245 



reach septicity ; i.e., all of the dissolved oxygen is depleted. Under such 

 anaerobic conditions the decay of the benthic sludge deposits can result 

 in the production of hydrogen sulfide, which has a very strong "rotten 

 egg^^ odor. This gas, highly soluble in water, causes a black discolora- 

 tion of bivalve shells and imparts an offensive taste and odor to their 

 flesh when water carrying it moves across shellfish beds. 



HEAVY METALS 



The heavy metal salts are fairly soluble and stable in solution. Con- 

 sequently, they will persist for extended lengths of time. Many of 

 these are highly toxic to the aquatic biota. Since many marine orga- 

 nisms accumulate and concentrate substances within their cell struc- 

 ture, the presence of these metals in small concentrations can have 

 deleterious effects. Table IV.5.1 lists the more common metals that are 

 of environmental concern (IV-5-1) . 



TABLE IV.5.1.— CHARACTERISTICS OF COMMON METALS OF CONCERN IN THE ESTUARINE ENVIRONMENT 



The toxic concentrations listed in the table represent the lowest 

 values for the particular species tested and not absolute minimums. 

 Also, these toxic levels do not consider the synergistic effect that may 

 occur with the presence of other metals. For example, the toxic effects 

 of mercuric salts are accentuated by the presence of trace amounts of 

 copper. The table does indicate the minute quantities of metal salts 

 that can damage an estuarine system. 



INORGANIC NUTRIENT SALTS 



Aquatic life forms require trace amounts of some minerals and 

 vitamins for growth and reproduction. Elimination of such materials 

 from the environment or their reduction below minimum levels can 

 limit the growth and reproduction of some biota. Conversely, an over- 

 supply of all necessary trace mineral salts and vitamins can retard 

 growth or stimulate it ; providing satisfactory conditions of tempera- 

 ture, salinity, and dissolved oxygen also exist. An oversupply of inor- 

 ganic nutrient salts, such as those of nitrogen and phosphorus, may 

 be associated with drastic shifts in the composition of the aquatic 

 community. 



There may be shifts in population as the growth of one kind of 

 life is stimulated more than that of others by additional nutrients. 



