G10 (II) Mercury in marine ecosystems

Mercury is characterized with a high affinity for particulate matter in the aquatic environment. It is adsorbed on the surface of organic matter rich particles or absorbed by the phytoplankton. In the course of sedimentation part of this metals is liberated into the water column as dissolved species. Those processes are specially intensive when suspended matter settling is hindered due to salinity or temperature gradients, influencing the density of seawater. Baltic Sea is characterized by both seasonal and permanent stratification. A seasonal thermocline is present at a depth of c.a. 10 meters, while a permanent halocline is usually located at the depth of to 50 meters. A limited water exchange with the North Sea, with no through mixing leads to hypoxia and anoxia in the bottom waters. Hence, density gradients in the Baltic Sea bottom waters are often accompanied by a change of redox conditions – which makes species transformation even faster. Profiles of mercury dissolved and particulate forms have been analysed in order to assess the magnitude of this process at different regions of the southern Baltic Sea prior to and during phytoplankton blooms in years 2008-2010. They are presented against background of particulate and dissolved organic carbon (POC, DOC) and SPM profiles. Suspended matter settling speed was calculated according to modified Navier-Stokes equation, to estimate the rate of mercury solid/dissolved partitioning. Mercury proved to be sensitive to the presence of phytoplankton, magnitude of solar radiation and water column stratification. It has been estimated, basing on high resolution profiles, that up to 60% of the initial metal concentration is removed to the water column after passing through a density anomally.

Signs of receipt and mercury dispersion in water column and atmosphere over hydrothermal vents of the underwater Pijp Volcano are revealed. It is established that mercury from hydrothermal vents is taken out mainly as a part of bubbles of hydrothermal gases, which penetrate through water column up to a sea surface. Mercury dispersion zone in water column and enrichment of sea waters is arose because of partial dissolution of gas bubbles. The atmochemical zone of Hg dispersion is expressed as low amplitude anomaly of mercury with two maxima is moved in a direction of a wind to the northwest from northern and southern tops of the volcano. The mercury consentration in bottom sediments near underwater volcano consists 20-60 ppb. In air Hg consentration is 1-2,2 ng/m3. In sea water Hg consentration is 1-5 nl/l. For the mercury investigations we used a mercury Zeeman atomic absorption spectrometer with high frequency modulation of light polarization RA-915+ (manufactured by Lumex Ltd, Russia) for direct on-line measurements of the mercury concentration in air and samples of water and bottom sediments with additional equipment.

Mercury is a highly toxic metal. It may damage cellular membranes, inhibit enzymes or damage DNA and RNA chains. It is easily bioaccumulated, and neurotoxic organomercurials are biomagnified in food chains. The most toxic and bioavailable form of mercury is methylmercury. Methylmercury is formed as a result of biological and chemical processes of oxidation and reduction, namely the biotransformation of inorganic and organic mercury species and the photochemical reactions involving organic compounds of mercury. Mercury methylation in bottom sediments is adequately described in the literature but a mathematical description, enabling evaluation of a given sedimentary environment readiness for methylmercury formation is lacking.

The study is presented, aiming at developing a quantitative measure of the phenomenon. For this purpose, an index – the ‘methylation potential (Pm)’ – is proposed describing potential for mercury methylation in the uppermost marine sediments. This index relates Pm to mercury bioavailability based on sequential extraction, and simple biogeochemical sediments properties (eg. organic matter). One advantage of the approach, are results indicating good correlation between Pm and mercury in fish from the study area.

Samples of surface sediments were collected from three areas in the southern Baltic and one area in the Greenland Sea. The Gdansk Basin and Vistula estuary (Baltic sea) were selected due to proximity to mercury point sources, while the Gotland Deep (Baltic) and Isfjord estuary (Greenland sea) - because they are distant from point sources. Sediments were analysed for total mercury (Hg_TOT) and three operationally defined mercury fractions: Hg_A – contained in pore waters, Hg_F – bound to fulvic acids and Hg_H – bound to humic acids. To validate methylation potential model, correlation between Pm and methyl mercury in sediments samples was tested and proved statistically significant.