106 



currents develop firmly attached communities where bottoms are hard, 

 and only microbial life where sediments are constantly in motion or 

 bein^ deposited. Where, however, such energy inputs do not dominate 

 the input of radiation solar energy, natural communities develop 

 compositions typical of Tropical, Temperate, or Arctic latitudes. 



Tropical systems (fig. IV.1.31) are subject to only slightly vary- 

 ing warm temperatures; light energy input is both greater and more 

 regular than in other latitudes. Within this general group there are 

 the sparse populations along coasts with deep clear water close inshore ; 

 the teeming and colorful populations of coral reefs; and the man- 

 groves and the submerged grasslands associated with shallow, nutri- 

 ent-laden water. Only the southern part of Florida and the islands are 

 of this type. 



Arctic systems are subject to wide fluctuation of sunlight and tem- 

 perature but ice is the key factor. Ecological systems develop in, on and 

 under the ice and in the fiords associated with glaciers. (Fig. IV. 1.32.) 

 Only a small part of Alaska includes estuarine systems of this type. 



Temperature systems are subject to moderate solar energy inputs, 

 temperatures that change regularly with the seasons, and generally 

 larger tide ranges and more wave action than either tropic or arctic 

 sj^tems. Most of the estuarine systems of the United States lie in the 

 temperate zone, and the balancing of solar energy input against me- 

 chanical energy input in this zone leads to a great variety of ecosystem 

 types, even within small geographic areas 



The tropical coral reefs have their counterparts in oyster reefs where 

 hard surfaces and constant currents exists, and where there is sufficient 

 particulaite food in the water. The mangroves and submerged grass- 

 lands also have their counterparts in extensive marches and submerged 

 algae and grass beds which are among the most productive parts of the 

 estuarine zone (fig. IV.1.33). 



There are also intertidal ecosystems of burrowing animals, such as 

 clams, where bottoms are soft (fig. IV.1.16) and of attached animals 

 and plants where they are not (fig. IV.1.34). The predominant influ- 

 ence 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 run- 

 off, characteristic plankton and attached algae communities develop 

 (fig.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 eco- 

 nomically significant animals. The great importance of such ecosys- 

 tems, however, lies in the fact that these communities form interme- 

 diate steps in the conversion of solar and gravitational energy to forms 

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

 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. 



The grouping of ecosystems outlined here describes a limited range 

 of recurring variation of chemical and physical properties to which 

 certain forms of life have adapted and on which they are now de- 

 pendent. The basic environmental needs for all living plants and 

 animals in such zones are zones of salinity consistently fluctuating 



