Sediment microstructure and resuspension behavior depend on each other

The critical shear stress of resuspension and rates of erosion for cohesive and loosely structured sediments must be obtained by direct measurements since there is no theoretical calculation. An in situ experiment on sediment resuspension was performed in a shallow lake (Langer See, NE Germany; area = 1.27 km2, zmax = 3.8 m) in summer 2006 using a hydrodynamically calibrated erosion chamber (O 20 cm). Shear velocity (u*) was incrementally increased in 11 steps (0–2.19 cm s−1) to initiate resuspension events. Entrainment rates (E) of suspended particulate matter (ESPM), total P (ETP), chlorophyll a (EChl a), and soluble reactive P (ESRP) were determined by mass balance. Two subsequent critical u* (0.53 cm s−1 and 1.48 cm s−1) support the ‘two-layered bed’ model of a fluffy surface aggregate layer (freshly deposited phytodetritus prone to resuspension) and an underlying more consolidated biostabilised layer. Patterns in ESPM (2–106 g m−2 h−1), ETP (11–532 mg m−2 h−1), and EChl a (3–24 μg m−2 h−1) revealed a sediment surface maximum of TP and Chl a and their theoretical vertical logarithmic decrease within 4 mm sediment depth, the maximum thickness of sediment layer entrained. The advective ESRP flux (17 mg m−2 h−1) was 43 times higher than the diffusive SRP flux (0.4 mg m−2 h−1). The TP and Chl a micro-profiles suggest that cohesive sediment bed formation is a function of both settling (fluff) and consolidation (biostabilisation). Thus, sediment microstructure and resuspension behavior depend on each other.