Quantification of phosphorus entrainment in a lowland river by in situ and laboratory resuspension experiments

Abstract

Hydrodynamics at the sediment water interface initiate and control the exchange of particles and associated phosphorus (P) between river sediment and the water column. Currently, no general analytical theory for cohesive sediment resuspension is available, so these properties must be inferred from direct measurements. In a moderately slow-flowing (annual mean velocity 0.1–0.3 m s-1) stretch of the lowland River Spree, Germany, an in situ experiment (INS) and a laboratory experiment (LAB) with surface sediment (0–3 cm) were conducted concurrently in May 2005 using an erosion chamber. In both approaches, the entrainment of particles and particulate P (PP) increased significantly as shear velocities were incrementally increased from 0.57 to 1.67 cm s-1. In repetitive INS runs, particle and PP entrainment rates at the lowest shear velocity applied differed by a factor of 5 and 11 (0.25 – 1.18 g m-2 h-1 and 4.1 – 45.9 mg m-2 h-1), respectively, due to in situ river bed heterogeneities, specifically the presence or absence of a high-porosity fluff layer. These rates were on average 48 and 3 times higher, respectively, than those of LAB runs (0.01– 0.02 g m-2 h-1 and 3.1– 14 mg m-2 h-1, respectively), suggesting the temporary availability of local, in situ, fluffy surface layers on the river bed that are not preserved during sediment preparation for LAB runs. Such a transient storage feature has not been described previously in the literature for river sediments. The entrainment of this easily resuspendable and P rich material at a low flow-generated shear velocity can lead to batch-wise P burdens in the water column and consequently to a displacement of eutrophication potential. Those events would be underestimated in LAB experiments devoid of this fluffy layer.