Spatio-temporal variability of surface sediment phosphorus fractions and water phosphorus concentration in Lake Peipsi (Estonia/Russia)

Spatial and temporal variations in the contents of total phosphorus and five P-fractions were investigated in the surface sediments of large and shallow Lake Peipsi (area 3555 km2, mean depth 7.1 m) and compared with total phosphorus (TP), dissolved inorganic P (PO4-P) and dissolved oxygen (O2) concentration in the overlying water column. The aim of the study was to determine the relationships between the sediment P and water P concentration dynamics. Samples from the uppermost 10-cm layer of the sediment and water samples from surface and near-bottom layers were collected simultaneously at six monitoring stations twice a year (in March and August) from 2004 to 2009. The results indicated that the concentrations of different P-fractions in the studied sediments varied greatly. Total P in sediment ranged from 470 to 1400 mg kg–1 dry mass (DM), while the loosely bound (labile) P (NH4Cl-P) was the smallest and the most variable fraction (range from 3.6 to 43 mg kg–1 DM). The metal oxide-bound P (NaOH-P) was also highly variable (range from 48 to 660 mg kg–1 DM), whereas the concentration of calcium-bound P (HCl-P) was relatively even (range from 300 to 550 mg kg–1 DM) in all the studied sediments. Redundancy analysis results revealed that different phosphorus fractions in sediment had statistically significant relationships with bottom-water variables like TP (in winter) and O2 (in summer). Our results show that the oxygen conditions have deteriorated in the lake during the last decades, which may have enhanced the release of P from the sediments.