9. Bioaccumu1ation and Trophic Transfer of Organic Contaminants
219
net movement of the contaminant between predator and prey, which appear to
have equal fugacity, becomes possible (LeBlanc, 1995; Gobas et aI., 1993a;
Thomann, 1989). In either mechanism, lipids become the driving force for the
bioaccumulation of contaminants from food.
Because lipids are the driving force behind trophic transfer (Gobas et aI., 1989),
it is important to account for the influence of both lipid in the food and lipid in the
organism, which may have contrasting effects. Organism lipid levels have been
previously discussed. How will contaminant transfer from a high-lipid food differ
from a low-lipid food, for instance? According to the fugacity concept, if the
primary phase responsible for dissolving the contaminant is lipid, then the
fugacity capacity of high-lipid food should be high, resulting in low fugacity at a
given food concentration. In other words, less contaminant should move into the
consumer's tissues from a high-lipid food than a low-lipid food. In a single study
of this issue, uptake of most hydrophobic contaminants from a low-fat diet
was much higher than from a high-fat diet (Gobas et aI., 1993b). Although the
fugacity-based explanation for this observation may be correct, in the case in
which the fugacities in the two food sources are equal, it may be true that the
digestibility of the high-lipid food is decreased and fugacity undergoes a greater
elevation for the low-lipid food during the transit through the GIT. Discerning the
role of food lipid in trophic transfer has been identified as a critical research need
(Clark and Mackay, 1991).
9.6.3. Factors Affecting Trophic Transfer
9.6.3.1. Assimilation Efficiency
Trophic transfer of contaminants is frequently assessed and quantified by using an
assimilation efficiency (AE) as the critical measurement. At its most basic level,
%AE is simply the ratio of the contaminant retained in the consumer's tissues
compared with the contaminant ingested (Harkey et aI., 1994b). A question in
anticipating how AE values will vary in nature is, how will AE be affected when
tracking the fate of contaminants with different hydrophobicities? Measurement
of the AE for organisms can be extremely difficult unless the food source can be
isolated and the exact contaminant concentration determined. For many selective
feeding organisms, such as benthic amphipods (e.g., Diporeia spp.), isolating the
food supply can be problematic and exact measurement of the AE difficult to
determine (Harkey et aI., 1994b). Thus, interpretation of measured AEs must be
performed with care.
Clearly, as the log Kow of a contaminant increases, there is a greater propensity
to partition into lipid-rich tissues. However, when the contaminant must be transferred from a lipid phase (food) instead of an aqueous phase, then the relationship
may be less straightforward, as described previously. Indeed, several studies
indicated that there is an inverse relationship between AE and contaminant hydrophobicity (Bierman, 1990; Muir and Yarechewski, 1988). Thus, as the hydrophobicity of the contaminant increases, the lower the tendency to move from food
lipid into the consumer's tissues.
219
net movement of the contaminant between predator and prey, which appear to
have equal fugacity, becomes possible (LeBlanc, 1995; Gobas et aI., 1993a;
Thomann, 1989). In either mechanism, lipids become the driving force for the
bioaccumulation of contaminants from food.
Because lipids are the driving force behind trophic transfer (Gobas et aI., 1989),
it is important to account for the influence of both lipid in the food and lipid in the
organism, which may have contrasting effects. Organism lipid levels have been
previously discussed. How will contaminant transfer from a high-lipid food differ
from a low-lipid food, for instance? According to the fugacity concept, if the
primary phase responsible for dissolving the contaminant is lipid, then the
fugacity capacity of high-lipid food should be high, resulting in low fugacity at a
given food concentration. In other words, less contaminant should move into the
consumer's tissues from a high-lipid food than a low-lipid food. In a single study
of this issue, uptake of most hydrophobic contaminants from a low-fat diet
was much higher than from a high-fat diet (Gobas et aI., 1993b). Although the
fugacity-based explanation for this observation may be correct, in the case in
which the fugacities in the two food sources are equal, it may be true that the
digestibility of the high-lipid food is decreased and fugacity undergoes a greater
elevation for the low-lipid food during the transit through the GIT. Discerning the
role of food lipid in trophic transfer has been identified as a critical research need
(Clark and Mackay, 1991).
9.6.3. Factors Affecting Trophic Transfer
9.6.3.1. Assimilation Efficiency
Trophic transfer of contaminants is frequently assessed and quantified by using an
assimilation efficiency (AE) as the critical measurement. At its most basic level,
%AE is simply the ratio of the contaminant retained in the consumer's tissues
compared with the contaminant ingested (Harkey et aI., 1994b). A question in
anticipating how AE values will vary in nature is, how will AE be affected when
tracking the fate of contaminants with different hydrophobicities? Measurement
of the AE for organisms can be extremely difficult unless the food source can be
isolated and the exact contaminant concentration determined. For many selective
feeding organisms, such as benthic amphipods (e.g., Diporeia spp.), isolating the
food supply can be problematic and exact measurement of the AE difficult to
determine (Harkey et aI., 1994b). Thus, interpretation of measured AEs must be
performed with care.
Clearly, as the log Kow of a contaminant increases, there is a greater propensity
to partition into lipid-rich tissues. However, when the contaminant must be transferred from a lipid phase (food) instead of an aqueous phase, then the relationship
may be less straightforward, as described previously. Indeed, several studies
indicated that there is an inverse relationship between AE and contaminant hydrophobicity (Bierman, 1990; Muir and Yarechewski, 1988). Thus, as the hydrophobicity of the contaminant increases, the lower the tendency to move from food
lipid into the consumer's tissues.