88 



PROBLEMS OF LAKE BIOLOGY 



Lake tlie increase was 285 per cent, or from 

 1,300 to 5,000 ni<»' per cnbic meter. The ac- 

 cumulation of material as the water slowly 

 flows doAvn the lake probably accounts for 

 this concentration at the outlet end. The 

 knowledge concerning the geographical dis- 

 tribution of organic matter is, as far as I 

 am able to determine, completely a blank 

 and here lies, I believe, another most impor- 

 tant field for future research — the deter- 

 mination of organic matter in lakes other 

 than those of the temperate zone, together 

 with tlie vertical distribution in such lakes. 



Diurnal Migrations of Zooplankton 



Vertical migrations of zoop'ankton have 

 been discussed in other papers and it will 

 suffice here to very briefly review some of 

 the recent work and trends in this field, par- 

 ticularly from the standpoint of the zo- 

 oplankton itself. The early ideas of a 

 simi)le vertical migration from bottom to 

 the surface during the night have been 

 largely exploded and in their place very 

 complex movements involving both up and 

 down migrations have been found to take 

 place in many instances. Very recently. 

 Langford (1938) has reported the results 

 of studies of the migrations of five groups 

 of plankton in Lake Nipissing. Here Di- 

 aptomus minntns showed a complex diurnal 

 movement due to differences in the move- 

 ments of the tw^o sexes. In intense light 

 botli sexes were at a maximum at the sur- 

 face. With decreasing light, the males 

 moved downward ; females remained and 

 onl}' gradually left the upper waters to ap- 

 pear in greatest abundance at dawn at the 

 thermocline. With the absence of light the 

 males again returned to the surface and 

 remained until dawn, after which they 

 moved down and were replaced at the sur- 

 face by the females. Later on the sexes 

 were equalized at the surface. Other work- 

 ers have also noted different migrations of 

 the sexes. Southern and Gardiner (1932), 

 however, found no difference in the move- 

 ments of the sexes of Diapfomns g^'acilis. 

 Male Cyclops showed greater abundance at 

 the bottom at all time, but especially be- 

 tween sunset and midnight. Here the fe- 



males were most active in diurnal migra- 

 tion. Kikuchi (1927) found that the fe- 

 males of Biaptoiniis pacificus were more 

 active in migration and were at a maximum 

 at lower levels than the males. Worthing- 

 ton and Ricardo (1936) found a case of 

 reverse migration in Lake Rudolph in East 

 Africa, where Diapfomns was near the sur- 

 face during the daytime and descended to 

 deeper water at night. Nicholls (1933) re- 

 ported similar reversals in the normal mi- 

 gration of Calanus finmarchicus in the 

 ocean. In each lake each species and the 

 sexes of each species seem to show individ- 

 ual reactions to environmental conditions. 

 In Lake Nipissing Diapiomus minutus also 

 showed great variation in the extent and 

 type of its migration during the season 

 (May to September). At times there was 

 no change in its position while at others a 

 definite movement toward the surface w^as 

 observed. Seasonal changes in the diurnal 

 migrations of plankton have not been fol- 

 lowed to any extent and further work might 

 possibly throw added light on the causes of 

 migrations. Langford believed that light 

 was the most important factor in causing 

 migrations, but that it undoubtedly was 

 modified by other environmental factors. 

 This was shown by the variations in migra- 

 tion at different seasons. In 1903 Juday 

 stated that the problem could be solved only 

 by complete observations covering a con- 

 siderable interval of time, and this opinion 

 has been amply justified in the period since. 

 The present situation in regard to our 

 knowledge of the migrations of zooplankton 

 is summed up by Worthington and Ricardo 

 (1936) who state in substance that the older 

 and simpler theories to explain diurnal 

 migrations did not completely explain all 

 movements. New work has been concerned 

 mainly with a measurement of physical con- 

 ditions in the water, such as light and tem- 

 peratures, and a laboratory study of the 

 behavior of different animals. Instantane- 

 ous readings of intensity and composition 

 of light and temperature can be made at 

 any depth and can be correlated with plank- 

 ton hauls with nets simultaneously. Labo- 

 ratorj' experimentation has shown that dif- 



