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teeming and colorful populations of coral reefs; and the mangroves 

 and the submerged grasslands associated with shallow, nutrient- 

 laden water. Only the southern part of Florida and the islands 

 are of this type. 



Arctic systems are subject to wide fluctuations of sunlight and 

 temperature but ice is the key factor. Ecological systems develop 

 in, on, and under the ice and in the fjords associated with glaciers. 

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



Temperate systems are subject to moderate solar energy inputs, 

 temperatures which cnange regularly with the seasons, and generally 

 larger tide ranges and more wave action than either tropic or 

 arctic systems. Most of the estuarine systems of the United States 

 lie in the temperate zone, and the balancing of solar energy input 

 against mechanical energy input in this zone leads to a great 

 variety of ecosystem types, even within small geographic areas. 



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 

 dependent. 



The basic environmental needs for all living plants and animals in 

 such zones are zones of salinity consistently fluctuating over a 

 limited range of concentration; solar energy; water temperature 

 variation; water quality and nutrients favorable to their 



