Epsilon Archive for Student Projects

Abstract

The limit for mercury in fish, set by health authorities, is exceeded in about half of the lakes in Sweden. Methyl mercury (MeHg) is the most toxic form of mercury and accumulates in fish. Therefore transformation of inorganic mercury (Hg) to methylmercury (MeHg) is an important process that we need to understand in order to possibly counteract the negative effects of mercury in the environment.
The seasonal dynamics of mercury biogeochemistry is likely of great importance for the accumulation of MeHg in the aquatic food web. In thesis the chemical speciation and transformations of Hg and MeHg in bottom waters and sediments is followed in Ängessjön, a small lake located on the east coast in the middle of Sweden. The lake is not directly affected by any industrial pollution and concentrations of Hg in the sediment reflect the atmospheric deposition in this area, as well as processes taking place along the hydrological pathways from soil to lake.
Samples from sediment, bottom- and streamwater were collected in average every second month during the period Feb. – Dec. 2007 at three different locations in the lake. The sediment cores were divided in three layers; 0-3, 3-10 and 10-20 cm. Pore water was extracted and redox sensitive measurements were conducted in a glove-box in an oxygen free atmosphere. Subsamples from the layers in the sediment were analyzed for Hgtot, MeHg, Ctot, Ntot, Stot, Fetot, pH, H2S(g). In incubation experiments using stable isotope labeling, potential methylation and demethylation rates were determined. In the extracted porewater the following parameters were analyzed; Hgtot, MeHg, DOC, Cl-, SO42-, Fe(II)/Fe(III), S(-II) and aromaticity of organic substances (SUVA).
During the progress of spring and summer sulphate concentrations decreased and iron (II) and manganese concentrations increased in the sediment pore water. This reflects an increasing activity of sulphate and iron reducing bacteria. A minimum redoxpotential was indicated in Jul. – Aug. The primary production of plankton showed an opposite trend with a maximum during mid-summer. Parallel to this decrease in redox potential and increase in primary productivity, determined potential methylation rates increased. During the period Aug. – Dec. sediments were re-oxidized and potential methylation rates went down to levels similar to the measurements in Feb.

 En ökad primärproduktion i Ängessjön under sommarmånaderna tillför färskt organiskt material i form av avdöda plankton och alger till främst ytsedimentet. Nedbrytningen av det organiska materialet förbrukar syre och ger upphov till suboxiska förhållanden i sedimentet under vår och sommar. Minskande halter av sulfat och ökande halter av Fe indikerar aktivitet av SRB och FeRB.
 De suboxiska förhållanden som råder under sommaren ökar metyleringshastigheten i framförallt det övre sedimentlagret där förutsättningarna för metylering är optimala bland annat genom en hög temperatur och tillgång på elektronacceptorer (sulfat och järn-III).
 Högre Hg/MeHg kvot i ytsedimentet (0-3 cm) indikerar en nettobildning av MeHg i detta skikt under hela året.
 Den potentiella metyleringshastigheten i Ängessjön är högst under sommarmånaderna när även produktionen i sjön är på topp. Detta möjliggör MeHg-ackumulering i näringskedjan under en period då tillväxthastigheten av organismer är hög.
 De relativt höga kvicksilverhalter som uppmätts i plankton utgör en risk för att kvicksilver ska ackumuleras uppåt i sjöns näringskedja. Överslagsberäkningar indikerar att halter av Hg i fisk bör ligga i nivå med livsmedelsverkets gränsvärde för regelbunden konsumtion.