Hypoxia in the Baltic Sea: Biogeochemical Cycles, Benthic Fauna, and Management

TLDR

Hypoxia in the Baltic Sea has expanded from <10 000 km² before 1950 to >60 000 km² since 2000, driven by increased nutrient inputs, with sediments shifting from nitrogen removal to release, equal water‑column and sediment denitrification, and phosphorus mainly buried in organic matter and Fe‑phosphate minerals, while benthic ecosystem services are lost. When oxic conditions return, a large fraction of organic phosphorus is remineralized and bound to iron oxides, which are then released back into the water column when hypoxia resumes. The study provides quantitative data on nutrient release and recycling under different conditions, informing decision‑support tools for the Baltic Sea Action Plan.

Abstract

Hypoxia has occurred intermittently over the Holocene in the Baltic Sea, but the recent expansion from less than 10 000 km2 before 1950 to >60 000 km2 since 2000 is mainly caused by enhanced nutrient inputs from land and atmosphere. With worsening hypoxia, the role of sediments changes from nitrogen removal to nitrogen release as ammonium. At present, denitrification in the water column and sediments is equally important. Phosphorus is currently buried in sediments mainly in organic form, with an additional contribution of reduced Fe-phosphate minerals in the deep anoxic basins. Upon the transition to oxic conditions, a significant proportion of the organic phosphorus will be remineralized, with the phosphorus then being bound to iron oxides. This iron-oxide bound phosphorus is readily released to the water column upon the onset of hypoxia again. Important ecosystems services carried out by the benthic fauna, including biogeochemical feedback-loops and biomass production, are also lost with hypoxia. The results provide quantitative knowledge of nutrient release and recycling processes under various environmental conditions in support of decision support tools underlying the Baltic Sea Action Plan.