446 



THE COMMUNITY 



torn. On the bottom the bacterial popula- 

 tion undergoes a dramatic increase. Drew 

 (1912) reported up to 160,000,000/ml. of 

 bottom mud o£F Andros Island, West 

 Indies; ZoBell (1946) found few bacteria 

 at 200 meters, but the number suddenly 

 increased to 9 X 10^ per gm. of mud on the 

 bottom oflE the coast of Southern Cahfomia. 



The vertical distribution of nonbacterial 

 plankton is marked in both fresh-water and 

 salt-water communities. In the discussion 

 of community stratification to this point, we 

 have attempted to present the reader with 

 a single principle at a time. Of course, 

 stratification is notably influenced by sea- 

 sonal, lunar, and day-night periodicities, 

 and these rhythms will be examined in the 

 chapter on periodism. Such forces markedly 

 affect vertical plankton gradients. 



Since the plankton consists of small or- 

 ganisms unable to move against waves or 

 currents, they drift through the water at 

 various levels. Sharp thermoclinal stratifi- 

 cation aside, vertical distribution of plank- 

 ton in fresh water is not so clearly defined 

 as in the sea, chiefly as a consequence of the 

 great depth of the latter. The early work 

 of Birge and Juday (1911) is still one of 

 the best sources of information on vertical 

 gradients, and has been recast successfully 

 with respect to quantitative and qualitative 

 plankton gradients by Welch (1935, Fig. 

 31). The subject is so complex that few 

 generalizations can be suggested. The com- 

 position varies not only with season, but 

 with time of day. local weather, and type 

 of lake (Welch, 1935; Prescott, 1939; 

 Chandler, 1942a). 



As to fresh-water phytoplankton, it may 

 be said that lakes large enough to be ther- 

 mally stratified tend to have a vertical 

 gradient in amounts, if not in kinds of, 

 phytoplankton during summer stagnation. 

 During the vernal and autumnal overturns 

 the organisms become thoroughly mixed by 

 the circulation of the water, and at these 

 limited periods this vertical gradient disap- 

 pears. The distribution gradient is the re- 

 sult of the need by chlorophyll-bearing or- 

 ganisms for effective light intensity and 

 quality, which consequently predetermines 

 the level at which they can exist. Even in 

 relatively large second order lakes, the ver- 

 tical gradient is not complete in shallow 

 areas. For example, the vertical distribution 

 in quality and quantity of phytoplankton is 



highly irregular in western Lake Erie 

 (Chandler, 1942a), since in shallow waters 

 (10 meters or less) wind action causes an 

 almost continual circulation from top to 

 bottom. Such shallow areas of second order 

 lakes duphcate the pattern in third order 

 lakes as a whole, save for periods of pro- 

 tracted calm, when regular stratification 

 may occur. Lakes with a deep hypolimnion, 

 as would be expected, have no green phy- 

 toplankton at deep levels. 



Usually each species of phytoplankter 

 has its own level of maximum population 

 density, its quantity diminishing both 

 above and below this zone. A few general- 

 izations can be made (Welch, 1935): (1) 

 Maximum populations of total chlorophyll- 

 bearing plankters are usually at a level 

 below the surface stratum; (2) the blue- 

 green algae and green algae usually have 

 their maximal concentration at a higher 

 level than the diatoms, which may be a 

 consequence of the greater specific gravity 

 of the diatoms. Much information regarding 

 such distribution in different lakes can be 

 obtained from Birge and Juday (1911, pp. 

 113-138 and Figs. 116-142). 



Zooplankton is also distributed vertically 

 in fresh-water communities, although iden- 

 tical patterns for any two lake communities 

 are to be expected no more frequently 

 than, say, identical patterns for two forest 

 communities. Indeed, when we attempt to 

 appreciate the numerous small differences 

 between broadly similar environments, the 

 degree of general convergence in vertical 

 distribution is notable. Welch (1935, p. 

 221) has suggested some tendencies in 

 vertical distribution of zooplankton: (1) 

 The Sarcodina are in greater abundance to- 

 ward the bottom of the vertical gradient; 

 (2) Dinoflagellata are in greater abun- 

 dance in the upper levels; (3) Ciliata, as 

 a class, are generally scattered over the 

 gradient; and (4) there is a differential 

 distribution between the naupUi and imag- 

 inal stages in Crustacea. 



Factors influencing the vertical distribu- 

 tion of zooplankton are separable into two 

 groups (Rylov, 1935): (1) physical fac- 

 tors, such as the mechanical effect of spe- 

 cific gravity and of current, temperature, 

 and light; (2) biological factors, such as 

 level of food and dissolved organic mate- 

 rials. Langford (1938) believed that light 

 was the most important influence in Lake 



