Enhanced sedimentary phosphorus release following a natural oxygenation event

Astrid Hylen1, Sebastiaan van de Velde2, Mingyue Luo3, Mikhail Y. Kononets4, Elin Almroth-Rosell5 and Per O J Hall4, (1)Universiteit Antwerpen, Department of Biology, Antwerpen, Belgium, (2)University of California, Riverside, Department of Earth and Planetary Sciences, Riverside, CA, United States, (3)Vrije Universiteit Brussel, Department of Chemistry, Brussels, Belgium, (4)University of Gothenburg, Department of Marine Sciences, Gothenburg, Sweden, (5)Swedish Meteorological and Hydrological Institute, Oceanography Research Dept., Norrköping, Sweden
Eutrophication is increasing oxygen depletion in coastal systems worldwide. A well-known example of this phenomenon is the Baltic Sea, which contains the world’s largest human-induced hypoxic-anoxic zone. Despite reduced nutrient inputs from land over the last decades, the size of this oxygen-depleted area has not decreased. The predominant cause is enhanced release of dissolved inorganic phosphorus (DIP) from sediments underlying anoxic bottom waters, which continues to stimulate primary production.

In 2015, a deep-water inflow from the North Sea ventilated parts of the hypoxic-anoxic areas in the Baltic Sea. This created an excellent opportunity to study in situ whether re-oxygenating the Baltic bottom water can stimulate sedimentary DIP retention. During four yearly expeditions in 2015-2018, we measured sediment-water DIP fluxes in situ using a benthic chamber lander and sampled the sediment for pore water DIP and solid phase phosphorus speciation.

Though a small amount of DIP adsorbed to iron oxides in the sediment while there was oxygen in the bottom water, most of this was released to the water column when anoxia returned the following year. Surprisingly, there was a transient but considerable increase in the efflux of DIP and other dissolved biogenic compounds from the sediment in the three years following the inflow. This increase was likely due to elevated rates of organic matter degradation, triggered by the oxygenation event. As a result, the net sedimentary DIP release was about 70-100 % per m2 higher over the three years following the transient re-oxygenation.

Contrary to previous assumptions, transient oxygenation of anoxic bottom waters can increase phosphorus release from the sediment. This enhanced recycling mechanism could play a role in the expected expansion of oxygen minimum zones in coastal areas worldwide.