How does the ocean get so much Mercury?

Mercury enters the oceans from:

* Natural sources * industrial processes * Coal burning Once in the ocean it becomes methylated in biological processes in the sediments and is bioaccumulated in the food chain until it accumulates in top predators (e.g. tuna) where it can reach unhealthy levels.

The average Hg concentration in seawater is 0.019 ug/L (Britush coastal waters)