Sources of faecal pollution in coastal recreational waters may be identified by analysing different host associated microorganisms or molecular markers. However, the microbial targets are often present at low numbers in moderately impacted waters, and often exhibit significant temporal and spatial variability in waters with fluctuating faecal loads. This patchy occurrence can limit successful detection of relevant targets in microbial source tracking studies. In this study, we explored the possibility for using the blue mussel (Mytilus edulis) as a biosampler for accumulation of faecal bacteria relevant for microbial source tracking. Non-contaminated blue mussels were transferred to three coastal recreational waters affected by faecal pollution of unknown origin. Molecular markers associated with animal and human waste were targeted by PCR and compared in seawater and mussel samples. The results demonstrated that transplanted mussels in simple enclosures accumulated and retained elevated levels of molecular markers associated with different types of faecal pollution. The targets included a novel putative human associated E. coli subgroup B2 VIII clone, and animal and human associated markers in enterococci (esp, M19, M66, M90, and M91). Human (sewage) associated markers including esp and M66 were sometimes not detectable in seawater samples despite known wastewater contamination, whereas the markers were detectable in mussels. We suggest that transplanted mussels should be considered as potential biosamplers in studies focusing on identifying source of faecal pollution in low or moderately impacted recreational waters. Bioaccumulation of molecular markers in mussels for several days may represent the water quality better than traditional grab samples from the water column.

Aims:  Micro‐organisms and molecular markers for microbial source tracking (MST) in coastal waters are often present at low numbers, and often exhibit significant variability in time and space. In this study, we investigated the uptake, accumulation, and persistence of human associated Enterococcus in the mussel Mytilus edulis. Methods and Results:  The human associated molecular markers esp in Enterococcus faecium, and M66 in Enterococcus faecalis were targetted by PCR in seawater and mussel samples from coastal sites affected by sewage contamination. Both native mussels and mussels transplanted from pristine to polluted sites were included. The results showed that the esp and M66 markers were often not detectable in seawater whereas mussels were enriched in the markers. Human associated E. faecalis accumulated rapidly in M. edulis, and reached maximum levels after 4–6 h with concentration 30–300 times greater than in the surrounding seawater. Enterococcus faecalis retained in M. edulis showed a survival comparable to planktonic E. faecalis in seawater with half lives of 30 and 22 h, respectively. Human associated markers remained detectable for 120 h in M. edulis after faecal contamination. Conclusions:  The study demonstrated that native and transplanted M. edulis can accumulate and retain human associated molecular markers relevant for MST. Significance and Impact of the Study:  Mussels should be considered as additional targets in MST studies in coastal waters.