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zone finds ultimate expression. Whether energy comes directly, as 

 in solar radiation stimulating photosynthesis, or whether it comes 

 indirectly, as with tidal flows or wind and rain pounding on the 

 shoreline, its absorption and conversion to other forms of energy 

 (such as food) are essential steps in the continuation of life in 

 the water, in the marshes, and on the land* 



Energy input from gravitational forces, as illustrated by tidal 

 action and river flow, depends primarily on local or regional 

 conditions, but direct energy input from solar radiation depends 

 largely on latitude, the tropics receiving much more energy per 

 acre than the arctic. The relative amounts of energy entering an 

 estuarine system govern the kinds of life found there, and natural 

 ecosystems show systematic variations related to the sources and 

 amounts of energy received. 



Estuarine zones with strong mechanical energy inputs from waves, 

 currents, tides, or river flows develop similar ecosystems no 

 matter whether in the tropics or the arctic. Where, however, such 

 energy inputs do not dominate the input of radiant solar energy, 

 natural communities develop compositions typical of Tropical, 

 Temperate, or Artie latitudes. 



Tropical systems are subject to unvarying warm temperatures; light 

 energy input is both greater and more regular than in other 

 latitudes. Within this general group there are the sparse popula- 

 tions along coasts with deep clear water close inshore; the teeming 



