96 



ANALYSIS OF THE ENVIRONMENT 



III. Tropical lakes: surface temperature al- 

 ways above 4° C. 



Order 1. Temperature of bottom water 

 near 4° C. throughout the 

 year; one circulation period 

 possible in winter, but gen- 

 erally none 

 Order 2. Temperature of bottom water 

 varies, but not far from 4° C; 

 one circulation period in 

 winter 

 Order 3. Temperature of bottom water 

 similar to that of surface 

 water; circulation at all 

 seasons 



From this classification, which sums up 

 in a general way a great deal of limnologi- 

 cal research, it is clear that temperate lakes, 

 especially of the second order, are regularly 

 stratified with respect to temperature. Third 

 order lakes generally are too shallow to 

 allow such stratification. In tropical lakes, 

 where the temperature never falls to 4° C, 

 if other conditions are favorable, there is 

 always a direct thermal stratification. Polar 

 lakes, where the temperature is always be- 

 low 4° C, show an inverse stratification. 



These essentials of thermal stratification 

 must be kept in mind when considering the 

 stratification of lake communities (p. 443), 

 and in many phases of the physical environ- 

 ment as well, notably as regards dissolved 

 chemicals (p. 202) and dissolved atmos- 

 pheric gases (p. 193). Despite earlier sug- 

 gestions by others (p. 41), we owe much 

 of the background to Birge, whose ecologi- 

 cal studies have extended over some fifty 

 years. He introduced the term "thermo- 

 cline" in 1897 and the terms "epilimnion" 

 and "hypolimnion" in 1910; the amounts of 

 heat acquired and lost by a lake over a 

 year's time were organized and presented 

 by him (1916) as the heat budget. The 

 study of heat budgets should be of great 

 use in the comparison of thermal stratifica- 

 tion in difiFerent lakes, but has been little 

 used (Rawson, 1939). 



Seasonal variation in the progress of 

 thermal stratification has been considered 

 for numerous lakes sufiicientlv to establish 

 its generality for second-order temperate 

 lakes over the world. A paired example 

 must sufiice: Lake Waskesiu, Saskatchewan, 

 and Lake Mendota, Wisconsin, among 

 others, were compared seasonally. In the 

 order listed, vernal overturn began in mid- 

 dle May, as opposed to middle April; sum- 

 mer stagnation with thermocline formation 



began the first week of June in the northern 

 lake, as against the end of May for the 

 more southern one; winter stagnation wdth 

 ice cover and reversed gradient became 

 established in middle November in Lake 

 Waskesiu and in middle December at Lake 

 Mendota (Rawson, 1939). These general 

 thermal cycles vary as much as one or two 

 months for the same lake in difiFerent years. 

 The lake cycle also varies with bottom 

 characteristics, altitude, and latitude, but 

 the process itself is universal for suitable 

 lakes and plays a major role in community 

 development both directly and indirectly. 



The rate of change of water temperature 

 may prove important in the organization of 

 the lake community. This has been studied 

 in Linsley Pond and Lake Quassapaug, 

 Connecticut, and derived for Lake Men- 

 dota, Wisconsin, from Birge's table of mean 

 temperatures, by Hutchinson (1941). Such 

 data indicate that the hypolimnion can be 

 divided into an upper clinolimnion, in 

 which the rate of heating falls exponentially 

 with increasing depth, and a lower bathy- 

 limnion, in which the rate of heating ap- 

 proaches a constant value independent of 

 depth. 



Not only is there a vertical gradient in 

 thermal stratification, but there is a horizon- 

 tal sjradient as well, at least in large lakes 

 such as Lake Michigan (Church, 1942). 

 Many factors are involved in establishing 

 such gradients; for example, radiation, 

 evaporation, conduction, mixing, chilling by 

 snow, hail or sleet, and condensation. Sea 

 water, unlike fresh water, continues to be- 

 come denser with cold until it freezes at 

 about —1.9° C, when the salt content is 

 35 "'',„. As the surface water evaporates, 

 it becomes more salty and therefore more 

 dense. Even though warmer than underly- 

 ing water, it sinks until its density matches 

 that of the colder, deeper water. At such a 

 depth there is a zone of rapid temperature 

 chance that may approach the abruptness 

 of Birge's thermocline; this is usually found 

 at depths between 50 and 150 meters. Since 

 ocean water does not show a change in den- 

 sity at 4° C, but continues to increase in 

 density tmtil the freezing point is reached, 

 the abyssal waters of the oceans are cooler 

 than those of lakes and ranee from about 

 — 1' C, in regions of cold currents to 

 slightly above zero elsewhere. 



Thermal stratification also occurs on land 

 with depth in the soil, and it is particularly 



