100 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



The general effect of such organisms is to roughen the surface of rocks 

 and thus favor the attachment of Bryozoa, serpudids, barnacles, etc. 



In the pockets that frequently occur in such areas, the tidal cur- 

 rents are retarded and the skeletons of animals may accumulate in 

 considerable quantities. Shells of barnacles, sea urchins, snails, 

 brachiopods, and bivalves frequently cover much of the surface and 

 exist at considerable depth, in some places overlying layers of mud 

 deposited earlier. Hence, the reaction of shelled animals is to produce 

 a hard bottom suitable for the attachment of sessile forms on top of 

 what was formerly mud or sand. In deeper pockets where the cur- 

 rent is retarded still more, there occurs a deposition of organic matter 

 derived from these animals, as well as from seaweeds. These are 

 merely early stages of a process leading eventually to a hard bottom 

 such as just indicated above. 



The attached plants that characterize faciations within these com- 

 munities are probably less important while in position than botanists 

 have been inclined to assume. They do produce comparatively dense 

 shade, but their effect upon the carbon dioxide and oxygen content of 

 the water is minimized by the rapid motion of the medium. Shade is 

 a much less important reaction in water than on land, and hence such 

 plants modify the habitat little with respect to animals, and the com- 

 munities are little influenced by them. Their chief effect is probably 

 exerted when they break loose and settle in pockets where they decom- 

 pose and add to the matter accumulated there. Furthermore, when 

 tissues of these plants are broken into fine material, they contribute 

 to the total of suspended organic matter, which is of great importance 

 in connection with the marine climate in different areas. 



Pelagic and Deep Benthic Areas 



Reactions on the Medium. The plankton and nekton exercise a 

 far-reaching control over the physical and chemical factors of the 

 marine climates. One of the most striking effects of the swimming 

 and floating organisms, reinforced by organic detritus and silt, is the 

 obstruction of light rays in the lower layers of the ocean and in bays 

 and inlets. 



The reactions upon the gases dissolved in salt water are, in part, 

 essentially the same as for fresh water. The resemblance extends to 

 the decomposition of material on the bottom, excess of carbon dioxide, 

 and deficiency of oxygen, etc. The occurrence of hydrogen sulphide 

 under conditions of poor circulation and deficient oxygen is more char- 



