2.1 Study system
The Baltic Sea is a shallow (mean depth 58 m) temperate regional sea,
which displays a strong salinity gradient from marine salinity (30 g
kg−1) at the connection to the North Sea in the west
to near freshwater (2 g kg−1) in the north-eastern
inner part (Meier et al. 2007).
The Baltic environmental situation entails strong fluctuations in
temperature and light availability, a horizontal salinity gradient and
strong vertical stratification, low oxygen conditions in the deep parts
of the basins (Carstensen et al. 2015), and an abundant nutrient supply
due to eutrophication (Gustafsson et al. 2012), with seasonal minima
when nutrients are taken up during phytoplankton blooms. Due to an
accumulation of anthropogenic pressures on a level that is expected for
other coastal seas, the system has been coined a “time machine for the
future coastal oceans” (Reusch et al. 2018).