COMMUNITY ORGANIZATION: STRATIFICATION 



445 



nourishment, but rather is to be considered 

 a specific physiological adaptation for a 

 few species. 



Dissolved and particulate organic and 

 inorganic substances affect the rate of pene- 

 tration, amount, and composition of light 

 in natural waters. This is the factor of tur- 

 bidity in the broad sense of the term, al- 

 though the suspended particulate state is 

 more commonly recognized. The reduction 

 in total light by turbid water is a fairly 

 obvious phenomenon, but it must be re- 

 membered that suspended and dissolved 

 substances have a selective action on Hght, 

 and may profoundly modify its character. 

 The resulting direct effects upon light, 

 their indirect effects through photochemical 

 reactions, and indirect effects upon the 

 oxidation-reduction cycle afford opportun- 

 ities for research. 



Light is often of limiting importance, 

 and, since utilization of hght is low, tur- 

 bidity becomes significant. The ways in 

 which turbidity affects the community in- 

 clude its action upon: (1) the composition, 

 size, duration, and occurrence time of phy- 

 toplankton pulses directly and, hence, zoo- 

 plankton indirectly; (2) rate of photosyn- 

 thesis by phytoplankton, at various depths; 

 (3) vertical stratification of the micro- 

 crustacea in particular; and (4) size of 

 catches of commercially important fishes— 

 for example, the sauger {Stizostedion 

 canadense) (Chandler, 1942). 



In summary, the fresh-water environ- 

 mental background is characteristically 

 stratified. This organization not only exhib- 

 its gradients with respect to such obvious 

 influences as water pressure, temperature, 

 and light, but also for many additional fac- 

 tors, including dissolved gases, dissolved 

 and particulate organic and inorganic ma- 

 terials, hydrogen ion concentration, and 

 oxidation-reduction potential. 



The salt water environment similarly ex- 

 hibits gradients. In addition to a rather uni- 

 form difference in chemical composition 

 and physical characteristics, fresh-water 

 and salt-water environments differ quantita- 

 tively. Such differences, as those of pres- 

 sure, currents, and tides exist principally 

 by virtue of the great differential in vol- 

 ume. 



The organisms composing fresh-water 

 and salt-water communities adjust to this 



stratified environment, both vertically and 

 horizontally. 



Available information on vertical distri- 

 bution of bacteria in inland water has 

 been summarized by Henrici (1939), and, 

 although the data are conflicting in certain 

 cases, a few generalizations are worthy of 

 notice here with respect to inland lakes of 

 Minnesota and Wisconsin. In lakes with a 

 rich epilimnial plankton "bloom," bacteria 

 are numerous at the surface. There is no 

 marked difference between the plate counts 

 of bacteria in the epilimnion and hypolim- 

 nion, except in strongly stratified lakes; in 

 these latter such differences as do exist in 

 the bacterial count are thought to be a 

 consequence of thermal stratification, and 

 hence fit our general concept of community 

 organization. Microstratification may be as- 

 sociated with shaip local differences in 

 vertical distribution. The most abundant 

 bacterial flora is that of the lake bottom. 

 Bacteria are always numerous there, espe- 

 cially at the mud-water interphase, and 

 they decrease regularly above and below 

 this level. This is to be expected in view 

 of the accumulation of organic materials 

 on the bottom. 



The vertical distribution of bacteria in 

 the sea is generally similar to that in fresh 

 water, and has been discussed succinctly 

 by ZoBell (1946). In general, where bac- 

 terial counts have been made, bacteria are 

 distributed vertically. Such distributions 

 are generally expressed in quantitative 

 terms of numbers of bacteria per milliliter 

 (ml.), and depth in meters or fathoms. 

 Species composition of the sample is less 

 often available. Few seasonal studies of 

 vertical distribution have been made, but 

 these suggest great seasonal variation (Zo- 

 Bell and McEwen, 1935). Only the most 

 general of statements are admissible. A 

 search of the literature shows that there is 

 seasonal variability, but details differ as 

 between different areas of the same sea at 

 the same depths, as well as between dif- 

 ferent seas. 



In general, the curve of bacterial popu- 

 lations follows that of the phytoplankton 

 for the first 100 meters, relatively few bac- 

 teria being found at the sea surface (1 to 

 200/ml.), gradually becoming more numer- 

 ous and reaching a maximum between 

 25 and 50 meters (500/ml.), then gradu- 

 ally decreasing in abundance to the bot- 



