IV- 79 

 The grouping of ecosystems outlined here describes a limited range 

 of recurring variation of chemical and physical properties to which 

 certain forms of life have adapted and on which they are now depen- 

 dent. The basic environmental needs for all living plants and ani- 

 mals in such zones are zones of salinity consistently fluctuating 

 over a limited range of concentration; solar energy; water tempera- 

 ture variation; water quality and nutrients favorable to their 

 propagation, growth, and survival; and, for some life forms, bottom 

 conditions suitable to their unique needs. 



Many forms of plant and animal life can tolerate salinity ranging 

 from ocean concentrations (35 parts per thousand) to practically 

 zero. Other life forms must have a much narrower salinity range in 

 which to live and reproduce. There are animals which require dif- 

 ferent salinities at different parts of their life cycle and which 

 migrate to find 1t. Figure IV. 1.36 shows the range of salinity 

 tolerance of some characteristic estuarine plants and animals. 

 Most of those with a limited salinity tolerance can also withstand 

 temporary exposure to salinities outside that range. 



Solar radiation governs the photosynthetic process by which plants 

 manufacture the basic food upon which all life ultimately depends. 

 The primary producers of food in the aquatic environment are the 

 microscopic plants upon which the succession of more advanced life 

 forms feed. Planktonic communities exist in all ranges of salinity 

 and temperature, but their composition may vary even with constant 



