15] VEGETATIONAL TYPES OF FRESH WATERS 477 



deep-dwelling cold-water Fishes and sometimes other types too, their 

 fish-productivity being poor at best, while succession proceeds to 

 peat bog. Eutrophic lakes usually also lack deep-dwelling cold- 

 water Fishes, though they are often suitable for Perch, Pike, Bass, 

 and other warm-water Fishes ; succession in them is to swamp or 

 marsh. Oligotrophic and dystrophic waters are often rich in 

 Desmids, eutrophic ones in Diatoms and Blue-green Algae. 



Among the various ways in which living organisms alter fresh 

 waters is in the matter of gas-content. In general, green plants 

 (except in non-photosynthetic periods) remove carbon dioxide and 

 add oxygen, while animals do the reverse, so that in the upper, well- 

 lit layers there tends to be a superabundance of oxygen and in the 

 deeper and darker layers more carbon dioxide than above. Such 

 considerations lead to the recognition of two types of waters, namely, 

 those where the gas-content is almost constant in all layers, and those 

 where it decreases markedly in the depths. The former are chiefly 

 masses of water in which vertical currents cause almost constant 

 mixing. In the latter it is common to recognize in summer in tem- 

 perate regions (i) a wind-stirred and largely homogeneous ' epilim- 

 nion ' or surface layer rich in oxygen (because of contact with the 

 air as well as photosynthesis), usually extending to a depth of 10-15 

 metres ; (2) a middle ' metalimnion ' or ' thermocline ' where the 

 temperature and oxygen-content decrease rapidly ; and (3) a ' hypo- 

 limnion ', underneath, where the water is virtually stationary and no 

 oxygen enters from above. Tropical lakes commonly difl^er from 

 those of temperate regions in that the shallower ones ' only stratify 

 for short periods, if at all, while deeper ones may have cyclical 

 stratification or, if very deep, e.g. Nyassa and Tanganyika, may turn 

 over very rarely or not at all ' (R. Ross in litt.). 



It is particularly in lakes sheltered from wind-disturbance in 

 temperate regions that the oxygen stratification tends to follow the 

 bottom contours, and the surface-waters in periods of quiescence are 

 often more alkaline than deeper ones. However, in autumn the 

 surface-waters cool and sink, carrying down dissolved oxygen, and 

 the deeper masses rise to take their place, so getting aerated by a kind 

 of reshuffling each year. The greater the amount of nutrient material 

 and accordingly of organic life in a lake, the faster does the oxygen 

 disappear in the depths during the quiescent period of summer. 

 Carbon dioxide, being complementary in metabolism, exhibits a 

 largely reverse trend, disappearing from the well-lit upper layers but 

 accumulating below\ In spite of the relatively small amount in the 



