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).