84 



PROBLEMS OF LAKE BIOLOGY 



there a few scattered winter observations, 

 most of which are not quantitative. It 

 seems to me that here is one of tlie most 

 important problems, the seasonal determi- 

 nation of zooplankton in various parts of 

 the world and the correlation of its distri- 

 bution with various physical and chemical 

 factors. 



In most lakes, there is, for most organ- 

 isms, a spring and fall maximum, but that 

 these maxima may vary somewhat in the 

 exact time at which they occur in different 

 lakes, and in the same lake for different 

 years, has been observed in a number of 

 instances. We may briefly illustrate this 

 by a comparison of the various groups in 

 Lake Mendota in the year 1916 (Birge and 

 Juday 1922) and Chautauqua Lake in 1936 

 and 1937. In the copepod group both lakes 

 had Cyclops and Diaptoynus as dominant 

 forms with Cyclops more numerous. In 

 Mendota the copepods continued in fair 

 abundance throughout the year with a max- 

 imum in the spring (April to June) and 

 again in November. The copepods in Chau- 

 tauqua were reduced to almost zero during 

 the winter and reappeared in fairly large 

 numbers in May. Another maxinuun oc- 

 curred in the fall in November. Nauplii 

 were present throughout the year in both 

 lakes but in Mendota their maximum num- 

 ber appeared in June and July with a very 

 short period of great abundance in Novem- 

 ber, while in Chautauqua there were two 

 maxima, one in late spring and again in 

 early fall. The Cladocera in Mendota ex- 

 hibited spring and fall maxima and in 

 Chautauqua their distribution was similar 

 to that of the Copepoda, although their 

 spring maximum did not start until June. 

 Kotifera were abundant the year around in 

 both lakes. In Mendota the maxima ap- 

 peared in June and January, while in 

 Chautauqua large numbers were found in 

 May and again in September, with only 

 average abundance in between. The Pro- 

 tozoa in Mendota showed spring and fall 

 maxima. In Chautauqua there were very 

 few protozoans and these were found in the 

 very early spring (March and April) and 

 again in September. 



The cause of seasonal changes seems to be 

 due to numerous factors, the most important 

 of which are the periods of spring and fall 

 overturn and the consequent mixing of the 

 bottom waters rich in organic matter with 

 the water of the entire lake. Spring and 

 fall rains and the resulting inwash, tem- 

 perature differences, light, dissolved oxy- 

 gen, and other environmental factors also 

 undoubtedly plaj* a part. Phosphorus and 

 silica have been thought to be limiting fac- 

 tors, but there is so much conflicting evi- 

 dence, particularly in the case of phos- 

 phorus, that it is doubtful if very much 



Fig. 1. Collecting lininologieal samples on Cliau- 

 tauqua Lake, February, 1937. 



significance can be ascribed to these two 

 factors. A coincidence between carbohy- 

 drates and crude proteins and the spring 

 and fall maxima of Cladocera has been 

 found in a Danish lake (Krogh and Berg 

 1931). Very likely seasonal distribution 

 will be found to be caused by different fac- 

 tors in different lakes although the most 

 important causes will probably be similar. 

 The fluctuation of zooplankton from year 

 to year has been reported by Richer (1938) 

 who found considerable variation in num- 

 bers over a ten year period in Cultus Lake. 

 This is possibh' due in this case to varia- 



