ZOOPLANKTON IN JMETABOLISM OF LAKES 



81 



more than a dozen endemic Cypridians were 

 fonnd ; in Africa in Lake Victoria there are 

 over 50 endemic Cichlidae. Tliere is thns 

 a o^enetical relation to be noticed in the 

 marked difference between tlie larg'er and 

 the more or less microscoi)i(' iidiabitants of 

 tropical fresh water. The earlier accepted 

 idea that the small plankters of tropical 

 fresh water were mainly cosmopolitan has, 

 however, been proved to be false. There 

 mnst be some other reason for this remark- 

 able difference. 



The geographical distribution of zoo- 

 plankton in temperate regions is by no 

 means completely known. There are cer- 

 tain crustaceans whose range has been 

 known to be restricted to the northern or 

 to the southern half of the temperate region 

 or to deep cold-water lakes when found in 

 more southerly regions. Others are cosmo- 

 politan and are found all over the world. 

 The rotifers in general are of this type. 

 The Diapiomns group, on the other hand, is 

 very restricted in the distribution of its 

 species, most species being endemic to the 

 region in which they are found. In White 

 Lake. North Carolina, a peculiar distribu- 

 tion of crustaceans has very recently been 

 reported by Coker ( 1938 ) . In this shallow, 

 clear-water lake, only two calanoid copepods 

 (no Cladocera or rotifers) live limnetically. 

 C (/clops )i(nu(s and other species were found 

 living near the bottom. These forms have 

 not previously been found south of New 

 York. Of the calanoid copepods, Di- 

 apio)iius )iiiiiufus was kiunvn mainly from 

 deep lakes. Similarly the ostracod, Ca)i- 

 dona crogniauiana, was originally found in 

 temporary shallow pools in Georgia, but 

 since then it has been found on the bottom 

 of the deepest parts of Lake INIendota, in 

 the St. Lawrence River, and in other places. 

 So that, as Coker points out (1938b), the 

 appearance of anomaly in many of the dis- 

 tribution records comes from an inadequacy 

 of knowledge of the fresh-water fauna of 

 this country. There is great need for more 

 study of the fresh-water crustaceans. 



In another type of environment, the shal- 

 low lakes of the arid regions, Hutchinson 

 (1937) has found a peculiar a ;!-;ociation of 



a l)ai)liiiid will) a lai'ge and a small Diap- 

 toiiiid. This situation has been round to be 

 characteristic of extreme cases of aridity in 

 various parts of the woi'ld. It is believed 

 tliat these three forms feed on the bacterial 

 and organic matter foiuid in the suspended 

 iiiild. 



The vertical distribution of zooplankton, 

 except in shallow lakes and at the time of 

 the si)ring and fall overturns, is vci-y com- 

 plicated and varies so nnich in different 

 lakes, Avith different oi-ganisms (and even 

 with the same species in different lakes) and 

 at different times of the year, that only gen- 

 eral statements can be made. Almost all 

 of these statements are subject to exceptions. 

 Welch (1935) gives a sunnnary of certain 

 tendencies in the vertical distribution of 

 zooi)lankton as follows: "(1) a greater oc- 

 currence of the Sarcodina in the lower 

 waters; (2) preference of the Dinoflagellata 

 for upper waters; (3) general scattering of 

 the Ciliata; (1) selection of different levels 

 by the young and adult stages of certain 

 Crustacea." Cold-water forms tend to keep 

 to the deeper waters of lakes unless the sup- 

 })ly of oxygen becomes depleted, when they 

 are driven into the u]iper regions. A study 

 of vertical distribution of plankton groups 

 in 60 samples taken in 36 lakes and reser- 

 voirs of New York State (Tressler and Bere 

 1931, 1935, 1936, 1937, 1938) shows no rela- 

 tion between the position of maxinnun abun- 

 dance of a group in one lake and that of the 

 same group in another lake. The Protozoa 

 had their average maximum abundance in 

 all the lakes at 5.3 meters, while the range 

 was between and 15 meters. The others ran 

 as follows: Copepoda, average, 7.1 meters, 

 i-ange, 0-30 meters; Nauplii, average, 9.9 

 meters, range 0-35 meters; Cladocera, aver- 

 age, 6.5 meters, range 0-25 meters, and 

 Rotifera, average 7.5 meters, range, 5-40 

 meters. The average depth of the lakes was 

 22.6 meters and the range, 5-50 meters. 

 These variations were caused by differences 

 in the species in the various lakes, differ- 

 ences in time of day and season at which the 

 sample was taken (all were taken between 

 June and SeptembeiO and to differences in 

 individual lake conditions. 



