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 (O 2 ) 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.

Seasonal oxygen, nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns

“…Strongly reducing conditions and dissolved sulfide appearing close to the sediment surface in the inner part of the estuary (Thang et al 2013) would have quickly consumed the oxygen present in the near bottom water. In turn, this would have limited the formation of iron oxyhydroxides and, therefore, limited the scavenging of dissolved P. The high fluxes (and the low N/P ratios) we measured in the inner Himmerfjärden can be thus related to high organic matter supply, high mineralization rates, and low retention of P by metal oxides as was suggested for other coastal areas in the Baltic Sea (Lukkari et al 2009). …”

Beyond the Fe-P-redox connection: preferential regeneration of phosphorus from organic matter as a key control on Baltic Sea nutrient cycles

“…Similarly, Gustafsson and Stigebrandt (2007) assume the uptake or release of P by sedimentary iron oxyhydroxides to account for any deviation in water column phosphate concentrations away from the value predicted by remineralization of organic matter in Redfield proportions. However, the behavior of phosphorus in Baltic Sea sediments has only recently begun to be studied in detail Lukkari et al, 2009;Mort et al, 2010), and a large amount remains to be learned about the response of phosphorus regeneration, and burial, to spatial and temporal hydrographic variability. In particular, the respective roles of the iron oxyhydroxide-P (or "Fe-P") interaction, and preferential remineralization of P during anaerobic degradation of organic matter, are poorly constrained.…”

Environmental Impacts—Marine Biogeochemistry

“…In the sediment pore water, PO 4 diffuses upwards into the oxic sediment layer and can be bound to Fe-oxyhydroxides at the surface (Mortimer 1971;Krom and Berner 1981). Fe-bound P is abundant in surface sediments overlain by oxic bottom waters in the Baltic Sea (Balzer 1986;Jensen and Thamdrup 1993;Lukkari et al 2009;Mort et al 2010). Furthermore, in oxic conditions, activity of benthic fauna oxygenates the sediment and enhances binding of PO 4 .…”

“…If vertical mixing in the water column supplies released PO 4 to the productive layer, it further enhances eutrophication, leading to yet another progression of anoxia. However, if the sea area has been anoxic for a long period, the amount of Fe-bound PO 4 sensitive to reduction-induced release may have already been diminished (Lukkari et al 2009;Jilbert et al 2011;Malmaeus and Karlsson 2012). Furthermore, under anoxia, regeneration of organic P may be enhanced (Ingall et al 1993;Ingall and Jahnke 1994), for example via release of organic P compounds from Fe-oxyhydroxides (Suzumura and Kamatani 1995) and release of PO 4 from microbial sources (Gächter et al 1988;Ingall and Jahnke 1994;Hupfer et al 2004).…”