The chemical character and behaviour of phosphorus in poorly oxygenated sediments from open sea to organic-rich inner bay in the Baltic Sea

Abstract

The chemical composition and vertical distribution of phosphorus (P) in poorly oxygenated sediments in a continuum extending from the open Baltic Sea towards an organic-rich inner bay were characterized by sequential extraction to examine the potential for release of sediment P. The chemical composition of P was related to chemical and physical characteristics of the sediments and the chemistry of pore water and near-bottom water to better understand the behaviour of P. Sediment P increased towards the inner bay, and the concentration of organic matter appeared to dictate its composition: the dominance of apatite-P turned to dominance of organic P (OP). Sediment P burial and, thus, release from sediment P reserves varied depending on the chemical composition of P. Dissolved species at the sediment–water interface suggested fluctuating redox conditions that affect P binding at short time scale. Redox-sensitive, iron (Fe)-bound P was usually relatively low because of poor oxygen (O2) conditions, which emphasized the role of OP in P release. The results indicate that, over the long term, the abundant organic P reserve may support a significant continuing P release from hypoxic sediments in the severely eutrophied Gulf of Finland (GoF) because capture of P into Fe oxyhydroxides at the sediment surface is restricted. The average long-term minimum annual rate of P release from poorly oxygenated sediments below about 60 m depth in the GoF was approximated on the basis of the vertical distribution of sedimentary P forms and estimates of sedimentation rate.