9. Bioaccumulation and Trophic Transfer of Organic Contaminants
223
dissolution in an aqueous phase. Lipids are then digested and absorbed across the
gut. The concentration and fugacity of the contaminants remaining in the gut then
increases, providing the diffusive force needed to transfer the contaminants, as
single molecules, into the organism. Thereafter, the contaminants may become
reassociated with the products of lipid digestion, particularly triglycerides or
lipoproteins, in which form they may circulate in the blood prior to deposition and
storage in adipose tissues. Although both diffusion and lipid co-assimilation are
components of the revised model, diffusion is identified as the rate-limiting step in
the transfer process. As a consequence, the actual magnification of the contaminant residue level is seen as taking place in the GIT, not in the consumer's tissues
(Gobas et aI., 1993b).
9.7.3. Current Issues in Biomagnification and Relationship
to Lipids
Despite the availability of a detailed mechanism to explain biomagnification and
an abundance of residue data demonstrating biomagnification for a limited group
of contaminants with fairly specific physicochemical requirements, the reality of
biomagnification continues to be questioned. LeBlanc (1995) has suggested that
much of what is misinterpreted as biomagnification is, in fact, simply bioconcentration. In support of this hypothesis, LeBlanc notes, in his limited survey, that
lipid levels tend to increase with each trophic levels on a wet weight basis, lipids
in phytoplankton average 0.5% (n = 1); in invertebrates, lipids average 1.8% (n =
8); and in fish, lipids average 5.4% (n = 10) (LeBlanc, 1995). Thus, contaminant
loads would be expected to be higher in each successive trophic level due simply
to passive uptake from water. Additionally, LeBlanc (1995) argues that the composition of lipid changes between trophic levels. If these compositional changes
cause the fugacity capacity of lipids at higher trophic levels to increase, then even
an increase in lipid levels would not be required to cause an increase in bioconcentration (or bioaccumulation). Finally, LeBlanc (1995) argues that differences
in lipid elimination mechanisms between trophic levels could account for elevated
contaminant loads in top trophic levels. That is, animal size tends to increase with
trophic level. In addition, elimination of contaminants from lipid storage (and
subsequent excretion from the body) requires that the contaminant be removed
from lipid storage and transferred to an excretory organ, most likely the gill
(respiratory organ) or liver (or functional equivalent). Liver cells have specific
membrane sites across which contaminants must pass, and these elimination sites
constitute a small fraction of the total organ surface area. As animal size increases,
the relative abundance of storage-to-elimination sites rises. Also, as organism size
increases, the relative size of the respiratory organ decreases. As a result, excretion via passive diffusion either via a liver function or through the respiratory
system is much lower in large animals than in smaller animals at lower trophic
levels. Similar findings of limited biomagnification were presented for freshwater
benthic organisms (Bierman, 1990).
223
dissolution in an aqueous phase. Lipids are then digested and absorbed across the
gut. The concentration and fugacity of the contaminants remaining in the gut then
increases, providing the diffusive force needed to transfer the contaminants, as
single molecules, into the organism. Thereafter, the contaminants may become
reassociated with the products of lipid digestion, particularly triglycerides or
lipoproteins, in which form they may circulate in the blood prior to deposition and
storage in adipose tissues. Although both diffusion and lipid co-assimilation are
components of the revised model, diffusion is identified as the rate-limiting step in
the transfer process. As a consequence, the actual magnification of the contaminant residue level is seen as taking place in the GIT, not in the consumer's tissues
(Gobas et aI., 1993b).
9.7.3. Current Issues in Biomagnification and Relationship
to Lipids
Despite the availability of a detailed mechanism to explain biomagnification and
an abundance of residue data demonstrating biomagnification for a limited group
of contaminants with fairly specific physicochemical requirements, the reality of
biomagnification continues to be questioned. LeBlanc (1995) has suggested that
much of what is misinterpreted as biomagnification is, in fact, simply bioconcentration. In support of this hypothesis, LeBlanc notes, in his limited survey, that
lipid levels tend to increase with each trophic levels on a wet weight basis, lipids
in phytoplankton average 0.5% (n = 1); in invertebrates, lipids average 1.8% (n =
8); and in fish, lipids average 5.4% (n = 10) (LeBlanc, 1995). Thus, contaminant
loads would be expected to be higher in each successive trophic level due simply
to passive uptake from water. Additionally, LeBlanc (1995) argues that the composition of lipid changes between trophic levels. If these compositional changes
cause the fugacity capacity of lipids at higher trophic levels to increase, then even
an increase in lipid levels would not be required to cause an increase in bioconcentration (or bioaccumulation). Finally, LeBlanc (1995) argues that differences
in lipid elimination mechanisms between trophic levels could account for elevated
contaminant loads in top trophic levels. That is, animal size tends to increase with
trophic level. In addition, elimination of contaminants from lipid storage (and
subsequent excretion from the body) requires that the contaminant be removed
from lipid storage and transferred to an excretory organ, most likely the gill
(respiratory organ) or liver (or functional equivalent). Liver cells have specific
membrane sites across which contaminants must pass, and these elimination sites
constitute a small fraction of the total organ surface area. As animal size increases,
the relative abundance of storage-to-elimination sites rises. Also, as organism size
increases, the relative size of the respiratory organ decreases. As a result, excretion via passive diffusion either via a liver function or through the respiratory
system is much lower in large animals than in smaller animals at lower trophic
levels. Similar findings of limited biomagnification were presented for freshwater
benthic organisms (Bierman, 1990).