IV-78 



The tropical coral reefs have their counterparts in oyster reefs 

 where hard surfaces and constant currents exist, and where there is 

 sufficient particulate food in the water. The mangroves and sub- 

 merged grasslands also have their counterparts in extensive marshes 

 and submerged algae and grass beds which are among the most product- 

 ive parts of the estuarine zone (Figure IV. 1.33). 



There are also intertidal ecosystems of burrowing animals, such as 

 clams, where bottoms are soft (Figure IV. 1.16) and of attached ani- 

 mals and plants where they are not (Figure IV. 1.34). The predomi- 

 nant influence of great amounts of river flow and the associated 

 rapid salinity changes and stratification also result in ecosystems 

 specific for different salinity zones or types of stratification. 

 Where there is little river runoff, characteristic plankton and 

 attached algae communities develop (Figure IV. 1.35). 



The ecosystems described relate primarily to organisms that tend to 

 stay in one place or move only short distances during their life. 

 Of these, the oyster, the clam, the crab, and the lobster are the 

 only economically significant animals. The great importance of such 

 ecosystems, however, lies in the fact that these communities form 

 intermediate steps in the conversion of solar and gravitational energy 

 to forms useful to mankind; upon them depend the great pelagic fisheries 

 which the estuarine zone nurtures. Without these communities mankind 

 would be without shrimp, salmon, and menhaden, as well as the oysters, 

 crabs, and lobsters which spend all of their lives there. 



