Mike is correct the dissolved copper binds with algae, organics, other living things such as zooplankton, bacteria AND most importantly carbonate ions (alkalinity- hardness) in the water. As the dead organisms die they release the bound copper to eventually go into the sediments. The more that is added over time,,, the more that accumulates in the bottom sediments. Certain types of sediment chemistries can re-dissolve some of the bound copper from the sediments.
