Phosphorus release from resuspended sediment in the shallow and wind-exposed Lake Arresø, Denmark

Abstract

Wind-induced sediment resuspension occurs frequently in the shallow and eutrophic Lake Arresø, Denmark. The impact of resuspension on internal phosphorus loading was investigated by laboratory experiments studying P-release from the undisturbed sediment surface and by experiments simulating resuspension events.

Phosphorus release from undisturbed sediment sampled in May and August was 12 mg and 4 mg m⁻² d⁻¹, respectively. During experimental simulation of resuspension, soluble reactive phosphate (SRP) increased by 20–80 µg l⁻¹, which indicates that a typical resuspension event in the lake would be accompanied by the release of 150 mg SRP m⁻². The internal P loading induced by resuspension is estimated to be 60–70 mg m⁻² d⁻¹, or 20–30 times greater than the release from undisturbed sediment.

SRP release during simulation of resuspension was mainly dependent on the equilibrium conditions in the water column and was basically independent of the increase in suspended solids and the duration of resuspension. A second simulation of resuspension conducted 26 hours later, did not result in any further release of SRP from sediment sampled in May. In contrast, there was an additional SRP release from sediment sampled in August, indicating that an exchangable P pool, capable of altering equilibrium conditions, is built up between resuspension events.

It is concluded that resuspension, by increasing the P flux between sediment and water, plays a major role in the maintenance of the high nutrient level in Lake Arresø. A relatively high release rate is maintained during resuspension because of the low Fe:P ratio and the high concentration of NH₄Cl-extractable P in the sediment.