ZOOPLANKTON IN METABOLIS.M OF LAKES 



83 



workers that distribution of i^lankton was 

 iiiiiforni over a body of water. This con- 

 ception of regularity in the horizontal dis- 

 tribution of plankton has for some time 

 been definitely disproved by many investi- 

 gators. In some small lakes, it is true, 

 seemingly uniform distribution has been re- 

 ported, but in the great majority of large 

 lakes the horizontal distribution is very 

 irregular. Recently, linear series of trap 

 samples taken from one end of a lake to an- 

 other have been carried out in one Ohio 

 lake (Buckeye) (Tressler, unpublished) and 

 in three New York lakes (Canada. Otsego 

 and Chautauqua) (Tressler and Bere 1935, 

 1936, 1938). These investigations have con- 

 firmed the irregular disti'ibution of zoo- 

 plankton. Within the past year some sim- 

 ilar studies on Lake Nipissing have been re- 

 ported by Langford (1938). Trap samples 

 at a series of depths were taken at 10 sta- 

 tions within a radius of three-fourths of a 

 mile. Ten samples were also obtained from 

 8 meters at one station. Diapfomus and 

 nauplii were more variable horizontally at 

 8 meters than in one spot. Cyclops, Epis- 

 chura, and Bosmina were not so much more 

 variable horizontally at 8 meters. No trend 

 was found when 10 stations were taken in 

 a linear series. Ricker (1937a) in Cultus 

 Lake, British Columbia, however, has ob- 

 tained different results. He found the 

 horizontal distribution to be approximately 

 the same in the innnediate vicinity of the 

 sampling station as over the entire lake. 

 Cyclops alone showed a more variable dis- 

 tribution. These conflicting results are 

 probably due to a difference in the lakes 

 themselves (Cultus is nnich smaller than 

 Nipissing) and also possibly to different 

 collecting methods employed (Ricker used 

 a closing net). 



Horizontal irregularities in plankton dis- 

 tribution may be caused by a number of 

 factors acting either singly or simultane- 

 ously. Welch (1935) states that wind ac- 

 tion is the most important and lists as con- 

 tributing causes inflowing streams, irregu- 

 larity of shore line, depth, general flowage 

 areas, currents, undertow currents, swarms. 



predators and, indirectly, diurnal migra- 

 tions. 



Another phase of horizontal disti-ibution 

 has been followed by Chandler (1937) who 

 determined the fate of lake plankton in 

 streams originating from lakes. It has 

 been well known that plankton decreased 

 in amount as it passed downstream, but 

 previous work had been done on streams 

 Avhich had other tributaries or which had 

 pollution areas. Chandler worked on 

 streams in Michigan which were without 

 tributaries or pollution areas. His i-esults 

 showed that the greatest decrease occurred 

 in areas which had a heavy growth of 

 vegetation. One such place showed a 70 

 jier cent decrease in 20 meters! When 

 the vegetation was removed, the decrease 

 in plankton was no longer conspicuous. It 

 was found that the vegetation strained out 

 the plankton or that the plankters actually 

 clung to the material on the stems or debris. 

 Here they apparently settled to the bottom. 

 Temperature, pollution, dilution from tribu- 

 taries, or change in chemical relations were 

 found not to be factors in this decrease in 

 the streams which were studied. 



Seasonal variation in the quality and 

 quantity of zooplankton is a subject regard- 

 ing which it is impossible to make general 

 statements. The total quantitative seasonal 

 variations are to be discussed later under 

 productivity; variations in the seasonal dis- 

 tribution of the various groups depend to a 

 large extent upon the species comprising 

 the group under consideration, and also 

 vary with tlie particular lake concerned. 

 Most of the studies of zooplankton through- 

 out the entire year have been made on a 

 few lakes in southern Wisconsin and IMich- 

 igan. Burkholder (1931) folh)wed the phy- 

 toplankton through the year in Cayuga 

 Lake and also followed to a certain extent 

 the chenucal relations. Recently a series 

 of samples collected monthly throughout 

 the year has been taken on Chautauqua 

 Lake (Tressler, Wagner, and Bere, unpub- 

 lished). With these exceptions, however, 

 most of the work has been undertakon in 

 the siunmer months only, with here and 



