Okay, well I did my own research and the references you guys post #5, 7, and 8 are books and not actual studies.

I did find one study which states that Kelp absorb mercury, but it doesn't seem to be available online, and might be originally written in Japanese. I wish we could see the actual paper, to see how effective Kelp is at absorbing mercury.

Anyway, here is the study...

Bladderwrack (Fucus vesiculosus), Kelp (Laminaria sp.) and Sargassum also contain Calcium and sodium alginates, that gel-like substance we’ve discovered binds to heavy metals such as lead, mercury, cadmium, barium, radium, plutonium, strontium and cesium in the gastrointestinal tract to form insoluble salts that are excreted in the stools (Tanaka, Y. et al. "Application of algal polysaccharides as in vivo binders of metal pollutants." Proc Seventh Int Seaweed Symp, 602-607, Wiley and sons, 1972.).