METALS - Marine Pollution: What Everyone Needs to Know - page 85

bound to the sulfide and is not available for bacteria to methyl-
ate. Nevertheless, mercury levels in the fish do exceed levels
that are considered safe, so fish advisories are posted through-
out the system warning people not to consume fish or crabs.
While there are other estuaries in the country that are highly
contaminated with toxic substances, this one was designated
by EPA at the time as the most Hg-contaminated one in the
nation.
How does the chemical form of the metal affect what it does?
Knowing the chemical form (species) of a metal is necessary
in order to understand its toxicity and the risk it poses. In
general for many metals, the free ion—for example, copper
with two positive charges (Cu 2+ )—is the most available and
toxic form of the metal found in the water. In aquatic environ-
ments copper exists in particulate, colloidal, and soluble states,
predominantly as metallic (Cu 0 ) and cupric copper (Cu 2+ ). It
forms complexes with both inorganic and organic molecules.
The toxicity of Cu is directly related to the free ion, as is the
toxicity of Cd, so measurements of total Cu or total Cd in the
water overestimate the amount that is bioavailable and poten-
tially toxic. Chromium 6+ is much more toxic than Cr 3+ . Organic
forms of mercury (e.g., methylmercury) and tin (e.g., tributyl-
tin) are far more toxic than the ionic forms.
Once metals are taken up into an organism, they may
be stored in granules within their cells or attached to
metal-binding proteins that keep the metals unavailable to
the animal (and out of trouble), but metals attached to these
proteins can get transferred to a predator that eats the organ-
ism. Feeding, or trophic transfer, is the most important way
that metals move up the food web into large fish and birds.
Plants generally pick up metals from the soil in which they
grow, and different species store different proportions in their
roots, stems, or leaves, and can pass metals along to animals
that consume them.