Tracing organic matter and organic phosphorus dynamics via surface sediments in a deep boreal lake

Organic phosphorus (P) is the predominant form of sedimentary P in many lakes; however, its role in P exchange at the sediment–water interface remains difficult to quantify. We investigated the dynamics of the organic P pool and its constituent species and their contribution to P release from the upper reactive sediment layer of a deep mesotrophic boreal lake. Using the non-carbonaceous mineral component as a reference, we estimated a 25% reduction in organic matter (OM) and a 63% decline in organic P (on the basis of sequential extraction) by 13-cm sediment depth compared with the surface. P release from the organic P pool was estimated to be at least four times higher compared with the combined pool of inorganic aluminum-, iron-, and calcium-bound P. Solid-state 31P nuclear magnetic resonance (NMR) spectroscopy revealed that the predominant organic P species in the studied sediment were monoester phosphates and DNA-associated P. We propose that estimating the temporal dynamics of OM and organic P pools in upper reactive sediment layers can be refined by referencing concentrations to a relatively stable sediment component, such as the mineral fraction, under the assumption of consistent inputs of both organic and mineral matter during the time span over which the studied sediment layer accumulated.