248 Oxygen and Carbon Dioxide 



(a) the rate of mixing of water units and (b) the difference in con- 

 centration of each characteristic. The rate of mixing, or austausch 

 coefficient, is a property of the water under the existing conditions of 

 turbulence, and it affects all characteristics that are being exchanged 

 between water masses. At the same time that oxygen is being trans- 

 ferred, heat may also be moved downward, phosphate moved upward, 

 and other exchanges carried out by the same eddy action between 

 the water masses. This is an illustration of the principle of common 

 transport: a current that transfers one property of the medium also 

 transfers other properties at the same time (Redfield, 1941). 



Eddy conduction is a process by which materials are transferred at 

 right angles to the direction of current. The transfer may take place 

 vertically, as in the situation just described, horizontally, or in some 

 other direction. The downward tiansfer of oxygen by eddy conduc- 

 tion is slow but it is commonly a thousand times greater than the 

 diffusion rate. Although eddy conduction helps furnish oxygen to 

 water layers at middepths wherever currents of different speeds or 

 directions are flowing, it cannot supply an adequate amount of this 

 essential material to the deepest parts of lakes or of the ocean. 



Fortunately for the abyssal animals oxygen can reach them by 

 another method, namely, the Jimss sinking or deep circulation of the 

 water. In the typical deep lake of the temperate region water 

 masses charged with oxygen at the surface sink to the bottom during 

 the spring and fall overturns, as described in relation to the tempera- 

 ture cycle. As a result of the extensive circulation of the lake at these 

 periods the water may become turbid and formerly this was often 

 noticeable in poorly filtered water supply systems. When muddy 

 water issued from the taps, the residents would nod knowingly and 

 say, "The pond is working." The limnologist, however, refers to 

 the periods of overturn that bring a fresh supply of oxygen to the 

 deeper layers after winter and summer stagnation as occasions when 

 "the pond takes a breath." 



An example of the seasonal changes in oxygen at various depths in 

 a temperate lake is taken from the classic work of Birge and Juday 

 (1914) (Fig. 7.4). During the period of this representative study 

 oxygen was abundant at all depths after the spring overturn, but a 

 rapid depletion began in May following thermal stratification. Oxy- 

 gen was almost completely absent at depths greater than 15 m during 

 July, August, and September. The fall overturn in mid-October 

 caused a sudden mixing and equalization of oxygen at all levels; in 

 succeeding weeks increasing amounts of oxygen were supplied 

 throughout the lake by deep circulation. In certain very deep lakes 



