Stable lead isotope ratios from distinct anthropogenic sources in fish otoliths: a potential nursery...

Comparative Biochemistry and Physiology Part A 127 (2000) 273–284

Stable lead isotope ratios from distinct anthropogenic sourcesin fish otoliths: a potential nursery ground stock marker

Khalil Spencer a, David J. Shafer b, Robert W. Gauldie b,*, Eric H. DeCarlo c

a Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, 1680 East West Road, POST 606,Honolulu, HI 96822, USA

b Hawaii Institute of Geophysics and Planetology, School of Ocean, Earth Sciences and Technology, Uni6ersity of Hawaii,2525 Correa Road, Honolulu, HI 96822, USA

c Department of Oceanography, School of Ocean, Earth Sciences and Technology, Uni6ersity of Hawaii, 1000 Pope Road, Honolulu,HI 96822, USA

Received 6 October 1999; received in revised form 29 July 2000; accepted 8 August 2000

Abstract

Variations measured in the lead (Pb) stable isotope ratios in otoliths of juvenile tropical reef fish Scarus perspiculatus,Abudefduf abdominalis and Dascyllus albisella reflect mixing of anthropogenic lead from the Kaneohe Bay watershed and‘background’ lead characteristic of the adjacent ocean. The otoliths and water samples collected in a transect across thebay demonstrated nearly identical Pb isotopic trends. The Pb isotopic composition of the watershed has a low206Pb/204Pb signature primarily reflecting past combustion of tetra-ethyl Pb additive in fuels. Ocean water notcontaminated by this watershed signature has a different, high 206Pb/204Pb isotopic composition, similar to previouslymeasured Asian anthropogenic aerosols and natural eolian dusts, where the anthropogenic signal dominates. Where ahistory of past anthropogenic Pb contamination exists, it may be possible to use the ratios of Pb stable isotopes in fishotoliths to reconstruct the nursery grounds of fish. © 2000 Elsevier Science Inc. All rights reserved.

Keywords: Mass spectrometry; 206Pb/204Pb; 208Pb/204Pb; 207Pb/204Pb; Stock marker; Nursery ground; Otoliths; Biological indicator;Fish; Scarus perspiculatus ; Abudefduf abdominalis ; Dascyllus albisella

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1. Introduction

Otoliths are calcium carbonate concretions lo-cated in the inner ears of fish. The general struc-ture of fish otoliths has been described in anumber of papers (Dunkelberger et al., 1980;Gauldie, 1993). Otoliths are widely used in esti-mating the age of fish. The trace elemental com-

position of otoliths has a complex relationship toboth the physiology of the fish and the ambientenvironmental conditions at the time of accretion,particularly temperature and salinity (Saitoh andYamada, 1989; Gauldie et al., 1995; Mugiya andYoshida, 1995). In addition, measurements ofotolith chemistry are affected by collection andpreparation (Proctor and Thresher, 1998). Thephysiological complexity underlying the incorpo-ration of trace elements into fish otoliths hasmade it difficult to utilize the chemical fingerprintof an otolith as a stock marker for much the samereasons why genetic, parasitic and other markershave proven difficult to implement. Essentially,

* Corresponding author. Present address: 76 Inglis Street,Seatoun, Wellington, New Zealand. Tel.: +64-4-388-5486.

E-mail addresses: [email protected] (K. Spencer),[email protected] (R.W. Gauldie), [email protected] (E.H. DeCarlo).

1095-6433/00/$ - see front matter © 2000 Elsevier Science Inc. All rights reserved.PII: S 1095 -6433 (00 )00260 -9

K. Spencer et al. / Comparati6e Biochemistry and Physiology, Part A 127 (2000) 273–284274

Fig. 1. Map of Kaneohe Bay, Oahu, with island map in smallinsert. Water samples were taken at the numbered locations on4 May 1999. Sites 1 and two were in Kaneohe Stream mouthand the South Bay, approximately 300 m offshore. Location 3is approximately 300 m south-southeast of Kapapa Island.Location 4 is near Buoy 7 in the main (‘Sampan’) channel.

One answer to this problem lies in finding astock character that differs qualitatively, notquantitatively, between stocks. In this study, ex-amined whether Pb stable isotopes, inherited fromwaters overprinted with an anthropogenic signa-ture, and subsequently recorded in otoliths, canuniquely identify spawning grounds. To assessthis possibility, we developed methods of measur-ing Pb isotopes in otoliths extracted from juveniletropical reef fishes caught in select locations ofKaneohe Bay, Oahu.

The otolith can incorporate and retain Pbwithin its crystal structure (Grady et al., 1989;Campana et al., 1995) because it is made of thecalcium carbonate mineral aragonite, which isisostructural with the Pb carbonate mineral cerus-site (Mason and Berry, 1968). Pb concentrationsin otoliths are about an order of magnitude lowerthan measured in aragonitic material such as reefcorals (Shen and Boyle, 1987). Measurements arepossible (Flegal and Stukas, 1987) as long ascontamination (‘blank’) is controlled and therange of isotopic variability is large enough toovercome limitations of measurement caused bysmall sample size and blank-corrections.

Because Kaneohe Bay (Fig. 1) is the centre ofdrainage for the Kaneohe township, significantamounts of Pb and other pollutants have washedinto the bay, especially in its southern portion(Hunter et al., 1995). The two principal sources ofanthropogenic Pb have been tetraethyl Pb, used asa gasoline additive in Hawaii until 1986, andlead-based house paint, which was phased out in1974. These two anthropogenic contributors over-print natural Pb concentrations in urban andsuburban Hawaiian watershed material, typicallyraising soil Pb by one to two orders of magnitudeand overwhelming natural isotopic compositions(Spencer et al., 1995; DeCarlo and Spencer, 1997).In addition, the background flux of Pb deliveredto the Central Pacific basin is thought to have anisotopic composition distinct from both the an-

variation in the measured stock character withinstocks has proven to be as great, or greater, thanthe variation in the measured character betweenstocks (Gauldie, 1991). Consequently, in practice,the stock ‘signal’ is difficult to distinguish fromthe background ‘noise’ in otolith chemistry. Thisis a common problem with stock markers becausethey are based on measurements of the quantita-tive differences in the regional intensities of thecharacter in question.

Table 1Collection locations of tropical reef fish (abbreviations used in text shown in ‘ ’)

Boats LiliPSpecies CocoI CheckR SampC

XXS. perspiculatus (‘Sp’) XA. abdominalis (‘Aa’) XXD. albisella (‘Da’) X X

K. Spencer et al. / Comparati6e Biochemistry and Physiology, Part A 127 (2000) 273–284 275

Fig. 2. The method of blank-correction illustrated for two otoliths. Location key follows Table 1. In each case, the isotopic compositionof the sample moves away from the blank, and the change is proportional to the difference in isotopic composition and relative amountsof Pb in the sample and blank. For example:

Corr. 206Pb/204Pb= (206Pb/204Pbmeas,sa* [Pb]meas,sa−206Pb/204Pb*[Pb]meas,blk)/[Pbcorr,sa]

Here, ‘meas’ is the actual isotopic composition and concentration measured in the sample or blank, and ‘corr’ is the actual Pbconcentration and isotopic composition of the sample with the ‘blank’ subtracted. Da-7 is corrected by a blank small enough in size(8 pg) and similar enough in isotopic composition that the correction is only somewhat larger than the statistical uncertainty on sampleand standard run statistics. Coconut Island sample Sp-1 is corrected by a blank of 10 pg, but which has a much higher 206Pb/204Pb ratio.Therefore, its blank-correction is nearly 4% different in 206Pb/204Pb. The resulting change in its isotopic composition is far outside ofcounting statistical uncertainty. For reference, the polygons enclose the total range in (uncorrected) isotopic compositions measured ateach geographic location. The blank-correction of Coco-Sp-1 moves it outside the box. The total range of Pb isotopic compositions ofcorals measured on Oahu corals is shown for scale.

thropogenic and natural signal measured instreams and sediments on Oahu (Flegal and Pat-terson, 1983; Maring et al., 1989). Therefore, thelocally-imposed anthropogenic perturbation to Pbisotopic compositions in the bay should bedistinct.

In Kaneohe Bay, watershed Pb mixes with sea-water Pb, derived primarily from natural andanthropogenic dusts and aerosols in global circu-lation (Turekian et al., 1977; Patterson and Settle,1987). The relative contributions of watershedand ‘background’ ocean Pb present in KaneoheBay reflect an interplay between delivery rates ofcontaminated watershed material into the bay bystorms and the flushing rate of the bay by tidaland wave-driven circulation (Bathen, 1968).

2. Materials and methods

2.1. Fish collections

Juvenile fish were captured using hand nets,

traps, and seines and placed live in plastic bagsfilled with seawater from the location where thefish were collected, and transferred within sealedbuckets to the laboratory. Fish were then immedi-ately quick frozen into blocks of sea water-ice.Three benthic species were selected for analysis in

Fig. 3. Pb concentration versus 206Pb/204Pb for pooled otolithsand blanks.


Fig. 4. (a and b) Schematic 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagram of otolith and water samplesfrom Kaneohe Bay, Oahu. Fish are shown as filled boxes. Water samples are shown as filled diamonds. Fish name and location keyfollows Table 1, water locations are numbered in accordance with Fig. 1. Errors are not shown in the interests of clarity. See Fig. 2and Table 2 for error information. The best-fit lines through the data are drawn for illustrative purposes only and does not indicate atemporal significance. These do, however, indicate that two possible mixing end-members correspond to isotopic compositions similarto the measured elsewhere on Oahu. We show the field of Pb isotope data measured on reef corals as an example of the extent of theanthropogenic signature present along the Oahu shorelines (Spencer and DeCarlo, 1997, and in press).

this study: the parrotfish, Scarus perspiculatus,sergeant major, Abudefduf abdominalis, anddomino damselfish, Dascyllus albisella. Fish werecollected at the following locations (see Refs inFig. 1).1. Lilipuna Pier (LiliP) is located at the end of a

roadway in heavily urbanised Kaneohe. Oys-ters collected previously near this locationshowed elevated levels of Pb, Cu, Cr, Ag, Zn,and As (Hunter et al., 1995).

2. Boats is a shoreline location approximately 3km north-west of Lilipuna Pier.

3. Coconut Island (CocoI) is a marine sanctuaryon which the University of Hawaii’s HawaiiInstitute of Marine Biology is located. Co-conut Island lies near the flight path of aircraftlanding at a nearby US Marine Corps AirStation.

4. Checker Reef (CheckR) is a reef flat locatedapproximately 1 km NW of Coconut Island.


5. Sampan Channel (SampC) is located between areef flat and Kaneohe Marine Corps BaseHawaii, approximately 4 km north of CoconutIsland, and near the opening of the bay to theocean.

The distribution of sample stations follows more-or-less the tidal flushing pattern of Kaneohe Bay(Bathen, 1968). The sequence of sampling stationsLiliP\Boats\CocoI\CheckR\SampC followsthe expected decline in Pb contamination betweennear inshore and the open sea (Table 1).

2.2. Water collection

As part of an ongoing watershed monitoringprogram, ultra-clean water samples fromKaneohe Bay were obtained during the 1999spring and summer seasons. Water samples wereanalyzed along a transect from the mouth ofKaneohe Stream to the area of the main channelbetween the bay and ocean. Kaneohe Stream(location 1) drains urban Kaneohe and is a majorsource of fresh water entering the more polluted

Fig. 5. (a and b) 207Pb/204Pb versus 206Pb/204Pb and 207Pb/206Pb versus 208Pb/206Pb diagram. Otoliths are shown as filled boxes andwater samples as filled triangles. On Fig. 5a, we have included data for contaminated urban Honolulu sediments from the Ala WaiCanal, shown as connected open boxes (Spencer et al., 1995). Two lines show permissible trajectories of anthropogenic aerosolsmeasured in the central Pacific during the 1970s and 1980s (Settle and Patterson, 1982; Flegal and Patterson, 1983; Maring et al., 1989).Data for central Pacific ocean pelagic sediments (Jones et al., 2000), which would define the field of ‘natural’ Pb isotopic compositionof central Pacific ocean surface waters were anthropogenic contaminants not present, are shown as open triangles. Otoliths and watersamples fall close to this trend. On Fig. 5b we include data from 5a and in addition, a field of Kaneohe Bay coral Pb isotopemeasurements (solid box, Spencer et al., in prep) and Chinese urban airborne Pb measurements (Mukai et al., 1993; Guyang urban airPb isotope data are highlighted in the dotted box) plotted on a 207Pb/206Pb versus 208Pb/206Pb diagram. We show this projection because204Pb was not reported by Mukai et al. In this projection, two Pb analyses from Chinese urban air samples (dashed polygon) haveisotopic compositions within the field of Kaneohe Bay otoliths and water samples.


Fig. 6. Schematic representations of Pb isotopic compositions,Pb isotope ratios, locations, and otolith Pb concentrations forsamples analyzed during this study.

distilled HCl and 18 MV water) followed by finalsoaking in 18 MV water left in until a sample istaken. Sample bottles were taken double-baggedand filled to the sampling site in a small motor-boat (using unleaded gasoline). While driving up-wind and into clean water unperturbed by theboat, sample bottles were placed on a 3 m longPVC tube by a gloved analyst, uncapped, andseveral times rinsed in seawater before collectinga sample. Samples were immediately rebagged,labelled, returned to the lab, and refrigerated.‘Raw acidified’ samples were immediately aci-dified to pH 1–2, while filtered samples werefiltered through 0.22 mm nucleopore membranefilters fitted to PMP towers that had been pre-cleaned in quartz-distilled acids and 18 MV water.Filters and towers were further precleaned bypassing an aliquant of 2% HNO3 (quartz stillHNO3 diluted with 18 MV H2O) followed by asmall amount of the sample through the filtertower and disposing it prior to collection. Allprocedures were carried out in HEPA modules inthe SOEST isotope laboratory or the Oceanogra-phy clean laboratory, which are filtered air labs.Procedural blanks (collection and filtration)ranged from 0.5 to 1.3 pg/ml). Our intrinsic waterblank, of the water stored in these bottles duringtheir trip to the field, is 0.2–0.5 pg/ml and wouldbe the lower limit of a blank measurement.

2.3. Otolith extraction and storage

Fish were thawed within sealed bags insidecollection buckets. All dissection and handlingtools were cleaned first with 18 MV water, fol-lowed by rinsing with quartz-distilled water(QED). Laboratory surfaces were cleaned withde-ionised water and covered with clean paper.Samples were dissected in a clean (Class-100)laboratory in a HEPA filtered air module. Alltools and specimen were handled with clean-roomgrade vinyl gloves.

The cranial cavity was opened from the top andthe sacculi containing the sagittal otoliths (sagit-tae) were carefully lifted out using teflon forceps.This approach reduced the potential for contami-nation through inadvertent contact with gills orthe digestive tract. Sacculi containing the sagittaewere placed into a drop of QED. Sagittae werequickly teased from the sacculus, transferred to aclean paleoslide, and immediately covered with acover glass. Paleoslides were placed into sealedstorage bags and stored in a clean cabinet.

south bay. Samples were also taken in the southbay (2); near Kipapa Island (3); and in SampanChannel (4). Sample locations are shown in Fig.1. We include these data in order to compareactual water measurements with otoliths.

Water was collected into HDPE, 500 ml jarsthat were pre-cleaned sequentially in reagent-grade HCl mixed with 18 MV water (mixed to 2N), followed by soaking for about 3 days inquartz-distilled 0.5 M HCl (made up with quartz-


2.4. Sample pooling

Samples were pooled by location and approxi-mate collection date, in order to produce a massof otoliths which would contain approximately100 pg of Pb. Attempts were made to use fishcollected within a few days of each other.

2.5. Pb separation

In the SOEST ultra-clean isotope laboratory,otoliths were transferred into Teflon beakers usingplastic tweezers, weighed, and then cleaned twicein QED with ultrasonification. Samples weredried and re-weighed and submerged in about 0.5ml of QED. A small amount of HNO3 was addedto dissolve the sample. Once dissolved, samples

were spiked with a 205Pb tracer (Todt et al., 1996).By using a nearly-pure 205Pb spike (B0.002%natural Pb), both isotopic compositions and Pbconcentrations could be analysed on sampleswithout splitting. Enough 205Pb spike was addedto obtain a measured 205Pb/204Pb ratio of approx-imately 1, thus causing only a small, correctableperturbation to the sample’s intrinsic isotopiccomposition.

Samples were then heated capped in warmHNO3 for at least 8 h to completely mix spike andsample, and taken to dryness. Next, a smallamount (0.3 ml each) of ultra-pure 6N HCl and7.5 N HNO3 was added and the sample cappedand heated in a filtered air container overnight (orabout 12 h) to decompose organic material, afterwhich the sample was again taken to dryness.

Table 2Pb isotopic results on otoliths and water from Kaneohe Bay

206Pb/204Pb 207Pb/204Pb 208Pb/204PbOtoliths Pb, pM/MCa Total [Pb], pM

15.6818.64 1.02Pool coconut Is. Sp 38.28 16.6Pool-Checkers Sp 0.88318.78 15.68 38.33 2.29Pool-Lilipuna pier Sp 4.3019.14 15.71 38.49 1.74

5.655.6538.62Pool-Lili Aa 15.7219.09Pool-coco Is. Sp-2 13.318.36 15.57 37.88 0.927Pool-Sampan Da 9.80 2.60

0.4442.8538.26Pool-Sam Da 2 15.6518.5918.59 15.64Pool Coco Sp-3 38.15 1.30 0.580

Pool Checkers Da-7 0.44418.81 15.67 38.32 1.4918.79 15.64Pool Checkers Da 5,9 38.32 6.04 3.1619.16 15.69 38.6Pool Boats 1,2 Aa 24.2 4.10

206Pb/204Pb 207Pb/204PbBay Water 208Pb/204Pb Pb, pM/Kg Pb, ppb0.021738.7715.71 0.004519.42SampC FA

19.34 15.73S. Bay FA 38.75 0.0116 0.002438.2615.6318.76Kaneohe str. FA 0.00240.0116

Kaneohe Str. RA 15.6118.49 38.15 3.33 0.690.0032Kapapa Is. FA 18.79 15.62 38.33 0.01540.00770.037238.15Kapapa Is. RA 15.6118.41

Procedural blank 0.000938.4515.6719.01 0.0043Small standard analyses on SRM 981a

207Pb/204Pb 208Pb/204Pb206Pb/204PbSmall standard analyses (33–66 pg of Pb)7/24/96 16.874 15.393 36.4098/5/96 16.887 15.406 36.4399/16/96 15.403 36.45716.8910/2/96 36.41115.39116.883

36.415.38516.88210/5/9610/24/96 16.868 15.398 36.408

16.86 15.3729/3/98 36.33616.884 15.3989/24/98 36.418

36.410MEAN 15.39316.8790.035Error 0.010 0.011

0.141% error 2 s 0.1940.122

a Values for SRM 981 of 206/204Pb=16.9356, 207/204Pb=15,4891, 208/204Pb=36.7006 are from Todt et al. (1996). A Daly biascorrection of 0.0005513/amu was applied as determined experimentally on standards.


Samples were then converted to a bromide and Pbwas separated using mixed HBr-HNO3 anion-ex-change chromatography.

For water samples, highly purified Fe andNH4OH solutions were prepared in order to co-precipitate Pb from large water samples (50–100ml), thus enabling us to collect several hundred pgor more of Pb while eliminating the soluble saltspresent in seawater. In this way, only 1–2 mg ofFe and its sorbed metals would need be passedthrough our anion-exchange microcolumns.

Fe was prepared by dissolving 99.999%(Aldrich) Fe in 2N HNO3. The Fe solution waspassed twice through a column containing Sr-specresin. We chose an acid strength that would max-imize the resin/acid distribution coefficient for Pb(Horowitz et al., 1992), thus optimizing thecleanup. Residual blanks of about 10 pg Pb/mgFe or smaller have been produced. The Fe solu-tions were analyzed several times to obtain theirPb concentration and isotopic composition, andthis Pb was subtracted quantitatively from ourresults.

NH4OH was prepared by isothermally diffusingammonia (from reagent-grade Fisher NH4OH)into quartz-distilled water. Blanks on this solutionare �1 pg/g, and since only a few tenths of g istypically used, no blank-corrections are made forthis reagent.

To coprecipitate Pb, approximately 2 mg equiv-alent solution of Fe solution and approximately100 g of seawater were weighed into an Teflon jar.The sample was then spiked with a 205Pb tracer.This mixture was allowed to equilibrate overnight.NH4OH was then added to the solution until apH of about 8 was obtained, and the sampleagain allowed to equilibrate overnight so that aprecipitate of Fe would form. The samples werethen carefully decanted until the solids remainedwith a small amount of liquid. This mixture wasthen centrifuged, washed in a small amount ofQED, and recentrifuged again. The precipitatewas dissolved in a small amount of HNO3 andredried, converted to a bromide, and Pb wasseparated using mixed HBr-HNO3 anion-ex-change chromatography.

Procedural blanks, or contamination checks,were performed with each set of Pb separations,spiking with 205Pb, and analyzed for both theblank’s magnitude and its isotopic composition.These blank analyses can potentially be used to

correct a sample’s isotopic composition and con-centration measurements. With the exception ofone Pb blank of 55 pg all ‘lab’ blanks rangedbetween 3 and 13 pg.

2.6. Mass spectrometry

Pb was measured on a micromass (VG-FisonsInstruments) Sector 54 mass spectrometerequipped with an ion-counting Daly, an ion-mul-tiplier that gives 100-fold greater sensitivity thanthe Faraday-cup measuring devices used on mostthermal-ionization mass spectrometers. Samplesand standards were loaded onto degassed Rhe-nium filaments with a silica gel-phosphoric acidmixture (loading blank 0.6 pg Pb), fumed todryness in a clean air module, and then loadedinto the mass spectrometer.

Standards (SRM 981) of 30 pg–1 ng wereanalysed to determine Daly bias and instrumentalfractionation. Instrumental fractionation wasmore noticeable on extremely small samples (lessthan 100 pg) compared to values on analyses oflarger standards of 500 pg or more. In addition,on small standards of 66 pg or less a small andreproducible shift in the 206Pb/204Pb ratio was seenrelative to the 207Pb/204Pb and 208Pb/204Pb ratio.This shift was consistent with a small (0.6 pg)loading blank having a 206Pb/204Pb ratio of about19.

Samples initially exhibited a strong hydrocar-bon signal upon being brought to run tempera-tures, which were typically of the order of 1240°C.In order to remove this potentially serious iso-baric interference (e.g. Flegal and Stukas, 1987),the hydrocarbon signal was monitored directlyusing the Daly, carefully scanning the mass range200–210 amu, and observing the spectral interfer-ence of hydrocarbons on ratios of the 205Pb and204Pb beams. Typically, the first block or two of15 ratios were discarded from both standard andunknown analyses as the hydrocarbon interfer-ences burned off. Otoliths were typically analysedfor 50–150 ratios, stopping when the isotope ra-tios began to fractionate. Water samples wereanalyzed for 120–150 ratios.

2.7. Otolith blank corrections and sample pooling:

With samples of such a small size, the amountof ‘blank’ or background Pb becomes a majorconsideration in calculating both the concentra-


tion and isotopic composition. Pairs of otolithsfrom single fish, for example, were found tocontain at most a few tens of picograms of Pb.Our best blanks, by comparison, contained 3.3and 6.6 pg. In instances when we analysed morethan one blank per group of samples, variationsof a factor of two in the quantity of Pb in eachblank was typical. This introduces a major andirreproducible correction, sometimes resulting innegative concentrations or bizarre isotopic com-positions inconsistent with known fields of Pbisotopic signatures in Hawaii and the NorthPacific. We therefore decided to pool groups ofotoliths to obtain better sample/blank ratios.The best sample/blank ratio was obtained byloading the maximum amount of material on asingle ion-exchange column (to maximize sampleto reagent). This was typically a sample of about35 mg of otolith, producing 80–100 pg of Pb. Ifthe blank was held to less than 10 pg, asample/blank ratio \8 could be maintained,greatly reducing the uncertainties produced by‘blank-correcting’ compositions and concentrat-ions. Therefore, only pooled data are reported inthis paper.

Nonetheless, because of the high blank/sampleratio, the uncertainty in each analysis is limitedby the uncertainty introduced by the blank-correction. Such a correction assumes that allsamples processed with a blank contain anidentical ‘blank’, an assumption we have demon-strated is questionable at best, based on ourreplicate blank measurements.

Fig. 2 shows two examples of blankcorrections. Coconut Island sample Sp.-1, whoseprocedural blank has a much different isotopiccomposition than the sample, has a blank-corrected isotopic composition that is lower in206Pb/204Pb by 0.09, or well outside of ourstandard’s external precision. Sample Da-7,whose blank has a similar isotopic compositionas the sample corrects to a 206Pb/204Pb ratio thatis only 0.02 lower, or nearly within our externalprecision.

While the small sample size and variable blanklimits our precision to less than that reported formore traditional measurements of larger samples,there was no evidence for a simple correlationbetween sample Pb content and isotopiccomposition, or blank magnitude and isotopiccomposition (Fig. 3). Therefore, contamination

in the laboratory did not produce the range ofisotopic compositions reported here.

3. Results

Pb concentrations measured on pools ofotoliths typically varied from about 2–12 ppb.One pooled sample contained 50 ppb Pb, whichwe interpret to result from contamination, al-though the isotopic composition lies on trend withthe data. It is possible that this resulted fromcontamination of the sample with the water orsediment the fish was caught in. Other concentra-tion differences may reflect water or fish chemistryor some un-corrected blank Pb.

In spite of the small amounts of Pb beingmeasured, we do not believe that our measure-ments simply result from a contamination vectorintroduced during sample collection and process-ing, as concentrations do not correlate with iso-tope ratios. In addition, the isotopic compositionsof the blanks are broadly similar to those of thesamples (Fig. 2), so it is unlikely that the trendresults from adding an extremely high or low206Pb/204Pb contaminant to the samples. Finally,we do not consider it likely that both watersamples and otoliths, collected at different timesand processed using different techniques, wouldproduce fortuitously similar trends.

Fig. 4a and b show otolith and water sampleisotopic compositions plotted as 208Pb/204Pb ver-sus 206Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pbdiagrams. Otolith and bay water isotopic compo-sitions lie on a mixing line defined by low andhigh 206Pb/204Pb end-members. The low 206Pb/204Pb end-members trend towards contaminatedsediments measured in urban Honolulu (Spenceret al., 1995; DeCarlo and Spencer, 1997). TheKaneohe Stream and Kapapa Is. raw-acidifiedwater samples (RA) are within the edge of theurban Honolulu sediment field, as is CocoI Sp-2.The high 206Pb/204Pb end member is present inwater samples taken elsewhere in Kaneohe Bay,suggesting this may reflect a ‘background’ Pbisotopic composition. The best-fit line through thedata is drawn for illustrative purposes only anddoes not indicate a temporal significance. It does,however, indicate that two possible mixing end-members correspond to isotopic compositionssimilar to that measured in watershed materialand Kaneohe Bay water itself.


4. Discussion

Both otoliths and bay water samples lie onnearly identical trend lines on lead isotope varia-tion diagrams. The general significance of theotolith and seawater trend lines for the isotoperatios of 208Pb/204Pb versus 206Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb can be evaluated by com-parison with the distribution of Pb stable isotopesfound on Oahu (Fig. 5a and b). We include fordiscussion the field of isotopic compositions possi-ble by mixing Pb isotope analyses collected in theCentral Pacific during the SEAREX expedition(Maring et al., 1989), and at Enewetak (Settle andPatterson, 1982; Maring et al., 1989). One mightexpect to find otoliths falling along these trends ifdiffuse background fluxes of Pb with these iso-topic compositions were contributing to the Pbbudget in the bay.

We can initially rule out Pb sources such asthose measured in central Pacific surface watersand air samples during the 1970’s and early1980’s, because these had isotopic compositionswith elevated 207Pb/204Pb at a given 206Pb/204Pb(Settle and Patterson, 1982; Flegal and Patterson,1983; Patterson and Settle, 1987; Maring et al.,1989). Likewise, natural volcanic rock Pb fromthe basalts that underlie Kaneohe town have iso-topic compositions with low 207Pb/204Pb and206Pb/204Pb; their data would plot off of Fig. 5a tothe left (Roden et al., 1994). These rocks thereforedo not appear to make a discernable contributionto the Pb isotopic compositions in Kaneohe Bay.The otoliths and water samples have Pb isotopiccompositions that can be broadly described asmixtures of low 206Pb/204Pb anthropogenic water-shed material and high 206Pb/204Pb lead similar tothat found in central Pacific eolian material(Spencer et al., 1995; Jones et al., 2000).

However, it is unlikely that these natural dustsalone are contributing to the Pb budget of thesurface ocean. Prehistoric surface Pb concentra-tions in the central Pacific were probably on theorder of a part-per trillion or less (Flegal andPatterson, 1983; Spencer and DeCarlo, 1997),while Kaneohe Bay concentrations are consider-ably higher at 3–5 ppt, suggesting an additionalcomponent with a similar Pb isotope signal ispresent. Kaneohe Bay’s high 206Pb/204Pb signatureis in fact similar in 207Pb/206Pb versus 208Pb/206Pbspace (Fig. 5b) to Chinese airborne particulate Pbmeasured in the Guyang region of China (Mukai

et al., 1993); however, these authors did not re-port 204Pb data. We therefore estimate that some60–80% of the lead in the surface water well awayfrom the influence of the Hawaiian Islands is ofanthropogenic origin, possibly of Asian origin,and would control the Pb isotopic signature ofotoliths in Hawaiian waters not contaminated bylocal watershed Pb.

Although otolith and water sample isotopiccompositions fall on nearly identical trends (Fig.6), there is no simple correlation between thelocations where fish were collected and the206Pb/204Pb ratio or otolith Pb concentration. Infact, some fish collected inshore fall at the higher206Pb/204Pb values, while those caught offshorehave lower 206Pb/204Pb values. This is consistentwith water measurements, because high206Pb/204Pb values were also measured in both theSouth bay and along the central (Sampan) chan-nel leading to the ocean, while lower 206Pb/204Pbvalues were measured immediately adjacent toKaneohe Stream and near Kapapa Island. Wesuspect an interplay of storm flushing, tidal cur-rents, and wave-driven currents in controllingmixing between water containing watershed-derived Pb and water containing open-ocean Pb.As these were juvenile fish, we assume they havenot moved very far from their spawning grounds.

Pb concentrations measured in many otolithshere are lower than reported in the literature(Grady et al., 1989; Campana et al., 1995). Thiscould be caused by unrecognized blank Pb beingreported in other studies, or by species differencesin the mechanism of Pb uptake. Most inorganicelements included in otoliths are derived from thewater (Farrell and Campana, 1996). Trace ele-ments move into blood plasma across the gills,enter the endolymph surrounding the otoliths,and finally become included in the otoliths. To aminor extent, some elements may be derived fromfood sources in the intestine (Farrell and Cam-pana, 1996; Gallahar and Kingsford, 1996). Forabsorption across the gills, elemental discrimina-tion can occur at interfaces between water-gill,blood-endolymph, and endolymph-otolith; how-ever, the interface where discrimination of Pboccurs is unknown. Nevertheless, the absoluteconcentration of Pb in the otolith is probably notan isometric measure of the absolute concentra-tion of Pb in the water.

Although petrol Pb additives were discontinuedin Hawaii in 1986, a sufficient quantity of water-


shed-sequestered anthropogenic Pb contaminantsare transported into Kaneohe Bay waters. Itwould be optimum to be able to measure the Pbstable isotope ratios of individual otoliths (orotolith cores from adult fish) to track nurserygrounds, but the limitations of present techniquesand the low levels of Pb isotopes constrain studiesto pooled samples. However, the key issue is thatthe characteristic anthropogenic Pb isotope ratiosare a qualitative rather than quantitative marker,so that the reliable detection of the presence ofdistinct Pb isotopes is all that is required fornursery ground discrimination.

Protection of nursery grounds of valuable fishspecies is a priority in fisheries management. Trac-ing adult fish to their respective nursery groundshas been a long-standing problem for many spe-cies of fish, including the tropical reef fishes ofOahu whose value to the aquarium collection andtourism industry is enormous. Pb isotope ratios inKaneohe Bay, which reflect mixing of a localanthropogenic component with seawater Pb, arepreserved in fish otoliths. This suggests that leadstable isotopes may be a useful nursery marker.

As time passes the mobilisation of petrol Pbcontaminants will decrease and eventually cease.But for the moment there is a window of opportu-nity to use a noxious chemical pollutant to protectthe same fish resources for which it once causedso much harm (Table 2).

Acknowledgements

This research was supported by a grant in aidfrom SEAGRANT, and by grants from the De-partment of Lands and Natural Resources, Stateof Hawaii. We thank Dr Charles Helsley of SEA-GRANT and Dr Kim Lowe of the Department ofLands and Natural Resources, State of Hawaiifor their continuing interest and support for thisresearch. The publication numbers for this paperare SOEST XXXX, HIGP XXXX.

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