A sediment trap flux study for trace metals under seasonal aspects in the stratified Baltic Sea (Gotland Basin; 57°19.20′N; 20°03.00′E)

A seasonal flux study was carried out in the eastern Gotland Basin, a stratified waterbody, to describe the vertical transport of trace metals (Al, Fe, Mn, Cd, Pb, Cu, Zn, Ni, As) in particulate phases through the pycnocline. A sediment trap was deployed between December 1999 and February 2001 in 120-m depth below the pycnocline. The trap was changed quarterly, sampling periods ranged between 6 and 8 days. To characterise the water column, vertical CTD profiles (suspended particulate matter (SPM), salinity, oxygen, hydrogen sulfide) and the determination of trace metals in the dissolved and particulate phases were performed parallel to the trap experiment. The leakage of the sampling bottles was between 9% and 15%. With the exception of nickel, the blank for the investigated metals in the sodium azide poisoned stock solution was in the range between 1% and 16% in relation to the mean trace metal concentrations in the supernatant. The mean resolution (annual average) of trace metals in the supernatant in relation to the total metal concentration in the sample bottle was 53% for Cd, 49% for Mn, 26% for Cu, 14% for Zn, 15% for Ni and 2% for Fe. The vertical distribution of Mn and Fe in the water column was mainly influenced by the seasonal variability of the distance between pycnocline and redoxcline. Concentrations of suspended particulate matter (SPM) in the water column ranged between 0.05 and 0.75 mg/dm3 with season and depth. During winter, the composition of the trap material is dominated by lithogenic fractions of aluminium and iron, while during summer, particulate organic material (POM) is the main compound with 63% of the total mass flux. Elevated trace metal contents (Zn, As, Fe, Mn, Pb) related to the particle mass were observed in suspended particulate matter (SPM) of the water column in February 2001, which was attributed to the lateral transport of material from the river Memel. A continuous increase of the enrichment factors (EF) in the trap material was noticed from December 1999 to November 2000 with highest values for the time period July 2000–November 2000 for Cd (280), Zn (87) and Ni (13) probably due to the low lithogenic Al content and the enhanced elimination of the metals by degradation products of cyanoblooms. Lead shows decreasing EF during the same time period which we think is a result of the “dilution effect” with particles of mainly biogenic origin. Total mass fluxes ranged between 1 and 200 mg/m2 day; significant seasonal variations were not observed. This result is discussed in connection with under/overtrapping and in relation to the change of the redox conditions in the depth horizon of the deployed trap. Significant correlation exists between the fluxes of Pb/Al, As/Al, Fe/Al, Cd/Corg, Cu/Corg, Zn/Corg and Ni/Corg.