454 



THE COMMUNITY 



gradually more steeply declivous. This 

 declivity is termed the "continental slope." 

 Below about the 2000 meter hne the ocean 

 floor descends into the abyssal zone of from 

 4000 to 10,000 meters. 



Eulittoral Zone 



The intertidal area, delimited by the high 

 and low water marks of spring tides, sup- 

 ports a rich and varied biota. These organ- 

 isms inhabit the httoral region. The area 

 occupied depends primarily upon the de- 

 gree of slope of a particular portion of the 



gradiented; both stratifications are especially 

 well shown by belts of algae on rocky 

 shores. In general, this sea-weed zonation 

 is characterized by three bands of vegeta- 

 tion, depending primarily on the physical 

 gradient imposed by tidal action (Russell 

 and Yonge, 1928): green seaweed {Entero- 

 morpha) growing in pools near or sUghtly 

 above high-water mark; brown seaweed 

 (Fucus), especially characteristic of the 

 intertidal limits; and red seaweeds in 

 shallow water oflFshore, or at the bottoms of 

 deep rock pools. 



-— :!_—_— -; 1000 



Fig. 157. Diagram of horizontal stratification in the major marine community. (After Sverdrup, 



Johnson, and Fleming.) 



continental shelf, and it may be relatively 

 wide or narrow. In general, there are three 

 types of littoral habitat: (1) rocky shores, 

 (2) sandy shores, formed by the deposition 

 of wave-eroded material, (3) muddy shores, 

 formed by deposition of silt from water- 

 borne runoff from the land. 



These three httoral types, and the many 

 intergradations between them, are subject 

 to drastic environmental changes. Tides 

 periodically cover and uncover their sur- 

 faces twice every day-night period, and cor- 

 responding alterations in temperature, 

 evaporation, light, and friction are features 

 of this stratum. Burrows are periodically 

 covered and uncovered; inhabitants of rock 

 pools at low tide may be subjected to high 

 temperatures in summer; and evaporation 

 increases the local salinity. 



As in the fresh-water littoral, the marine 

 httoral is also vertically and horizontally 



Chapman, (1941) discussed these marine 

 algae, in general terms, for British coasts, 

 and the basic zonation on rocky shores is 

 stated to be as follows: (1) upper Enter o- 

 morpha-Urospora-Codioltim belt; (2) Pel- 

 vetia canaliculata belt, extending from high- 

 water mark shoreward through the spray 

 zone; (3) Fucus spiralis-F. platy carpus belt, 

 just seaward of the second stratum; 

 (4) Ascophyllum nodosum; (5) Fucus 

 vesiculosus; (6) lower fucoid belt, char- 

 acterized by numbers of Fucus serratus 

 (this sixth belt may be dominated by the 

 red algae in other areas, or followed by 

 Porphyra, Laurencia, or Gigartina; still 

 another variation is the development of 

 epiphytic algae (Cladophora, Lomentaria) 

 on Laurencia in summer); (7) the belt of 

 Himanthalia. The essential characteristics 

 of this algal stratification are repeated in 

 the horizontal gradient of plants on rockv 



