Legacy Phosphorus in Sediments of Lowland Waterways

Abstract

Riverbed sediments in agricultural landscapes are loaded with phosphorus (P). They may act as a source or sink for riverine P, possibly causing harmful algae blooms and eutrophication in streams and receiving water bodies, including coastal waters. In this study, we aimed at identifying the labile, moderately labile, and stable P fraction (Hedley fractionation) in sediments of a northeastern German river basin (3000 km2). A non-metrical multidimensional scaling (NMDS) was used to identify the most significant environmental predictors of the P fractionation in sediments. The total P contents of the sediments varied over a wide range (698 ± 701 mg P kg−1 sediment−1), spanning from 98 to 2648 mg P kg−1 sediment−1. Adjacent agricultural reference soils had markedly lower total P contents of 354 ± 132 mg P kg−1 soil−1, ranging from 146 to 483 P kg−1 soil−1. There were almost no differences between the P contents of the top (0–2 cm) and the bottom (2–10 cm) layer. The dominant P fractions were the moderately labile (NaOH-P) and the stable (H2SO4-P) fractions, which accounted for more than 50% of the total P at each sampling point. The NMDS revealed that iron and aluminum contents, as well as land use, are significant predictors for the P fractionation of the sediment. The sediment P-composition reflects the P-status of the agriculturally used mineral soils. However, the size of the contributing catchment as well as the length of the water way have no effects on sediment P. In conclusion, sediment P stocks, though variable, may impede the good ecological status of river waters for decades, especially in lowland basins where hydraulic conditions and a very low stream velocity often create low redox and P dissolution conditions in sediments.