Dynamics of P, Fe and Mn along a depth gradient in the SW Baltic Sea

Diffusive, advective and resuspension fluxes of both dissolved and suspended P, Fe and Mn were measured along a depth transect (16 to 47 m) in the SW Baltic Sea. Sediments were covered by a thin fluff layer, which was functioning as a source of nutrients to the water column. As compared with the diffusive transport, pore water advection can release as much phosphate and redox sensitive elements to the overlying water as 10–1000 days of diffusive flux at undisturbed conditions. Linear correlations between suspended P and Fe suggest that phosphate binding Fe-oxides are the source for suspended P and Fe. There was no P accumulation in shallow water. In the Arkona basin (47 m), gross sedimentation rates of P were 55 P mmol m–2 y–1 and the DRP diffusion rate back to the water column was 10 P mmol m–2 y–1 indicating that Arkona sediments are accumulating P. However, episodic resuspension may strongly enrich the water column in this nutrient. The potential resuspension of P varied with the season and a maximum rate of 8 mmol P m–2 was registered in June.