McKay, Donald. "Front matter" Multimedia Environmental Models ...

8.9.4 Model
Available on the website are Windows- and DOS-based Fish models that calculate
the fish fugacity, concentration and the various flux terms from input data on
the chemical’s properties, concentration in water, and various physiological constants.
They include a “bioavailability” calculation in the water by estimating sorption
to organic matter in particles. The models are particularly useful for exploring
how variation in K OW and metabolic half-life affect bioaccumulation, and they show
the relative importance of food and water as sources of chemical. An overall residence
time is calculated that indicates the time required for contamination or decontamination
to take place.
8.9.5 Food Webs
The fish bioaccumulation model can be applied to an aquatic food web starting
with water and then moving successively to phytoplankton, zooplankton, invertebrates,
small fish, and to various levels of larger fish. Each level becomes food for
the next higher level. If K OW is relatively small, i.e., 10 7 , the E A term becomes small, uptake is slowed, and the growth
and metabolism terms become critical. Association with suspended organic matter
in the water column becomes important, i.e., “bioavailability’ is reduced. A falloff
in observed BCFs is (fortunately) observed for such chemicals; thus, there
appears to be a “window” in K OW of about 10 6 to 10 7 in which bioaccumulation is
most significant and most troublesome. Chemicals such as DDT and PCBs lie in
this “window.” This issue has been discussed in detail and modeled by Thomann
(1989).
Several features of food web biomagnification are worthy of note. Humans
usually eat creatures close to the top of food webs, and strive to remain at the top
of food webs, avoiding being eaten by other predators. Fish consumption is often
the primary route of human exposure to hydrophobic chemicals. Creatures high
in food webs are invaluable as bioindicators or biomonitors of contamination of
lakes by hydrophobic chemicals. But to use them as such requires knowledge of
the D values, especially the D value for metabolism. A convincing argument can
be made that, if we live in an ecosystem in which wildlife at all trophic levels is
thriving, we can be fairly optimistic that we humans are not being severely affected
by environmental chemicals. This is a (selfish) social incentive for developing,
testing, and validating better environmental fate models, especially those employing
fugacity.
A food web model treating multiple species can be written by applying the
general bioaccumulation equation to each species (with appropriate parameters).
The final set of equations for n organisms has n unknown fugacities that can be
solved sequentially, starting at the base of the food web and proceeding to other
species, with smaller animals becoming food for larger animals.
©2001 CRC Press LLC