The increase in ionic strength as a consequence of salinisation can also disrupt chemical equilibria between dissolved and particulate phases, either through changes to ion activity co-efficients or through salt ions blocking mineral surface adsorption sites (Chang 1977; Stumm and Morgan 1996). The activity co-efficient of phosphate decreases with increasing salinity, suggesting that phosphate should be more soluble in saline systems than in freshwater systems. Surface chemistry may also be disrupted with increasing salinity as cations present in salt compete with other ions for adsorption sites on particle surfaces. For example, Seitzinger et al . (1991) have shown that the concentration of exchangeab le ammonium in freshwater sediments is significantly greater than in marine sediments. They attributed this difference to cations out-competing ammonium ions for adsorption sites on the sediment.