Phosphorus fractions and profiles in the sediment of shallow Danish lakes as related to phosphorus load, sediment composition and lake chemistry

Depth profiles of sediment dry weight (DW), loss on ignition (LOI), total iron (Fe), total nitrogen (TN), calcium (Ca), total phosphorus (TP), NH4Cl-P, NaOH-P, HCl-P and Res-P were measured in 32 shallow meso- to hypertrophic Danish lakes and the data compared with that for external phosphorus load (Pex), lake water phosphorus and phytoplankton biovolume. Surface sediment TP was highly and positively correlated to Pex (g P m−2 y−1) and Fe (mg Fe g−1 DW) (TP = 0.92 + 0.23 Pex + 0.031 Fe, r2 = 0.68, P < 0.0001), but not or only weakly related to the other parameters analyzed. NaOH-P in the sediment was significantly and positively correlated to Fe (r2 = 0.43) and TP (r2 = 0.75), while HCl-P was correlated to Pex (r2 = 0.68), TP (r2 = 0.28) and to phytoplankton biovolume (r2 = 0.35), but not to Ca. On average 4% of the surface sediment TP was NH4Cl-extractable, 35% NaOH-extractable and 20% HCl-extractable, with the remaining 41% being residual phosphorus (Res-P). At sediment depths below 50 cm the Res-P fraction was greater (63% of TP), while there was still considerable NaOH-P (21% of TP), thus indicating a significant permanent deposition of iron-bound phosphorus. In most sediments a higher surface Res-P and NaOH-P were recorded as compared to depths below 20–30 cm. The presence of a NaOH-P gradient, even in lakes in which P-lake and anthropogenic impact were low, suggests that post-depositional mobility of phosphorus may occur. The Res-P gradient indicates that a major part of the organically bound phosphorus in the upper 10–20 cm of the sediment eventually is mineralized and dissolved.