Pool of mobile and immobile phosphorus in sediments of the large, shallow Lake Peipsi over the last 100 years.

Temporal variations in sediment phosphorus (P) composition and mobility were estimated in surface sediments of accumulation (core PS509 44 cm) and erosion (core PS2009 30 cm) areas of the shallow, large Lake Peipsi sensu stricto (s.s.; 2611 km(2), unregulated water level). The P pool in sediments including buried and mobile P is evaluated for the first time, which will provide baseline data for the future modelling of internal loading in L. Peipsi. Five sedimentary P fractions (including inorganic and nonreactive P) were separately quantified: loosely adsorbed and pore-water P (NaCl-P); redox-sensitive fraction P (NaBD-P); P bound to oxides of non-reducible Fe and Al (NaOH-P); calcium-bound P, mainly from apatite minerals (HCl-P) and refractory, mainly organic P (Res-P). Concentrations of P fractions varied during the 100 years with the highest values around 2007-2008 and 1923-1935. The P in "active" layers that are available for bacteria and algae or those undergoing changes and diagenetic transformations in the sediment could follow sediments with the water content of ∼88%. Potentially mobile P is not decreased in the sediments deposited 50-100 years ago and makes up ca 63 mg m(-2) y(-1) (with range 8.3-23.7% of the total P (TP)) in the accumulation area, and ca 0.047 mg cm(-3) (with range 1.3-22.4% of TP) in the erosion area. The result shows that 13-60% of TP contained in the surface sediment (from 34 to 398 mg P m(-2) y(-1)) has been remobilised during accumulation and could be exported to the overlying water.