FOOD RELATIONS 297 



compared with saturation. This indicates a very large consumption 

 by dead and living organisms, which constitutes an important reaction 

 on marine climates. The amount of dissolved oxygen usually varies 

 inversely with the carbon dioxide pressure. 



The suli)hur compounds of the sea, such as hydrogen sulphide, sul- 

 phur dioxide, and colloidal sulphur, are much more important than 

 in fresh water; free sulphurous acid is often found below 75 meters 

 in salt water. The extent to which the various products of metabolism 

 and decomposition accumulate or are carried away from an area of 

 abundance by currents constitutes one of the outstanding features of 

 a marine as well as a fresh-water hydroclimate. The accumulation 

 of all life products in situ is of prime importance on land, while the 

 extent to which they are moved in the sea and large lakes is the fore- 

 most limiting factor. There is little doubt that the total effect of 

 organisms on a marine or fresh-water hydroclimate is comparable in 

 magnitude to that of plants on land. 



Certain physical factors or complexes have little effect on aquatic 

 organisms. Pressure is of some small significance, but necessarily 

 operates in the same fashion in both fresh and salt water (Regnard, 

 1891). Tides do not constitute a climatic complex for subtidal ani- 

 mals, but merely produce a special climate for intertidal ones; they 

 have no counterpart in fresh water. The thermocline phenomena of 

 lakes (Birge, 1903) also characterize baj^s and fjords with threshold 

 outlets (Murray and Hjort, 1912:257), producing somewhat similar 

 results. 



FOOD RELATIONS 



One other important difference between terrestrial climax com- 

 munities and aquatic communities with climax characteristics lies in 

 the presence of food in the form of detritus and plankton distributed 

 throughout the water medium. The communities that are not clearly 

 a part of a terrestrial sere, such as most marine communities and 

 those of permanent streams and large lakes, lack the growth of rooted 

 vegetation that forms the primary food supply for practically all 

 herbivorous land animals. Food supplies in water have their basis 

 in (1) microscopic plants, (2) detritus from organisms in the plank- 

 ton and from the rooted vegetation along the shores, and (3) possibly 

 dissolved organic matter. 



Some microscopic animals may possibly absorb dissolved organic 

 matter. The earlier views of Frankland and Armstrong (1874; cf. 

 Birge and Juday, 1926) and of Piitter (1908) have been revived by 

 Krogh (1934) for the sea, and by Birge and Juday (1934) for fresh 



