Excessive levels of essential metals, however, can be detrimental to the organism. The soluble forms are generally ions or unionized organometallic chelates or complexes. The behavior of metals in natural waters is a function of the substrate sediment composition, the suspended sediment composition, and the water chemistry. Sediment composed of fine sand and silt will generally have higher levels of adsorbed metal than will quartz, feldspar, and detrital carbonate-rich sediment. The water chemistry of the system controls the rate of adsorption and desorbtion of metals to and from sediment.
Adsorption removes the metal from the water column and stores the metal in the substrate.
Desorption returns the metal to the water column, where recirculation and bioassimilation may take place. Often, metals will be driven off into the overlying water. A decreased redox potential, as is often seen under oxygen deficient conditions, will change the composition of metal complexes and release the metal ions into the overlying water. A lower pH increases the competition between metal and hydrogen ions for binding sites. Heavy metals in surface water systems can be from natural or anthropogenic sources.