Assimilation efficiencies of chemical contaminants in aquatic invertebrates: A synthesis

Assimilation efficiencies of contaminants from ingested food are critical f or understanding chemical accumulation and trophic transfer in aquatic inve rtebrates. Assimilation efficiency is a first-order physiological parameter that can be used to systematically compare the bioavailability of differen t contaminants from different foods. The various techniques used to measure contaminant assimilation efficiencies are reviewed. Pulse-chase feeding te chniques and the application of gamma-emitting radiotracers have been inval uable in measuring metal assimilation efficiencies in aquatic animals. Unif orm radiolabeling of food is required to measure assimilation, but this can be difficult when sediments are the food source. Biological factors that i nfluence contaminant assimilation include food quantity and quality, partit ioning of contaminants in the food particles, and digestive physiology of t he animals. Other factors influencing assimilation include the behavior of the chemical within the animal's gut and its associations with different ge ochemical fractions in food particles. Assimilation efficiency is a critica l parameter to determine (and to make predictions of) bioaccumulation of ch emicals from dietary exposure. Robust estimates of assimilation efficiency coupled with estimates of aqueous uptake can be used to determine the relat ive importance of aqueous and dietary exposures. For bioaccumulation of met als from sediments, additional studies are required to test whether metals bound to the acid-volatile sulfide fraction of sediments can be available t o benthic deposit-feeding invertebrates, Most assimilation efficiency studi es have focused on chemical transfer in organisms at the bottom of the food chain; additional studies are required to examine chemical transfer at hig her trophic levels.