海水沉积物中总汞甲基汞测定

Baseline
Total,methyl and organic mercury in sediments of the Southern Baltic
Sea
J.Bełdowski a ,M.Miotk a ,M.Bełdowska b ,J.Pempkowiak a ,⇑
a Institute of Oceanology,Polish Academy of Sciences,P.O.Box 197,Sopot,Poland b
Institute of Oceanography,Gdansk University,Al.Pilsudskiego 46,Gdynia,Poland
a r t i c l e i n f o Article history:
Available online 31July 2014Keywords:Patchiness Speciation
Organic mercury Extraction
a b s t r a c t
Distribution of sedimentary mercury in the Southern Baltic was investigated.Sediment samples were collected from the Southern Baltic in the period from 2009to 2011,and concentrations of sedimentary total mercury (average 102ng/g,range 5.8–225ng/g)and methyl mercury (average 261pg/g,range 61–940pg/g)were measured in the manner that the influence of both patchiness and seasonal changes were assessed.Moreover,sedimentary mercury extracted with organic solvent-the so-called organic mercury was also analyzed (average 425pg/g,range 100–1440pg/g).There is a statistically significant dependence between organic mercury and both methyl mercury and total mercury concentrations in the sediments.Methyl mercury contribution to total mercury varied from 0.12%to 1.05%,while organic mercury contributed to 2%of total concentration on average.The area studied,although mercury concentrations exceed threefold the geochemical background,can be regarded as moderately contaminated with mercury,and methylmercury.
Ó2014Elsevier Ltd.All rights reserved.
Mercury has been a subject of environmental chemistry interest for several decades (Pacyna et al.,2006).Although all chemical forms of mercury are toxic,public health concerns are focused on methylmercury (MeHg).
Nowadays loads of mercury discharged to the environment from anthropogenic sources exceed several times these from natu-ral ones (Pacyna et al.,2006).Much of the mercury originating from both anthropogenic and natural sources is,eventually,brought to the marine environment.There,owing to its affinity to particulate matter,mercury is readily scavenged from the water column (Laurier et al.,2003)and deposited to bottom sediments (Cossa and Gobeil,2000)in particular in estuaries and coastal areas (Boening,2000).
Distribution of mercury in marine sediments is influenced by physical transport,sediment texture,mineralogical composition,reduction/oxidation status of sediments,adsorption and desorp-tion processes and organic matter content (Boening,2000;French et al.,1999).Red-ox conditions are of particular interest as,in the reducing environment,mercury is readily transferred into organo-mercurial species (e.g.methylmercury-MeHg)that are both mobile and highly bioavailable.Thus,under specific condi-tions a fraction of mercury deposited to sediments re-enters the overlying water and constitutes threat to living organisms.As a result,sediments can act as both sink and source for mercury in aquatic environment (Zoumis et al.,2001).
The Baltic Sea is a land locked basin surrounded by highly industrialized catchment area.Mercury concentrations in the sur-face Baltic sediments exceed the geochemical background by a fac-tor of three to five (Beldowski and Pempkowiak,2009).According to the recent pollution load compilation (HELCOM,2011)the input of mercury to the Baltic Sea had efficiently decreased within the previous two decades.Despite this,no corresponding decrease is observed in biota mercury concentrations (Saniewska et al.,2014).One of possible reasons is the re-emission of the previously accumulated mercury from sediments,in particular within sedi-mentation basins,due to anoxic conditions prevailing there (Bełdowski et al.,2009).
Mercury in the Baltic sediments has been a subject of investiga-tions for several decades (Saniewska et al.,2010;Beldowski and Pempkowiak,2009;Bełdowski and Pempkowiak,2007;Borg and Jonsson,1996;Kannan and Falandysz,1998;Pempkowiak,1991;Pempkowiak et al.,1998).Concentration of the metal is well char-acterized (Bełdowski and Pempkowiak,2007;Borg and Jonsson,1996),as is the mechanism of mercury transport to the deposi-tional basins of the sea (Bełdowski and Pempkowiak,2007).How-ever,MeHg in the Baltic sediments have been seldom studied.So far just one report indicated the presence of MeHg in the Baltic
/10.1016/j.marpolbul.2014.07.0010025-326X/Ó2014Elsevier Ltd.All rights reserved.
⇑Corresponding author.
E-mail address:pempa@iopan.gda.pl (J.Pempkowiak).
Sediments(Kannan and Falandysz,1998).The authors of the report measured substantial contribution of MeHg to total mercury bas-ing on several results of methyl mercury in the Baltic sediments. Thus little is known regarding both contemporary concentrations of MeHg in the sediments and factors affecting the concentrations. This,at least partly,may be caused by relatively extensive analyt-ical procedure required to quantify sedimentary MeHg(Liang et al., 1994).Recently the so called organic mercury was suggested as a mercury fraction closely related to MeHg in fresh-water sediments (Boszke and Kowalski,2008).The authors used methylene chloride to separate organic mercury from sediments.Other organic sol-vents were also used for the purpose:toluene(Miller et al., 1995),and chloroform(Eguchi and Tomiyasu,2002;Tomiyasu et al.,2000).There are no reports regarding the usefulness of organic mercury as a substitute for MeHg in studies of marine sed-iments contamination.
The aim of this study was to investigate concentrations of total mercury,and selected mercury fractions:total organic mercury and methylmercury in sediments of the Southern Baltic Sea and to compare the results with concentrations measured in other marine coastal areas.As sampling stations characterized by vary-ing red-ox conditions,sediment texture and organic matter con-tent were collected in close proximity to one another,factors affecting sedimentary mercury concentration and speciation were assessed too,as were relations between the measured mercury fractions.
The Baltic Sea is a semi-enclosed water body surrounded by highly industrialized countries.Two main features characterize hydrology of the sea.Firstly,the surface water is brackish as a result of a large riverine input and the limited exchange of water with the North Sea.Secondly,there is a permanent halocline at a depth of about70m.The sub-halocline waters in the central basins are depleted of dissolved oxygen or even anoxic.Geochemical cycles in the Baltic have been strongly influenced by human activ-ities since the beginning of the20th century(Borg and Jonsson, 1996;Pempkowiak,1991).Much of the anthropogenic load is car-ried to the Baltic Sea with the river runoff.Subsurface groundwater discharge plays substantial role in case of nutrients and organic matter,and a minor role in the case of mercury(Szymczycha et al.,2013).The major rivers entering the Baltic can be divided into two broad categories:those separated from the sea by a lagoon,and those entering the sea directly.The lagoons act as traps for suspended and dissolved riverine loads(Borg and Jonsson, 1996;Pempkowiak et al.,2000).Samples for this study have been collected from the Southern and central Baltic.This area is,on average,quite shallow–mean depth being50m(Voipo,1981). Southern and central Baltic consists of series of deep basins sepa-rated by sills.Sedimentation regime in deep basins(>80m)may be considered as stable(Zaborska et al.,2014).In the intermediate areas(>50m)accumulation type of bed prevails,whereas in shal-lower regions erosion or no-accumulation bottoms predominate (Feistel et al.,2008;Voipo,1981).Sediments consist of silt and mud in the Gdan´sk Deep,the Bornholm Deep and the Gotland Deep-clay in the Słupsk Channel,and sand with occasional silt deposits in the Bay of Gdan´sk,the Słupsk Sill and the Pomeranian Bay(Feistel et al.,2008).For this study,three sedimentation basins of the Southern Baltic were sampled–Bornholm,Gdan´sk and Got-land Deeps,and two shallow areas adjacent to river mouths–the Gdan´sk Bay,close to the Vistula mouth and the Pomeranian Bay, close to the Odra mouth during cruise on the R/V Oceania in Spring 2009and2010from the Gdansk Deep,the Gotland Deep,the Pom-eranian Bay and the Gdan´sk Bay and in Autumn2009from the Gdan´sk Deep and the Gotland Deep.Location of the sampling sta-tions is shown in Fig.1.
Samples were collected with a gravity corer.The top three cen-timeters of stratified sediments were sampled by cutting it away with a plastic spatula,mixed,transferred into polyethylene bags and stored frozen(À20°C)until analyses in laboratory.
Before mercury analysis all the samples were homogenized under laminarflow hood and aliquots were taken for determina-tions of moisture,organic carbon andfine grain fraction contents. Moisture was used to calculate dry mass of sample,and all results are reported as mass per dry weight.Fine grained fraction (<0.067mm)content was determined by anic carbon content in sediments was determined after removal of carbonates (2M HCl)using an Elemental Analyzer Flash EA1112Series com-bined with the Isotopic Ratio Mass Spectrometer IRMS Delta V Advantage(Thermo Electron Corp.,Germany)and presented as percentage in the bulk of the dry sample.Quality control was car-ried out with standard materials supplied by the Thermo Electron Corp.The methodology used proved satisfactory accuracy and pre-cision(average recovery99.1±2.0%).
Total mercury determination was performed via sample (500mg)pyrolysis in a stream of oxygen(Leco AMA254,Czech Republic).The AMA254technique of direct combustion features a combustion/catalyst tube where sediment decomposes in an oxygen-rich environment and removes interfering elements.Both recovery and precision given as Relative Standard Deviation proved satisfactory(97%±3%RSD)basing on a reference material analysis (NIST2584).
Extractable fraction of mercury(organic mercury)was deter-mined according to procedure described for river sediments (Boszke et al.,2007).In short,a sediment sample(5.0g)was twice extracted with chloroform,reextracted by aqueous sodium thio-sulphate solution(0.01M;10mL)From the aqueous layer an ali-quot of5mL was collected,placed in a measuringflask(50mL) and treated with20l L65%HNO3,7.5mL33%HCl and5mL of a 1:1solution of0.0033M KBrO3and0.2M KBr to oxidize all mer-cury species to Hg(II).Resulting solutions were analyzed by means of atomicfluorescence spectrophotometry on automated Tekran 2600(Canada)apparatus,according to EPA1631method(EPA, 2002).
Methylmercury has been determined in the Josef Stefan Insti-tute laboratories in Ljubljana(Slovenia),using the procedure developed by Liang et al.(1994)and used successfully by others (Logar et al.,2002;Quevauviller et al.,1998).Methylmercury defined by this procedure includes all monomethyl mercury spe-cies found in sediments(e.g.CH3Hg+,CH3HgCl,CH3HgOH,and CH3-HgS-R),which are amenable to complexation and extraction as CH3HgBr.In short,300mg sample of wet sediment was sequen-tially eluted with2.5ml of1.5M HBr solution and1ml of1M CuSO4.Then MeHg was extracted into methylene chloride.20ml of deionized water were added,and the organic fraction was evap-orated after dilution to a known volume with reagent water,fur-ther analysis was carried out by aqueous phase ethylation,and then analyzed using the GC/pyrolysis/CVAFS technique in a Brooks and Rand Model1Detector equipped with a gas chromatography column and a high temperature(300°C)desorption unit.All sam-ples were analyzed in triplicate,and blank samples were run for every six samples.Recovery and precision of measurements were assessed by the use of certified reference material(NIST2584for total mercury and BCR580for organic/methyl mercury).Those were equal to98%and3%RSD,for total mercury,while for HgOrg and MeHg RSDs did not exceed7.4%while recovery equaled91%.
The common problem with mercury analysis in marine sedi-ments is the random component,associated with the so called ‘‘patchiness’’–mosaic properties of sediments,which vary,even on a very local scale.The Baltic sediments were reported to be characterized with substantial patchiness(Zaborska et al.,2014; Zalewska and Suplinska,2013;Winterhalter,2001),which may cause the measured concentration of mercury to be non-represen-tative for a given area,if it is based on a single sample analyses.
J.Bełdowski et al./Marine Pollution Bulletin87(2014)388–395389
Therefore in this study,cores were collected in triplicate,within a one square km area.Moreover in order to assess possible seasonal variability,samples were collected in three seasons(Spring2009, Autumn2009and Spring2010)in the area of Gdan´sk and Gotland Deeps,and in two seasons in the remaining stations(Bornholm Deep,Pomeranian Bay and Gdan´sk Bay).
Results of the so designed exercise are presented in Fig.2,sep-arately for each station,error bars represent seasonal differences.
Since variability in stations close to shore,especially near the Vistula mouth(V)is visibly greater than in the accumulation basins,near shore areas and deeps(accumulation basins)will be discussed separately.
Seasonal variability for total mercury in the accumulation basins(GD,BO,and GO)within the same station varied in the range from5%to33%,except one case in Bornholm Deep,where it reached67%.The variability related to patchiness ranged from 9%to34%.Thus the average uncertainty,given as Relative Standard Deviation,attached to a singular total mercury result is in the range of20%.For organic mercury seasonal variability varied from 4%to32%,while the spatial variability ranges from4%to31%. Methyl mercury spatial and temporal distributions were similar, amounting to4–36%RSD for seasonal and2–34%RSD for spatial differences,resulting in15%average uncertainty.Thus the average concentrations of mercury obtained in this study can be regarded as representative for the sediments of the Southern Baltic accumu-lation basins.The uncertainty is less than20%of the average values (Fig.2).
Different situation is observed in the coastal areas–there sea-sonal variability given as RSD reaches103%,115%and121%respec-tively for total,organic and methyl mercury,while patchiness related uncertainty reaches77%,44%and45%for the respective forms.This translates to an average uncertainty of44%for all stud-ied mercury species.Such differences might be attributed to both the dynamics of shallow sediments and variable riverine mercury input.The latter directly controls composition of marine sediments in those areas(Huzarska,2013).Especially pronounced differences observed close to the Vistula mouth might be caused by theflood in May2010,the biggest one since1850.At the time of theflood exceptionally large quantities of mercury were transported with the run-off(Saniewska et al.,2014;Wielgat-Rychert et al.,2013).
Observed seasonal variability and patchiness is not limited to mercury species and results from heterogeneity of sediment and environmental conditions.The former could be characterized by organic matter content,granulometry and oxidative state–factors that strongly influence sedimentary mercury concentration (Pempkowiak et al.,1998).Variability of thefine fraction contribu-tion,organic carbon content and redox potential is presented in Table1.
Organic carbon variability in the whole data set was similar for accumulation and coastal areas,and varied from5.56%to13.58%, whilefine fraction contribution was markedly more variable closer to the coast.Redox potentials varied in the range from3.9%to309% of the average.Thus the observed heterogeneity in mercury con-centrations(Fig.2)can be attributed to sediment texture differ-ences(in coastal areas)and combined effect of both organic carbon and redox conditions variabilities.
Concentrations of total mercury(THg),organic mercury (HgOrg)and methylmercury in marine sediments from the South-ern Baltic varied in the range5.8–225(average:103)ng gÀ1dry weight,90–1240(320)pg gÀ1dry wt.and60–940(230)pg gÀ1
dry Fig.1.Distribution of sediment sampling stations.
wt.,respectively (Fig.3).The highest levels of THg and MeHg were found in sediments from the Gdansk Deep and the vicinity of the Vistula mouth,respectively.
The lowest concentrations of both THg and MeHg were found in sediments collected close to the Odra mouth located in the Bay of Pomerania.This can be attributed to the morphology of the Odra
river estuary,where the Szczecin Lagoon acts as ‘filter’for the river run-off discharged to the Pomeranian Bay (see Fig.1).In the case of the Vistula River estuary,the morphology is different –the river run-off and the loads of chemicals it carries are discharged directly
to the Gulf of Gdan
´sk (Pempkowiak et al.,2000).Craig (1986)reported concentration ranges of 200–400ng g À1THg for uncontaminated marine sediments,whereas heavily pol-luted sediments in urban,industrial or mining areas can contain up to 100l g g À1of total mercury.Sedimentary levels of THg and MeHg reported in the literature are presented in Table 2.Our results indicate that the mercury concentrations averages and ranges,determined in sediments from the study area,are lower than those reported in other areas (Covelli et al.,2001;Jin et al.,2012;Kannan and Falandysz,1998;Mzoughi et al.,2002;Spada et al.,2012)with the exception of the level reported by Asmund and Nielsen (Asmund and Nielsen,2000)who indicated back-ground mercury levels of 24ng g À1in sediments from the Green-land Shelf.The highest values were recorded in sediments from Gulf of Trieste (the Adriatic),influenced by the contaminated river Soca/Isonzo,for centuries draining the cinnabar-rich deposits of the Idrija mining district,in the Northwestern part of Slovenia (Covelli et al.,2001).
MeHg concentrations measured in the study area are character-istic of anoxic polluted sediments (Kwokal et al.,2002).Concentra-tions measured within this study are lower than these reported for period 1992–1994by Kannan and Falandysz (1998)by a factor of two for the same region,a feature difficult to explain taking into account that both seasonal and spatial variability of mercury con-centrations do not exceed 20%of the average,and the fact,that total mercury concentration observed in this study are comparable to the values observed in 1993–1995(Pempkowiak et al.,1998).This suggests a change in methylating potential of the Baltic sediments since 1990s,which could be attributed i.e.to the overall improvement of oxic conditions on the bottom (Feistel et al.,2008).The contribution of methylmercury to total mercury in sedi-ments of the study area ranged from 0.14%to 1.05%which falls within the range reported in the literature for marine environ-ments (Cossa et al.,1996;Mason et al.,1994).This might suggest a low methylation potential of marine sediments in the study area.Correlation analyses showed that THg was strongly correlated with MeHg in sediments (Spearman R =0.82,p <0.01).The established relation is calculated for all analyzed samples,except three samples collected close to the Vistula mouth,(Fig.4),since the samples in question were collected shortly after major flood in May,2010.The flood has introduced large loads of both mercury and organic matter to the Baltic Sea (Saniewska et al.,2014),and may well explain the elevated concentrations of both THg and MeHg there.
In the remaining areas,reducing conditions were observed in sediments (the measured red-ox potential was in the range from À122to 66mV).The measured redox potential indicates that the conditions in sediments were appropriate for the sulfate reducing bacteria to reduce sulfate to sulfide.As a result labile mercury forms are transformed to mercury sulfide (Bełdowski and Pempkowiak,2007)that is sparingly soluble in aqueous solution.Once deposited as HgS,mercury is presumably not available for methylation (Boening,2000).However,bioturbation or physical mixing can introduce oxygen to sediments that leads to oxidation of HgS and thus remobilize a fraction of HgS (Stein et al.,1996).Moreover,even within the same location,the percentage of methyl mercury varied to some extend suggesting that other factors such as organic matter and microbial activity may influence or even play a significant role in the methylation process (Bełdowski et al.,2009)Most pronounced differences were observed in the Born-holm Deep (0.21–1.03%)and in the vicinity of river mouths (Vistula –0.14–0.61%;Odra –
0.64–1.05%).
Fig.2.Concentrations of total mercury (A),organic mercury (B)and methylmer-cury (C)in sediments of the Gdansk Deep (GD),Gotland Deep (GO),Bornholm Deep (BO),Gdansk Bay (V)and Pomeranian Bay (O).Bars represent the three seasonal samplings (Spring,2009;Autumn,2009;Spring,2010).Error bars represent one standard deviation of three samples collected within 1km 2at a given location.
Table 1
Median values in the studied sediments and ranges of organic carbon (Corg)concentration (mg/g),fine fraction contribution (<0.063)(%)and redox potential (Eh)(mV).
Corg (mg/g)
<0.063(%)
Eh (mV)
GD 7.9(6.2–9.1)92.61(89.65–94.87)À76(À129À+9)GO 11.8(10.8–12.9)87.94(83.58–91.51)+5(À15À+15)BO 7.7(6.3–9.1)62.72(59.41–66.72)+47(+30À+66)V 4.4(3.6–5.2) 4.23(2.54–5.99)+158(+76À+236)O
2.5(2.1–
3.0)
0.93(0.49–1.31)
+234(+221À+245)
Bulletin 87(2014)388–395391
Organomercury compounds are those in which mercury is bonded directly to the carbon atom e.g.CH3Hg(I)and C2H5Hg(I) (Hintermann,2010).Several extracting agents were used for sepa-rating organomercury compounds from sediments so far:toluene (Miller et al.,1995),chloroform(Eguchi and Tomiyasu,2002; Tomiyasu et al.,2000)and dichloromethane(Renneberg and Dudas,2001).However,with the use of these solvents not only the organomercury compounds but also a fraction of the mercury complexed by organic ligands is extracted(Eguchi and Tomiyasu,2002).In this study chloroform was used as an extracting agent to separate this operationally defined fraction of mercury,called here organic mercury(HgOrg).Obviously organic mercury com-prises methyl mercury as defined in this study.The average con-centration of mercury in this fraction in the study area is 0.42ng gÀ1(range0.10–1.44ng gÀ1).Concentrations of organic mercury in the range:0.9–26ng gÀ1were determined in the mar-ine bottom sediments from the Yatsushiro Sea in Japan(Tomiyasu et al.,2000),in the soil samples from the area strongly polluted
Table2
Total mercury and methylmercury levels reported for sediments in different coastal seas.
Study area THg conc.(ng gÀ1)MeHg conc.(pg gÀ1)Source
Average Range Average Range
Gulf of Trieste5240100–23,30016900200–60,100Covelli et al.(2001)
Adriatic sea
Lagoon of Bizerte13010–650530nd–3200Mzoughi et al.(2002) Mediteranean off Tunisia
Gulf of Taranto2770360–7730108001000–40,000Spada et al.(2012)
Ionian Sea
Jade Bay10835–243––Jin et al.(2012)
North Sea
Vistula Mouth7117–15335468–940This study
(Gulf of Gdansk)
Odra mouth96–137561–94This study
(Pomeranian Bay)
Gdan´sk Deep315220–420––This study
Gulf of Gdansk16437–88064535–1700Kannan and Falandysz(1998) Baltic Sea
Gdan´sk Basin17628–473Bełdowski and Pempkowiak(2007) Gdan´sk Deep190130–370Pempkowiak et al.(1998) Bornholm Deep64Bełdowski and Pempkowiak(2007) Bornholm Deep5625–84This study
392J.Bełdowski et al./Marine Pollution Bulletin87(2014)388–395
with mercury near the cinnabar mine and processing plant: 1–28ng gÀ1(Bloom and Katon,2000;Bloom et al.,2003;Miller et al.,1995),or near a chlor-alkali producing plant:9ng gÀ1 (Bloom and Katon,2000;Bloom et al.,2003).Much higher concen-trations of organic mercury were found near a gold mine: 1000ng gÀ1(Miller et al.,1995).
Contribution of HgOrg to THg in the study area amounts to2%, on the average.In general,the contribution of HgOrg to THg in sed-iments of other marine areas is low.For example,in bottom sedi-ments from the Minamata Bay(Japan),the contribution varied from1%to4%.On occasion it can be much higher,however–e.g. in the Kagoshima Bay(Japan)where submarine fumaroles affect mercury speciation the range was from7%to37%,(Eguchi and Tomiyasu,2002;Sakamoto et al.,1995).In the samples of soil and sediments from the regions polluted with mercury e.g.near cinnabar mines or near chlor-alkali plants,the contribution of organic mercury to the total mercury concentration was very low (Bloom and Katon,2000;Bloom et al.,2003;Martian-Doimeadios et al.,2000;Miller et al.,1995;Renneberg and Dudas,2001).
There is a linear dependence between methyl mercury and organic mercury in the analyzed sediments.Relationships pre-sented in Fig.5shows that in the study area it is possible to estimate methylmercury using the organic mercury concentration,with a limited certainty,however.The contribution of methylmercury to organic mercury concentrations in sediments ranges from42%to 77%,while the dependence sets the contribution of MeHg to HgOrg at63%.
In the Southern Baltic,accumulation areas of sediments are located in the so-called deeps,at the depths greater than80m. As the halocline,in the Southern Baltic,persists at the depth of 60–70m the sediments of the accumulation areas are anoxic, and characterized by low red-ox potentials.Relationships between MeHg,HgOrg and redox potentials are linear with high Spearman rank coefficients(R=À0.75for MeHg and R=À0.71for HgOrg), especially for MeHg(Fig.6)and both are statistically significant with p<0.01.This suggests,that although both forms of mercury are closely related,organic mercury fraction includes also species that do not comprise mercury bonded to organic carbon atoms, i.e.mercury complexed by humic substances.Such complexes may comprise up to40%of THg in the Baltic sediments (Bełdowski and Pempkowiak,2007).Obviously not all of the com-plexes are extracted with organic solvent,as the contribution of organic mercury fraction to total mercury is just2%.
High concentrations of methyl mercury and organic mercury in the vicinity of the Vistula mouth in sediment samples collected in 2010were measured.These are associated with relatively high val-ues of the redox potential(76–115mV)and thus seem to contra-dict the conclusion.However,mercury could be methylated in the inundated areas,and brought to the Baltic Sea as methyl mer-cury(Saniewska et al.,2014),during the extremeflood in2010. Methylation of mercury in inundated areas was reported in other areas of the world(Heaven et al.,2000).
To characterize concentrations of sedimentary mercury in a given area one sample collected at a single sampling point is insuf-ficient as substantial variability at short distances needs to be taken into account.This study proved that in the Southern Baltic the short distance spatial variability,as characterized by the Rela-tive Standard Deviation,is close to20%of the average(n=3).The highest variability was observed in coastal areas,which results most probably from variable riverine input of mercury,and inho-mogeneity of sediments in the shallow areas.
Concentrations of total mercury in the Baltic surface sediments exceed the background values by a factor of three(Pempkowiak, 1991).Despite this sediments of the Southern Baltic can be regarded as moderately contaminated with mercury on the back-ground of sedimentary concentration in other coastal areas.
The obtained results characterize distribution of mercury spe-cies in sediments of the Southern Baltic Sea.The highest concentra-tions of THg were found in the stations located in the Gdansk Deep. MeHg concentrations,measured in this study,are characteristic of anoxic polluted sediments world-wide.The so called organic
J.Bełdowski et al./Marine Pollution Bulletin87(2014)388–395393
mercury that is operationally defined as mercury extracted with organic solvents is well correlated to methylmercury.The latter constitutes,on average some62%of organic mercury.Methylmer-cury originates from the in situ methylation of inorganic mercury as there is a highly significant linear dependence with red-ox potential.However,there are indications that methyl mercury is also brought to the Southern Baltic with river run-off.
In the coastal areas,surface sediments showed high levels of MeHg,most likely,as a response to the extremeflood that affected the region shortly before sampling.Since majorfloods occur every few years,it seems that methyl mercury originating fromfloods does not persist for long time periods(several years)(Huzarska, 2013;Wielgat-Rychert et al.,2013).According to previous studies, most mercury in Baltic sediments is converted to either insoluble HgS or mercury(II)complexed to refractory organic substances (Bełdowski et al.,2009).This suggests that the historical study (Kannan and Falandysz,1998)showing methylmercury concentra-tions twice higher than the contemporary ones,most likely,repre-sented either specific situation that is not typical of the region,or reflects a change that has occurred in the region since1992–94 until present day.Gdan´sk Deep is characterized by the highest con-centration of total mercury,organic mercury and methyl mercury from all studied accumulation basins(Gdan´sk,Gotland and Born-holm Deeps).This can be attributed to the influence of the Vistula River–the second largest river in the Baltic Sea.Thus it seems,that river borne mercury contamination of surface sediments must be considered,in the Baltic Sea,on equal terms with the atmospheric input of this metal.
Acknowledgements
This study is a part of the Institute of Oceanology Polish Acad-emy of Sciences statutory activities-theme2.2.Mercury analyses were partially performed in the frame of National Science Center project number2011/01/B/ST10/07697.
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