VEGETATIONAL TYPES OF FRESH WATERS 473 



terrestrial ones. However, a marked vertical distribution occurs, the 

 vegetation in sufficiently deep waters being divided into zones ac- 

 cording to the different depths. In this deUmitation light is usually 

 the main factor, though the temperature of water-masses and the 

 local chemical composition and especially aeration of the water may 

 also be important. The zones, at various depths in water, that are 

 characterized by different forms or abundance of life, largely repre- 

 sent stages of decreasing intensity of light. They range from a 

 relatively bright surface ' euphotic ' zone in which the light is suf- 

 ficient for the normal development of large plants, through a dim 

 ' dysphotic ' zone in which photosynthesizing small Algae and even 

 Mosses may still flourish, to a dark and relatively deep ' aphotic ' 

 zone in which only non-photosynthesizing organisms can exist. 

 Owing to the varying turbidity of waters due to suspended particles, 

 and to the different penetration of the sun's rays at different angles, 

 the limits of these zones, which are themselves imprecise, lie at very 

 different depths in different instances. 



In summer, lakes in temperate regions experience a marked rise 

 in temperature, particularly on and near the surface, but this is 

 followed by a decrease in autumn. Such fluctuations engender 

 vertical convection currents and eddies which lead ultimately to a 

 temperature of low value throughout, while at the same time they 

 are important in aerating the deeper layers {see p. 477, and cf. Fig. 

 165). These last may vary very little (less than 4° C.) in temperature 

 between summer maximum and winter minimum. Tropical lakes 

 may also show a surface temperature fluctuation with changing 

 weather, but here a reduction in temperature of merely 1-2° C. is 

 reported to bring about a circulation similar to that which is effected 

 in cooler regions by a winter cooling of some 20° C. 



Limnology is the study of inland waters, including the environ- 

 ment and all inhabiting organisms and their interrelationships. In 

 it a distinction is made between the benthos (of organisms growing 

 on or in the bottom material ; these may be described as 'benthic'), 

 the freely floating plankton, and the swimming nekton. The dis- 

 tinction is, however, somewhat artificial as many organisms are 

 border-line cases. The plankton and benthos of inland waters are 

 said to be limnetic, in contradistinction to those of the open sea 

 which are said to be pelagic. These terms are not much used in 

 limnology, where the region of open water is commonly distin- 

 guished as the pelagial. In shallow coastal waters the plankton 

 is apt to be mixed with forms belonging to the benthos, and 



