Bioavailability in Ecological Risk Assessment

Within the ecological risk-assessment framework, bio-availability processes are taken into account in exposure intake equations. Ecological risk assessment is complex as numerous organisms and physical-chemical processes must be considered to predict the impact of contaminants on the ecosystem. Within contaminant exposure and intake equations, typically two pathways are considered; direct contact with the environment and dietary intake. The influence of basic partitioning processes for metals or organic contaminants between the different environmental phases has been used to estimate exposure. These processes were reviewed in the section titled 'Effects of environmental chemistry on contaminant bioavailability'.

Dietary intake pathways incorporate all dietary exposure. Accumulation of contaminants from water into organisms is called bioconcentration. Bioconcentration is defined as the partitioning of a contaminant from the aqueous phase into an organism; typically, this occurs when uptake is greater than elimination. A consequence of contaminant storage by an organism is bioaccumulation. Bioaccumulation is the total amount of contaminant in the organism, the route of uptake including all forms of exposure such as dietary, water, and dermal. Depending on the storage mechanism, bioaccumulated contaminants may be transferred to higher-trophic-level organisms through predator-prey interactions. Biomagnification can occur when the concentration of the original contaminant available from the environment is less than the concentration found in the animal. Biomagnification occurs when bioaccumulation causes an increase in tissue concentration from one trophic level to the next. In contrast, biodilution, regulation of contaminant uptake through the food web, has been observed for some contaminants, especially metals (such as cadmium). Organism distribution and metabolism processes are of great importance in determining the overall effect of contaminants to an organism, and bioaccumulation and biomagnification processes relate to contaminant distribution on ecosystem-level scales. Approaches to determine the bioavailability of contaminants in lower-order animals, such as invertebrates, are rare. Generally, they are assumed to be 100% bioavailable. To efficiently generate quantitative exposure estimates and to accurately characterize risks posed by contaminants, bioavailability needs to be considered in the risk-assessment process.

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