Phosphorus entrainment due to resuspension in a lowland river, spree, NE Germany - A laboratory microcosm study

Resuspension of benthic phosphorus (P) often constitutes a high percentage of the annual P flux in lowland rivers. To study P entrainment at controlled shear velocity (u*) sediment from lowland River Spree of slower flowing (0.1-0.3 m s(-1)) stretch Kossenblatt (KOB) and of faster flowing (0.5-0.7 m s(-1)) stretch Freienbrink (FRB) was incubated in a microcosm at incrementally enhanced u* (0.34-1.9 cm s(-1)). Particle and P entrainment rates as well as the number of particle-associated bacteria of fine- grained mud-like KOB sediment were much higher (16.7 g m(-2) h(-1), 104.9 mg P m(-2) h(-1), 15.47 10(6) cells ml(-1)) than those (4.3 g m(-2) h(-1), 2.1 mg P m(-2) h(-1), 3.06 10(6) cells ml(-1)) of coarser sandy FRB sediment. The microcosm used so far in marine research is suited to compare riverine resuspension suggesting the lower u* the more particles are deposited and the more P can be retained (KOB >> FRB). Conversely, correspondingly more and easier particulate P and bacteria can again be remobilised (KOB >> FRB) if u* increases. The general relationship found for u* and the entrainment of particulate P and bacteria as well as their decelerated and selective deposition where bacteria may stay longer in the water implies a temporarily enhanced P bioavailability, turnover and subsequent P transformations.