Of the many different toxic compounds present in aquatic ecosystems, the heavy metals
are considered by some to be the most hazardous. It is clear that the anthropogenic input
of heavy metals is much higher than the natural input. For example, anthropogenic input
is almost double the natural input of mercury, and for copper, lead, and zinc it is greater
by an order of magnitude. This is typical for the global distribution of heavy metals in
the hydrosphere. Thus, compared to other toxicants, they are of prime interest,
particularly in view of their high toxicity in relation to aquatic organisms (hydrobionts).
The environmental impact of metals is of particular concern because, unlike organic
compounds, they cannot be subject to chemical degradation beyond the elemental state;
they can only be redistributed between abiotic and biotic components and interact with
their components.