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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 

 arctic latitudes. 



Tropical systems are subject to unvarying warm temperatures; light 

 energy input is both greater and more regular than in other altitudes. 

 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 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 tem- 

 perature 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, tem- 

 peratures which change 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 eco- 

 system 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 propagation, growth, 

 and survivial ; and, for some life forms, bottom conditions suitable to 

 their unique needs. 



The dependence of fish and shellfish on the estuarine zone is gov- 

 erned by particular environmental requirements for reproduction, pro- 

 tection, food supply, or a combination of these. Estuarine dependent 

 species are of three types : 



1. Species restricted to estuaries 



Amon^ the relatively few species of fish and shellfish that complete 

 their entire life cycle in the estuarine zone is the Atlantic (American) 

 oyster. It will die after long exposure to freshwater although it can 

 stand limited periods of such exposure and can thrive in relatively 

 high salinity water. The spotted sea trout occupies the estuary for all 

 its life purposes and only occasionally leaves the estuary under unusual 

 extremes of salinity and temperature. 



