fish, decreases with increasing salinity 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 
Note: Top: mercuric complex as a percent of the total mercury (10* 9 M); 
B °^rrvr a tHe activity of the mercur ic ion. All other trace metals remain bound 
to EDTA throughout the salinity range. SW represents seawater, salinity 33ppt. 
42 
