216
P.E Landrum and S.w. Fisher
or from food are more relevant to long-term ecosystem dynamics. Dietary accumulation requires contaminant desorption from a lipophilic site in food into water
in the gut, then subsequently to a lipophilic phase in the intestinal membrane. This
two-phase process is less kinetically favorable and less predictable than a simple
water-to-lipid partitioning. In addition, long-term assays are needed to quantify
bioaccumulation and trophic transfer. However, it now appears that bioaccumulation is a dominant process at lower trophic levels, where benthic invertebrates
play an ecologically pivotal role in removing contaminants from storage in sediments and funneling the contaminants into aquatic food chains (Landrum and
Robbins, 1990; Swartz and Lee, 1980). Subsequent trophic transfer of contaminants following lower-tier food chain introduction may account for as much as
90-99% of the contaminant load in top predators (Thomann, 1989; Rubenstein et
aI., 1984; Thomann and Connolly, 1984). It is now acknowledged, in a large
number of studies that have used organisms as diverse as invertebrates and fish,
that diet is the primary source of contaminant for highly lipophilic (log Kow >5),
nonmetabolized chemicals (Landrum et aI., 1991; Rasmussen et a!., 1990).
Establishing the reality of trophic transfer of contaminants in food chains has
important implications for hazard assessment. For example, if the primary source
of contaminant accumulation is not water. as suggested by the proponents of
bioconcentration, then merely removing contaminants from the water column
cannot necessarily be equated with a reduction in hazard. Exposure of living
organisms to contaminants will continue as long as contaminants are present in
any biologically active medium. In addition, contaminants can move from aquatic
to terrestrial environments via consumption of contaminated prey. Food chain
length is also important when trophic transfer is accompanied by biomagnification. Lengthening the food chain by introducing species can exacerbate this problem (Rasmussen et aI., 1990). Thus, exposure of humans and fish-eating wildlife
is, thus, an ongoing health concern (Rasmussen et aI., 1990).
9.6.2. Role of Lipids in Food Chain Accumulation
9.6.2.1. Fugacity Model
In theory, the type and amount of lipid in an animal should determine the extent to
which a lipid-soluble contaminant will accumulate at each trophic level in a food
chain in relation to the chemical activity of the contaminant in the source compared with the organism (the sink) in question. However, to accurately assess the
contaminant movement via ingestion, it is necessary to account for lipid levels in
both the food (the source of contamination) and the predator (the sink). Additionally, several factors, which do not directly relate to lipid levels, are potentially
very important in determining food chain accumulation. These include feeding
rates, digestibility of prey, gut retention time, length and morphology of the
digestive tract, the rate of fecal egestion, and the growth rate and age of the
organism (Sijm et aI., 1992; Clark and Mackay, 1991; Thomann, 1989; Serafin,
1984). These factors may obscure the role of lipids in trophic transfer.
P.E Landrum and S.w. Fisher
or from food are more relevant to long-term ecosystem dynamics. Dietary accumulation requires contaminant desorption from a lipophilic site in food into water
in the gut, then subsequently to a lipophilic phase in the intestinal membrane. This
two-phase process is less kinetically favorable and less predictable than a simple
water-to-lipid partitioning. In addition, long-term assays are needed to quantify
bioaccumulation and trophic transfer. However, it now appears that bioaccumulation is a dominant process at lower trophic levels, where benthic invertebrates
play an ecologically pivotal role in removing contaminants from storage in sediments and funneling the contaminants into aquatic food chains (Landrum and
Robbins, 1990; Swartz and Lee, 1980). Subsequent trophic transfer of contaminants following lower-tier food chain introduction may account for as much as
90-99% of the contaminant load in top predators (Thomann, 1989; Rubenstein et
aI., 1984; Thomann and Connolly, 1984). It is now acknowledged, in a large
number of studies that have used organisms as diverse as invertebrates and fish,
that diet is the primary source of contaminant for highly lipophilic (log Kow >5),
nonmetabolized chemicals (Landrum et aI., 1991; Rasmussen et a!., 1990).
Establishing the reality of trophic transfer of contaminants in food chains has
important implications for hazard assessment. For example, if the primary source
of contaminant accumulation is not water. as suggested by the proponents of
bioconcentration, then merely removing contaminants from the water column
cannot necessarily be equated with a reduction in hazard. Exposure of living
organisms to contaminants will continue as long as contaminants are present in
any biologically active medium. In addition, contaminants can move from aquatic
to terrestrial environments via consumption of contaminated prey. Food chain
length is also important when trophic transfer is accompanied by biomagnification. Lengthening the food chain by introducing species can exacerbate this problem (Rasmussen et aI., 1990). Thus, exposure of humans and fish-eating wildlife
is, thus, an ongoing health concern (Rasmussen et aI., 1990).
9.6.2. Role of Lipids in Food Chain Accumulation
9.6.2.1. Fugacity Model
In theory, the type and amount of lipid in an animal should determine the extent to
which a lipid-soluble contaminant will accumulate at each trophic level in a food
chain in relation to the chemical activity of the contaminant in the source compared with the organism (the sink) in question. However, to accurately assess the
contaminant movement via ingestion, it is necessary to account for lipid levels in
both the food (the source of contamination) and the predator (the sink). Additionally, several factors, which do not directly relate to lipid levels, are potentially
very important in determining food chain accumulation. These include feeding
rates, digestibility of prey, gut retention time, length and morphology of the
digestive tract, the rate of fecal egestion, and the growth rate and age of the
organism (Sijm et aI., 1992; Clark and Mackay, 1991; Thomann, 1989; Serafin,
1984). These factors may obscure the role of lipids in trophic transfer.