Assessing microplastic contamination on the Finnish coastline applying the mussel caging approach

Transplantation of bioindicator organisms, a widely used method in monitoring of chemical pollution, was tested to be used for the monitoring of concentrations of microplastics (MP) and their potential biological effects. Previous data on chemical contaminants and selected biomarker responses measured in mussels (Mytilus spp.) were combined with new MP observations in mussel soft tissues after a two-month caging period along the coastline of Finland (Baltic Sea). The number of MP observed in the tissues was on average 20.0 ± 41.1 per mussel and 168.9 ± 236.8 per g mussel dry weight. Polyethylene terephthalate and polyethylene were the two main polymer types observed. The Integrated Biomarker Index was elevated in mussels at the caging site with the highest tissue concentrations of MP. However, the relationship between tissue concentrations of MP and biomarker responses was not clear at all sites. Further, variability in tissue concentrations of MP between the sites was high without a clear link to local contamination, suggesting that the mussel caging approach should be studied more before accepting it as a monitoring method for detecting spatial variability in concentrations of MP.