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

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.