80 



PROBLEMS OF LAKE BIOLOGY 



The distribution of zooplaiikton forms de- 

 pends upon the type of environment to 

 which the particular organism is suited, the 

 species in question, and to many other fac- 

 tors. The same species may also show con- 

 siderable variation in distribution in one 

 lake from its distribution in other lakes. 

 There are, however, certain general state- 

 ments wliich may be made for the distribu- 

 tion of zooplankton and it is these that we 

 shall now consider, together with some more 

 recent observations. 



Most of the limnological investigations 

 have been carried on in temperate lakes, 

 while very little work has been attempted 

 on tropical waters and practically none on 

 the lakes of the far north, particularly in 

 North America. In Europe, most of the 

 lakes of Norway, Finland, and the Baltic 

 countries yield a good plankton crop, and 

 if it is assumed that conditions in fresh- 

 water lakes are similar to those of polar 

 seas, then it may be said that the polar 

 waters contain a more abundant plankton 

 than those in the tropics and more than most 

 temperate waters. Johansen (1931) and 

 others have made collections in the region 

 of Hudson Bay and in the Northwest Ter- 

 ritories (Marsh 1920) and reported several 

 kinds of plankton crustaceans, some of 

 which were distinctly northern forms, while 

 others are found all over the continent. No 

 quantitative work was done, and, as far as 

 the writer has been able to determine, none 

 has been attempted on the lakes of ex- 

 tremely northern regions. Here is certainly 

 an unpioneered field for future limnological 

 research. 



In tropical lakes there are somewhat more 

 data to work with. The recent publications 

 of Ruttner (1931) on the lakes of Java and 

 Sumatra, of Worthington and Ricardo 

 (1936) on the East African Lakes, and of 

 others, have shown that many tropical lakes 

 support a fairly generous plankton popula- 

 tion. In Lake Edward, in East Africa, for 

 example, eutrophic conditions were found, 

 although most of the life was concentrated 

 above the thermocline, and in Lake Bunyoni 

 there was an abundant macro-plankton 

 fauna although little micro-plankton. Dr. 



Woltereck has been kind enough to supply 

 me with a summary of conditions in the 

 lakes of the Philippines, Celebes, Bali, 

 and Java which were obtained during his 

 Wallacea Expedition in 1932 and which 

 have not as yet been published. He states 

 that in the warm waters of tropical lakes, 

 in general, one finds no greater abundance 

 of individuals or of species than in tem- 

 perate lakes. This is in great contrast to 

 the conditions in the tropical rain forests in 

 Celebes, for example, where over 1,000 dif- 

 ferent species of trees are to be found. For 

 most of the groups of animal and plant 

 plankton there is often a considerably 

 smaller number of species in tropical lakes 

 than in temperate waters. Diatoms, Cyano- 

 pliyceac, and Desmids are just as difiPeren- 

 tiated as they are in the temperate regions, 

 although cosmopolitan species are fewer in 

 number than the tropical forms. This 

 is also true of the Rotifera and to a certain 

 extent of the Ostracoda. The copepods play 

 a dominant part in tropical lake plankton, 

 much more than do the Cladocera, which 

 when compared with temperate lakes are 

 much reduced in numbers. In the tropical 

 lakes one finds many endemic forms of 

 Diaptomidae while the Cyclopidae are much 

 less differentiated (Brehm 1937). The 

 Cladocera are found in tropical waters in 

 limited numbers. There are a number of 

 species of Diaphanosoyiia, few Bosmina and 

 almost no Daphnids. (Daphnids were 

 found only on the Island of Flores in the 

 tropics and subtropically in Hawaii.) It 

 will be seen that Asia shows no parallel in 

 the tropics to the very great variety of 

 Daphnia piilex, D. longispina and D. longi- 

 remis in North America and of Daphnia 

 cucullata or of Bosmina coregoni in Europe. 

 The picture, strange to say, completely 

 changes if w^e consider the larger plankton, 

 especially the Decapoda, which are partially 

 littoral and partially pelagic in the tropics 

 and enormously differentiated. In Towoeti 

 Lake alone were found seven endemic deca- 

 pods (Woltereck 1937). The same is true 

 of the pelagic fresh-water fish of tropical 

 lakes and for the non-pelagic Mollusca. In 

 Lanao Lake, in the southern Philippines, 



