A chemical journey through food webs: Improving predictions of bioaccumulation patterns.
Persistence, bioaccumulation and toxic properties of chemicals (“PBT”) have been raising regulatory flags for the past quarter century. Since the beginning the metric used to evaluate the bioaccumulation (“B”) property has been the bioconcentration factor (BCF), which is the ratio of chemical concentrations in biota to surrounding water at steady state. International standard protocols have been developed for the calculations of BCFs based on measured concentration data in fish and in the water that contained the chemical during an exposure experiment. Chemicals with BCF>5000 are labeled as a bioaccumulative (B) substance in many regulatory frameworks (EU REACH, US EPA, Environment Canada, Stockholm POPs Convention etc.).
A recent study by IVM researchers in cooperation with Prof. Frank Gobas of Simon Fraser University in Vancouver, Canada has closely examined the relationship of laboratory-derived BCF data to the actual bioaccumulation observed in a real-life food web. The overall bioaccumulation in food webs can be expressed with the ‘trophic magnification factor‘ (TMF). The Western Scheldt Estuary served as the study area, where benthic and pelagic food chains, sediment, water and suspended particulate matter were sampled and chemically analysed. A test set of environmental contaminants included the ‘classics’, such as pesticides and PCBs, but also ‘emerging’ chemical classes, such as phosphorous-based flame retardants never before measured in food chains.
This study set out to figure out how reliable the BCF is for predicting if bioaccumulation at different trophic levels of a food web and under what conditions BCF might under- or overpredict the field observations. It turns out that the BCF can do all three: underestimate, overestimate or get it more or less right depending on a variety of factors.
For some of the ‘classic’ contaminants like PCBs that biomagnify in food webs the BCF can be a good predictor and remains a useful parameter in bioaccumulation assessments. But when does the BCF fall short of predictions that are accurate enough to protect species in the environment from building up too much chemical residue in their bodies?
When higher predators are able to biotransform a substance, the BCF tends to overestimate the trophic magnification (i.e. false positives). Some polycyclic aromatic hydrocarbons and brominated flame retardants fall into this category. Sometimes low bioavailability contributes to low trophic magnification, especially in combination with metabolism. Much attention is being paid to the persistence (P) of chemicals as an indicator of risk that is perhaps more applicable and useful in a regulatory setting than B. For substances with low persistence, metabolites can be formed, some of which are also known to bioaccumulate. Hence next generation chemical risk assessments - if focusing strongly on persistence - may do well to include the persistence of metabolites.
Another important conclusion can be drawn from this study because it included air breathing species, which eliminate chemicals from their bodies to the air according to different rules than water breathers. In several cases, such as for perfluorinated chemicals, the relatively low BCF (in fish) leads to an underestimation of the transfer of these substances up the food chain to higher predators.
This study demonstrates the complexity of the routes and fates of chemicals in the real biological systems that chemical risk assessment and regulations are intended to protect. Since this study showed that BCF-based predictions are not always accurate enough in predicting bioaccumulation throughout the food chain, regulators should consider the TMF and air-breathers in their assessments rather than solely relying on the fish BCF. The TMF is closer to the “real thing” and it is more information-rich, while the BCF is a “surrogate” for bioaccumulation in ecosystems.