PART VIII — AQUATIC ECOSYSTEMS 



4. DYNAMICS OF LAKES 



Lake Circulation Patterns 



Lakes are large bodies of water 

 which would he mostly stagnant ex- 

 cept for the "stirring" influence of 

 wind on their upper surface. In rare 

 cases, lakes are part of a river system 

 and the flow of water through them 

 drives a pattern of circulation, while 

 small heated ponds can have their 

 own thermally induced circulations. 

 In most other lakes, however, includ- 

 ing the North American Great Lakes, 

 wind stress is the prime mover of any 

 "circulation" (i.e., more or less or- 

 ganized motions) and "mixing" (i.e., 

 random motions leading to the dis- 

 persal of an admixture). 



Man uses lakes for several pur- 

 poses. Most important, perhaps, is 

 the "aesthetic" use (building a house 

 on a lakeshore), closely coupled to 

 a "recreational" use (swimming, 

 boating, etc.); lakes are also used 

 as a source of food (fisheries), of 

 fresh water supply, as a sink for 

 waste materials (sewage), and for 

 waste heat (power generation). Some 

 lakes also serve as waterways. These 

 uses conflict to some degree, and 

 optimizing the use of, say, the Great 

 Lakes is not a simple problem. For 

 example, in many places around the 

 Great Lakes, the only present alterna- 

 tive to using the lakes as a waste-heat 

 sink for power generation is to build 

 cooling towers, which would increase 

 the costs of power generation quite 

 appreciably. At the same time, it is 

 not certain whether or how far the 

 discharge of large quantities of warm 

 water into the Great Lakes would 

 have undesirable consequences for 

 some other use of these lakes. 



Conflicts between different lake 

 uses are alleviated by lake "circula- 

 tion" and "mixing." For example, 

 none of the Great Lakes is in any 

 sense "polluted" as a whole at pres- 

 ent, although the water near the 



shores certainly is in many places. 

 The difficulty is that the pollution 

 is usually concentrated in an "in- 

 fluence zone" near an effluent source, 

 which is usually located at the shore. 

 If all waste matter and waste heat 

 discharged into a lake were mixed 

 with its entire body of water, there 

 would be far less interference with 

 other lake uses — although there are 

 clearly limits to the advantage to be 

 obtained in this manner. 



The main cause of circulation and 

 mixing in most lakes is the stress 

 that the wind exerts at the air-water 

 interface. The actual patterns of cir- 

 culation are also determined by the 

 shapes of the basins, the thermal 

 (density) structure of the water, and, 

 for large lakes, the rotation of the 

 earth. The problem is basically one 

 of physical oceanography (or physical 

 "limnology," to be precise, although 

 the behavior of lakes is usually dis- 

 cussed in the oceanographic litera- 

 ture). However, heat by solar radia- 

 tion, evaporation heat loss (both af- 

 fecting density structure), and wind 

 stress are inputs the knowledge of 

 which comes from meteorology. 



Evaluation of Current Knowledge 



Generally speaking, problems of a 

 meteorological nature are better ex- 

 plored than those of the oceano- 

 graphic kind. Most existing knowl- 

 edge on physical limnology was 

 developed in connection with bio- 

 logical studies, witness the highly 

 authoritative Treatise on Limnology 

 by Hutchinson. Indeed, several emi- 

 nent workers in physical limnology 

 started their careers as biological 

 limnologists. Inevitably, then, the 

 character of existing knowledge re- 

 flects a certain bias toward problems 

 of biological importance. For ex- 



ample, the annual cycle of tempera- 

 ture distribution in lakes (which has 

 a direct bearing on life processes) is 

 well explored, while the dynamics of 

 medium- and large-scale motions is 

 poorly understood. "Meteorological 

 inputs" are also better known. While 

 it would be a gross exaggeration to 

 say that the problem of predicting 

 wind stress over a water surface is 

 solved, we can make a much closer 

 estimate of this stress than of the 

 speed of the current produced by it. 



Wind Mixing — In greater detail, 

 the "wind mixing" of the top layers 

 of lakes, their yearly cycle of "over- 

 turn," and similar "local" phenomena 

 are fairly well documented, even if 

 the basic mechanics of these processes 

 (e.g., the formation of steep "steps" 

 in the thermocline) are only now be- 

 ginning to be investigated. Inspiring 

 fundamental work in this area has 

 recently been reported from the Med- 

 iterranean and the Great Lakes and 

 from laboratory simulation. These 

 studies have been complemented by 

 results obtained through computer 

 modeling in connection with the nu- 

 merical forecasting of ocean circula- 

 tion. The small-scale structure of 

 turbulence, of internal waves, their 

 "breaking" and interaction with tur- 

 bulence (leading to vertical mixing, 

 particularly across the thermocline) 

 are highly relevant to the mixing 

 problem and are under investigation 

 in a few places. 



Wave-Like Motions — Among the 

 large-scale motions in lakes, the best 

 understood are the "seiches," or regu- 

 lar surface oscillations, usually started 

 by bursts of wind. Perhaps the most 

 prominent example is provided by the 

 seiches in Lake Erie, which acquire 

 economic importance due to their 

 effect on the output of the Niagara 

 power plant. 



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