fish, decreases with increasing saUnity of the water (45). One possible 

 explanation for such a phenomenon is the decrease in metal ion activity 

 resulting from the formation of ion pairs with the major anions of seawater. 

 Figure 4-1 shows, for example, how the speciation of mercury and the activity 

 of the mercuric ion vary in function of salinity in the medium F/2. As salinity 

 increases the bromide complexes of mercury replace the EDTA chelate as the 

 major species, followed by the chloride complexes as the salinity approaches 

 that of seawater. In natural systems, in the absence of strong chelating agents, 

 the same phenomenon would extend to other metals such as lead and 

 cadmium. Table 4-2 illustrates this point by giving the major species of the 

 various metals in Aquil where EDTA has been reduced to 10' M. Besides the 

 chloride complexes, a number of carbonate (Cu, Pb), sulfate (Zn, Mn, Co) and 

 hydroxide (Zn, Pb, Co, Cr) complexes become significant. Because the kinetics 

 of formation of the various inorganic complexes of metals are typically fast 

 (43), equilibrium is a good assumption in this instance, and the thermodynamic 

 calculations should give accurate values of metal activities. 



The role of carbonate complexation in decreasing the toxicity of metals in 

 unchelated media has been verified for copper on Daphnia magna (3), for 



T Salinity (%o) 

 SW 



Figure 4-1. Chemical speciation of mercury in F/2 medium as a 



function of salinity at pH = 8.1. 



Note: Top: mercuric complexes as a percent of the total mercury (10'^M); 

 Bottom: the activity of the mercuric ion. All other trace metals remain bound 

 to EDTA throughout the salinity range. SW represents seawater, salinity 33%©. 



42 



