Trace metal speciation in the Baltic sea

Sammanfattning: Physicochemical speciation of metals in natural waters is very important for understanding their distribution, mobility, bioavailability and toxicity. To be able to understand the behaviour of an aqueous element and the transformation between chemical and physical species, there is a need for reliable methods that enable measurements of specific fractions of metals. Many different techniques are used for metal speciation, of which many suffer from problems. Ultrafiltration has frequently been used to study speciation of metals in natural waters. A possible alternative or complement to ultrafiltration is the technique of diffusive gradients in thin films (DGT), a novel technique which provides an in situ measurement of labile metal species. DGT accumulates metals in a time- integrated way and produces a mean concentration over the chosen deployment period. DGT- labile metals may be regarded as a measure of the bioavailable amount, since the DGT simulates the diffusion process that occurs when a metal is diffusing into a cell membrane. This thesis is focused on the DGT technique for sampling and determination of labile species in the Baltic Sea. The aim of this study was to compare the trace metal speciation methods; DGT, 1 kDa ultrafiltration, 0.22µm membrane filtration and unfiltered water, to study the dynamics for the DGT labile fractions to find out which mechanisms that control the labile fraction. In 2003 and 2004, DGT and 1 kDa ultrafiltration were simultaneously applied at two sampling stations in the Baltic Sea with different salinity and trace metal concentrations. Baltic Sea concentrations of Mn, Zn and Cd measured by DGT during 2004 were similar to the concentrations measured in 1 kDa ultrafiltered samples, especially for Mn. Cu and Ni, showed noticeably higher concentrations in ultrafiltered water than DGT-labile concentrations. This indicates the existence of low molecular weight Cu and Ni species, small enough to pass through the 1 kDa, but can also be a sign of high degree of organic complexation which will lead to an underestimation in the DGT labile fraction. The dynamics of DGT-labile trace metals during 2004 show quite large variations during the season at 0.5 to 40 meters depth. From May to August, Cu, Cd and Mn drop about 35, 50% and 60% respectively. Ni decreased about 25% late April to late June but was slightly recovered at late season. The only elements that showed good correlation between DGT-labile species to dissolved phase (0.22µm filtrate) was Mn and Cd. DGT labile Mn is probably controlled by oxidizing bacteria during most of the sampling period, and DGT labile Co, Cd and to some extent Zn seem to follow this process. It should be noted that Mn is closely correlated to P, a relationship which need further investigations. Cu and Ni are controlled by other processes, where influence from primary production may be one. This is the first comparison of DGT and 1 kDa ultrafiltration regarding trace metals in brackish waters. Strong correlations between the methods imply that DGT can be a simple alternative to an ultrafiltration procedure. It is also the first study on trace metals in the Baltic Sea where measurements were performed at high temporal resolution during several months.