Forms and Mobility of Sediment Phosphorus in Shallow Eutrophic Lake Võrtsjärv (Estonia)

The surficial sediment (0–10 cm) of shallow eutrophic Lake Võrtsjärv (Estonia) was characterized by an acid insoluble residue of 50% dry weight and low nutrient, Fe and Mn content. Among phosphorus (P) fractions (H ieltjes and L ijklema , 1980), NaOH‐NRP amounted on an average to 50%, HCl‐RP to 30%, NaOH‐RP to 16%, and NH 4 Cl‐RP to 4% of their sum. Seasonal changes in sediment P content were inconsistent with mass balance calculations and could be attributed to sediment redistribution caused by decreasing water level. High Fe/P ratio (26–30) and the aeration of surficial sediment by frequent resuspension kept phosphate adsorbed. Low pore‐water SRP (commonly <10 μg l −1 ) usually prevented phosphate release from surficial sediment. However, a storm in September 1996 (max. wind speed 16 m s −1 ) which coincided with the extremely low water level in the lake (mean depth 1.44 m), denuded deeper anoxic sediment layers and caused a SRP release of 193 mg P m −2 d −1 .