305 



Maintenance of estuarine ecological systems is dependent on an ef- 

 fective flow of energy and mineral cycles; it is these factors that most 

 fundamentally determine the important features of system yield, sys- 

 tem 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 entering from fresh water 

 inflows are the most important energy bases. Both the ecosystem com- 

 ponents and overall energy flows primarily originating jfrom these 

 sources must, be maintained without acute shortages or excesses. If the 

 balance of cycling fails, estuarine ecological systems become less effec- 

 tive in processing food energies and are subject to replacement, either 

 as a whole, or by substitution 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 finfish or shell- 

 fish 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 demanding special adap- 

 tive 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. 



In general, estuarine system diversity and organization is highest 

 where energy inputs are high and stresses are low as in many relatively 

 unmodified temperate and tropical estuarine areas. Conversely, those 

 systems where stresses are high and sources of energy low are charac- 

 terized by low species diversity and relatively simple organization as 

 in the case of artic systems or those greatly modified by man. Thus, the 

 relative diversity and organization of estuarine systems are due to 

 both "natural" occurrences, such as sunlight, temperature fluxes and 

 nutrient inflows, as well as those associated with man's activities such 

 as modification of circulation regimes, addition of pollutants to the 

 water, and thermal waste heating. 



Estuarine systems in the United States are almost universally 

 stressed by both natural and man-induced processes. The relative mix- 

 ture of man-caused and naturally occurring stresses, and their respec- 

 tive roles in estuarine modification, are presently little known, and 

 difficult to separate. It is certain, however, that the stresses resulting 

 from man's present and potential activities in the estuarine zone play 

 a decisive and increasing role in the foreseeable future operation of 

 estuarine ecologies. Therefore, the following discussion focuses uj)on 

 man's activities as they relate to modification of existing estuarine 

 systems. 



man's activities and esttjakine system stress 



Part IV, chapter 4 showed the presently identifiable trends associated 

 with population and economic development and with specific activities 

 affecting the estuarine zone. At present, however, the rate of change 

 effected by these trends on estuarine ecological systems is little known. 

 The most recent work by ecologists is generally concerned with identifi- 

 cation of system types, the development of general theory, and the 

 measurement of system characteristics and operating phenomena. 

 Much is known about certain elements of estuarine ecological systems, 

 such as temperatures, salinities, abundance of certain biotic commu- 



