42 THE LIFE OF DEVILS LAKE 



3. There is no close correspondence between temperature and 

 plancton abundance. It is true that in winter most species are 

 either absent from, or very rare in the collections; but from spring 

 to autumn there may occur several maxima and minima of plancton 

 numbers, which are apparently wholly unrelated to temperature. 

 These naturally differ for different species and different years, but 

 in general, occur, one in early summer, and the other in early 

 autumn. The causes are obscure and are probably manifold. In 

 spring there is usually a considerable influx of drainage water, 

 carrying with it dissolved materials, including much organic matter, 

 from the soil. Following this influx, and with increasing light and 

 rising temperature, there is a great development of both algae and 

 bacteria, especially the latter, accompanied in turn by a great 

 increase in animals, especially the rotifers. The maximum of early 

 summer is succeeded by a more or less well marked period of 

 depression during July, which may be succeeded by other periods 

 of maxima and minima in August. In the fall there generally 

 occurs a high point in the distribution curves accompanying a falling 

 temperature and decrease of light. These waves of production are 

 irregular and inconstant and their explanation is obscure. It is 

 not impossible that in midsummer the temperature is too high for 

 the successful development of many of the organisms in the lake; 

 or that, with their development in early summer the dissolved mate- 

 rials in the lake necessary for plant growth are largely consumed, 

 with consequent inhibition of development; it is further possible 

 that the great growth of bacteria may be responsible for the produc- 

 tion of some toxins inimical to the growth of other forms. The flrst 

 of these hypotheses seems the more likely, but none of them are 

 entirely satisfactory and the reason remains obscure. Similar 

 periods of maxima and minima in plancton production have been 

 described by other writers, but it is difiicult to compare the sea- 

 sonal distribution of plancton in different lakes because of 1) dif- 

 ferences in its environment and 2) differences in its component 

 species. A detailed comparison of my own work with that of others 

 would require far too long a discussion. It must suffice to indicate 

 some of the more important results. 



1.) In studying the distribution curves for any lake, one Js 

 at once struck with the numerous minor irregularities which they 

 display. Take for example, the curves illustrating the vertical 

 distribution of the Crustacea and nauplii in Lake Mendota on 

 September 8, 1896 (pi. XLII, Birge, 1897). According to these 

 there were more adult Crustacea by about 35% at the 6 than at 

 the 4 m. level, and slightly more at 3 than at 2 m. ; in spite of 

 the fact that they were decreasing rapidly from above downward, 

 except in the first half metre. The nauplii vice versa, while 



