A sediment trap and radioisotope study to determine resuspension of particle reactive substances in the sound between Sweden and Denmark

Abstract

Sediment traps and the radioisotopes 137Cs, 210Pb, 7Be, 234Th and 228Th were used to study downward fluxes and resuspension of sediments in the shallow sound (Øresund) between Sweden and Denmark connecting the Baltic Sea with the North Sea. The water transport through Øresund covers approximately 30% of the total water transport between the North Sea and the Baltic Sea. The heavy boat traffic and the densely populated surroundings coupled to a shallow sound makes resuspension events an important factor in transporting released pollutants. Over the time period studied mass-fluxes recorded by the sediment traps ranged between 175 g m −2 day −1 near bottom during winter time to less than 1 g m −2 day −1 during late spring when the water column was stratified. A combination of sediment traps at different levels in the water column and radioisotope ratios shows that resuspended seabed sediments accounts for a large portion of the collected trap material, ranging from nearly 100% during the winter period to some 50% during stratified periods in late spring. In general the flux of all radioisotopes studied showed a linear relation with the mass-flux. The correlation of flux with organic carbon was better than with mass-flux. A comparison between atmospheric fluxes of 7Be and 210Pb and the fluxes of these isotope measured in the sediment traps shows that the agreement for 7Be is within a factor of two while for 210Pb the sediment trap fluxes are totally dominated by resuspended matter. The fraction of resuspended sediments in the traps may be quantified partly by using the 7Be/ 210Pb or 228Th/ 232Th ratios.