Although inorganic Hg species have severe health effects on humans, methylmercury is thought to be the highly toxic species involved in bioaccumulation in aquatic food chains. (pnas.org)
I fear that many hidden tripping points have already been passed eg increase in disease, demise of soil micro-organisms, slowing down of oceanic currents, melting ice, sea level rise, decline of aquatic food chains. (reneweconomy.com.au)