16 



In the estuarine zone these two different solutions meet and mix. 

 Salt concentrations range from those of the oceans to the almost un- 

 measurable amounts present in some rivers. Where little stratification 

 exists, sea salt dominates mineral concentrations in estuarine waters; 

 in stratified systems, however, the small amounts of minerals entering 

 in the fresh water may be as important in some parts of the estuarine 

 zone as the much larger concentrations from the sea are in others. 



THE LIFE 



The governance of the dominating environmental factors, as modi- 

 fied by estuarine shape and water quality, result in an input of energy 

 into individual estuarine systems, and it is in the variety and diver- 

 sity of estuarine life that the input of energy to the estuarine 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 otlier forms of energy (such as food) 

 are essential steps in the continuation of life in the water, in the 

 mashes, and on the land. 



Energy input from gravitational forces, as illustrated by tidal ac- 

 tion and river flow, depends primarily on local or regional conditions, 

 but direct energy input from solar radiation depends largely on lati- 

 tude, 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 in- 

 puts do not dominate the input of radiant solar energy, natural com- 

 munities develop compositions typical of tropical, temperate, or arctic 

 latitudes. 



Tropical s}^stems are subject to unvarying warm temperatures; 

 light energy input is botli 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 mangroves and the sub- 

 merged 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. 



Temperature systems are subject to moderate solar energy inputs, 

 temperatures which change regularly witli 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 



