The report summarises the long-term changes in the stable isotope composition of Blue mussels (Mytilus edulis) from Kvädöfjärden (1981-2017) and the spatial variation in the isotope composition in Blue mussels and the Baltic clam, Limecola balthica (syn. Macoma balthica), from 14 stations throughout the Baltic Proper (coast and offshore stations) and the west coast of Sweden.

Time series and the spatial variability of analysed isotopes in Blue mussels and the Baltic clam are presented.

The δ13C values were rather stable over time and the year-to-year variation corresponds well to observations in the Askö area, suggesting the two reference stations are rather similar regarding ultimate carbon sources. However, the pattern for δ15N increased over time in Kvädöfjärden (although not during the last 25 years). The difference in δ15N between years in Kvädöfjärden was sometimes equivalent to one trophic level (i.e. a trophic shift of 3.4 per mill).

Spatial comparisons of the isotope composition in mussels revealed large differences (equivalent to the trophic shift) among stations and between species, highlighting the need for site-specific isotope baselines when normalizing contaminant data to a specific trophic position according to the WFD (Water Framework Directive).