speciation of dissolved iodine in the waters of a humic-rich estuary

Ž .Marine Chemistry 69 2000 179–192www.elsevier.nlrlocatermarchem

Speciation of dissolved iodine in the watersof a humic-rich estuary

Perran L.M. Cook a,), Peter D. Carpenter a,1, Edward C.V. Butler b,2

a Department of Applied Chemistry, RMIT UniÕersity, GPO Box 2476V, Melbourne, Victoria 3001, Australiab CSIRO Marine Research, Marine Laboratories, GPO Box 1538, Hobart, Tasmania 7001, Australia

Received 20 April 1999; accepted 10 November 1999

Abstract

This paper reports the findings of a study into iodine speciation in the humic-rich waters of the Huon Estuary, Tasmania,Australia. Water samples were taken from the estuary at various locations during both summer and winter. The samples wereanalysed for a range of parameters including iodate, iodide, total iodine, nutrients, chlorophyll a, dissolved organic carbonŽ .DOC and salinity. Total iodine behaved conservatively within the estuary irrespective of season. Iodate concentrationsvaried linearly with salinity but became undetectable in the low salinity end of the estuary. Iodide concentrations showed nocorrelation with salinity, but showed a positive correlation with total dissolved phosphorus during summer suggesting iodideconcentrations within the estuary are controlled by biological activity during, or immediately after, periods of highproductivity. ‘‘Organic’’ iodine is produced within the estuary and the fraction of iodine present in this form increases in thelow salinity waters where DOC concentrations are highest. q 2000 Published by Elsevier Science B.V. All rights reserved.

Keywords: iodine speciation; iodate; iodide; humic substances; estuaries; nutrients; DOC

1. Introduction

Iodine exists in the ocean at a total concentrationŽ .of about 0.5 mM Wong, 1991 . In seawater with a

Ž y.pH of 8.1 and a p´ of 12.5, iodate IO is the3

thermodynamically favoured form of iodine. At ther-modynamic equilibrium, the ratio of iodate to iodide

) Corresponding author. Present address: School of Chemistry,University of Tasmania, GPO Box 252-75, Hobart, Tasmania7001, Australia. Tel.: q61-3-6226-2174; Fax: q61-3-6226-2858;E-mail: [email protected]

1 E-mail: [email protected] E-mail: [email protected].

13.5 Žhas been calculated to be 10 see Spokes and.Liss, 1996, for example . However, reducing pro-

cesses combined with the slow oxidation kinetics ofŽ .iodide Wong, 1991; Truesdale, 1995 mean that the

observed ratio is usually between 5–10 in oceanicŽ .surface waters Luther and Cole, 1988 . Such devia-

tions from thermodynamic equilibrium are often evenmore pronounced in estuaries due to their high bio-geochemical activity. Iodate has been observed to

Žbehave conservatively in estuaries Smith and Butler,.1979; Takayanagi and Cossa, 1985 , although this

may have been on account of the short flushingŽtimes of the estuaries at the time of study Ullman et

.al., 1988 . On occasions that iodate has been re-ported to behave non-conservatively, it has become

0304-4203r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved.Ž .PII: S0304-4203 99 00104-8

( )P.L.M. Cook et al.rMarine Chemistry 69 2000 179–192180

undetectable below salinities between 15 and 25ŽLuther and Cole, 1988; Ullman et al., 1988; Carpen-

.ter et al., 1991; Luther et al., 1991 .The reasons for the observed non-conservative

behaviour of iodate in estuaries are still unclear.There is now much evidence to suggest that biologi-cal activity plays a major role in the reduction of

Žiodate Elderfield and Truesdale, 1980; De LucaRebello et al., 1990; Brandao et al., 1994; Truesdale,˜

.1994; Moisan et al., 1994;, Farrenkopf et al., 1997 .Possible abiotic processes of iodate removal includechemical reduction by reduced species such as HSy,

2q 2q ŽMn and Fe Luther and Cole, 1988; Ullman et.al., 1988; Carpenter et al., 1991 and sedimentary

Ž .processes Francois, 1987; Ullman et al., 1988 .Humic substances can react with iodine species

such as iodate, to form electrophilic organic iodineŽ .compounds Francois, 1987 . It has also been discov-

ered that a fraction of iodine present in freshwaters isŽbound up within the humic fraction Reifenhauser¨

.and Heumann, 1990 . The reduction of iodate in thehumic-rich estuarine waters of southwestern Tasma-

Ž .nia in Macquarie Harbour Carpenter et al., 1991Žand Port Davey Butler and Plaschke, unpublished

.results has led to speculation that humic substancesplay a role in iodine speciation in these waters.

Iodide is often operationally defined as the differ-ence between total iodine and iodate. Since theadvent of the square-wave cathodic stripping voltam-

Ž .metry method of Luther et al. 1988 , only twopublished surveys of directly measured iodide con-

Žcentrations in estuaries have been undertaken Luther.et al., 1991; Abdel-Moati, 1999 . Luther et al.’s

survey found iodide behaved conservatively in thesurface waters of Chesapeake Bay. It also found thatas much as 70% of the iodine in the Bay existed inan ‘‘organic’’ form. This compares with the usual

Ž .5% in ocean waters Wong, 1991 and 15% in theŽ .Yarra Estuary Smith and Butler, 1979 . As such,

Žthere is a dearth of information on iodide directly.measured and organic iodine distribution in estuar-

ine waters.This study has determined iodine speciation

Ž .iodide, iodate and total iodine and a range of otherŽ Žparameters salinity, DOC, season, biomass as indi-

. .cated by chlorophyll a and nutrients with the viewto relating these to possible interconversion pro-cesses for iodine species. The study site was the

Huon River estuary in southeastern Tasmania, whichderives much of its run-off from buttongrass moor-land and wet scrub founded on peat soils, resulting inthe waters of the estuary containing high concentra-

Žtions of humic substances or coloured dissolved.organic matter — CDOM .

2. Materials and methods

2.1. Study area and sampling

The Huon Estuary is a drowned river valley andbegins at Ranelagh, south of Hobart, stretching for

Ž39 km southeast to Huon Island Fig. 1, and Gal-.lagher, 1996 . The estuary has an estimated mean

Žflushing time of 7 days Hunter, personal communi-.cation , with a riverine input averaging ca. 90 cumecs.

Ž .It is a microtidal mean tidal amplitude ca. 1 m ,salt-wedge system, with a tendency to become par-tially mixed near the mouth. The waters of theestuary are generally close to oxygen saturation apartfor oxygen-depleted subhalocline waters at the top of

Ž .the Egg Islands Fig. 1 , influenced by anoxic sedi-ments. Port Cygnet is a somewhat isolated marineside-arm of the waterway, with small and sporadicdischarge from two rivulets. The CSIRO MarineLaboratories in Hobart are currently completing a

Fig. 1. Map of the Huon Estuary, southeastern Tasmania, withŽ .sampling locations indicated v HES 4; ` HES 5 .

( )P.L.M. Cook et al.rMarine Chemistry 69 2000 179–192 181

3-year interdisciplinary environmental study of theHuon Estuary.

Samples from the Huon Estuary were taken aspart of the spatial surveys HES 4 during the austral

Ž .summer 24–28 February 1997 and HES 5 duringŽ .the austral winter 16–20 June 1997 . Samples were

Ž .collected in 5-l 0.5 m long Niskin bottles. A 1-lsubsample was then taken into an acid-washed LDPEbottle and the sample stored on ice until returned tothe laboratory. This sample was then filtered under

Ž . Ž .vacuum F125 Torr using polycarbonate Sartoriusfiltration units, which were rinsed with ultra-pureŽ .Milli-Q, Millipore water between samples. Thefilters used were acid-washed 0.45 mm cellulose

Ž .acetatercellulose nitrate Millipore HA . The filtratefor determination of iodine species was then trans-

ferred to acid-washed bottles and stored at 48C in thedark. Samples were analysed for iodine species within

Ž . Ž .5 HES 4 and 2 HES 5 months of collection.Iodine speciation has been found to be preserved

Ž .over this interval Campos, 1997 .

2.2. Sample analysis

Iodate was determined directly using differentialŽ . Žpulse polarography DPP Herring and Liss, 1974 as

.modified in Butler, 1981 and iodide by square-waveŽ . Žcathodic stripping voltammetry SWCSV Luther et

.al., 1988 . A slight modification of the SWCSVmethod was the addition of 0.003% Triton X-100 tothe cell rather than 0.001%. This was to reduceinterference from humic substances. The conditions

Ž . Ž .Fig. 2. The relationship between iodate = surface, q subsurface and total iodine ` surface, v subsurface with salinity in the waters ofŽthe Huon Estuary for summer and winter subsurface values have been omitted from the linear regression analysis, as have zero iodate

.values .


were as follows: DPP polarograms were run fromy0.6 to y1.5 V at a scan rate of 8 mVrs; forSWCSV deposition time 30 s, quiescent time 5 s,deposition potential y0.1 V, scan from y0.1 toy0.7 V. ‘‘Total iodine’’ was determined as iodate

Žafter hypochlorite oxidation Takayanagi and Wong,. Ž .1986 . Luther and Cole 1988 report that this method

of oxidation recovers 100% of iodine in iodoaceticacid, but only 50% of the iodine in thyroxine. Theseresults imply that this method provides an empiricalmeasure of total iodine in estuarine waters; UVphoto-oxidation followed by DPP will mineralisemore refractory organic iodine compounds, where

Ž .they exist Butler, 1981; Wong and Cheng, 1998 .Polarography and voltammetry were performed

using a Metrohm 693 VA processor connected to aMetrohm 694 VA stand. Reproducibility experiments

were conducted using seawater diluted 1:1 with ul-tra-pure water, the results were 2% RSD for iodate at

Ž .170 nM ns7 and 8% RSD for iodide at 40 nMŽ . Ž .ns7 . Luther and Campbell 1991 have quoted adetection limit of 20 nM for iodate using DPP; inthis work, the detection limit would have been be-tween 20 and 50 nM depending upon concentrationof humic substances and instrumental performance.

Ž .Luther et al. 1988 quote a detection limit of be-tween 0.1 and 0.2 nM for iodide using a 180-sstripping step, once again this was slightly higherŽ .approximately 1 nM in waters containing highhumic concentrations. As there are no previouslypublished studies of iodide measurements using the

Ž .method of Luther et al. 1988 in humic-rich waters,recovery trials were conducted. Water from the HuonRiver was spiked at 80 and 40 nM with iodide.

Ž .Fig. 3. The relationship between iodide ` surface, v subsurface and salinity in the waters of the Huon Estuary during summer andwinter. The marked data points indicate samples taken from Port Cygnet.


Subsequent determinations yielded 110% recoveriesfor both samples. ‘‘Organic’’ iodine concentration isdefined as the difference between the concentrationsof total iodine and the sum of iodate and iodideŽw yx w yx.IO q I . Salinity, DOC, chlorophyll a and3

total phosphorus were measured using standard pro-cedures as part of the Huon Estuary Study currentlybeing conducted by CSIRO Marine Research, HobartŽ .CSIRO Marine Research, unpublished information .

3. Results

3.1. Total iodine and iodate

Total iodine was found to behave conservativelywith salinity in the surface waters for both summer

and winter; the slope was invariant with season inŽ .surface waters Fig. 2 . Iodate also showed a linear

relationship with salinity down to a ‘‘threshold’’salinity where iodate became undetectable. This

Ž .threshold salinity was different for winter 8 andŽ .summer 12 — bestfit line intercept , as were the

absolute iodate concentrations.Subsurface concentrations of iodate and total io-

dine were discrepant, with values on occasion fallingbelow the expected line of dilution, especially in

Ž .winter Fig. 2 . This removal of iodine did notappear to be associated with any marked change inany of the other measured parameters.

Whilst iodine species in the riverine end-memberwere not determined during the survey periods, ariver water sample taken in July 1997 had no iodine

Žspecies detected -50 nM total iodine, -1 nM

Fig. 4. The relationship between iodide and total dissolved phosphorus in the surface waters in the Huon Estuary during summer and winter.The marked data points indicate samples taken from Port Cygnet.


.iodide . Given the iodine-poor status of TasmanianŽsoils grazing lands along the lower reaches of the

Huon have a high incidence of sheep goitre, due tolow iodine levels in the soils; Statham and Bray,

.1975 and the relatively pristine nature of much ofŽ .the catchment area Gallagher, 1996 , it is likely that

this situation would prevail throughout the year. Alsoas a result of the pristine nature of the catchments,the waters of the Huon River are also low in conduc-

Ž .tivity, nutrients and SPM Butler et al., 1999 .

3.2. Iodide

Iodide concentrations were only loosely relatedwith salinity; iodide levels tended to be marginallyhigher at the marine end of the estuary than at the

Ž .freshwater end Fig. 3 . The summer sampling foundsporadic high values of iodide. The two highest were

Ž .from the Port Cygnet arm Fig. 1 . Iodide concentra-tions were closely correlated with total dissolved

Ž 2 .phosphorus in summer R s0.922 , but the relationŽ 2 . Ž .collapsed in winter R s0.102 Fig. 4 . No similar

seasonal correlation with chlorophyll a was noted inthese samples; summer — R2 s0.113, winter —

2 Ž .R s0.004 results not plotted . The two outliers inFig. 4 with high iodide levels in summer were thesame Port Cygnet samples remarked upon for Fig. 3.

3.3. Organic iodine

Organic iodine concentrations remained fairlyconstant throughout the estuary during both summer

Ž .Fig. 5. The relationship between organic iodine ` surface, v subsurface concentration and salinity in the waters of the Huon Estuaryduring summer and winter.


Ž .and winter Fig. 5 . However, percentage of organiciodine showed a strong relationship with salinity for

Ž .both the summer and winter sampling runs Fig. 6 .In summer, organic iodine levels appeared to beslightly higher in subsurface waters. When organiciodine is represented as a proportion of the totaliodine present, it can be seen that almost all of theiodine present in the low salinity samples existed asorganic iodine; this decreases to 10–20% organic

Ž .iodine in the marine samples Fig. 6 . DOC andpercentage of organic iodine were closely correlated

Ž 2 .in summer R s0.947 while a similar, but looserŽ 2 . Ž .relation existed in winter R s0.610 Fig. 7 . In

both instances, the greater the DOC concentration,the greater was the proportion of organic iodineformed. Subsurface samples did not show the samerelation between DOC and %organic iodine. In both

summer and winter, the proportion of organic iodineŽ .formed was higher in subsurface waters Fig. 6 .

Low iodate concentrations also appeared to becorrelated with a concomitant increase in the propor-tion of organic iodine present for both summer and

Ž .winter Fig. 8 .

3.4. Proportions of iodine species within the estuary

In summer, the proportion of iodine representedby each species within the estuary was quite closely

Ž .correlated with salinity Fig. 9 . The apparent inter-conversion of iodate to organic iodine was clearlyevident. During winter, there was much more scatterin the relation of each iodine species with salinity,but the trends seen during the summer samplingwere still apparent with iodate less abundant at lower

Ž .Fig. 6. The proportions of iodine present as ‘‘organic’’ iodine ` surface, v subsurface within the Huon Estuary for summer and winterŽ .subsurface organic iodine values have been omitted from the linear regression analysis for winter .


Ž .Fig. 7. The relationship between DOC and the proportion of organic iodine ` surface, v subsurface present in the waters of the HuonŽ .Estuary during summer and winter surface samples only .

salinities while organic iodine became enriched. Theproportion of iodide again remained fairly constantthroughout the estuary in both seasons.

4. Discussion

The Huon Estuary is an example of a ‘biochem-ical’ estuary in the terminology of Sharp et al.Ž .1984 . Influences upon nutrients and other tracesolutes by biota, typically phytoplankton, dominate

Ž‘geochemical’ processes flocculation, adsorption,.etc. that usually arise from high concentrations of

suspended particulate matter. The reasons for this arethat the estuary is microtidal and the tributary streamsdo not themselves have high loads of suspended

Ž .material Butler et al., 1999 . Nevertheless, a factorin addition to biochemical processes does play a rolein estuarine chemistry of the Huon River. Humic

substances that are brought in by terrestrial run-offare this additional influence.

We discuss the iodine speciation below as a resultof these estuarine conditions, and with respect tobroad seasonal factors as seen in the results from thesummer and winter surveys.

4.1. Total iodine and iodate

Since total iodine was conservative summer andwinter, absence of iodate in lower salinity waters onboth occasions must be balanced with the productionof one or more reduced forms of iodine in the watercolumn. Direct determination of iodide indicates itsconcentration is too low to be the form supplantingiodate. Intermediate oxidation states — moleculariodine and hypoiodous acidrhypoiodite — are too

Žreactive in the presence of organic matter Truesdale.and Luther, 1995 to be significant. Organic iodine,

Žin possibly a variety of forms see Reifenhauser and¨


Fig. 8. The relationship between the proportion of organic iodine and iodate concentrations for summer and winter.

.Heumann, 1990 , must be the dominant iodine in theupper estuary. Its source and possible processes offormation are addressed later.

The removal of iodate from the water columndoes not seem to be directly associated with lowoxygen concentrations, with the exception of stationN2 for the summer survey. Reducing sediments andrelatively low dissolved oxygen concentrations un-derlying this station most likely caused the anoma-lously low iodate concentration seen in summer. Thisstation has some parallels with oxygen-deficient holes

Žin the estuary of the Yarra River Butler and Smith,.1985 , but the subhalocline waters are not isolated at

station N2 in the Huon Estuary. Nor does the deple-tion of oxygen approach oxygen-deficient conditions.We note reduction of iodate, but no change in totaliodine concentration; in contrast to the isolated bot-tom waters of the Yarra Estuary, where iodine en-tered from anoxic sediments.

Indeed, it is interesting to note that for subsurfaceŽwaters of the upper Huon Estuary stations R1, N2

.and L1 in the winter survey, both total iodine andŽiodate fell markedly below the line of dilution Fig.

.2 . There is a net removal of iodine from the watercolumn. As this occurred during winter, biologicalactivity is an unlikely explanation. Possible explana-tions for these observations are the following. Geo-chemical reactions within the reducing sediments ofthe upper estuary may be a sink for iodine duringthis time of the year. Irregular storm events mayresult in episodic increase in the input of suspended

Žparticulate matter to the estuary in excess of the lowambient concentrations observed during the surveys

.— Table 1 , or sediment resuspension, with theŽsubsequent sorption of iodine species Neal and

.Truesdale, 1976 . Once settled, the iodine would beretained within the sediment through diagenetic recy-

Žcling processes Ullman and Aller, 1980; Elderfield


Ž .Fig. 9. The proportions of each iodine species within the estuary. ` iodate, v iodide, q organic iodine, = total iodine for summer andwinter. The lines for iodate and organic iodine were produced from logarithmic and exponential regression analysis, respectively, whilst theline for iodide was produced from linear regression analysis of the data.

.et al., 1981; Upstill-Goddard and Elderfield, 1988 .It is also conceivable that the subsurface samplescontained a refractory fraction of iodine, and hence,

Žthe total iodine measurements via hypochlorite oxi-.dation might not have included all forms of iodine

present in these samples.

4.2. Iodide

Iodide was present within the estuary — summerand winter — at relatively uniform concentrationsŽ .5–40 nM over the estuarine gradient, perhaps in-

Ž .creasing marginally toward the marine end Fig. 3 .The coupling observed between iodide and total

Ž .dissolved phosphorus Fig. 4 suggests that iodideconcentrations within the estuary are biologicallyrather than physically mediated. This contention is

supported by the fact that neither iodide nor totalŽ 2dissolved phosphorus are correlated with salinity R

.for P vs. Salinitys0.231 , hence, a process otherthan simple mixing was active. De Luca Rebello et

Ž . Ž .al. 1990 and Brandao et al. 1994 have suggested˜that zooplankton can be responsible for regenerationof iodine as iodide; zooplankton have also beenfound to be capable of releasing up to 40% of thetotal phosphorus present in an aquatic systemŽ .Pintocoelho et al., 1997 . The relationship betweeniodide and total dissolved phosphorus then brokedown in winter, perhaps as a result of differing ratesand pathways for recycling of each element.

Ž .The two outliers in Fig. 4 summer occurred inPort Cygnet, where microalgal blooms had recentlyoccurred to cause marked depletion of nitrate. Ele-vated iodide concentrations in marine waters may be


Table 1Summary of iodine speciation and all other relevant data for the summer and winter sampling campaigns in the Huon Estuary

aSample Depth Salinity Iodate Iodide Total I Org I % Total Dissolved TSS DOC Chl aw x w x w x w x w x w x w x w xnM nM nM nM Org I dissolved O mM mgrl mgrl mgrl2

w xP nM

( )Summer HES 4 samplesB5 0s 32.5 286 15 378 77 20 133 243 0.7 1342 0.89F2 0s 29.7 237 26 350 87 25 245 244 1.3 2342 1.15F3 0s 30.1 247 33 337 57 17 269 240 1.2 2155 1.15H1 0s 30 231 30 337 76 23 257 239 1.2 2131 1.40H2 0s 24.8 177 23 291 91 31 189 244 1.8 3602 1.26H3 0s 25.1 160 13 288 115 40 151 239 1.6 3382 1.04I1 0s 25.8 196 19 300 85 28 138 243 2.3 2871 1.55I3 0s 20.1 107 – 226 – – 89 249 1.7 – 0.56L1 0s 8.5 N.D. 18 94 76 81 154 267 3.6 7470 11.94N1 0s 7.23 N.D. 8 84 76 90 54 235 3 – 5.81V3 0s 32.4 249 103 401 49 12 118 242 1.7 1414 0.96X3 0s 32.3 230 67 386 89 23 142 243 1.4 1664 0.82F3 3m 31.9 265 16 384 103 27 1992 239 1 2172 1.13H2 2m 31.2 252 12 363 99 27 309 237 1.7 2158 1.84H3 3m 32.7 293 52 411 66 16 284 228 0.9 1503 1.23F2 2m 32.9 296 38 430 96 22 294 244 0.9 1627 1.30L1 2.2b 30.83 219 11 358 128 36 2410 183 3.1 1891 –N2 3b 25.3 107 22 282 153 54 171 119 1.3 – 1.57R5 – – – – – – 70 – 1 – –

( )Winter HES 5 samplesAl 0s 34 354 40 424 30 7 320 259 0.8 – 0.26E5 0s 24 204 50 281 27 10 310 280 1.9 – 0.11F1 0s 25 210 11 295 74 25 380 279 1.4 4197 0.24F2 0s 26 202 15 306 89 29 410 262 1.7 – 0.41F3 0s 15 125 26 199 48 24 200 296 2.5 8335 0.23H1 0s 9.3 62 13 125 50 40 120 307 3 11,012 0.05H2 0s 9.5 81 7 120 32 27 120 309 2.7 – 0.07H3 0s 9.7 36 18 121 67 55 140 305 1.6 10,954 0.07I1 0s 8.1 N.D. 12 81 69 85 0 312 2.7 11,825 0.13I3 0s 23 190 11 256 55 21 350 283 1.9 – 0.17J1 0s 9.5 62 19 130 49 38 120 302 2.9 10,000 1.56L1 0s 4.9 N.D. 12 54 42 78 90 320 2.6 12,435 0.10N1 0s 4.4 N.D. 12 – – – 90 323 2 – 0.10V3 0s 21 151 9 252 92 37 280 293 2 – 1.04X1 0s 29 254 28 333 51 15 400 271 1.1 – 0.37X3 0s 22 176 16 269 77 29 310 298 1.8 5497 0.44H2 2m 28 240 34 317 43 14 450 268 1.1 – 0.29X3 5m 32 347 47 – – – 200 261 1.1 – 0.27L1 4b 33 168 29 278 81 29 270 242 1.2 – 0.20N2 3b 23 152 19 236 65 28 340 210 1.7 – –R1 5b 29 190 32 292 70 24 420 205 1.6 – 0.17R5 – – – – – – 120 – 1.5 – –

assSurface, msmiddepth, bsbottom.N.D., not detected.–, sample not analysed.

Žlinked with regenerated biological production Tian.et al., 1996 , where nitrogen is assimilated in forms

other than nitrate. These are the same samples re-marked upon in Fig. 3 their anomalous behaviour


here reinforces the notion that a different biologicalprocess was active.

4.3. Organic iodine

Organic iodine was present in the estuary for bothsummer and winter samplings, in the order of 30–150nM. The fraction of this iodine increased as DOC

Ž .concentration increased Fig. 7 . The fact that totaliodine in the Huon River water was below the

Ž .detection limit -50 nM demonstrates that no sig-nificant quantities of organic iodine were broughtinto the estuary via the river as has been suggested in

Ž .the Nile estuary Abdel-Moati, 1999 . This seems tosuggest an interconversion process, where iodate isreduced to organic iodine within the estuary. Luther

Ž .et al. 1991 also found organic iodine generally onlypredominated when iodate was not detected.

Differences in behaviour of organic iodine insurface and sub-surface samples suggest differentpathways for its formation might operate in thesetwo layers. Each layer is considered separately in thefollowing.

4.3.1. Formation of organic iodine in subsurfacewaters

Ž .Luther and Campbell 1991 only found organiciodine species in the suboxic and anoxic waters ofthe Black Sea. Whilst low oxygen conditions wererare in the subsurface waters of the Huon estuary, it

Ž .can be seen Table 1 that during the summer thetwo samples with DO concentrations below 200 mMhave the highest recorded organic iodine concentra-tions. Thus, suggesting low DO levels may promotea more rapid pathway for organic iodine formation.The fact that DO concentrations were still high inmost of the subsurface samples means that lower DOstill does not explain why organic iodine made up alarger fraction of iodine in the subsurface samples.

The net removal of iodine from the subsurfaceŽ .waters of the Huon Estuary during winter Fig. 2

and associated formation of a high proportion oforganic iodine suggests that perhaps the sedimentmay play a role, not only in removing some iodineŽ .see Section 4.1 , but also in the interconversion ofiodine species in the subhalocline waters. This maylead to the periodic release of organic iodine, whichcan be formed by interaction with sedimentary hu-

Ž .mic substances Francois, 1987 . Therefore, it may

be that inorganic iodine species are periodically re-moved from the water column with a small concomi-tant release of organic iodine. This scenario seemspossible given the undulating nature of the river bedand the presence of the halocline. This could resultin saltwater becoming isolated for short periods of

Ž .time as indicated by low dissolved oxygen levels ,and thus, acting as reactors which periodically re-lease organic iodine to the subsurface waters.

4.3.2. Organic iodine in surface samplesIn contrast to the findings of Luther and Campbell

Ž . Ž .1991 , Luther et al. 1991 found organic iodinelevels to be highest in waters with near oxygensaturation, it was suggested that iodine was retainedin a reduced state in these surface waters via acycling between organic iodine and iodide. It waspostulated that the initial reduction of iodine specieswas carried out by plankton through incorporation ofiodine into organic matter. Remineralisation of theorganic matter by bacterial processes would liberateiodide:

R–I ™ RU q Iy. 1Ž .Ž . Remineralisation processesOrganic matter

The liberated iodide may then form the reactiveiodine intermediates in the presence of light andhydrogen peroxide:

Ž .Light Organic matter RyI ™ I ,HOI ™ RI 2Ž .2

Hydrogen peroxide

HOI would then rapidly react with organic matterŽ .present to form organic iodine compounds RI

Ž .Truesdale and Luther, 1995 .This hypothesis can be broadly supported here by

our observation that as the fraction of organic iodineincreased, the concentration of iodate in the water

Ž .column decreased Fig. 8 . Also in agreement withŽ .Eq. 1 , is our notion that iodide production is possi-

bly mediated by heterotrophic activity. Iodide mayalso be produced photochemically from iodate in the

Ž .presence of humic acid Spokes and Liss, 1996 . Theabundance of humic acids within the estuary alsomakes this a likely pathway for iodide production.Either way, once formed iodide may be converted to

Ž .HOI according to the first stage of Eq. 2 , onceagain the abundance of light-absorbing humic acidsis likely to mediate this pathway. HOI will thenrapidly react with organic matter, in this case humicmaterials to form organic iodine as illustrated in the


Ž .second stage of Eq. 2 . The tight coupling betweenpercentage of organic iodine and DOC in the surfacewaters of the Huon Estuary during summer can beseen in Fig. 7. This strongly suggests that humicsubstances, in conjunction with light play an impor-tant role in maintaining iodine in a reduced state andthe formation of organic iodine in the surface watersof the Huon Estuary.

4.4. Proportions of iodine species within the estuary

The loss of iodate and subsequent increase in theproportions of iodide and organic iodine within the

Ž .estuary Fig. 9 illustrate how the Huon Estuary actsto convert iodate to the reduced iodide and organiciodine species. This process is apparent in bothsummer and winter with both plots in Fig. 9 havingthe same shape. In summer, the process of intercon-version of iodate to organic iodine is much moreactive, with the best fit lines for the two speciescrossing at a salinity of 20, this compares to a wintervalue of 12. This decrease in ‘‘crossover’’ salinity ismost likely because of reduced biological activityand light levels. Iodide is a small fraction of theiodine species present, and has no definite trend withsalinity, attesting to its dynamic nature possibly as anintermediate in organic iodine formation, and a prod-uct of heterotrophic remineralisation.

5. Conclusions

The high concentrations of humic substances en-tering the Huon Estuary from tributary streams ap-pear to profoundly influence the speciation of iodine.In low salinity waters of the estuary, almost all of theiodine is organic iodine. We do not ascribe thisformation of organic iodine solely to the large input

Ž .of DOC, because as Luther and Campbell 1991recorded, low oxygen conditions also appear to en-hance its production. Some depletion of oxygen oc-curs in the subhalocline waters of the upper HuonEstuary.

Previous estuarine studies of iodine have notlooked at seasonal influences upon the halogen andits speciation. A preliminary investigation of iodinebehaviour in summer and winter in the Huon EstuaryŽ .a ‘‘biochemical’’ estuary find differences between

the two seasons for the interconversion of iodate andorganic iodine; iodide seems to be relatively invari-ant with season and salinity. These differences are

Žmanifest in the ‘‘threshold’’ salinity salinity above.at which iodate becomes undetectable and ‘‘cros-

Žsover’’ salinity salinity at which the concentrations.of iodate and organic iodine are equal . Seasonality

might not be the only influence here; riverine dis-charge is another possible factor.

Further research into the estuarine chemistry ofiodine should look more closely at the influence ofseason, and associated biological activity. There isalso scope for more investigation of the in situ

Žextracellular processes they are likely to several,and probably a mix of straight chemical and biologi-

.cally mediated mechanisms that produce organiciodine in estuarine and other aquatic systems.

Acknowledgements

Thanks to Dr. Peter Iles and Dr. Alan Fowless fortheir initial assistance with the electrochemical tech-niques. Salinity measurements were done by ValLatham, nutrient measurements by Val Latham, KateBerry and Donna Roberts, and chlorophyll a mea-surements by Lesley Clementson, as part of theCSIRO Huon Estuary Study. DOC determinationswere done by Neale Johnston and Tom Trull of theAntarctic Cooperative Research Centre. We alsothank various members of the Huon Estuary Studyteam for the taking of samples during HES 4 andHES 5. We are grateful for the input of HenryElderfield, Victor Truesdale and another anonymousreferee, who together substantially improved an ear-lier version of this paper. The Huon Estuary Study ispartly funded by the Fisheries Research and Devel-opment as project 96r284.

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speciation of dissolved iodine in the waters of a humic-rich estuary

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