The Vertical and Horizontal Distribution of Selected Trace Metals in the Baltic Sea off Poland

Abstract

Concentrations of dissolved and particulate heavy metals (Fe, Co, Ni, Cd, Cu, Pb, Mn, Zn) were measured in the Baltic Proper off Poland in September 1996. Water samples were collected along two offshore horizontal transects from the mouths of the rivers Vistula and Odra and two vertical profiles in the Gdańsk and Bornholm Deeps. Particulate/dissolved metal concentrations in the following ranges were measured (mgkg−1/μgl−1): Cd—0·89–8·91/0·003–0·022, Pb—10·2–114/0·004–0·0088, Cu—20·2–214/0·14–0·80, Ni—27–71/0·59–1·63, Mn—53–7590/0–971, Zn—124–3460/0·30–1·62, Co—not analysed/0·003–0·235, Fe (%)—0·14–3·29/0–190, Al (%)—0·11–3·92/not analysed.Horizontal concentration gradients were found in the Gulf of Gdańsk, which were attributed to the inflow of metals with the Vistula runoff. The lack of gradients in the Pomeranian Bay was explained by the Odra runoff having to pass through the Szczecin Lagoon, which acts as a trap for the suspended and dissolved loads carried by the river. The steep vertical gradients of both dissolved and particulate metals were put down to bioaccumulation at the surface, the presence of accumulated colloids above the halocline and redox phenomena in the suboxic zone below the halocline. These last involve diffusion of Fe(II) and Mn(II) from anoxic sediments, their oxidation to Fe(III) and Mn(IV) and possible formation of poorly soluble sulphides.