IV-490 



Maintenance of estuarine ecological systems is dependent on an 

 effective flow of energy and mineral cycles; it is these factors 

 that most fundamentally determine the important features of system 

 yield, system stability and water quality, rather than the presence 

 of large, visible forms of life. In estuaries, the sun operating 

 plant production processes and the mineral and organic fuels enter- 

 ing from fresh water inflows are the most important energy bases. 

 Both the ecosystem components and overall energy flows primarily 

 originating from these sources must be maintained without acute 

 shortages or excesses. If the balance of cycling fails, estuarine 

 ecological systems become less effective in processing food energies 

 and are subject to replacement, either as a whole, or by substitu- 

 tion of their parts. 



A stress on an estuary is a process which drains available energy. 

 Stress can be either direct as in the case of harvesting fin fish 

 or shellfish from the system, or indirect as happens when increased 

 turbidities shade out light or when some substance such as phenol 

 is added to the aquatic system, either causing mortality or demand- 

 ing special adaptive work on the part of surviving organisms to 

 sustain life. Energy drains on existing organisms may also occur 

 when excesses of nutrients added to the system deplete the 

 available oxygen necessary for respiration. 



