G10 



DATA OF GEOCHEMISTRY 



Stratification is commonly the result of surface 

 heating by the sun. The warm surface water is lighter 

 than the deep water beneath it, and so resists the 

 tendency of the wind to stir it into the depths. Whether 

 or not a permanent stratification is set up depends on 

 the temperature range involved, upon the rate at which 

 the lake warms up at the beginning of the summer in 

 temperate lands or the sunny dry season in the tropics, 

 upon the wind strength, and upon the size and shape 

 of the lake. The wind is able to work effectively on 

 a lake several miles long and may mix it to a depth of 

 some tens of feet. If the total depth of the lake is not 

 greater than that, it will not stratify. On the other 

 hand, a small farm pond sheltered by thick woods may 

 be mixed only to a depth of few inches. 



A thermally stratified lake may be considered as two 

 compartments: an upper freely circulating epilimnion 

 and a lower, nearly stagnant hypolimnion. The zone 

 of rapidly changing temperature that separates the two 

 compartments is commonly known as the thermocline 

 or clinolimnion. 



An homologous stratification may be set up by differ- 

 ences in salt content of the deep and shallow water in a 

 lake. This may happen if salt springs flow into the 

 bottom, in a coastal lake if sea water at spring high 

 tide flows over the sill that separates the lake basin from 

 the sea, or as a result of internal biological processes. 

 It may also occur in semiarid lakes if a shift in drainage 

 of a nearby river causes it to pour fresh water over the 

 salt water in the lake basin. Stratification because of 

 salt differences is known as meromixis and usually lasts 

 for many years or even indefinitely. Its principal 

 difference from thermal stratification is this perma- 

 nence, for thermal stratification breaks down every 

 time the surface water cools enough for the wind to 

 mix it into the depths. The chemical results of 

 meromixis are cumulative and are usually more pro- 

 nounced than those of seasonal thermal stratification. 



The chemical differences that develop between sur- 

 face and deep water in a stratified lake are of biological 

 origin. In the upper zone light is plentiful and photo- 

 synthesis is actively carried on. This removes carbon 

 dioxide from the water and adds oxygen to it. Dif- 

 fusion from the atmosphere tends to restore gaseous 

 equilibrium at the water surface, and turbulent mixing 

 tends to carry this water down into the depths, so that 

 there is no permanent change in the gas content of the 

 epilimnion. On a bright day, however, when the plant 

 community is actively photosynthesizing, temporary 

 changes will occur, as mentioned above in connection 

 with the diurnal gas changes of natural waters. The 

 amount of oxygen found at a depth of a few feet under 

 such conditions may be substantially more than the 

 water would hold if saturated at atmospheric pressure. 



Although such water is sometimes said to be supersat- 

 urated with oxygen, it is not actually supersaturated 

 at the ambient pressure. When the oxygen content 

 exceeds the saturation value under the ambient con- 

 ditions, bubbles form. This phenomenon is com- 

 monly observed in dense plant beds growing in the 

 upper few meters of clear productive lakes. 



Conditions in deep water are rather different. Light 

 is scarce and photosynthesis much reduced. Respira- 

 tion, however, continues apace, not only the respiration 

 of the animal community, but also the respiration of the 

 host of reducing organisms, particularly bacteria, that 

 are engaged in breaking down the organic substance 

 that settles from the productive epilimnion. 



Oxygen is used up and carbon dioxide is produced in 

 the hypolimnion. There is no possibility of rapid 

 replenishment by diffusion from the atmosphere, which 

 is sealed off by the thermocline, and the gas changes are 

 cumulative. In meromictic lakes the gas changes will 

 accumulate for many years. 



The change from oxidizing to reducing conditions 

 leads to the appearance of much nitrite, ammonia, 

 hydrogen sulfide, and ferrous iron in the water. It 

 also causes the release from the bottom sediments of a 

 considerable quantity of phosphorus and silica. The 

 seasonal cycle of events has been studied by Mortimer 

 (1941-42) in Esthwaite Water, a productive lake in the 

 English lake district that stratifies very strongly 

 during the summer and to some extent also during the 

 winter. Some of these results are shown in figure 6. 



If a strong wind blows across a stratified lake, the 

 light surface water of the epilimnion will tend to pile 

 up on the downwind side of the lake. This may be so 

 pronounced as to strip all of the epilimnion from the 

 upwind side, exposing hypolimnetic water of very 

 different chemical composition. After the wind stops 

 blowing a standing wave of very great amplitude will 

 exist at the boundary between light and dense water, 

 and this wave may continue to oscillate for many days. 



It is evident that any system showing as many tem- 

 poral and spatial variations in chemical content as a 

 deep lake will be inadequately represented by the chemi- 

 cal analysis of a single sample taken at some point on the 

 surface. In a lake with strong meromixis such a 

 sample will not even provide a rough idea of the mean 

 composition of the water. Limnologists are aware 

 of this state of affairs, but their chemical analyses are 

 usually very incomplete; geologists who are, in general, 

 more scrupulous about including all the major ions 

 in their chemical analyses, tend to sample lakes as if 

 they were temporally and spatially homogeneous. 



With these general words of warning about the state 

 of present knowledge of the chemistry of lakes and 

 rivers, we may proceed to an examination of the data. 



