e-mail: milena.horvat@ijs · m. horvat department of environmental sciences, jozef stefan...

Mercury behaviour in estuarine and coastal

environment

M. Horvat

Department of Environmental Sciences, Jozef Stefan Institute,

Jamova 39, Ljubljana, Slovenia

E-mail: [email protected]

Abstract

General facts about the global mercury cycle with particular emphasis on the coastal and oceanenvironment are summarized. In the coastal environment the largest source of mercury isriver-born particulate bound species. This portion of mercury is unreactive and is quicklyburied in nearshore sediments. Only a small fraction of reactive mercury (ionic mercury insolution that is immediately available for reaction) originates from river inputs. The mostimportant source of reactive mercury in the coastal and oceanic environment is throughatmospheric input and via upwelling. Biologically-mediated processes, mainly connected toprimary production, are responsible for active redistribution of reactive mercury. In thisprocess a large part of reactive Hg is reduced to elemental mercury which is returned to theatmosphere by evasion, while the rest is scavenged by particles and transported to deeperoceanic waters. Because of the active atmospheric mercury cycle oceans acts as a source and asink of atmospheric mercury and the global oceanic evasion is balanced by the deposition.Current studies show that methylated species are primarily formed in the deeper ocean and themam source of monomethylmercury (MMHg) compounds in coastal areas is throughupwelling of oceanic waters and from in-situ methylation in coastal waters. All theseenvironmental processes occur at extremely low concentration levels of mercury species;however MMHg in marine organisms accounts for a high proportion of this toxic compoundsowing to its property for bioaccumulation and biomagnification. Coastal areas on the localscale may account for geochemical differences that significantly influence the conversionbetween various Hg species. In order to assess the impact of mercury in contaminated and non-contaminated coastal areas on man and his environment, it is of the greatest importance tounderstand these processes. The paper also identifies uncertainties and gaps in currentknowledge of mercuy cycling

1 General facts

Mercury and its compounds are extremely hazardous. The most toxic aremonomethylmercury (MMHg) compounds which represent a health risk,particularly to the foetal neurosystem . The risk to public health is evidenced infish consumption regulations that have been issued in Canada, Scandinavia, by

Transactions on Ecology and the Environment vol 14, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541

78 Water Pollution

more than 30 states, the US FDA, the World Health Organization (WHO) andnumerous other governments. In the USA, the Clean Air Act Amendmentsrequire an assessment of health risk to humans and wildlife caused by Hgemissions. Also, the potential adverse effects of atmospheric Hg deposition oncoastal waters is contained in a planned Protection Agency report to Congress .

In the environment mercury can exist in a number of different physical andchemical forms with a wide range of properties. Biogeochemical conversionbetween these different forms provides the basis for mercury's complexdistribution pattern in local and global cycles and for its biological enrichmentand effects. The most important chemical forms are: elemental mercury (Hg°),divalent inorganic mercury (Hg(II)), monomethylmercury (MMHg), anddimethylmercury (DMHg). There is a general biogeochemical cycle by whichthese different forms may interchange in the atmospheric, aquatic and terrestrialenvironments.

Most studies of Hg cycling have, so far, been made in terrestrial systems, withthe biogeochemistry of Hg in fresh water systems , even though it is wellknown that the primary exposure of humans to MMHg is through theconsumption of marine fish and fish products . In principle, Hg cycles interrestrial and marine aquatic systems are similar with some distinct differences.The most important feature in both systems is the in-situ bacterial conversion ofinorganic Hg species to the more toxic MMHg, which concentrates in fishmuscle?".

The key feature that influences mercury distribution in aquatic environmentsis the high stability of its associations with sulphur and carbon, the stability of itsvolatile elemental form and its strong affinity to particles. Consequently, mostinorganic and organic Hg appears to be bound to particles, colloids and highmolecular weight organic matter, where it is probably coordinated with sulphurligands on particles (distribution coefficients are in the 10 -10 ml/g range). Inturbid rivers most mercury, therefore, is transported by suspended matter. Onlya small part of Hg in fresh, estuarine and sea water is likely to be present indissolved form **"**.

After entering the atmosphere mercury exchanges and cycles through theatmosphere to be deposited in the ecosystem, almost exclusively as Hg(II).When Hg enters a surface (soil or water) Hg(II) can be methylated to MMHg.This is the first step in aquatic and terrestrial bioaccumulation processes. Themechanism of synthesis of MMHg is not very well understood. The main factorsthat affect the levels of MMHg in fish are the dietary trophic level of the species,the age of the fish, microbial activity and the mercury concentration in the upperlayer of the local sediment, its dissolved organic carbon content, salinity, pH,and redox potential . Recent work suggest that sulphato-reducing bacteria arethe most important methylating agents along with environmental conditionsexisting in transition regions between oxygenated and anoxic conditions .Methylation-demethylation reactions are assumed to be widespread in theenvironment and each ecosystem attains its own steady state equilibrium withrespect to the individual species of mercury. However, owing to


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bio accumulation of MMHg, methylation is more prevalent in the aquaticenvironment than demethylation.

Once MMHg is formed, it enters the food chain by rapid diffusion and tightbinding to proteins in aquatic biota, and attains its highest concentrations in thetissues of fish at the top of the aquatic food chain due to biomagnificationthrough the trophic levels. For example, most predatory fish species showMMHg values > 1 ppm, while concentrations in water are commonly < Ippt(e.g. amplification factor of about 10 ' . Although MMHg is the dominantform of mercury in higher organisms, it represents only a very small amount ofthe total mercury in aquatic ecosystems and in the atmosphere.

2 Global mercury cycle

Mercury vapour is released into the atmosphere from a number of naturalsources (e.g. the ocean surface and other water surfaces, soils, minerals, andvegetation located on land, forest fires and volcanoes) and throughanthropogenic emissions (metal extraction processes, agricultural uses, paints,waste disposal, burning of fossil fuels, smelting of ores and other industrialminerals, and from power plants burning fossil fuels for electricity generation).Recent studies ^ show that human activity contributes about 50 to 75% (e.g.3500 to 4500 tons) of the total yearly input from all sources (7000 tons/y)(Figure 1). About half of the anthropogenic emissions (approx. 2000 tons/y)appear to enter the global Hg cycle, while the other half is deposited locally. Asa consequence human activities have tripled the concentrations of Hg in theatmosphere and in the surface ocean. It is estimated that 60% (approx. 5000tons) of the total Hg is deposited on terrestrial environments (30% of thesurface of the Earth) and the remainder to the ocean. This is due to the oxidationof Hg in the abundant terrestrial aerosols. The ocean receives about 90% of itsHg through wet and dry deposition as Hg(II) and the remaining (200 tons/y)from river inflows. The particulate scavenging and removal to the deep ocean isequal to the riverine Hg flux (200 tons/y). Due to biological reduction ofdeposited Hg(II) in the mixed layer of the ocean and its evasion most Hg°deposited (2000 ton/y) is re-emitted to the atmosphere. This active process andthe minimal removal of Hg to the deep ocean makes terrestrial systems thedominant sink.

The model presented in Figure 1 is based on rather limited data anduncertainties may account for a factor of two or more. The main reasons for thisuncertainty are the limited data for Hg concentrations and speciation inprecipitation, both temporally and spatially. Almost no data are available for theSouthern Hemisphere, Asia and the Arctic coastlines. Tropical regions also needto be assessed, as the impact of current activities (e.g. biomass burning and goldextraction ) on the global Hg cycle and budget may be significant. The flux ofMMHg to the oceans is also unknown. In particular, coastal rain could be animportant source of MeHg.

Another reason for uncertainty is the limited data available on evasion of Hgfrom the ocean surface. This process is controlled by the ability of the system to


80 Water Pollution

reduce reactive Hg. Current studies show that this process is directly connectedto primary productivity. It has also been shown that there are ocean areaswhere's atmospheric deposition exceed the evasion ( e.g. the Siberian Arctic)and other where evasion exceeds the deposition (e.g. the productive zones atlower latitudes). Probably, net atmospheric transfer of mercury to higherlatitudes is occurring. More studies need to be done in productive and non-productive ocean areas in order to understand better global Hg transport andcycling.

The current model also does not take into account some other potentialsources of Hg in the ocean. These are hydrothermal inputs and oil and gasdeposits. It is assumed that most inorganic mercury from hydrothermal ventswould be trapped on particles and precipitated. However, the extent to whichthese deposited minerals are remobilized is unknown. Many more studies needto be done, particularly in regions where such activity occurs (Pacific rim,Aleutian islands, Mediterranean, etc.). Oil and gas deposits also contain elevatedHg concentration and the flux from such deposits may be significant in coastaloceans and shelves where these deposits formed seeps into the ocean.

The mercury flux to the deeper ocean only accounts for about 10% of theocean input**. Some recent studies suggest that this flux may be much smaller,but no reliable study has been done so far. Sedimentation as well asremobilization of Hg from sediments in different geological parts of the deepocean and continental shelves should be studied to improve these figures.

It is important to note that due to extremely low mercury concentrations inwater samples (Table 1) the measurement (sampling and analysis) protocol mustbe carefully designed, in particular, if speciation of mercury forms is intended.Due to possible erroneous analytical data caused by sample contaminationand/or speciation change during storage of samples a number of studies in thepast should, therefore, be treated with great reservation. Due to remarkableimprovements in analytical techniques over the last 10 years the reliability ofdata have improved significantly . However, further development andimprovements of techniques for Hg speciation, particularly for identification ofHg-organic associations, are needed.

3 Mercury in the oceanic environment

Concentration levels of dissolved Hg in the oceans waters are very low (Table1), usually below 1 ng/1, of which a portion can be present as dissolvedchlorocomplexes and the remaining is bound to particles, which are not abundantin the ocean environment, except in highly productive zones. Other forms of Hgin ocean waters are the elemental Hg form, particularly in highly productivesurface waters, and the organic mercury forms (MMHg and DMHg) which arefound in deeper waters . It was shown that in the ocean


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HAIR 25 Mmol

EP 98% Hg'2% Hg,

DEPOSITION

Kg'

Hg(ll)

Figure 1. The current global Hg cycle, (adapted from Fitzgerald and Mason**.(1 Mmol is equal to 200 tons of Hg)

environment DMHg is the dominant methylated compound in contrast tofreshwater where MMHg predominates. Even more, it was hypothesized thatDMHg may be the primary source of MMHg in the oceans, which means thatthe mechanisms of MMHg production may be different from fresh water

1 8 74systems. But this still needs further investigation ' .

The present oceanic data are very sparse. As already mentioned one of themost important components of the Hg cycle the ocean environment is the in-situproduction and water-air transfer of elemental Hg*'**' \ In this process most ofthe deposited Hg(II) is reduced to Hg° and returned to the atmosphere and onlya small fraction of Hg is trapped in the ocean. One part of this fraction isremoved by sedimentation, while the other enters the Hg cycle including in-situmethylation. It was estimated that only 2% of the mercury entering the oceans isenough to account for the significant accumulation of MMHg in fish*. Theimportance of Hg° in controlling the production of MMHg in oceanenvironment is clearly evidenced from Figure 2.

4 Mercury in coastal areas

Mercury concentrations in coastal waters tend to be higher than in the open sea,which is due to the greater abundance and deposition of particulate mercury(Table l/\ Particulate Hg dominates Hg partition due to the high suspendedsolids load. Dissolved Hg concentrations are low and vary in the range from 0.3to 1.6 ng/1. Particulate Hg concentrations are rather variable ranging from 0.05Hg/g to 1 |ig/g in non-contaminated areas and reach up to several |ig/g incontaminated coastal regions**. Hg enrichment in particles is proportional totheir organic carbon content, particularly in areas where phytoplankton isabundant. MMHg compounds can be found as up to 3% of total particulate Hg,while dissolved MMHg is present at very low concentrations (< 0.DMHg is undetectable in coastal waters.


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It is estimated that the Hg flux through rivers represents about 10% of theglobal Hg input to the oceans. This figure needs further refinement as it iscalculated on the basis of limited data. For example there is no information oninputs through the largest rivers, such as the Amazon, and others in the SouthernHemisphere, and Asian and Siberian rivers. Very little is known about mercurybehaviour in rivers, even though rivers represent an important mechanism for thetransport of Hg species to other ecosystems. Also the biogeochemistry of Hg ina river system may be different from that in other water bodies.

Table 1. Mercury concentrations in ocean and coastal waters

Riversnon-contaminated

contaminated

Coastal areasnear shoreoff shore

contaminated area

OceanNW AtlanticNE Pacific

Mediterranean Sea

Dissolved Hg(0.45 jim filter)ng/1 (range)

0.3- 1.0"2_4^»

0.5-2.0 *0.2 - 0.4 *0.6 - 2.3 *

0.6-1.30.2 - 0.6*""0.1-0.8 °

Participate Hgpg/g (range)

0.044- 1.40"1.2-30

0.04- 1.88 "

0.1-2.14

low abundanceof participates

Other Hg compounds

MMHg: 1-10% of paniculate Hg< 1 % of dissolved Hg

Hg° : < 0.2 ng/1DMHg : not detected

MMHg: 3% of part. Kg"5-50 pg/1 incontaminated areas

DMHg: up to 58 pg/1 "'

DMHg: < 1 0% of total Hg *-"'**"Hg°: 40 - 80 pg/1 *"'*MMHg: below detection limits

Estuaries are very important parts of the ocean margin and only a few studiesof Hg behaviour have been done so far ,28,3132 Mercury cycling in estuaries ismore complex and variable due to differences in chemical and physical gradients.The concentrations of Hg are more variable than in other coastal areas becauseof a direct relationship to the source of contamination. Mercury behavioursignificantly differs from one estuary to another. In some estuaries dissolved Hgbehaves conservatively (it is not removed from water), while in other it behavesnon-conservatively due to coagulation of colloids and subsequent sedimentation.Colloidal ligands, particularly those with thiol functional gropus play animportant role in mercury cycling in estuaries ". Elemental Hg is present outsidethe turbidity plumes and is related to the presence of phytoplankton . Maximumconcentrations of MMHg occur in hypoxic conditions and in the fluvial part thatis rich with organic particulates. DMHg was also detected at very lowconcentrations (<20 pg/1) .

In order to calculate accurately the input of Hg from rivers and coastal areasmore work needs to be done to understand processes in estuaries, particularly,the relationship between Hg and colloidal and participate phase, and the fate ofHg during estuarine mixing.


!Hg'!<

\

-*!"

, \

wet anddry

deposition

r

*N|(CHj

i

^. \,-, k 7+iW •_ .!H

^ ^ ^ i

)2Hg!

r*

g 1 " Hg

dry (?) andevasion wet

deposition

i* /* t

|(CH Hgi

run-off

Hgp - paniculate HgHg - divalent, reactive HgCHgHg - monomethyl Hg(CH Hg - dimethyl Hg

burial

Fig. 2: Cycling of mercury in coastal and ocean environment.


84 Water Pollution

4.1 Mercury in coastal sediments

In general, sediments are considered to be an ultimate sink of particulate Hg.However, they also play an important role in mercury cycling (Figure 2). It iswell documented that MMHg and DMHg can be produced in marinesediments . As important factor influencing MMHg production is theconcentrations of sulphides . Net methylation rates were favoured in anaerobicoxic sediments. With the increasing sulphide concentration MMHg is lost bydismutation - a process by which MMHg is transformed into DMHg and HgSvia decomposition of dimethylmercury sulphide. This process has beenexperimentally proven in natural environments such as highly reducing estuarinesediments and flood plain soils of polluted rivers . Such reactions could alsogenerate gaseous Hg°, due to reduction of Hg(n) ^ Both elemental Hg andDMHg are in gaseous form and are lost from anoxic coastal sediments byvolatilization (Figure 2). MMHg in sediments accounts for less than 1% of totalHg and originates from settling particles and from internal production associatedwith the sulphato-reducing bacterial activity *^\ The diffusion of MMHg andHg(II) from pore to overlying waters is generally low . It is governed bymolecular diffusion and biological mobilization by trophic transfer.

5 Bioaccumulation

It is well known that mercury is an accumulative toxic trace metal. The mercuryconcentration in an organism depends on environmental factors such as itsconcentration in seawater, its position in the food chain, and in particular, onthe chemical species of Hg to which the organism is exposed. The uptakeefficiency of inorganic Hg is much smaller (less than 10%) than for MeHg (near100%). Phytoplankton and seaweeds constitute the first level of the food chainand take up inorganic and organic mercury directly from seawater. Assuming anaverage concentration of Hg in seawater, the concentration factor is about 5000-10 000. Uptake at higher trophic level may occur primarily through the foodchain. However, other mechanisms should also be investigated such as directuptake of MMHg and DMHg through gills. This process may account forconsiderable uptake in open ocean fish, especially long-lived species and marinemammals Current understanding of the marine food chain is limited and manymore studies need to be done. Even though numerous papers on mercury levelsin marine biota were published, typical mercury concentrations are difficult toidentify . Various organisms have differerent mercury concentrations, andbiological tissues within the same organism differ considerably. Highest Hglevels are found in long-lived fish in which its concentration frequently exceeds500 ng/g, FW. Consumption of 200 g of such fish will result in an intake of 100j g of mercury (predominantly in methylated form). This represents one-half ofthe provisional tolerable weekly (PTWI) intake in humans^. Problemsassociated with Hg in the marine environment are directly related toaccumulation of MMHg in the aquatic food chain; however this process in themarine environment is poorly understood. Even more, the budget for MMHg in


Water Pollution 85

the oceans is rather uncertain, and much more work needs to be done toimprove the accuracy of these figures, which are essential for accurateassessment of mercury impact on man and ecosystems.

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