Abstract
The study aimed to determine the level of mercury (Hg) and its labile and stable forms in the surface sediments of the Baltic Sea. The work considers the impact of current and historical sources of Hg on sediment pollution, together with the influence of different environmental parameters, including water inflows from the North Sea. Surface sediments (top 5 cm) were collected in 2016–2017 at 91 stations located in different areas of the Baltic Sea, including Belt Sea, Arkona Basin, Bornholm Basin, Gdańsk Basin, West Gotland Basin, East Gotland Basin, and the Bothnian Sea. Besides, the particulate matter suspended in the surface and near-bottom water was also collected. The analysis of total Hg concentration and individual Hg forms in collected samples was carried out using a 5-step thermodesorption method. This method allows for the identification of three labile and thus biologically available, fractions of Hg, which are mercury halides, organic Hg, mercury oxide and sulphate. Two stable fractions, mercury sulphide and residual Hg, were also determined. The highest Hg concentrations, reaching 341 ng g−1, were measured in the highly industrialised Kiel Bay, which was additionally a munition dumping site during and after World War II. High Hg level, ranging from 228 to 255 ng g−1, was also recorded in the surface sediments of the Arkona Basin, which was a result of the cumulative effect of several factors, such as deposition of Hg-rich riverine matter, favourable hydrodynamic conditions and military activities in the past. The relatively elevated Hg concentrations, varying from 60 to 264 ng g−1, were found in the Gdańsk Basin, a region under strong anthropopressure and dominated by soft sediments. The sum of labile Hg in sediments was high and averaged 67% (with the domination of organic Hg compounds), which means that a large part of Hg can be released to the water column. It was found that the water inflows from the North Sea intensify the remobilisation of Hg and its transformation into bioavailable labile forms. As a consequence, the load of Hg introduced into the trophic chain can increase. Despite the significant reduction of Hg emission into the Baltic in the last decades, surface sediments can be an important secondary Hg source in the marine ecosystem. This is especially dangerous in the case of the western Baltic Sea.
Highlights
Owing to its high toxicity, mobility and stability, mercury (Hg) is considered one of the most dangerous elements found in the environment (UNEP 2013, 2019)
The main aim of this study was to determine the level of total Hg concentration and the proportions of its labile and stable forms in the surface sediments of the Baltic Sea, taking into account the differences resulting from diverse anthropopressure and varied environmental conditions
These results were similar to those measured in the surface sediments of the Baltic Sea in 1993 by Perttilä et al (2003), which had ranged from 13 to 406 ng g−1, as well as those observed in 2001–2008 by HELCOM (2010), which had ranged from 40 to 300 ng g−1
Summary
Owing to its high toxicity, mobility and stability, mercury (Hg) is considered one of the most dangerous elements found in the environment (UNEP 2013, 2019). Even low levels of Hg in the body can lead to disruptions of important biochemical processes, irreversible disorders in the nervous system and brain functions (Axelrad et al 2007). Even small doses of Hg led to changes in their physiology, negatively influencing, e.g., food intake and digestion processes, respiration and metabolism, the reproduction and development of organisms. Given the fact that the most common cause of Hg poisoning is the consumption of fish and seafood (Sunderland 2007; Rice et al 2014; Mosa and Duffin 2017), the examination of Hg level and bioavailability in the marine environment is of particular importance
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