IV-5 

 SECTION 1. GENERAL DESCRIPTION 



The estuarine zone is best characterized as a region of constantly 

 recurring change. The constancy of change and the dynamic equili- 

 brium associated with the changes comprise the visible features of 

 the estuarine environment. The obvious complexity of structure, 

 movement, and life in the estuarine zone hides the inherently simple 

 basic causes of the existence and character of the estuarine environ- 

 ment. 



All life is dominated by gravity and by the sun's radiant energy, 

 but the effects of these are especially apparent in the estuarine 

 zone. The earth's gravity pulls the rivers down to the sea; at sea 

 level the gravitational attraction of the earth itself reaches a 

 dynamic balance with the gravitational attraction of the sun and the 

 moon. The results of this are the unique estuarine water movement 

 patterns caused by the differences in density between fresh river 

 water and salt ocean water, and the tidal ebb and flow which is 

 noticeable only in the estuarine zone. 



All forms of life on earth depend on the sun as their ultimate source 

 of energy. This energy is incorporated into plant material which in 

 turn supports all animal life. Plants need water and light to grow. 

 There is a profusion of both in the estuarine zone together with a 

 plentiful supply of dissolved nutrients derived from both land and 

 sea. These conditions make coastal areas the most productive environ- 

 ments in the world, and as a result very specialized biological 

 communities have developed in the estuarine zone. Such communities 



