An in situ experiment to investigate the modification of particulate matter and urea above a benthic sandy silt community in the Baltic Sea

Abstract

During two cruises to the Baltic (Mecklenburg Bight) in September 1993 and November 1994 bottom water and sediment samples were taken from 5 stations on a 2.0 km long transect above a benthic sandy silt community. Profiles of total particulate matter, particulate organic carbon, chlorophyll equivalents and urea were taken in the benthic boundary layer (5–40 cm height above sea floor) on the downstream stations across an area occupied by macrofauna feeding at the sediment-water interface at 26 m water depth. Particulate matter concentration profiles varied under the two different flow conditions in September (u _* = 0.7 cm s⁻¹) and November (u _* = 0.2 cm s⁻¹). In September 1993 resuspension of total particulate matter (TPM) of 22 to 130 mg m⁻² h⁻¹ occurred while particulate organic carbon (POC) and chlorophyll (CPE) were deposited with a rate of 9 mg m⁻² h⁻¹ and 0.11 mg m⁻² h⁻¹ respectively at the station of highest macrofauna abundance. In November 1994 physical sedimentation and biological deposition of up to 388 mg m⁻² h⁻¹ TPM, 7.4 mg m⁻² h⁻¹ POC and 0.07 mg m⁻² h⁻¹ CPE occurred. Urea was released into the water column. Data suggest that in shallow water environments local sediment and benthic boundary layer characteristics prevent large scale calculations of fluxes of particulate matter.