Factors affecting bioaccumulation of cyclic volatile methyl siloxanes in a sub-arctic bentho-pelagic food web

Abstract

Cyclic volatile methylsiloxanes (cVMS) are used in personal care products and are emitted to aquatic environments through wastewater effluents. Due to the high hydrophobicity of cVMS, sediments are expected to be an important source for exposure in benthic and bentho-pelagic food webs. However, details of exposure pathways have yet to be fully understood. The main objective of this study was to evaluate the role of sediments in the bioaccumulation of cVMS in Nordic environments, using a north-Norwegian lake (Lake Storvannet) as a case-study. Concentrations of cVMS were measured in benthos (chironomid larvae and the pea clam Pisidium sp.) and fish (three-spined sticklebacks [Gasterosteus aculeatus], Arctic char [Salvelinus alpinus], and brown trout [Salmo trutta]) from the lake. Existing measurements of polychlorinated biphenyls (PCBs 101, 118, 138, 153, 180) were used to evaluate a new bentho-pelagic version of the bioaccumulation model ACC-HUMAN (including a benthic filter feeder and a benthic predator/deposit feeder). The model predictions of PCB concentrations in char and trout were in better agreement with measurements (predictions were within a factor of 4 to measured concentrations) compared to predictions with the pelagic-only model (factor of 8 between predictions and measurements). Predictions improved most for char, which feed mainly on benthos in this lake. This suggests that the bentho-pelagic model is a better representation of the Storvannet ecosystem than the pelagic-only model. The model was next applied to evaluate the fate of cVMS in the Storvannet food web. Decamethylcyclopentasiloxane (D5) was the dominant cVMS congener in the lake, with measured concentrations of 2.0 ± 0.04, 17.8 ± 0.7, 3.2 ± 2.3, 3.8 ± 2.8, and 0.8 ± 0.6 μg/g lipid in chironomid larvae, Pisidium sp., sticklebacks, char, and trout, respectively. Concentrations were lower, and sometimes below limits of quantification, for octamethylcyclotetrasiloxane (D4) and dodecamethylcyclohexasiloxane (D6). Predicted cVMS concentrations in benthos were underestimated compared to measurements (ratio of 0.04 to 0.4 between predictions and measurements), while predicted cVMS concentrations in fish were in good agreement with measurements (ratio of 0.5 to 2.5). Model predictions were highly sensitive to the partition coefficients between organic carbon and water (KOC) and its temperature dependence. This is because the model predicts benthic organisms to be the main source of cVMS for fish, and KOC controls bioavailability of cVMS for benthos in the model. The model furthermore predicted considerable losses of cVMS through ventilation for fish, resulting from higher fugacity in the benthic-feeding fish than in lake water. As a result, model predictions were relatively insensitive to the biotransformation rate constants in fish.