The coupling of long-term trace metal trends to internal trace metal fluxes at the oxic–anoxic interface in the Gotland Basin (57°19,20′N; 20°03,00′E) Baltic Sea

Abstract

The Gotland Basin is a well-stratified water body with lower saline surface waters and a halocline between 70 and 90 m. Since 1996, a continuous oxygen depletion started in the deep water of the Gotland Basin after the major saltwater inflow 1993/1994. Since 1996, the deep water became anoxic during periodical intervals below 150 m up to the bottom (238 m), and since 1999, a permanent redox interface developed in the range of the 150 m depth horizon. This paper presents results on the trace metal speciation between dissolved and particulate phases during long-term changes, which will be explained in relation to the upward penetration and the seasonal movement of the redox interface. Trace metal trends for surface and deep waters were observed between 1993 and 2002. In the water body “Below Halocline”, a decrease of the dissolved concentrations of Cd diss (−7.6% year −1), Cu diss (−8.3% year −1), Zn diss (−15.9% year −1) and Pb diss (−7.6% year −1) were observed since 1995, due to the formation of sulfide species and their burial in the surface sediments. “Above Halocline”, slightly increasing concentrations were observed for Cd diss (+6% year −1) between 1994 and 1999 and for Zn diss between 1994 and 1997, which has been attributed to the enrichment from riverine and atmospheric input in surface waters. Since 1999, the system is controlled and dominated by the vertical fluxes due to the establishing stagnant anoxic deep water body influencing the removal of trace metals from surface waters by time delay. Significant negative trends for Cd diss (−11% year −1) and Zn diss (−10% year −1) were observed “Above Halocline” since 1999 and 1997, respectively. Pb diss shows a negative trend of −6% year −1 “Above Halocline” between 1993 and 2002. Additional to the decrease of Pb use in gasoline, this result is attributed to the specific geochemistry and short residence time of Pb in connection with the strong affinity and export by particles of mainly lithogenic origin. For Cu diss, no trend was observed for surface waters. In this context, the relation of trace metal export through the redox interface and the linkage of trace metal dynamic due to eutrophication (movement of the redox interface) has been investigated on a seasonal scale. Highly resolved vertical profiles of trace metals (Pb, Cd, Zn, Cu, Co, Fe, Mn) have been carried out between December 1999 and February 2001 on a seasonal base. Calculations of dissolved trace metal fluxes using the vertical turbulent mixing coefficient according to the seasonal movement of the redoxcline gave information about the diffusive flux and quantitative transport of trace metals into the anoxic water body. For Cd diss, Cu diss and Zn diss, elevated downward fluxes were observed during winter season, and by a factor 1.5–2 reduced fluxes during summer. Co diss, Fe diss and Mn diss show no seasonal flux variations. About 25% of Cd, 12% of Cu and 8% of Zn which has been supplied by rivers and by the atmosphere are eliminated by diffusive exchange, due to sulfide precipitation, across the oxic–anoxic interface in the Gotland Basin. For dissolved lead, the diffusive exchange is negligible.