The chemical character and burial of phosphorus in shallow coastal sediments in the northeastern Baltic Sea Kaarina Lukkari Æ Mirja Leivuori Æ Henry Vallius Æ Aarno Kotilainen Received: 28 July 2008 / Accepted: 22 March 2009 / Published online: 8 April 2009 Ó Springer Science+Business Media B.V. 2009 Abstract The chemical composition and vertical distribution of sediment phosphorus (P) in shallow coastal sediments of the northeastern Baltic Sea (BS) were characterized by sequential extraction. Different P forms were related to chemical and physical properties of the sediments and the chemistry of pore water and near-bottom water. Sediment P composition varied among the sampling sites located in the Archipelago Sea (AS) and along the northern coast of the Gulf of Finland (GoF): the organic rich sites were high in organic P (OP), while apatite-P dominated in the area affected by sediment transpor- tation. Although the near-bottom water was oxic, the sediments released P. Release of P was most pronounced at the site with high sediment OP and reduced conditions in the sediment-water interface, indicating that P had its origins in organic sources as well as in reducible iron (Fe) oxyhydroxides. The results suggest that even though these coastal areas are shallow enough to lack salinity stratification typical for the brackish BS, they are vulnerable to seasonal oxygen (O 2 ) depletion and P release because of their patchy bottom topography, which restricts mixing of water. Furthermore, coastal basins accu- mulate organic matter (OM) and OP, degradation of which further diminishes O 2 and creates the potential for P release from the sediment. In these conditions, an abundance of labile OP may cause marked efflux of P from sediment reserves in the long-term. Keywords Baltic Sea Á Coastal sea Á Fractionation Á Organic matter Á Phosphorus Á Sediment Introduction Like many coastal sea areas, the Gulf of Finland (GoF), which is a continuum of the Baltic Proper (BP) in the northeastern Baltic Sea (BS), and the Archipelago Sea (AS), which contains thousands of small isles southwest of Finland, are eutrophied. Coastal sea areas receive most of the nutrients discharged from the terrestrial environment via rivers and estuaries (and groundwater discharge), and algal blooms often cause the most harm in coastal areas, hindering their recreational and economical use. In the GoF and AS, the main sources of phosphorus (P) are agriculture, municipal wastes, and fish farming (Bonsdorff et al. 1997; HELCOM 2003). However, P is also released from reserves accumulated in bottom sediments (e.g. Emeis et al. 2000). P bound to Present Address: K. Lukkari (&) Á M. Leivuori Finnish Institute of Marine Research, Erik Palme ´nin aukio 1, P.O. Box 140, 00251 Helsinki, Finland e-mail: kaarina.lukkari@ymparisto.fi H. Vallius Á A. Kotilainen Geological Survey of Finland, P.O. Box 96, 02151 Espoo, Finland 123 Biogeochemistry (2009) 94:141–162 DOI 10.1007/s10533-009-9315-y