PHYSICAL AND CHEMICAL FACTORS IX THE METABOLISM OF LAKES 



19 



factor for life than lias been supposed, but 

 1li;il it may be of considerable value as an 

 indicator of certain environmental condi- 

 tions. 



Under usual conditions, excluding the ef- 

 fects of bogs, volcanic action, or great accu- 

 mulation of alkaline salts, the pH of nat- 

 ural lake waters tends to lie in the range 

 6.0 to 8.5. As Juday ci aJ. (1935) have 

 pointed out with respect to the lakes of 

 northeastern AYisconsin, the annual changes 

 are usually less than three units of pH and 

 the greatest difference from surface to bot- 

 tom is also less than three units. Appar- 

 ently few aquatic organisms have difficulty 

 in tolerating pH ditferences over this or 

 even wider ranges. Many of them can also 

 tolerate rapid changes over wide ranges, 

 Wiebe (1931a). There is, however, con- 

 siderable evidence that some organisms re- 

 act to rather slight differences, and others 

 tend to thrive only in hard or in soft waters. 



The application of pH determinations as 

 an index of more fundamental environmen- 

 tal conditions is considered of much impor- 

 tance, for example, by Juday ef al. (1935), 

 Strom (1930), and Xaumann (1932). 

 Their opinions are in most cases accom- 

 panied by a Ava riling that pH must be rec- 

 ognized for what it is, namely, the result 

 of a number of underlying chemical condi- 

 tions, and it must, therefore, be considered 

 along with other physical and chemical con- 

 ditions in the lake. Since the basic factors 

 which determine pH are often inconvenient 

 or impossible to determine we rarely know 

 its exact meaning. 



Increased acidity of bottom waters as an 

 indication of stagnation and the i^resence of 

 free carbon dioxide is a commonplace ob- 

 servation. To consider a more involved ap- 

 plication, pH along with the color of the 

 water may tell us wliether a reduction of 

 dissolved oxygen in the hypolimnion in a 

 given lake is chiefly due to the decomposi- 

 tion of allochthonous or autochthonous ma- 

 terials, thus providing important informa- 

 tion as to the productivity of the lake. 



Although we have confined our comments 

 to the more usual range, i.e.. pH 6.0 to 8.5, 

 we do not infer that lower and higher val- 

 ues are not of great significance. On the 



contrary extreme acidity or alkalinity may 

 become the dominant ecological influence. 

 Naumann (1929) uses the terms acido- 

 trophy and alkalitroiihy with essentially 

 this significance. 



CALCIUM 



Calcium is of fundamental importance 

 in plant nutrition both directly and 

 through its interrelations with carbon di- 

 oxide. In a lake it undergoes a more or less 

 active circulation and may be lost either 

 temporarily or permanently by precipita- 

 tion as lime or marl. 



The amount of calcium in different lakes 

 varies widely and a number of classifica- 

 tions have been suggested. Ohle (1931) 

 considers lakes with less than 10 mg of cal- 

 cium per liter as poor, those with 10 to 25 

 as medium and those with more than 25 

 rich. Yoshimura (1932) suggests that each 

 of these groups has a more or less charac- 

 teristic degree of calcium stratification 

 (Fig. 4). In the soft water group (poor 



PoijrjK 



Fig. 4. Stratification of calcium in soft and hard 

 water lakes. (From Yoshimura, 1932.) 



in calcium) the stratification is slight, 

 although there may be some increase in the 

 deeper water. Most of the lakes of north- 

 eastern Wisconsin studied by Juday ct al. 

 (1938) fall in this group and show similar 

 distribution of calcium. In the middle 

 group there is a considerable increase in 

 calcium below the thermocline (cf., Presque 

 Isle Lake, Wisconsin) and in the hard 

 water group a very great increase of cal- 

 cium in the deep water. Kicker (1937), 

 however, shows that Cultus Lake has a hard 

 water (calcium 32 mg per liter) but prac- 

 tically uniform distribution throughout the 

 year. Naumann (1932) makes the useful 



