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Speciation of trace metals in the Baltic Sea with focus on the Euphotic zone
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Physicochemical speciation of iron (Fe) and the trace metals Cd, Cu, Co, Mn, Ni, and Zn were performed at four different locations in surface waters of the Baltic Sea. Measurements were performed during several months of the growth season at each station to obtain a detailed picture of the temporal variation in relation to phytoplankton growth. The main target was to understand the speciation of iron, and to evaluate if Fe was limiting in primary production. A methodological aim of the thesis work was focused on comparison between trace metal speciation methods; where the DGT method was calibrated to other methods and also to use the DGT method find out which mechanisms that control the labile fraction. Other methods, such as CSV, CL-FIA and Fe isotope measurements (MC-ICP-MS) were used to further evaluate the changes in the Fe fractions.Concentrations of Mn, Zn and Cd measured by DGT during 2003 and 2004 were similar to concentrations measured in <1 kDa samples, but Cu and Ni, showed noticeably higher concentrations in ultrafiltered water than DGT-labile concentrations. This indicates the existence of un-labile low molecular weight Cu and Ni species, small enough to pass through the 1 kDa filter. It can also be a sign of a high degree of organic complexation which will lead to an underestimation in the DGT labile fraction. The temporal variations 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. Data from this investigation show a temperature dependency in the labile Mn concentration, which indicate a bacterial driven oxidation of dissolved to particulate Mn. During this process, trace metals in the surface water, like Cd, Zn and Co are scavenged, along with P. Ni and Cu seem to be regulated by other processes.Total Fe in the Baltic Sea euphotic zone decrease by more than one order of magnitude from the Bothnian Sea to the central Baltic proper. The Baltic Sea system is forming a natural well defined Fe gradient for studying physicochemical speciation of Fe and other trace elements and the role of iron for primary production at different total iron concentrations. From measurements with high temporal resolution from the euphotic zone, significant variations in the physicochemical speciation of Fe were observed, including the iron isotopes. To evaluate which elements were depleted with regard to cyanobacteria demand, internal elemental ratios were measured during three growth seasons. Fe:C within cyanobacteria did not indicate lack of iron, whereas dropping P:C ratios were indicating P-limitation at the peak of the bloom. This pattern was consistent for all studied locations. The study also showed that the levels of <1kDa are sufficient for Fe-replete phytoplankton growth. Also, a relatively high standing concentration of Fe(II) was measured, large enough to cover the demand for iron by cyanobacteria. Data from this Baltic Sea study suggest that iron isotope measurements provide new information on iron cycling in coastal areas. At the Landsort Deep, vertical mixing was a probable cause of the enrichment of light Fe during spring and fall. The Bothnian Sea lacks anoxic waters, which also is reflected in positive δ56Fe within the whole measured period (April-August). The study showed that Fe/Ti or Fe/Al ratios close to average crust material do not necessarily indicate that the suspended phase mainly reflects detrital particles. Both positive and negative iron isotope values have been measured although the sample has a Fe/Ti or Fe/Al ratio close to average crust material. Furthermore, a δ56Fe value around zero does not necessarily mean that the sample consists of mainly rock fragments, as it usually is a mixture of iron particles with positive and negative δ56Fe values.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2009. , 33 p.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Research subject
Applied Geology
Identifiers
URN: urn:nbn:se:ltu:diva-18188Local ID: 758e7970-9303-11de-8da0-000ea68e967bISBN: 978-91-86233-90-7OAI: oai:DiVA.org:ltu-18188DiVA: diva2:991195
Note
Godkänd; 2009; 20090827 (jgn); DISPUTATION Ämnesområde: Tillämpad geologi/Applied Geology Opponent: Dr Peter Croot, Leibniz Institut Für Meereswissenschaften, Tyskland Ordförande: Professor Johan Ingri, Luleå tekniska universitet Tid: Fredag den 25 september 2009, kl 10.00 Plats: F 341, Luleå tekniska universitetAvailable from: 2016-09-29 Created: 2016-09-29Bibliographically approved

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