Environmental Engineering Reference
In-Depth Information
and make them more available to biota. Another thorny issue
is where to put the contaminated sediments once they have
been dredged up from the bottom. Solving these problems is
a major reason for long delays in dredging for deepening ship
channels and for cleanups of toxic hot spots. Organisms can
take up or bioaccumulate chemicals from the environment.
Once taken up into the body, the chemicals can exert toxic
effects.
How do chemicals get into marine animals?
Aquatic animals take pollutants into their body through the
skin, gills, and digestive tract, and excrete them in their waste
or expel them through the gills. When the rate of uptake is
greater than removal, the chemical builds up in the body.
Chemicals that have low solubility in water and bind to sedi-
ments tend to accumulate to greater concentrations in organ-
isms, especially in their fatty tissues. Chlorinated hydrocarbon
pesticides, polychlorinated biphenyls (PCBs), and methyl-
mercury are among those toxic substances with low water
solubility that concentrate in organisms and are not readily
metabolized or excreted.
Contaminants are transferred through food webs from
prey to predator (trophic transfer), and some chemicals tend to
become more concentrated during this process—a phenome-
non called biomagnification. Persistent organic chemicals like
PCBs and DDT, as well as methylmercury, tend to build up or
biomagnify as they go from prey to predator, causing the larg-
est, long-lived top predator to have the highest levels (Figure
1.3). An animal in a polluted area accumulates toxic chemicals
from each item of contaminated food that it eats; concentra-
tions are higher in consumers than in their food, and are high-
est in the top carnivores such as large fish, fish-eating birds,
marine mammals, and humans. Because of biomagnification,
methylmercury levels can be quite high in large carnivorous
fish like swordfish and tuna, even though they live in the open
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