Material transport from the nearshore to the basinal environment in the southern Baltic Sea -: I.: Processes and mass estimates

Processes involved in erosion, transport and deposition of cohesive materials are studied in a transect from shallow (16 m) to deep (47 m) water of the SW Baltic Sea. The wave- and current-induced energy input to the seabed in shallow water is high with strong variability and suspended matter concentrations may double within a few hours. Primary settling fluxes (from sedimentation traps) are less than 10 g m (-2) day (-1), whereas resuspension fluxes (evaluated from sedimentation flux gradients) are 15-20 times higher and the residence time for suspended matter in the water column is 1-2 days. Settling velocities of aggregates are on average six times higher than for individual particles resulting in an enhanced downward transport of organic matter. Wave-induced resuspension (four to six times per month) takes place with higher shear stresses on the bottom than current-induced resuspension (three to five times per month). The short residence time in the water column and the frequent resuspension events provide a fast operating benthic-pelagic coupling. Due to the high-energy input, the shallow water areas are nondepositional on time scales longer than 1-2 weeks. The sediment is sand partly covered by a thin fluff layer during low-energy periods. The presence of the fluff layer keeps the resuspension threshold very low (<0.023 N m (-2)) throughout the year. Evaluated from 3-D sediment transport modeling, transport from shallow to deep water is episodic. The net main directions are towards the Arkona Basin (5.5 x 10(5) t per year) and the Bornholm Basin (3.7 x 10(5) t per year). Energy input to the bottom in deep water is low and takes place much less frequently. Wave-induced resuspension occurs on average once per month. Residence time of particles (based on radioactive isotopes) in the water column is half a year and the sediment accumulation rate is 2.2 mm year (-1) in the Arkona Basin. (C) 2002 Elsevier Science B.V. All rights reserved.