30 



PLANKTON OF WISCONSIN LAKES 



and through the month of May there was a steady increase in the 

 amount of organic matter which culminated in a maximum of 400.2 

 milligrams on June 1, 1917 ; after this date the observations were dis- 

 continued. 



While the various curves show that there are considerable variations 

 in the organic matter of the net plankton from year to year, yet they 

 all agree in bringing out the fact that the annual cycle consists of four 

 phases which correspond more or less closely to the four seasons of the 

 year. Beginning in the spring, there is an increase during the latter 

 part of April or early in May, at which time the organic matter rises 

 more or less rapidly to a vernal maximum in late May or early June. 

 There are considerable variations in the amount and in the duration of 

 this increase from year to year. In two of the five years for which 

 complete records were obtained, that is, in 1912 and 1915, the quantity 

 of organic matter rose to maxima of 574.4 milligrams and 647.1 milli- 

 grams per cubic meter of water respectively. In the other three years 

 the maxima lay betwen 400.0 and 462.0 milligrams. The period during 

 which the organic matter remains near the maximum amount varies 

 from a few days, as in 1912, to four weeks, as in 1913 and 1914. That 

 part of the curve covering the vernal period in 1912 consists of a sharp 

 peak ; those for 1915 and 1916 have fairly broad apexes, while those for 

 1913 and 1914 have a more or less irregular plateau covering a period 

 of about four weeks. 



This vernal pulse is followed by a decrease during June to a general 

 summer minimum about the first of July. From this time to the middle 

 of September the organic matter may remain uniformly low as in 1911 

 and in 1916, or it may show a more or less marked summer increase as 

 in 1912, 1913, and 1915. This summer increase was most marked in 

 1913; in fact, the organic matter rose to a higher point at this time 

 than it did during the vernal maximum. The curve for 1912 shows two 

 secondary peaks during the summer, one the first of August and the 

 other the first of September; that for 1915 also shows a secondary 

 peak about the first of September. On the whole, however, the period 

 from July 1 to September 15 may be regarded as one in which the 

 average amount of net plankton is relatively low. 



The third phase of the annual cycle comprises the autumn of the year, 

 during which there is a rather rapid increase, beginning about the third 

 week in September. This rise terminates in a maximum usually about 

 the middle or last of October, but the different years show important 

 differences both with respect to the rapidity and the extent of the in- 

 crease. In 1916, for example, the autumnal rise was much more grad- 

 ual than in the other years and did not reach its maximum height until 

 almost the middle of December; this maximum was the largest found 



