What affects the concentration of mercury contamination?
Oxygenation. Biological uptake and Bioaccumulation. Biomagnification.
If the contaminant is naturally decaying one under aerobic circumstances (oxygenation occurs naturally from air to surface water), you can expect that contaminant concentration will decrease with respect to time.
Another mechanism is bioaccumulation. Contaminant will be taken by aquatic creatures (such as phytoplankton).
Concentration of contaminant in phytoplankton will be lower than concentration of contaminant in zooplankton and fish due to bioaccumulation.
There are other ways I did not mention in here. You can visit the following website to get some details (Aquatic Ecology Lab, 2017):