Sampling procedure and sites description
Västervik Gåsfjärden (57°34.35’N, 16°34.98’E) is a semi-enclosed inlet on the Swedish coast of the Baltic Sea. It has been exposed to varying mining activity from the Solstad copper mine from the 17th to the early 20th century (Söderhielm & Sundblad, 1996). We have previously documented detailed sedimentological features of Gåsfjärden and inferred past environmental changes from this data (Ning, Ghosh, et al., 2016; Ning et al., 2018; Ning, Tang, et al., 2016). In this study, we use the unpolluted Gropviken (58°19.92N 16°42.35’E) inlet as a reference site since it also has hypoxic and laminated sediments (Karlsson et al., 2010), but to our knowledge it has not been exposed to mining activity. The sites are ~100 km apart and have similar bathymetry and a cross-section of one to three km. Salinity was between 6 and 7 at the time of sampling, with a thermocline around six meters depth and a 10-20% decrease in oxygen concentration towards the bottom. The sediment is generally not bioturbated, and lamination patterns suggest frequent hypoxia periods (Ning, Ghosh, et al., 2016). Throughout this manuscript, we refer to Västervik Gåsfjärden [VG] as the ’mining inlet’ and Gropviken [GP] as the ’reference inlet’, while strains are named using the respective acronym.