Plain Language Summary
Hypoxia in the coastal ocean is expanding worldwide, and inputs of nutrients from waste water and agriculture are mainly to blame. Nutrients feed plankton blooms, which then consume oxygen as they decay. Because much of this decay takes place at the seafloor, sediments play an important role in deoxygenation, and in the recycling of nutrients in coastal regions. It is known that the amount of iron oxides in sediments has a strong effect on nutrient recycling during algal decay. Iron oxide-rich sediments in healthy oxygen-rich areas can trap phosphorus, a key nutrient element from decaying algae. In contrast, iron oxide-poor sediments in deoxygenated areas release phosphorus back to the water to fuel more algal growth. Our study shows that changes in the distribution of iron oxides between deep and shallow areas of the Baltic Sea led to self-sustaining variability (oscillations) in oxygen stress on decadal timescales during past intervals in the Sea’s 8000-year history. We use a model to demonstrate that under certain conditions of climate and nutrient pressure, such variability may occur naturally, and therefore may influence the future recovery of the Baltic Sea from its present nutrient-rich state.