REACTION IX WATER 99 



Reactions in the Sea 



Reactions in marine habitats closely resemble those of fresh water 

 for the most part, but they are on a much larger scale, owing to the 

 extent and age of the oceans. They also exhibit certain qualitative 

 differences, due to the presence of halides in particular, and are much 

 influenced by currents, tides, and upwelling. Accumulation of effects 

 is regulated by the presence or absence of these to such a degree as to 

 give further warrant to the grouping of marine reactions on the basis 

 of tidal, benthic, and pelagic climaxes. Manifestly, these differ from 

 one another more in degree than in kind of reaction, though each pos- 

 sesses one or more typical effects, such as deposit in the case of 

 benthos. As dominants, the animals are the chief reactors of the 

 ocean, plants assuming this role relatively rarely. 



Tidal Areas 



Belt between Mean High and Mean Low Tide. Rocky intertidal 

 areas, and areas covered with coarse gravel or strewn with boulders, 

 are occupied by definite communities, in which barnacles are the chief 

 dominants, associated with sea mussels, sea anemones, and often brown 

 and red seaweeds. These react by virtue of attachment and density, 

 holding water on the rocky surfaces and thus reducing the danger of 

 drying during periods of exposure to the sun. 



Owing to the water-holding capacity of the bottom materials of 

 sandy and muddy shores, the water withdrawal at low tide has a 

 lesser effect than on rock. This is primarily influenced by the degree 

 of water movement. Strands beaten by waves do not permit the 

 accumulation of the typically small reactions, but in quiet backwaters, 

 where mud and organic matter accumulate, fiddler crabs and various 

 other invertebrates burrow in the substratum, or work it over in ways 

 that differ little from reactions on soil; these communities represent 

 succession to land. However, any area alternately exposed and sub- 

 merged is of the nature of an ecotone between marine and terrestrial 

 connnunitics. 



Littoral Benthic Belt. In protected mud-bottomed bays the 

 processes are similar to those in lakes. However, on tide-swept rocky 

 bottoms in the littoral belt (0-200 meters deep), great numbers of 

 dominants occur in the form of large showy echinoderms, mollusks, 

 coelenterates, and crustaceans. Relatively few of these are perma- 

 nently attached to the rock bottom, and dredging and sampling show 

 comparatively little skeletal material not belonging to living animals. 



