Abstract. The anthropogenically forced expansion of coastal hypoxia is a
major environmental problem affecting coastal ecosystems and biogeochemical
cycles throughout the world. The Baltic Sea is a semi-enclosed shelf sea
whose central deep basins have been highly prone to deoxygenation during its
Holocene history, as shown previously by numerous paleoenvironmental studies.
However, long-term data on past fluctuations in the intensity of hypoxia in
the coastal zone of the Baltic Sea are largely lacking, despite the
significant role of these areas in retaining nutrients derived from the
catchment. Here we present a 1500-year multiproxy record of near-bottom water
redox changes from the coastal zone of the northern Baltic Sea, encompassing
the climatic phases of the Medieval Climate Anomaly (MCA), the Little Ice Age
(LIA), and the Modern Warm Period (MoWP). Our reconstruction shows that
although multicentennial climate variability has modulated the depositional
conditions and delivery of organic matter (OM) to the basin the modern
aggravation of coastal hypoxia is unprecedented and, in addition to gradual changes
in the basin configuration, it must have been forced by excess human-induced
nutrient loading. Alongside the anthropogenic nutrient input, the progressive
deoxygenation since the beginning of the 1900s was fueled by the combined
effects of gradual shoaling of the basin and warming climate, which amplified
sediment focusing and increased the vulnerability to hypoxia. Importantly,
the eutrophication of coastal waters in our study area began decades earlier
than previously thought, leading to a marked aggravation of hypoxia in the
1950s. We find no evidence of similar anthropogenic forcing during the MCA.
These results have implications for the assessment of reference conditions
for coastal water quality. Furthermore, this study highlights the need for
combined use of sedimentological, ichnological, and geochemical proxies in
order to robustly reconstruct subtle redox shifts especially in dynamic,
non-euxinic coastal settings with strong seasonal contrasts in the bottom
water quality.
Impacts of changing society and climate on nutrient loading to the Baltic Sea