220
P.P. Landrum and S.w. Fisher
9.6.3.2. Miscellaneous Factors Affecting Assimilation
The relationship between contaminant characteristics and AE can be modified by
a variety of factors. The feeding rate and the amount of food ingested, for instance,
can exert considerable influence on AE. When identical amounts of contaminants
were fed to guppies, Poecilia reticulata, in two different volumes of food, the AE
appeared to be lower in the case in which more food was used (Clark and Mackay,
1991). In reality, the AE declined because fecal egestion increased when a higher
food volume was used. Thus, the thermodynamic tendency for the contaminant to
move from food into the organism was not altered between exposures, but the
processing of the food was quicker in the case of the high food volume, resulting
in diminished contact time in the gut and an apparent reduction in absorption
efficiency. Gut retention time is a key factor in determining absorption. Bruner et
al. (1994b) found that AEs of several contaminants from sediment into zebra
mussels were much lower than AE of the same contaminants from algae in part
because the residence time of the sediment in the GIT was much lower.
Gut morphology can also be important in determining AE. For birds, the
proximal part of the GIT appears to absorb more contaminants (Serafin, 1984).
This may indicate a reduction in absorption efficiency in the distal portions of the
GIT. Because the fugacity of the contaminant may be highest when the greatest
digestion and removal of lipid has occurred in distal portion of the GIT, AE may
decrease as a function of intestinal length.
In short, AE is known to be critical in assessing trophic transfer. Organism lipid
levels are clearly important in determining AE. However, a variety of other factors
that are not related to the adiposity of the organism is also influential. These
interacting factors may obscure the relationship between AE and lipids.
9.7. Biomagnification and Organism Lipids
9.7.1. Is Biomagnification Real?
The issue of biomagnification has been a thorny one for years and has always
revolved around lipid levels of each food chain element. An early report of
biomagnification was the description of DDT levels in a Lake Michigan food
chain (Harrison et aI., 1970). In a simple food chain consisting of sediment,
amphipods, fish, and herring gulls, they detected DDT concentrations of 14, 410,
3,000-6,000, and 99,000 ppb, respectively. The increase in DDT concentration
with each trophic level was attributed to the fact that biomass conversion at each
trophic link was <50% whereas DDT transfer was close to 80%. The concentration of DDT, thus, increased with each successive food chain link. Additional
concentrating mechanisms were later identified. These included resistance to
metabolism, high lipid solubility, and increasing lipid levels with each trophic link
(Bierman, 1990).
Early reports of biomagnification were contested as a greater mechanistic understanding of the processes involved in accumulation evolved. In particular, the
P.P. Landrum and S.w. Fisher
9.6.3.2. Miscellaneous Factors Affecting Assimilation
The relationship between contaminant characteristics and AE can be modified by
a variety of factors. The feeding rate and the amount of food ingested, for instance,
can exert considerable influence on AE. When identical amounts of contaminants
were fed to guppies, Poecilia reticulata, in two different volumes of food, the AE
appeared to be lower in the case in which more food was used (Clark and Mackay,
1991). In reality, the AE declined because fecal egestion increased when a higher
food volume was used. Thus, the thermodynamic tendency for the contaminant to
move from food into the organism was not altered between exposures, but the
processing of the food was quicker in the case of the high food volume, resulting
in diminished contact time in the gut and an apparent reduction in absorption
efficiency. Gut retention time is a key factor in determining absorption. Bruner et
al. (1994b) found that AEs of several contaminants from sediment into zebra
mussels were much lower than AE of the same contaminants from algae in part
because the residence time of the sediment in the GIT was much lower.
Gut morphology can also be important in determining AE. For birds, the
proximal part of the GIT appears to absorb more contaminants (Serafin, 1984).
This may indicate a reduction in absorption efficiency in the distal portions of the
GIT. Because the fugacity of the contaminant may be highest when the greatest
digestion and removal of lipid has occurred in distal portion of the GIT, AE may
decrease as a function of intestinal length.
In short, AE is known to be critical in assessing trophic transfer. Organism lipid
levels are clearly important in determining AE. However, a variety of other factors
that are not related to the adiposity of the organism is also influential. These
interacting factors may obscure the relationship between AE and lipids.
9.7. Biomagnification and Organism Lipids
9.7.1. Is Biomagnification Real?
The issue of biomagnification has been a thorny one for years and has always
revolved around lipid levels of each food chain element. An early report of
biomagnification was the description of DDT levels in a Lake Michigan food
chain (Harrison et aI., 1970). In a simple food chain consisting of sediment,
amphipods, fish, and herring gulls, they detected DDT concentrations of 14, 410,
3,000-6,000, and 99,000 ppb, respectively. The increase in DDT concentration
with each trophic level was attributed to the fact that biomass conversion at each
trophic link was <50% whereas DDT transfer was close to 80%. The concentration of DDT, thus, increased with each successive food chain link. Additional
concentrating mechanisms were later identified. These included resistance to
metabolism, high lipid solubility, and increasing lipid levels with each trophic link
(Bierman, 1990).
Early reports of biomagnification were contested as a greater mechanistic understanding of the processes involved in accumulation evolved. In particular, the