82 



PROBLEMS OP LAKE BIOLOGY 



Variations in the vertical distribution of 

 individual species in different lakes has been 

 shown to be the case in several instances. 

 In Lake Nipissino', Langford (1938) found 

 that Diaptomus oregonensis migrated from 

 the lower strata of the hypolimnion after 

 midsummer and sought the upper regions. 

 This was correlated with oxygen depletion 

 and increase in carbon dioxide in the bottom 

 layers. These movements led to a series of 

 experiments using a small vertical gradient 

 tank designed by Dr. Fry. It consisted of 

 a small aquarium, 15" high, in which verti- 

 cal stratification was produced by cooling 

 and heating devices and changes in carbon 

 dioxide and dissolved oxygen were produced 

 at will. Samples of water at various depths 

 could be Avithdrawn for analysis ; thermom- 

 eters in the walls gave temperatures at 

 various levels and the plankton could be 

 counted at any level by means of a mag- 

 nifying glass. The results showed that 

 Daphnia mag7ia and Cyclops hicuspidatus 

 moved from a region of high carbon dioxide 

 to a lower one and from a low concentration 

 of oxygen to a higher one. The movements 

 counteracted such factors as higher tem- 

 perature and a positive geotropism in the 

 case of C. 'hicuspidatus. These movements 

 were comparable to those of immature 

 Diaptomus oregonensis in the lower strata 

 of Lake Nipissing. An increase in carbon 

 dioxide of 12-15 ppm and a decrease in 

 oxygen to 4.3 ppm seemed to start these 

 movements. It should be pointed out here 

 that these movements were not complete 

 evacuations of the lower depths, and this 

 was also true in the experiments ; some in- 

 dividuals remained under conditions from 

 which others migrated. 



In each lake, or perhaps in each type of 

 lake, every organism has its own preferred 

 level. This is true to a certain extent in 

 groups of plankton, is true of species, and 

 Woltereck (1932) has shown that, in the 

 Wisconsin lakes with stratified water, each 

 stratum has its own peculiar race of 

 Daphnids. 



The factors influencing the vertical dis- 

 tribution of zooplankton are divided into 

 two groups by Rylov (1935) as follows: 

 (1) mechanical factors, including the spe- 



cific gravity of the plankter and the effect 

 of currents, and (2) biological factors, in- 

 eluding temperature, light, food relation- 

 ships, pH, dissolved salts, and dissolved 

 organic matter. Langford (1938) believed 

 that light was the most important factor 

 in Lake Nipissing but that it was qualified 

 by other factors such as food, chemical con- 

 stituents, temperatures, in some cases the 

 wind, gravity, age and sex of the plankter, 

 specific differences, and finally by actual 

 depletion at a certain level or levels due to 

 consumption by some plankton feeder, such 

 as the Cisco. Not enough is known about 

 the food of zooplankton to state definitely 

 how nuich this factor influences vertical dis- 

 tribution. Many plankters feed upon phy- 

 toplankton which are more or less confined 

 to the upper layers, but if Flitter 's (1909) 

 theories are right, and there has been some 

 evidence favoring them lately, the presence 

 or absence of phytoplankton need not deter- 

 mine the presence or absence of food since 

 organic matter may be used. The influence 

 of carbon dioxide and oxygen has already 

 been discussed. Temperature seems to be a 

 controlling factor in some cases; for ex- 

 ample, Diaptomus oregonensis and Limno- 

 calanus seem to prefer cooler waters and 

 Epischura warmer. The thermocline is also 

 a barrier to migration in some forms, al- 

 though if the recent evidence of the periodic 

 dissolution of the thermocline in Lake Win- 

 dermere given by Ullyott and Holmes 

 (1936) holds for other lakes, the thermo- 

 cline may not always be the formidable 

 barrier it has been thought to be. Whether 

 higher or lower temperatures decrease or 

 increase the sensitivity to light of an or- 

 ganism is a disputed question. Langford 

 (1938) found that higher temperatures les- 

 sened sensitivity in nauplii and Epischura, 

 while Welch (1935) states that the opposite 

 is true. In Lake Nipissing on a few occa- 

 sions the wind acted in making the distri- 

 bution uniform for most organisms and in 

 depressing the thermocline, thus scattering 

 man.y of the plankton groups. Gravity 

 affects all organisms heavier than water 

 and most forms have to actively exert them- 

 selves to remain in a fixed position. 



It was generally believed by the earlier 



