittoral; the region of 

 shore-lino fluctuation, eulittoral; and the permanent.) 

 submerged shore Line, sublittoral, above the level 

 of light compensation. Microscopic organisms among 

 the sand grains at the wator's edge arc referred to as 

 the psammon or psammolittoral. Also a specialized 

 insect fauna including mostly beetles (Hetero< eridae) 

 and fly larvae (Psychodidao, Tipulidae, lloloidae, 

 etc.) occurs in mud flats. 



The benthos or bottom community contains a rich 

 but specialized insect fauna and plays an important 

 part in the economy of lakes. During the process of 

 outrophication, bottom materials accumulate. The 

 process has been worked out in detail by Doovoy 

 (1942) for Linsley Pond (intro. fig. 22). During an 

 early oligotrophic stage the water was probablj clear, 

 and Chironomids of the genus T any tarsus predominated. 

 Later, during the period of maximum production, plank- 

 ton blooms reduced the transparency, and Chironomids 

 of the genus Endochironomus were abundant. Finally, 

 during the present period, the pond has become 

 shallow and relatively stable with Chironomus and 

 Chaoborus as the dominant benthic insects. This 

 sequence was discovered by boring with a peat sam- 

 pler and analyzing fragments of microfossils at various 

 levels in the bottom of the lake. 



The precise effect of Chironomid and Chaoborus 

 larvae on the metabolism of a lake is not known. 

 However, it has been demonstrated by Walshe (1951) 

 that Glyptotendipes and other larvae live in tubes in 

 the bottom mud. They spin nets (intro. fig. 23) to 

 catch food which is carried in the currents of water 

 that are constantly circulated by undulatory move- 

 ments of their bodies. As a result of this activity by 

 millions of larvae in the bottom of most lakes, stag- 

 nation is reduced at the mud-water interface and 

 nutrients are made available for general circulation. 

 Oxygen, in particular, is at a premium at tho bottom 

 of lakes, and Mortimer (quoted in Ruttner, 1953) has 



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Intro, fig. 22. Schematic bottom profiles and a summary of ecological conditions in Linsley 

 Pond., a, during an early oligotrophic period; b, at the time of maximum organic production; and 

 c, at present. Estimated transparencies are shown by Secchi discs at the left of each figure 

 (Deevey, 1942). 



