Application of an enzyme digestion method reveals microlitter in Mytilus trossulus at a wastewater discharge area

Highlights: • Elevated microlitter levels were found in wastewater recipient blue mussels. • Non-synthetic fibres dominated all samples. • Enzyme digestion method recommended for bivalve tissue digestion. • Mussel dissection prior to digestion exposes samples to airborne contamination. - Abstract: The ingestion of microlitter by blue mussels (450) was studied at a wastewater recipient area in the Baltic Sea. The mussel soft tissues were digested using enzymatic detergents and the detected litter particles characterized with FT-IR imaging spectroscopy. Microlitter concentration in seawater and WWTP effluent were also measured. Microlitter was found in 66% of the mussels. Mussels from the WWTP recipient had higher microlitter content compared to those collected at the reference site. Plastics made up 8% of all the analysed microlitter particles. The dominating litter types were fibres (~90% of all microlitter), 42% of which were cotton, 17% linen, 17% viscose and 4% polyester. The risk of airborne contamination during laboratory work was lowered when mussels were digested with their shells on instead of dissecting them first. The approach was found applicable and gentle to both non-synthetic and synthetic materials including fragile fibres.