103 



The turbidity of ocean water is generally low except where it meets 

 the shore; there the amount of turbidity is a direct reflection of the 

 intensity of wave action and the nature of bottom material. 



Dissolved oxygen is essential for all aquatic life. The amount of 

 dissolved oxygen present in surface ocean water is very close to 

 the total amounts the water can contain. Since this saturation con- 

 centration depends on both temperature and salt concentration, the 

 w^arm, saline waters, of the Gulf contain far less oxygen than the cold, 

 relatively fresh waters off the Alaskan coast. 



The natural quality of water free from human impact in the rivers 

 entering the estuarine zone depends primarily on the nature of the 

 ground over which they flow. Minerals enter the water by dissolving 

 from soil and rock as the water flows over it or carries it along. Water 

 flowing over limestone or other sedimentary material usually has 

 greater concentrations of dissolved minerals than water flowing over 

 volcanic rock and sand. Insoluble minerals are carried along as sedi- 

 ments, some dissolving slightly and others settling out in quiet reaches 

 of the rivers or in the estuarine zone. 



Decaying plant and animal materials also dissolve into the flowing 

 streams. These materials use oxygen in the decaying process and in 

 some streams, particularly in swampy areas, very low- dissolved oxy- 

 gen concentrations are normal. Dissolved organic material frequently 

 has a very intense yellow-black color which may make a water body 

 appear jet black. This condition is common in the estuarine zones of 

 the south Atlantic and Gulf regions. 



Variable as estuarine water quality and water circulation are, estu- 

 arine waters in each of the estuarine regions have typical character- 

 istics for different morphological categories. Table IV. 1.9 outlines 

 such typical natural estuarine zone conditions. 



SEcnoN" 5. The Life Energy and Life in the Esttjaeine Zone 



It is in the variety and diversity of estuarine life that the input of 

 energy to the estuarine zone finds ultimate expression. Whether energy 

 comes directly, as in the 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 condi- 

 tions, but direct energy input from solar radiation depends largely on 

 the latitude, the tropics receiving 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. Exposed ocean beaches at all 

 latitudes have communities of burrowing animals such as snails, 

 worms, clams, and crabs. Rocky sea fronts develop communities of 

 attached algae and moUusks (fig. IV.1.30). Channels with strong 



42-847 O— 70 8 



