Phosphorus speciation in sediments from the Baltic Sea, evaluated by a multi-method approach

Purpose Phosphorus (P) is a crucial element for living organisms at both land and sea, but simultaneously, it can cause environmental problems especially in marine ecosystems. The pathway of P from soils through riverine and coastal systems to sea sediments has not been clarified to date. Thus, the main aim of this study was to characterize P species in sediments along a gradient from the coast of Northern Germany to the Baltic Proper. Materials and methods Six sediment samples along a transect from river outlets into the Baltic Sea in northeastern Germany to the deep basins at the Baltic Proper were analyzed. Four complementary methods were applied to explore the different sedimentary P compounds: (i) the sequential P fractionation to extract P from sediment pools of different solubility; (ii) P K-edge X-ray absorption near edge structure (XANES) spectroscopy as an element-specific method for speciation analyses of complex environmental samples; (iii) P-31 nuclear magnetic resonance (P-31 NMR) spectroscopy as a capable technique distinguishing especially different organic P compounds based on their characteristic resonance frequencies and (iv) scanning electron microscopy (SEM) combined with energy-dispersive X-ray microanalyses (EDX) to identify certain solid particles and their elemental composition. Results and discussion Sequential P fractionation revealed a decrease in labile P pools (resin-P; NaHCO3-P) along with an increase in more stable P fractions (H2SO4-P; residual-P) from near-coastal sediments towards the Baltic Proper with increasing water depths of sediment deposits. In comparison, XANES analysis indicated a decline of Fe-associated P compounds in favor of Ca-bound P with increasing distance of sediments from the coastline into the Baltic Sea. Results of P-31 NMR spectroscopy showed that the variety of different mono- and diester P compounds decreased with increasing distance from the coast and that high proportions of orthophosphate (ortho-P) were present especially in greater water depths. The SEM-EDX analysis supported most of these results by retriving Fe phosphate particles especially in the sediments near the coast. Conclusions The integration of several P-specific methods enabled improved insights into P speciation. A trend towards more stable Ca-P compounds towards the Gotland basin was found with sequential fractionation and P-XANES. In the future, different types of sediments will be analyzed with respect to their interactions with adjacent soils to find out a common principle of P-transformations and derive approaches for capturing P before entering susceptible marine systems.