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.