Forms and mobility of sediment phosphorus in shallow eutrophic Lake Vortsjarv (Estonia)

The surficial sediment (0-10 cm) of shallow eutrophic Lake Vortsjarv (Estonia) was characterized by an acid insoluble residue of 50% dry weight and low nutrient, Fe and Mn content. Among phosphorus (P) fractions (HIELTJES and LIJKLEMA, 1980), NaOH-NRP amounted on an average to 50%, HCl-RP to 30%, NaOH-RP to 16%, and NH4Cl-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 FelP ratio (26-30) and the aeration of surficial sediment by frequent resuspension kept phosphate adsorbed. Low pore-water SRP (commonly <10 mu g 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).