Polar Bears are more an more affected by mercury pollution. How is this happening?

First of all, it's important to establish from where the Polar Bear is feeding.The result was that from 2 cardinal food chains the Polar Bear procures it's nutrients, mainly:phytoplankton (microscopic plants which are floating on the ocean's surface) and algae.

According to studies, the phytoplankton is the principal source of nutrients for Polar Bear, but also having a higher amount of mercury than algae.

The dependency of the Polar Bear of ingestion of increasingly large phytoplankton quantities is the result of global warming, thus, the amount of mercury ingested is worrisome. 

It is not negligible the fact that ice and snow have their proven contribution to deposition of mercury, substance which is found in environment, naturally, as well as from sources generated by mankind. The mutation of mercury into the poisonous methylmercury is happening in microorganisms which, after that, are stored in fish and animals.

The dependency of the Polar Bear of ingestion of increasingly large phytoplankton quantities is the result of global warming, thus, the amount of mercury ingested is worrisome.