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Trace elements in ships' ballast water as tracers ofmid-ocean exchange
Kathleen R. Murphya,b,⁎, M. Paul Fieldc, T. David Waiteb, Gregory M. Ruiza
aSmithsonian Environmental Research Center, PO Box 28, Edgewater MD 21037, USAbThe University of New South Wales, School of Civil and Environmental Engineering, NSW 2052, AustraliacInstitute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, USA
Article history:Received 28 March 2007Received in revised form23 November 2007Accepted 1 December 2007Available online 30 January 2008
Recent regulation mandates that ships conduct mid-ocean ballast water exchange (BWE)prior to discharging foreign ballast in U.S. territorial waters. We investigated the utility ofdissolved concentration measurements for 6 elements (Ba, P, Mn, U, V and Mo) in the ballasttanks of ships operating in the North Pacific and Atlantic oceans as tracers of mid-oceanBWE. Relatively conservative elements Mo, U and V provided little additional informationbeyond that obtained from salinity, whereas nonconservative Ba, P and Mn offered greaterresolution. The utility of Ba, P and Mn was further examined in the context of three criteria:(1) stability, or whether tracers maintain stable concentrations in ballast tanks over time; (2)fidelity, or the degree to which tracer concentrations in ballast tanks faithfully reflectconcentrations at their ocean source; and (3) predictability, or the degree to which ballasttanks have a predictable and restricted range of tracer concentrations following BWE. Wefound that inwater held in ballast tanks over time, average stability increased forMnbPbBa,as reflected by decreasing coefficients of variation (30%N21%N3%) and fidelity increased inthe same direction. While Ba and P usually increased discrimination at high salinities, Mnwas typically the most sensitive indicator of BWE and the presence of residual port water inpartially exchanged tanks. Ba, P and Mn in tanks exchanged in the Atlantic exhibiteddifferent concentration ranges compared to tanks exchanged in the Pacific, suggesting thatif trace elements are to be used to verify BWE, criteria for discriminating between exchangedand unexchanged ballast tanks may need to be basin-specific.
Keywords:Trace elementsTracersBallast water exchangeSeawaterBiological invasions
1. Introduction
Ships transporting ballast water between geographicallyisolated ports contribute to the spread ofmany aquatic speciesbeyond their natural range limits. Such biological invasionsthreaten the biodiversity of coastal environments, resulting inthe homogenization of biota and sometimes causing severeeconomic, social or ecological impacts through the introduc-tion of harmful species or ecosystem-modifiers (Carlton, 1985;Ruiz et al., 1997; Ruiz and Carlton, 2003; Wonham and Carlton,
mental Research Center, P
rphy).
er B.V. All rights reserved
2005). To reduce the inter-coastal transfer of invasive species,ships can implement mid-ocean ballast water exchange(BWE). During BWE, ships replace coastal ballast water,usually loaded during cargo operations in port and containingpotential invasive species, with oceanic ballast water. Thistreatment replaces coastal organisms with oceanic speciesthat are poorly adapted for survival in coastal environments,thereby reducing the likelihood of establishment (Verlinget al., 2005). BWE is being promoted at national (USCG, 2004)and international (IMO, 2004) levels and is expected to remain
O Box 28, Edgewater MD 21037, USA. Tel.: +61 2 9385 5778; fax: +61 2
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the dominant ballast treatment method until better techno-logical solutions are widely available (Hunt et al., 2005).
Since 2004, the United States Coast Guard (USCG) hasrequired BWE for all ships prior to discharging foreign ballastwater in U.S. territorial waters (USCG, 2004), yet the require-ment cannot be adequately enforced due to the lack of reliablemethods for determining compliance. One approach toverifying BWE is to compare concentrations of naturally-occurring chemical tracers in ballast tanks with their knowndistributions in the open ocean. Salinity is a primary tracerbecause salinities in the open oceans are high (typically 32–37)whereas salinities in coastal environments are often loweredby rainfall and river runoff. Although low salinity is anunambiguous indicator of coastal sources, many ports havehigh salinities overlapping with ranges in the open oceans forall or part of an annual cycle (Murphy, 2007). Allowing for the(legally tolerated) presence of up to 5% residual freshwater in aballast tank exchanged in the open ocean (IMO, 2004; USCG,2004), a salinity of ~30 appears to be an appropriate lowerthreshold for considering ballast water to be of uncertain BWEstatus. Thus, secondary tracers are required to distinguishcoastal from ocean sources when salinities in ballast tanksexceed such a threshold value.
A preliminary study of predominantly high-salinity ballastwater indicated that chromophoric dissolved organic matter(CDOM), radium isotopes and certain trace elements (includ-ing Ba, P and Mn) aided discrimination between coastal andoceanic sources (Murphy et al., 2004a). Amore extensive studyof CDOM in ballast water frommultiple sources in Asia, NorthAmerica and Europe found that a single fluorescence intensitythreshold separated most high-salinity coastal and oceanicballast water (Murphy et al., 2006). The utility of sensitive,short lived isotopes of radium (223Ra and 224Ra) in thisapplication is constrained by logistical challenges surroundingthe collection and processing of samples (Murphy et al.,2004a). A rigorous evaluation of trace elements as ballastwater indicators is yet to be performed. Initial resultssuggested that Ba, Mn and P were more sensitive tracers ofballast water source than salinity, but it was unclear whetherthe results were robust or could be extended into new oceanregions; furthermore, questions were raised regarding thereliability and stability of trace element measurements inballast tanks (Murphy et al., 2004a).
The distribution of trace elements in seawater is geogra-phically variable with respect to open ocean and coastalenvironments. U, V and Mo occur in rivers and coastal regionstypically at lower concentrations than in the open ocean,where higher concentrations are maintained by long oceanicresidence times (5×104–7.6×106 years) relative to the mixingtime of the oceans (~103 years) (Collier, 1984; Palmer andEdmond, 1993; Sohrin et al., 1998). Because Mo, U and V havelow particle reactivity and nearly always vary conservativelywith salinity, they offer limited sensitivity when salinitydifferentials are small, and concentrations in high-salinitycoastal environments are similar to ranges in the open oceans(U: ~3.3 µg/L; V: ~1.6 µg/L; Mo: ~9–11 µg/L) (Sohrin et al., 1987;Delanghe et al., 2002, this study).
In contrast, Ba, P and Mn tend to occur near the coast athigher concentrations than in the surface open ocean due tothe predominance of terrestrial sources over oceanic sinks, as
reflected by their short residence times of 10,000, 69,000 and60 years, respectively (Chan et al., 1977; Martin and Knauer,1980; Broecker and Peng, 1982). A combination of terrestrialand anthropogenic sources, water column recycling, sedimentdiagenesis, aeolian inputs, sorption to particles, biologicaluptake, co-precipitation, biological decay and redox cyclingcontribute to potentially large oceanic gradients relative toconservative elements (Dehairs et al., 1980; Broecker and Peng,1982; Sunda et al., 1983; Sunda and Huntsman, 1990; Benitez-Nelson, 2000; Karl and Björkman, 2002). As a first approxima-tion, typical coastal:oceanic concentration ratios are on theorder of N2:1 for dissolved Ba (Hanor and Chan, 1977; Shawet al., 1998), N10:1 for total dissolved P (Froelich et al., 1982;Baturin, 2003), and N20:1 for dissolved Mn (Bruland andFranks, 1983; Shiller, 1997; Wells et al., 2000) with greaterratios typically correlating with greater salinity differentials.Elevated levels of Ba, P and Mn in ships' ballast tanks aretherefore potential indicators of coastal (unexchanged) ballastwater.
Despite the potential complexity of reactions that couldaffect trace element speciation and behavior in ballast tanks,previous studies have documented occurrences of conservativebehavior of dissolved Ba (Coffey et al., 1997; Taylor et al., 2003),Mn (Moore et al., 1979; Muller et al., 1994; Hatje, 2003) and P(Shammon and Hartnoll, 2002) in natural environments,including in estuaries that experience large physico-chemicalgradients. Furthermore, deviations from conservative mixingfor these elements are often attributed to sediment interactions(Hanor and Chan, 1977; Laslett and Balls, 1995), particularly inlow-oxygen or low-salinity conditions (e.g. Coffey et al., 1997).Ballast water typically experiences relatively stable salinity,oxygen and pH conditions (Gollasch et al., 2000;Wonham et al.,2001; Drake et al., 2002, Ruiz unpublished data), thus Ba, P andMn could plausibly act as stable and sensitive tracers of mid-ocean ballast water exchange.
In this study we examine whether Ba, P and Mn aresensitive and reliable tracers of BWE, or conversely, whetherchemical or biological processes in ballast tanks preclude theirwidespread use as tracers of ballast water origin. The utility ofBa, P and Mn is examined with respect to three criteria: (1)stability, or whether tracers maintain stable concentrations inballast tanks over time; (2) fidelity, or the degree to whichtracer concentrations in ballast tanks faithfully reflect con-centrations at their ocean source; and (3) predictability, or thedegree to which BWE results in a predictable and restrictedrange of tracer concentrations in ballast tanks.
2. Experimental methods
2.1. Sampling
Ballast water samples were collected during eight cruises in2001–2004 on commercial ships operating in the north Pacificand Atlantic oceans (Fig. 1 and Table 1). Tanks were ballastedinitially at the port of departure or at a previous port of call andexchanged in the open ocean. Implementation of BWErequired between 2 and 15 h, depending upon tank volume(~260–11,500 m3) and method of exchange (flow-through (FT)takes approximately three times longer than empty-refill (ER)),
Fig. 1 –Sources of ballastwater (loading and exchange locations) in the Pacific Ocean (upper plot) andAtlantic Ocean (lower plot).Symbols signify cruises: LF ( ), AS ( ), K1 ( ), K2 ( ), SF ( ), LA ( ), BN ( ), Fos ( ).
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as the ship tracked a linear distance of ~30–480 nautical miles.A total of 42 ballast tanks were sampled that contained ballastwater from various geographic locations. Approximately halfof the ballast tanks were inspected while empty prior to thebeginning of experiments and none were seen to containappreciable quantities of residual sediment. Nontoxic Rhoda-mine WT dye (Bright-Dyes) was added to ballast tanks at thebeginning of two cruises (SF, LA) to enable BWE efficacy to beindependently quantified.
Overall, 239 ballast water samples and 150 seawater(shipside) samples were collected. For ballast tanks, samplingdepths were 1–3 m with additional samples from 10–12 m indeep tanks (cruises SF, LA and BN). Seawater (shipside)samples were collected via the ship's engine cooling system,which constantly circulates ambient seawater from a depth ofapproximately 4–7 m through steel pipes at flow rates of~200 m3/h. Sampling was via a short length of tubing attachedto the pipe as near as possible to the seawater intake andupstream of the engine machinery. Equipment potentiallycontacting samples was acid washed in 1 N HCl before eachcruise and flushed with N10 water volumes prior to collectingsamples.
Ballast water samples were collected via acid-cleanedplastic pumps (Wilden: Pro-Flo P.025) and tubing (Cole-Parmer:Chemfluor 367). Equipment andmethodologies are detailed inearlier publications (Murphy et al., 2003; Murphy et al., 2004a).Salinity, temperature and oxygen measurements wereobtained using either (a) a dissolved oxygen and conductivitymeter (YSI-85) at discrete depths (cruises LA, SF, K1, K2 andBN); or (b) a CTD (Hydrolab MS4) to obtain profiles through theaccessible tank water column (cruises Fos, LF and AS).Instruments were precalibrated with distilled water andNIST-traceable seawater conductivity standard (YSI 3169,50,000 µScm−1). A failure in the CTD motherboard 5 days intocruise LF resulted in falling salinity readings; these data wereback corrected with an estimated accuracy of ±1 salinity unit.
Filtration on cruises SF, LA, Fos was via individual 0.22 µmsyringe filters and on cruises LF, AS, K1, K2, BN via 0.45 µmhigh-capacity inline polypropylene capsule filters (OsmonicsInc., Memtrex™). Different inline filters, replaced severaltimes during each voyage, were used for ballast versusseawater samples and refrigerated when not in use. Thecutoff for ‘dissolved’ concentrations is operationally definedand the larger filter size used in this study (0.45 µm) would nothave excluded colloidal material (Nakatsuka et al., 2007).Potential sampling artifacts were investigated on cruise LF,during which ballast water samples were collected by bothpump+inline filter (used for multiple samples) and syringesampler+syringe filter (used once only). No significant differ-ences between replicate samples collected by the twomethods were found despite small sample variances (Murphyet al., 2004a), indicating that both procedures were equallyreliable and that the elements measured were present mostlyin dissolved form or as species of size b0.22 µm.
Filtered samples were frozen after collection and shippedto the laboratory at the end of the cruise where they wereacidified to pH b2 by addition of 2 mL L−1 12 M HNO3 (Optimagrade, Fisher Scientific) upon thawing. This protocol of samplepreservation has been tested and proved reliable when at seaacidification was logistically difficult or impractical.
2.2. Trace element analysis
The concentrations of Ba, Mn, Mo, P, U and Vwere analyzed onan Element-1 high resolution inductively coupled plasmamass spectrometer (ThermoFinnigan, Bremen, Germany) atRutgers Inorganic Analytical Laboratory, Institute of Marineand Coastal Sciences, Rutgers the State University of NewJersey. Samples were diluted 10 fold with 10% V/V ultra-pureHNO3 then analyzed in low and medium resolution usingpublished techniques (Field et al., 1999). Replicate analysis ofNASS-5 (North Atlantic Surface Seawater, certified reference
Table 1 – Experimental design indicating numbers of (i) samples in the assessment of stability; (ii) pair-wise comparisons inthe assessment of fidelity, and (iii) different ballast water masses (source/tank combinations) in the assessment ofpredictability
Coastal ballastwater source
Cruisedates
Ballast tanks #BWEloc.
BWEtypes
Salinity
# andtype
Samp.depth (m)
Tank vol.(×102 m3)
(Avg.) before/afterBWE
(i) (ii) (iii)
North Pacific OceanSF San Francisco (USA) Nov 5–13, 2000 3 w 1,12 113.8–114.9 4 ■▼▲ 21.1/32.0 – – 7LA Los Angeles (USA) Dec 9–12, 2000 2 w 1,11 67.5 3 ■▲ 33.3/32.8 – – 4LF Haramachi (Japan) Jun 5–16, 2003 8 w 1–3 18.8–48.1 3 ■▲ 33.2/33.0 30 6 12
Haramachi/unknown 2 w 1–3 19.1 1 ■▲ 33.4/32.6 8 2 4AS Yokkaichi (Japan) Aug 5–17, 2003 2 w 1–3 2.8–4.5 2 ■▼ 27.2/32.8 10 1 3
North Atlantic OceanFos Fos Sur Mer (Mediterr. Sea) Jun 13–25, 2001 8 w 1–3 6.2–18.0 7 ■▼◪ 37.6/37 62 16 24
BN Rotterdam (Netherlands) Sep 10–23, 2004 4 w2 w
1,121
87.6–116.7 21
■▲ 32.8/36.5 29 12 12
Ballast tank types were wing (w) or double-bottom (d). BWE types indicate that tanks sampled were exchanged by empty-refill (ER) method (▼),exchanged by flow-through (FT) method (▲), partially (1/3) FT-exchanged ( ), partially (2/3) FT-exchanged (◪) or unexchanged (■).
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material) indicate good precision and accuracy (10%) for allelementsof interest (Table 2). Note that ourMndeterminationsin NASS samples exceeded certified reference values by ~9%across all runs.
Sporadic analytical errors affected P and V determinationsin 2003 (CV N35%); samples that appeared anomalous (N=13from cruises LF and AS) were rerun in 2004. A number ofsamples from cruises K and BN also appeared to be anomalousand 64 were rerun. Data for anomalous samples were replacedupon reanalysis if significantly different or averagedwherenotdifferent. Thereafter, a small number of individual data pointsthat differed by N2 standard deviations from the means of theremaining (true) replicate samples were excluded only wherethere were N ≥ 3 replicates from the same time and place; thisaffected b1% of the dataset for any element. Erroneous datawere difficult to diagnose in tanks with small sample N; in
Table 2 – Summary of trace element determination in NASS sa
Element Resolution Reference
Value 2σ Type M
(µgL−1) (µ
138Ba Low 5.1 p55Mn Medium 0.919 0.057 v98Mo Low 9.6 1.0 c31P Medium 17.7 p 2238U Low 2.6 i51V Medium 1.2 i
Reported NASS reference data (determined value and standard deviation,(2007).
these situations we took the approach of retaining apparentoutliers due to the lack of sufficient evidence to justifyremoving them.
3. Results
3.1. Tracer sensitivity
Fig. 2 illustrates the relative sensitivity of trace elements asballast water indicators for each ballast tank during this study,calculated from the ratio of (percent) change in mean tracerconcentration to the (percent) change in mean salinity due toBWE. Where tracers exhibited essentially stable or increasingconcentrations over time (i.e. Mn on cruises LF and Fos,together with all other tracers on all other cruises; Section 3.2),
mples and detection limits (DL) during the analytical period
σ) are either certified (c), informative (i) or proposed (p) by Field et al.
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mean initial tracer concentrations were calculated from allsamples collected prior to BWE. Where concentrationsdecreased over time (i.e. Mn on cruises BN and AS), meaninitial tracer concentrations were calculated from samplescollected immediately prior to BWE, thus testing the “worst-case” scenario for tracer sensitivity. Final tracer concentra-tions represent means of all samples collected from the sametank following BWE. Sensitivity ratios determined accordingto this method are approximate due to measurement error,which can substantially influence ratios particularly when theeffect of BWE on salinity or tracer concentrations is small. Themethod ismost useful for illustrating consistent differences inthe relative performance of the six tracers.
Physico-chemical conditions in the ballast tanks aresummarized in Table 3. In general, the conservative elementsMo, U and V exhibited comparable sensitivity to salinity, as isindicated by a sensitivity ratio close to 1 (Fig. 2). Of the three, Vusually exhibited the greatest relative change following BWE.On cruise LF, all trace elements including Mo, U and Vperformed better than salinity. This result is attributed tothe CTDmalfunction (Section 2.1) which reduced the precisionand accuracy of salinity measurements on cruise LF. If therelative change in salinity had in fact been similar to that ofthe conservative elements, then the relative sensitivity of alltracers on this cruise should be reduced by a factor of ~10.
In the majority of cases especially at higher salinities, Ba, PandMnweremore sensitive than salinity and the conservativeelements. Ba exhibited typically 3–10 times greater relativechange than salinity, except on cruise AS (salinity ~27 beforeBWE), where it performed similar to or worse than conserva-tive tracers. P offered no discrimination additional to salinity
Fig. 2 –Percent change in trace element concentrations relativeduring this study.
on cruises AS and SF, but was more sensitive than salinity onall cruises where initial salinity N30. Mn was most sensitiveoverall, exhibiting 10–30 times greater relative change thansalinity, Mo and U on cruise Fos and 10–20 times greaterrelative change thanMo, U, V andBa on cruise LF. On cruise AS,Mn exhibited more than 5 times greater relative change thansalinity and the conservative elements, despite decreasing inconcentration by 3–10 fold prior to BWE (Section 3.2). Since ourgoal is to study tracers that can offer additional information tosalinity, the remainder of this paper will focus upon assessingthe suitability of Ba, Mn and P for tracing ballast water sources.
3.2. Stability
Tracer stability was examined for all unexchanged ballasttanks that were sampled on at least three occasions eachseparated by a day or longer, and for which replicatemeasurements existed for multiple sampling occasions.Overall, 18 ballast tanks from four cruises (Fos, LF, AS, BN)are included in the analysis of stability (Figs. 3 and 4). Errorbars are for true (independent) replicate samples and thusindicate reproducibility for the entire sampling and analysisprocess.
Ba concentrations were stable ±0.25 µgL−1 over the periodof sampling, with the exception of cruise BN (Fig. 3A). Trendsfor Mn were variable (Fig. 3B); Mn concentrations decreasedsignificantly over time on cruise AS (ANOVA by tank:F10,15=162.3, pb0.0001) and cruise BN (ANOVA by tank:F6,28=13.8, pb0.0001) but were quite stable on cruises Fos,and LF, varying by no more than ±2.5 µgL−1 in each tank.Phosphorus exhibited highly fluctuating concentrations in
to percent change in salinity following BWE of ballast tanks
Fig. 3 –Stability of dissolved Ba and Mn in unexchangedballast tanks sampled repeatedly over time (mean±SE).Symbols signify cruise: AS ( ), BN ( ), LF ( )and Fos ( ).
Table 3 – Average salinity (Salav), temperature (Tav) anddissolved oxygen (DOav) and minimum dissolved oxygen(DOmin) in ballast tanks during this study
BWE treatments indicate that tanks sampled were exchanged byempty-refill (ER) method (▼), exchanged by flow-through (FT)method (▲), partially (1/3) FT-exchanged ( ), partially (2/3) FT-exchanged (◪) or unexchanged (■).
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ballast tanks held over time (Fig. 4); on cruise AS, Pconcentrations in individual tanks differed by as much as20 µgL−1 on consecutive days. On cruise Fos, there was ageneral trend of increasing P from ~6 to ~11 µgL−1, and oncruise LF, P concentrations fluctuated by ±1 µgL−1 in sometanks and ±6 µgL−1 in others.
The relative standard deviation (i.e. coefficient of variation)of elemental concentrations in true replicate samples gives anindication of the relative importance of sampling/analyticalerror, with higher values of Rcv indicating lower overallmeasurement precision. In Table 4, Rcv is defined as the CVfor replicate samples (N=2–4) collected in the same tank at thesame time, averaged across individual cruise datasets. Rcv forthe four cruises in increasing order was AS~LF~Fos≪BN. Thelow Rcv for cruise BN reflects intermittent problems with ananalytical run in 2004, affecting ~20% of ballast water samplesfrom this cruise. For cruises except Cruise BN, Rcv in increasingorder was Ba~Mo~UbV≪MnbP. On Cruise BN, inter-repli-cate variability was up to 3× higher than on other cruises, withRcv in increasing order MobUbBa~V≪Mn~P.
Total measurement variability in individual ballast tanksover a cruise summarizes information on element stability+precision. Tcv in Table 4 is defined as the CV for all samples(N=2–16) collected in an individual tank, averaged across tanksfor individual cruise datasets. Low values of Tcv indicate thatan elements overall stability in ballast tanks on a cruise washigh, and that the sampling and analysis process resulted inprecise estimates of concentration. Tcv is increased by changesin concentrations in ballast tanks over time, sample contam-
ination, poor analytical reproducibility, or any combination ofthese factors. Indicating decreasing stability, Tcv for the fourcruises increased for LF~FosbASbBN. On cruises except cruiseBN, Tcv for the elements increased for Ba~Mo~UbV≪P~Mn.Values of Tcv on cruise BNwere 3–10× higher than on the otherthree cruises, increasing for MobUbVbBa≪P≪Mn.
3.3. Fidelity
Tracer fidelity was assessed for all tanks in which BWE coin-cided geographically with the collection of one ormore shipsidesamples, by comparing the mean concentrations of tracers inballast tanks (N=2–24, residence time b80 h) with shipsidesamplemeans (N=1–8). Tracerswith high fidelity should exhibitclose to aperfect 1:1 correlationbetween the twomeasurements(Fig. 5). Ba exhibited greatest fidelity overall, with mostmeasurements from fully exchanged ballast tanks (i.e. triangu-lar symbols) lying close to the line of perfect fit (Fig. 5A). Meansfor partially exchanged ballast tanks on Cruise Fos lie to the leftof the 1:1 correspondence line, indicating higher concentrationsin the ballast tanks than in the ocean; but are located pro-gressively closer to the line after successive exchanges. Thegreatest deviations were exhibited by three tanks during initialballasting onCruise BN forwhichBa concentrations in the tankswere lower than measurements in the port. Since very fewsamples from the port were available for comparison (N=3), lowsampling/analytical precision and/or inaccurate characteriza-tion of the water entering these ballast tanks could account for
Table 4 –Mean coefficients of variation (CV: σ/ x−×100%) fortrace elements in ballast tanks sampled over time
TCV is the mean CV for all samples from each tank, averaged acrosstanks on each cruise. RCV is the mean CV for replicate samplescollected at the same location and time. The average number ofsamples in each case is given by Nav.
Fig. 4 –Stability of dissolved P in unexchanged ballast tankssampled repeatedly over time (mean±SE). A) cruises AS( ) and BN ( ); B) cruise LF; and C) cruise Fos. InB) and C), symbols indicate individual ballast tanks.
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this deviation; furthermore, no significant deviations areassociated with the mid-ocean exchange of the same tanks(blue triangles). Considering only samples from fully exchangedballast tanks after removing the six data points associated withthe initial ballasting of tanks on cruise BN, the equation of thestraight line for the remaining data points (mean±standarderror) has a slope of 0.88±0.06 and x-intercept of 1.24±0.4 µgL−1
(R2=0.85). This indicates that overall, concentrations of Ba infully exchanged ballast tanks were slightly greater thanconcentrations in the ocean where the ballasting occurred,particularly when oceanic concentrations were lowest.
Manganese fidelity was comparatively poor (Fig. 5B), with ahigh degree of scatter in measurements around the 1:1correspondence line. For most tanks, the deviations were inthesamedirectionas those seen forPandBa (e.g. tanksoncruiseFos during initial ballasting, tanks on Cruise BN during FTexchange), whereas for others, deviations in the oppositedirection were observed (e.g. ER exchanged tanks on cruises K1and K2). Considering all samples from fully exchanged ballasttanks again after removing the six data points associated withthe initial ballasting of tanks on cruise BN, the equation of thelineof best fit (R2=0.49) has a slope of 1.01±0.2 and x-intercept of0.23±0.33 µgL−1. This indicates generally poor correspondencebetween the two measurements, although with no particularbias toward higher concentrations in either ballast tanks or theocean.
Concentrations of P in ballast tanks corresponded well toconcentrations in the ocean during BWE, however, the greaternumber of points lying slightly to the left of the 1:1correspondence line suggests a bias toward higher concentra-tions in ballast tanks than in the ocean where BWE took place(Fig. 5C). With few exceptions (initial ballasting on cruise BNand ER exchange on cruise K1) deviations from the 1:1 linewere in the same direction as for Ba, but greater in magnitude,reflecting typically greater differences in end-member con-centrations. Considering only samples from fully exchangedballast tanks after removing the six data points associatedwith the initial ballasting of tanks on cruise BN, the equationof the straight line for the remaining data points has a slope of0.97±0.05 and x-intercept of 3.59±1.01 µgL−1 (R2=0.90), con-firming the bias toward slightly elevated concentrations in theballast tanks relative to the locations where ballast water wasdrawn.
3.4. Predictability
Tracer predictability was assessed by examining the effect ofBWE on the range of tracer concentrationsmeasured in ballasttanks. The multivariate distribution of Ba, P and Mn inexchanged and unexchanged ballast tanks during this studyis illustrated in Fig. 6. Overall, smaller concentration rangeswere measured in exchanged ballast tanks (triangles) than inunexchanged tanks (diamonds). Also in exchanged ballasttanks, a smaller range of concentrations was observed incruises that took place in the Atlantic (light blue symbols)compared to the Pacific Ocean (black and yellow symbols).
With few exceptions, Ba concentrations in unexchangedtanks containing coastal ballast water exceeded 9 µgL−1
(Fig. 6). Exceptions were the tanks ballasted in Hawaii oncruise K2 (~5.9 µgL−1) and two of ten tanks ballasted in Japanon cruise LF (~5.5 µgL−1). In tanks exchanged in the Pacific andAtlantic open oceans, mean Ba was ~4.6–8 and ~5.3–7 µgL−1,respectively. Partially exchanged tanks contained Ba at levelsintermediate to coastal and oceanic concentrations. Whereasit appears possible to discriminate exchanged from unex-changed ballast tanks on both Atlantic cruises on the basis ofBa alone (i.e. Ba b~7 ppb indicates fully exchanged ballasttanks), Ba concentrations in nine tanks containing ballast
Fig. 6 –Concentrations (µgL−1, mean±SE) of dissolved Ba, Mnand P in ballast tanks during nine research cruises.Treatments are unexchanged (R. ), exchanged by FT method(Δ); exchanged by ER method (∇); partially (1/3rd) exchanged( ); and partially (2/3rd) exchanged (◪). Colors indicatelocations where tanks were ballasted in the Pacific Ocean(Hawaii [pink], Korea [purple], Japan [orange], USA WestCoast [red], open N. Pacific [yellow], Gulf of Alaska [black]) orAtlantic Ocean (Rotterdam [navy], Mid Atlantic [light blue],Mediterranean Sea [green]). Polygons enclose means inballast tanks exchanged at least 200 nautical miles fromshore in the Pacific (red) or Atlantic (blue) Oceans.
Fig. 5 –Correspondence between dissolved Ba, Mn and Pconcentrations in ballast tanks and in the ambient oceanduring BWE (µgL− 1, mean±SE). Treatments are initialballasting (●), exchanged by FT method (▲); exchanged byER method (▼); partially (1/3) exchanged ( ); and partially (2/3) exchanged (◪). Colors indicate cruise (LF [pink], AS[purple], K1 [red], K2 [orange], BN [blue], Fos [green]).
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water from Pacific ports overlapped with ranges in tanks thatwere exchanged in the open ocean.
P concentrations in unexchanged ballast tanks varied widelyfrom ~4–10 µgL−1 in tanks ballasted at the port of Fos Sur Mer onthe Mediterranean Sea to N100 µgL−1 in tanks ballasted in SanFrancisco Bay. In tanks exchanged in the Atlantic Ocean, Pconcentrations occupied a tight range between 3 and 6 µgL−1
(cruises BN and Fos), whereas in tanks exchanged in the PacificOcean P variedwidely (~6–60 µgL−1) and entirely overlappedwithranges in coastal ballast water from both oceans. Mn concentra-tions in unexchanged ballast tanks varied between ~1 and70µgL−1 (Atlanticports) or between~2and80µgL−1 (Pacific ports).
In exchanged ballast tanks, Mn concentrations were typically~0.2–2 µgL−1 (Atlantic cruises) and b0.1–1 µgL−1 (Pacific cruises).
Fig. 6A shows the joint distribution of Ba and P inexchanged and unexchanged ballast tanks. Ba and P arecorrelated in tanks exchanged in the Pacific (r=0.68, N=85,pb0.001) but not the Atlantic. When tanks are classifiedseparately according to ocean basin, exchanged and unex-changed tanks occupy predominately separate positions inmultivariate space. In Fig. 6A, tanks exchanged in the PacificOcean fall within the red polygon and tanks exchanged in theAtlantic Ocean within the blue ellipse, except for four ballasttanks containing low Ba and P coastal water from Honoluluand Haramachi (Japan).
Fig. 6B shows the multivariate distribution of Ba and Mn inballast water. Ba and Mn were weakly correlated in tanksexchanged in the Atlantic (r=0.37, N=42, pb0.05), but not the
19S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 1 – 2 6
Pacific. In Fig. 6B, tanks exchanged in the Pacific Ocean fallwithin the red polygon and tanks exchanged in the AtlanticOcean within the blue ellipse, except for five cases of elevated(N1–3 µgL−1) Mn; these were in two tanks on cruise AS(~1.5 µgL−1), one tank on cruise LA (~4.3 µgL−1) two tanks oncruise SF (~6 µgL−1).
4. Discussion
4.1. Tracer utility
Many metals are sensitive tracers of terrestrial and anthro-pogenic influences (for example, many transition elementsincluding iron and heavymetals), however, most are unsuitedto measurement in ballast tanks due to the unacceptably highrisk of contamination from ship structural materials. Dis-solved concentrations of Ba, P and Mn were examined aspotential tracers of BWE in this study because (1) it wasconsidered likely that accurate measurements could be madein ballast tanks due to relatively low contamination risk; (2)concentrations of these elements (together with Mo, U and V)can be determined simultaneously and rapidly at low(oceanic) levels without pre-concentration using ICP-MS(Field et al., 1999, 2007); (3) the elements were consideredlikely to behave conservatively in ballast tanks; and (4) themultivariate distribution of these elements in coastal waters islikely to differ from their distribution in the open oceans.Dissolved (b0.45 µm) concentrationswere considered themostappropriate measure because particulate concentrations inthe open oceans are low (Dehairs et al., 1980; Jeandel et al.,1987; Sunda and Huntsman, 1990; Yoshimura et al., 2007) andtotal concentrations in ballast tanks could vary strongly as afunction of sediment settling and resuspension. Cycling oftrace elements between particulate and dissolved phasesaffects dissolved concentrations, but does not preclude theuse of dissolvedmeasurements as long as the scale of changesdue to cycling is small relative to the difference in concentra-tions between oceanic and coastal ballast waters.
4.1.1. Mo, U and VMo and U exhibit conservative concentrations throughouttheir oceanic profiles (Palmer and Edmond, 1993; Sohrin et al.,1998). In this study, Mo and U offered comparable resolutionfor tracing ballast water sources to salinity measured in-situ.Vanadium occasionally offered additional resolution, possiblydue to its slight surface depletion in the open oceans (Collier,1984). Although salinity can be measured more easily andprecisely than Mo, U and V, particularly under controlledconditions in the laboratory, in the absence of salinity data allthree elements could assist in discriminating coastal fromoceanic ballast water.
4.1.2. BaBa was a stable and faithful tracer of the mid-ocean exchangeof ballast waters in this study, as indicated by stable Baconcentrations in 14 (of 17) ballast tanks containing port waterheld over time (Tcv below 3.5%, Table 4), together with anoverall strong linear correlation between the concentrations ofBa in ballast tanks and concentrations in the ocean where the
ballast water was sourced. Much lower stability was observedon cruise BN (Tcv ~20%) however, this correlated with highvariability in Mo, U and V (Tcv N10%), suggesting an externalexplanation for this variability, such as sampling/analyticalerror or water-mass mixing in ballast tanks during cruise BN.Themuch higher Rcv values for all three elements on cruise BN(Rcv ~14%) relative to other cruises (Rcv b3%) indicates lowermeasurement precision and suggests that the sampling/analytical explanation is most likely. Overall, these resultssuggest that Ba cycling in ballast tanks is unimportant on thetimescales of this study.
Ba exhibited close to 1:1 correspondence between concen-trations measured in ballast tanks and shipside samplesduring ballasting, despite often low sample replication. Weattribute the anomalously poor correspondence between Ba(and other tracers) in the port of Rotterdam compared toballast tanks at the beginning of cruise BN to the collection oftoo few port samples to accurately characterize the watermasses that entered the ballast tanks, exacerbated by the factthat tanks were particularly large (~104 m3) and loaded overseveral days. Our assessment of tracer fidelity for Ba and othertracers would have benefited from more frequent shipsidesampling during ballasting — particularly in ports and in theNorth Pacific ocean where tracer concentrations were mostspatially and temporally variable.
Barium was usually a more sensitive tracer of BWE thansalinity and the conservative elements particularly at highersalinities, but almost always a less sensitive tracer than Mnand P (Fig. 2). Ba concentrations in shipside samples collectedN100 miles offshore in this study averaged 4.4–6.6 µgL−1 in theNorth Pacific and 5.8–6.7 µgL−1 in the North Atlantic, corre-sponding well with previous measurements in the NE Pacific,NE and NW Atlantic and western Mediterranean Sea (Bernatet al., 1972; Chan et al., 1976, 1977; Nozaki et al., 2001) (Table 5).In unexchanged ballast tanks with salinity N30.0, average Baconcentrations were ~5.6–10 µgL−1 in tanks ballasted in theNorth Pacific and 9–14 µgL−1 tanks ballasted in the NorthAtlantic. These small differences betweenmeasured ranges ofBa in ballast tanks sourced from high-salinity coasts versusthe open ocean illustrate the limited sensitivity of this tracer.While improved data quality in future studies should enhancethe resolution of Bameasurements in ballast water, Bamay beof greatest utility when used in combinationwith other ballastwater tracers.
4.1.3. PPhosphorus concentrations were relatively dynamic in ballasttanks held over time, with Tcv ranging from 11–32% (Table 4).These high values of Tcv resulted from fluctuating P in sometanks together with lower measurement precision for Pcompared to most other elements (Rcv ranging from 7–26%).In oligotrophic oceanic waters near Hawaii and in the Atlantic,low P concentrations greatly increased measurement uncer-tainty. Even so, P remained a much more sensitive tracer ofBWE than salinity or the conservative elements at highersalinities (Fig. 2).
While some P variability was attributable to measure-ments, the fluctuating P concentrations observed in severalballast tanks is suggestive of nutrient recycling. Dissolved P inseawater is present primarily as the orthophosphate anion
Table 5 – Concentration (mean±σ) of total dissolvedbarium, manganese and phosphorus (µgL−1) in shipsidesamples during this study [TS: number of samples],compared to previous observations in these regions(references are indicated in parentheses in units of µgL−1)
Ba Mn P, DOP† orphosphate‡
North Pacific OceanNorthwestern(40–60°N,130–180°E)
Samples were from the open ocean N200 nautical miles offshoreexcept in the English Channel and Mediterranean Sea.
20 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 1 – 2 6
HPO42− and dissolved organic phosphorus (DOP) (Karl and
Björkman, 2002). Both the organic and inorganic componentsof the total dissolved P pool are available to phytoplanktonand other microorganisms for growth (Cotner and Biddanda,2002; Bjorkman and Karl, 2003) and subsequently recycledback to phosphate by digestion, decay and hydrolysis. Puptake by autotrophs is dependent upon solar radiation butheterotrophic bacterial uptake apparently is not (Roberts andHowarth, 2006); the dark ballast tank environment might thusbe expected to favor heterotrophic microbial uptake of P.Laboratory investigations of the microbial degradation oforganic detritus (Cooper, 1935; Ogura, 1975; Newell et al.,1981; Suzumura and Ingall, 2004) indicate that a relativelylabile proportion can be mineralized within days, suggestingthat P fluctuations in ballast tanks probably reflected in partthe relative rates of P sequestration versus liberation bymicrobes.
Total dissolved P in exchanged ballast tanks ranged widelyon Pacific cruises (6–52 µgL−1) compared to the Atlantic cruises(3–7 µgL−1), reflecting the much greater surface ocean varia-bility of P along the North Pacific cruise tracks compared to theNorth Atlantic cruise tracks. Our oceanic P measurements inshipside samples (Table 5) correspond well with publisheddata from the NE and NW Atlantic oceans (Butler et al., 1979;Wu et al., 2000; Sohm and Capone, 2006) and NE Pacific (Ridal,1992; Wu et al., 2000), and in the NW Pacific and Mediterra-nean Sea overlap with phosphate ranges according to theWorld Ocean Atlas (Garcia et al., 2006).
P concentrations in ballast tanks were strongly correlatedwith concentrations in shipside samples during ballasting,however, overall, P concentrations were typically slightlygreater in ballast tanks than in the open ocean where BWEtook place. This is partly a reflection of differences in end-member concentrations coupledwith BWE efficiencies near orbelow theoretical (95%) levels and partly a reflection of the factthat fewer samples were collected from the ocean during BWEthan from any ballast tank.
Fig. 5 probably underestimates source variability, espe-cially for tanks exchanged in the highly variable subpolarNorth Pacific (cruises LF and AS). For most ballast tanksexhibiting deviations from 1:1 correlation, BWE efficiency wasunknown,making it impossible to draw definitive conclusionsabout the origin of additional (~1–10 µgL−1) P. However, thereis some suggestion that ballast tanks may themselves be aminor source of P, possibly via the resuspension of sedimentsor organic detritus during BWE. Particle resuspension inballast tanks during ballasting or in inclement weather couldaffect the speciation of particle-reactive elements like P andMn (VanCappellen and Wang, 1996; Pan et al., 2002).
4.1.4. MnMn exhibits high potential for tracing terrestrial water massesdue to large coastal versus oceanic concentration differences(Bruland and Franks, 1983; Shiller, 1997), and was almostalways the most sensitive tracer of BWE in this study (Fig. 2).Despite this, the performance of dissolved Mn as a ballastwater tracer was reduced in this study by several factors.These included (1) several instances where dissolved Mnconcentrations in ballast tank decreased significantly overtime (low stability); (2) poor correspondence between Mn
21S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 1 – 2 6
concentrations in ballast tanks compared to concentrationsmeasured in shipside samples during exchange (low fidelity);and (3) higher sampling and/or analytical uncertainty com-pared to U, V, Mo and Ba, which impacted the performance ofMn according to all criteria assessed in this study.
An essential nutrient for phytoplankton growth, Mn isaccumulated in phytoplankton cells under low-light condi-tions (Bruland et al., 1991), released during biological decay(Collier and Edmond, 1984) and oxidized by bacteria in the dark(Sunda et al., 1983). In six ballast tanks on cruise AS, dissolvedMn decreased by 5–10% day−1, which is comparable toparticulate formation rates measured by Sunda and Hunts-man (1988) in dark-stored deep ocean samples of ~4.8% day−1
and exceeds typical rates of particulate formation in light-exposed samples of ~1–2% day−1 (Sunda and Huntsman, 1988,1990; Moffett, 1997). In natural environments, Mn-oxidizingbacteria that are suppressed by light catalyze the slowchemical oxidation of Mn(II) species to insoluble Mn oxidesat night time, whereas by day photo-mediated reactionsdissolve the oxides and regenerate Mn(II) (Hem, 1963;Stumm and Morgan, 1981; Sunda and Huntsman, 1988, 1990).In ballast tanks, the absence of light may favor the bacterially-mediated transformation of dissolved Mn into insolubleoxides which settle from the water column or are excludedby 0.45 µm filters.
A number of factors could have contributed to the observedinter-cruise (and inter-tank) variability in dissolved Mnstability during this study. Mn-oxidizing activity of temperatecoastal waters can vary strongly by season, an effect some-times attributed to seasonal changes in temperature andprimary productivity (Sunda and Huntsman, 1987; Moffett,1997). In this study, stability did not correlate with watertemperature, however, the two cruises that exhibited decreas-ing dissolved Mn over time also had ~3× greater concentra-tions of fluorescent dissolved organicmatter than cruiseswithstable Mn (Murphy et al., 2006), suggesting a possible link withprimary productivity. Furthermore, significant increases inorganic matter fluorescence have been observed in ballasttanks held over time (Murphy et al., 2004b), with fluorescencecharacteristics consistent with recent, microbially-producedhumic material (Murphy et al., 2008). Plankton compositionand survival in ballast tanks is known to be highly variable(e.g. Verling et al., 2005), however, the only published time-series of bacterial populations in ballast tanks (Drake et al.,2002) did not include community composition, and it remainsfor future research to determine whether ballast tank envir-onments favor the survival of particular microorganismspecies.
Another possible source of between-cruise variability inMnbehavior is suggested by the observation that rates ofparticulate formation in dark-stored samples are depressedby pre-exposure to sunlight (Sunda and Huntsman, 1988). Thiseffect may be due to the light-mediated production ofhydrogen peroxide, an agent recognized to be capable ofinducing the reductive dissolution of Mn(IV) oxides (Szymczakand Waite, 1989). If photo-generated reactive oxygen speciessuch as hydrogen peroxide and superoxide are present inballast tanks, transformations might be expected to beongoing until redox-active agents such as these are consumed(Waite et al., 1988; Rose and Waite, 2006).
In ballast tanks where dissolved Mn decreases over time, itsresolution as a BWE tracer is a decreasing function of voyageduration. Itmay be possible in future studies to circumvent thisproblem by measuring total (dissolved+particulate) Mn which,due to much higher particulate Mn in coastal versus oceanicenvironments, would also greatly magnify differences betweencoastal and oceanic concentrations relative to dissolvedmeasurements (Bruland and Franks, 1983; Moffett, 1997; Wellset al., 2000). A trade-off to this approach is likely to be themuchgreater dependence of measurements on the quantity andbehavior of entrained sediments, which can build up tosignificant levels in ballast tanks (Bailey et al., 2003; Duggan etal., 2005). Sediments retained in exchanged ballast tanks couldgreatly elevate total suspended Mn and P beyond oceanic levelsand increase the variability of whole-tank concentrationestimates as a result of resuspension, scavenging, and settling(VanCappellen and Wang, 1996; Pan et al., 2002).
Decreasing concentrations in ballast tanks held over timereduces sensitivity but does not eliminate dissolved Mn as aballast water tracer as long as the resulting changes inconcentration are small relative to the difference betweenoceanic and coastal ballast waters. This was the case for theballast tanks examined in this study except on cruise BN. Onother cruises, Mn concentrations were always significantlylower following BWE, even where Mn decreased by 50–90%.Thus Mn concentrations were typically below 1 µgL−1 in tanksexchanged in the Pacific ocean and below 3 µgL−1 in tanksexchanged in the Atlantic. There were only five cases whereMn remained significantly elevated above these levels follow-ing BWE, and all are potentially explained by differences inend-member concentrations together with b100% BWE effi-ciency: (1) Four tanks on cruises AS and SF with high pre-BWEMn concentrations (30–60 µgL−1) and post-exchange concen-trations of 1.5–6 µgL−1, since 95% BWE efficiency accounts forMn concentrations elevated by ~1.5–3 µgL−1 above oceaniclevels, and (2) One tank on cruise LA,where low BWE efficiency(75% according to independent determination; see below)accounts for Mn concentrations of ~3.3 µgL−1 following BWEof tanks with initial concentrations of 12–16 µgL−1. On cruiseLA where Mn decreased by ~75%, BWE resulted in a salinityand Mo reduction of b10%, and reductions in Ba and P of~30% and ~40% respectively, demonstrating the muchgreater sensitivity of Mn to coastal influences. The fidelityresults for Mn may therefore reflect relatively large coastalversus oceanic concentration differentials and variable BWEefficiency together with lower measurement precision forthis element.
Dissolved Mn in open ocean shipside samples in this studyranged from 0.2–3 µgL−1 in the North Pacific and 0.1–0.5 µgL−1
in the North Atlantic (Table 5). Mn in shipside samples fromthe NE Atlantic and English Channel are comparable withprevious reports (Bruland and Franks, 1983; Shiller, 1997;Statham et al., 1998); however Mn in NE Pacific samples areelevated 2–10 times relative to previous measurements in thesame region (Martin et al., 1985; Martin et al., 1989). Some ofour oceanic Mn data may therefore be overestimated;especially since Mn concentrations in oceanic shipsidesamples often approached detection limits of ~0.2 µgL−1.Recent modifications to analytical protocols have reduced Mndetection limits in ballastwater to below 0.01 µgL−1 (Field et al.,
Fig. 7 –Mean annual phosphate distribution in the surface global oceans (µgL−1). Data are from theWorldOceanAtlas (Garcia et al.,2006) mapped in Ocean Data View (Schlitzer, 2007).
22 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 1 – 2 6
2007). A larger database of surface ocean Mn distribution isnow required particularly for the North Pacific, in order tobetter define the expected range Mn concentrations inexchanged ballast tanks and reduce the influence of outliers.
4.2. Implementation
The optimal implementation of tracer techniques for verifyingmid-ocean ballast water exchange requires at minimum a two-stage procedure. In the first stage, the ballast water shouldexceed a salinity criterion, since low salinity in ballast tanks isan unambiguous indicator of coastal sources. Only afterexceeding the salinity criterion would confirmation be neededthat additional tracers lie within the expected ranges forexchanged ballast tanks. From an enforcement standpoint, ahigh-salinity/ocean-referenced approach is critical for threereasons. First, salinity screening eliminates the unnecessaryimplementation of more costly and time-consuming techni-ques. Second, verification should ultimately be possible in theabsence of either information regarding, or comparative sam-ples from, the original coastal source port. Instead, tracerconcentrations should be compared with ranges for the surfaceopen oceans, where variability is far less than at the coasts andwhere the position of BWE is known from declarations on theship's compulsory ballast water reporting form (USCG, 2004).Third, in delineating the boundaries for acceptablemultivariatetracer concentrations in exchanged ballast tanks, the opportu-nity exists to incorporate statistical uncertainties in existingknowledge together with management tolerances toward falsedeterminations of different types.
While ICP-MS enables sensitive and accurate multi-elementmeasurements in the laboratory, it is ultimately desirable toassess the BWE status of ships in real-time in order for it to bepossible to intercept high-risk ballast water before it is dis-charged. Colorimetric and chemiluminescence methods arewidely used in field measurements of phosphate and Mn (e.g.Chiswell et al., 1990;Okamuraet al., 1998; Yaqoobet al., 2004), butare non-trivial and unsuitable for non-scientists. Automated in-
situ instruments increase thepracticality of fieldmeasurements,while reducing the opportunity for sample contamination.Prototype in-situ instruments have beendeveloped for quantify-ing Mn and P using spectroscopy (Klinkhammer, 1994; Adornatoet al., 2007) and voltammetry (Tercier-Waeber et al., 1998), butmany technical andpracticalhurdleswouldneed tobeovercomebefore their deploymentwouldbe feasible in a regulatory setting.
4.3. Improved predictability
The discrimination of ballast water sources under this frame-work will rely upon knowledge of the spatial and temporaldistribution of target tracers in the open oceans. Developingglobal scale tracers for oceanic environments requires exten-sive investigation and sampling directed at existing gaps inspatial and temporal coverage. There is a need for rapid,accurate and precise analytical techniques with appropriateQA/QC management. Overall measurement precision forelements examined in this study was generally high (Rcv
b10%), with the exception of a subset of samples primarilyfrom Cruise BN (Rcv 9–30%, Table 4). Recent improvements toour methods for ICP-MS measurement of BWE tracers haveresulted in three fold improvements in detection limits,accuracy and precision (Field et al., 2007). Even so, tracerconcentrations in shipside samples from this study generallycompared well to previous studies that used orthodoxsampling methods (Table 5) suggesting that it is possible tocollect uncontaminated samples of these elements in theoceans via the water-cooling pipes of commercial ships.
Several existingdatabases thatmap theglobaldistributionofP are freely-available. For example, the 2005World Ocean Atlasand World Ocean Database available via NOAA-NODC includemean dissolved inorganic phosphorus (phosphate) data for theglobal ocean with 1° grid resolution (Garcia et al., 2006), and theGlobal Open Ocean DOP database (NN139,000 measurements)records dissolved organic phosphorus (Karl and Björkman,2002). Shiller (1997) mapped surface Mn distributions in theNorth Atlantic. We are aware of no extensive databases of
23S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 1 – 2 6
surface distributions of Mn in the Pacific or Ba in either oceansouth of the Arctic circle. Many ships that travel to the US westcoast from Asia follow a Great Circle route transecting thesubpolar (western subarctic and Alaskan) gyres (Miller et al.,2004), where surface tracer distributions are influenced byEkman upwelling of nutrient-rich deep water (Tomczak andGodfrey, 2003; Garcia et al., 2006). The high-P oceanic samplesfrom this study also contained higher Ba relative to theoligotrophic North Pacific south of 40°N. It is clear from theglobal distribution of phosphate (Fig. 7) that P andpossibly otherBWE tracers will be more variable in tanks exchanged in theNorth Pacific than in the North Atlantic, suggesting thatdifferent levels of complexity would be associated with BWEverification in either ocean.
4.4. Exchange efficiencies
A significant complication to verifying BWE by natural tracermethods is that in practice, lower than theoretical exchangeefficiencies are frequently demonstrated by vessels that haveimplemented ballast water exchange to theoretical 95% effec-tiveness, particularly when ships use the flow-throughexchange method (Murphy et al., 2004a; Ruiz et al., 2005; Ruizand Smith, 2005). This is apparently due to a combination offactors including engineering constraints (Hall and Wilson,2006;Wilsonetal., 2006), complicatedphysics, andhumanerror,since appropriate pumping times are difficult to calculate ifpumpcapacities are not accurately known (T. Snell, pers. comm.).Regulators may thus be forced to allow for b95% exchangeefficiency until technological advances enable better flushing ofballast tanks, or until guidelines on the best-practice imple-mentation of BWE under current technology are available. Ifregulators have to accommodate lower exchange efficienciesand hence a greater contribution of residual port water to theoverall chemical signature of ballast tanks, the overall sensitiv-ity of the method for distinguishing between high-salinitycoastal and oceanic ballast waters will be reduced.
Currently, themost reliableway to determine BWE efficiencyfor a given ballast tank is through experimentation and directempirical measures. Deploying artificial tracers to track themovement of water in tanks allows control over detectionsensitivity relative to natural tracers. In the earliest cruises ofthis study, fluorescent dye was added to tanks prior to BWEallowing independent calculation of BWE efficiency (75%efficient by FT exchange on cruise LA, 93% efficient by FTexchange and 98% efficient by ER exchange on cruise SF)(Murphy et al., 2004a). Artificial dye tracers were discontinuedonsubsequent cruisesdueto interferencewithanotherpotentialballast water tracer (chromophoric dissolved organic matter;Murphy et al., 2006) and concerns about the possible contamina-tion of low-level trace elementmeasurements. Fluorescent 1 µmplastic beads have been deployed in other ballast water studies(Ruiz unpublished, N=13 cruises), but have tended to stick tosurfaces and settle from the water column over time.
5. Conclusion
Results from sampling the ballast tanks of ocean-going cargoships indicate that Ba, Mn and P are sensitive and useful
tracers of high-salinity coastal ballast water sources. Criteriafor effective ballast water tracers include relatively stableconcentrations over time, concentrations which accuratelyreflect the entrained source waters and a predictable range ofconcentrations in exchanged ballast tanks. In this study,dissolved Ba and P performed well according to these criteriaand were more sensitive tracers of source than salinity andthe conservative elements Mo, U and V. Dissolved Mnexhibited lower stability and fidelity, however, Mn remainedthe most sensitive tracer of coastal influences except in asmall number of tanks where initial concentrations were lowand decreased by an order of magnitude during a 7-day cruise.
We conclude that several trace elements aid the discrimina-tion of coastal from oceanic ballast water sources and showpromise for verification, for which applications may be mostlimited by the opportunity for real-time determinations. Ourresults also suggest that commercial vessels may offer anexceptional and unexploited opportunity for reliable observa-tions (measures) of some trace elements, given the number andglobal reach of operating ships, trade routes, and frequency oftransits. Efforts currently underway to gather data fromadditional ships, ports and oceanic regions across greatertemporal and spatial scales will assist in characterizing thefull range of signals expected in exchanged ballast tanks andwill further test the utility of trace elements for discriminatingballast water sources.
Acknowledgement
We thank SERC staff and volunteers for sampling assistanceand the Rutgers Inorganic Analytical Laboratory for analy-tical services. L. Kalnejais and anonymous reviewers pro-vided invaluable comments on earlier versions of thismanuscript. NYK Bulkship (USA) LTD., Gateway MaritimeCorp./Sincere Industrial Corp., Matson Navigation Company,Bergesen DY ASA., Sea River Maritime, the Alaska TankerCompany, BP Amoco PLC and Krupp Seeschiffahrt GmbHgenerously provided experimental platforms. This researchwas funded by the US Coast Guard's Research and Develop-ment Center and the Columbia River Aquatic Nuisance Spe-cies Initiative (CRANSI).
R E F E R E N C E S
Adornato LR, Kaltenbacher EA, Greenhow DR, Byrne RH.High-resolution in situ analysis of nitrate and phosphate in theoligotrophic ocean. Environ Sci Technol 2007;41:4045–52.
Aminot A, Kerouel R. Dissolved organic carbon, nitrogen andphosphorus in the N–E Atlantic and the N–W Mediterraneanwith particular reference to non-refractory fractions anddegradation. Deep-Sea Res, Part I, Oceanogr Res Pap2004;51:1975–99.
Bailey SA, Duggan IC, van Overdijk CDA, Jenkins PT, MacIsaac HJ.Viability of invertebrate diapausing eggs collected fromresidual ballast sediment. Limnol Oceanogr 2003;48:1701–10.
Baturin GN. Phosphorus cycle in the ocean. Lithol Miner Resour2003;38:101–19.
Benitez-Nelson CR. The biogeochemical cycling of phosphorus inmarine systems. Earth-Sci Rev 2000;51:109–35.
24 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 1 – 2 6
Bernat M, Church T, Allegre CJ. Barium and strontiumconcentrations in Pacific and Mediterranean sea waterprofiles by direct isotope dilution mass spectrometry. EarthPlanet Sci Lett 1972;16:75–80.
Bjorkman KM, Karl DM. Bioavailability of dissolved organicphosphorus in the euphotic zone at station ALOHA, NorthPacific Subtropical Gyre. Limnol Oceanogr 2003;48:1049–57.
Broecker WS, Peng TH. Tracers in the Sea. NY: Lamont-DohertyGeological Observatory; 1982.
Bruland KW, Franks RP. Mn, Ni, Cu, Zn and Cd in thewestern NorthAtlantic. In: Wong CS, Boyle EA, Bruland KW, Burton JD,Goldberg ED, editors. Trace Metals in Seawater. New York:Plenum Press; 1983. p. 395–414.
Bruland KW, Donat JR, Hutchins DA. Interactive influences ofbioactive trace metals on biological production in oceanicwaters. Limnol Oceanogr 1991;36:1555–77.
Butler EI, Knox S, Liddicoat MI. Relationship between Inorganicand Organic Nutrients in Sea-Water. J Mar Biol Assoc UK1979;59:239–50.
Carlton JT. Transoceanic and interoceanic dispersal of coastalmarine organisms: the biology of ballast water. Oceanogr MarBiol Ann Rev 1985;23:313–71.
Chan LH, Edmond JM, Stallard RF, Broecker WS, Chung YC, WeissRF, et al. Radium and barium at GEOSECS stations in theAtlantic and Pacific. Earth Planet Sci Lett 1976;32:258–67.
Chan LH, Drummond D, Edmond JM, Grant B. On the barium datafrom the Atlantic Geosecs expedition. Deep-Sea Res1977;24:613–49.
Chiswell B, Rauchle G, Pascoe M. Spectrophotometric methods forthe determination of manganese. Talanta 1990;37:237–59.
Coffey M, Dehairs F, Collette O, Luther G, Church T, Jickells T. Thebehaviour of dissolved barium in estuaries. Estuar Coast ShelfSci 1997;45:113–21.
Collier RW. Particulate and dissolved vanadium in the NorthPacific Ocean. Nature 1984;309:441–4.
Collier R, Edmond J. The trace-element geochemistry of marinebiogenic particulate matter. Prog Oceanogr 1984;13:113–99.
Cooper LHN. The rate of liberation of phosphate in sea water bythe breakdown of plankton organisms. J Mar Biol Assoc UK1935;20:197–202.
Cotner JB, Biddanda BA. Small players, large role: microbialinfluence on biogeochemical processes in pelagic aquaticecosystems. Ecosystems 2002;5:105–21.
Dehairs F, Chesselet R, Jedwab J. Discrete suspended particles ofbarite and the barium cycle in the open ocean. Earth Planet SciLett 1980;49:528–50.
Delanghe D, Bard E, Hamelin B. New TIMS constraints on theuranium-238 and uranium-234 in seawaters from the mainocean basins and the Mediterranean Sea. Mar Chem2002;80:79–93.
Drake LA, Ruiz GM, Galil BS, Mullady TL, Friedman DO, Dobbs FC.Microbial ecology of ballast water during a transoceanic voyageand the effects of open-ocean exchange. Mar Ecol, Prog Ser2002;233:13–20.
Duggan IC, van Overdijk CDA, Bailey SA, Jenkins PT, Limen H,MacIsaac HJ. Invertebrates associated with residual ballastwater and sediments of cargo-carrying ships entering the GreatLakes. Can J Fish Aquat Sci 2005;62:2463–74.
Fang TH. Phosphorus speciation and budget of the East China Sea.Cont Shelf Res 2004;24:1285–99.
Field MP, Cullen JT, Sherrell RM. Direct determination of 10 tracemetals in 50 mL samples of coastal seawater using desolvatingmicronebulization sector field ICP-MS. J Anal At Spectrom1999;14:1425–31.
Field MP, LaVigne M, Murphy KR, Ruiz GM, Sherrell RM. Directdetermination of P, V, Mn, As, Mo, Ba and U in seawater bySF-ICP-MS. J Anal At Spectrom 2007;22:1145–51.
Froelich PN, Bender ML, Luedtke NA, Heath GR, DeVries T. Themarine phosphorus cycle. Am J Sci 1982;282:474–511.
Gollasch S, Lenz J, Dammer M, Andres HG. Survival of tropicalballast water organisms during a cruise from the Indian Oceanto the North Sea. J Plankton Res 2000;22:923–37.
Hanor JS, Chan LH. Non-conservative behavior of barium duringmixing of Mississippi River and Gulf of Mexico waters. EarthPlanet Sci Lett 1977;37:242–50.
Hatje V. Particulate trace metal and major element distributionsover consecutive tidal cycles in Port Jackson Estuary, Australia.Environ Geol 2003;44:231–9.
Hem JD. Chemical equilibria and rates of manganese oxidation. U.S.Geol Surv Water-supply Pap 1963;1667A:1–64.
Hunt CD, Tanis DC, Stevens TG, Frederick RM, Everett RA.Verifying Ballast-Water Treatment Performance. Environ SciTechnol 2005;39:321A–8A A-pages.
IMO. (International Maritime Organisation). InternationalConvention for the Control and Management of ShipsBallast Water and Sediments. BWM/CONF/36, 16 February2004. London: International Maritime Organisation; 2004.
Jeandel C, Caisso M, Minster JF. Vanadium behaviour in the globalocean and in the Mediterranean sea. Mar Chem 1987;21:51–74.
Karl D, Björkman K. Dynamics of DOP. In: Hansell DA, Carlson CA,editors. Biogeochemistry of marine dissolved organic matter.Academic Press; 2002. p. 249–366.
KlinkhammerGP. Fiber optic spectrometers for in-situmeasurementsin the oceans— the ZAPS probe. Mar Chem 1994;47:13–20.
Laslett RE, Balls PW. The behaviour of dissolved Mn, Ni and Zn inthe Forth an industrialised, partially mixed estuary. Mar Chem1995;48:311–28.
Martin JH, Knauer GA. Manganese cycling in Northeast Pacificwaters. Earth Planet Sci Lett 1980;51:266–74.
Martin JH, Knauer GA, Broenkow WW. VERTEX: the lateraltransport of manganese in the northeast Pacific. Deep-Sea Res1985;32:1405–27.
Martin JH, Gordon RM, Fitzwater S, Broenkow WW. Vertex:phytoplankton/iron studies in the Gulf of Alaska. Deep-SeaRese, Part A, Oceanogr Res Pap 1989;36:649–80.
Miller AW, Ruiz GM, Lion K. Status and trends of ballast watermanagement in the United States. Second biennial report ofthe national ballast information clearinghouse (January 2002 toDecember 2003). Submitted to the United States Coast Guard.Smithsonian Environmental Research Center, Edgewater,Maryland, 2004, pp. 1–32.
Moffett JW. The importance of microbial Mn oxidation in theupper ocean: a comparison of the Sargasso Sea and equatorialPacific. Deep-Sea Res, Part I, Oceanogr Res Pap1997;44:1277–91.
Moore RM, Burton JD, Williams PJL, Young ML. The behavior ofdissolved organic material, iron and manganese duringestuarine mixing. Geochim Cosmochim Acta 1979;43:919–26.
Muller FLL, Tranter M, Balls PW. Distribution and transport ofchemical constituents in the Clyde Estuary. Estuar Coast ShelfSci 1994;39:105–26.
Murphy KR. Naturally occurring chemical tracers in seawater andtheir application to verifyingmid-ocean ballast water exchange.PhD Thesis, Sydney: School of Civil and EnvironmentalEngineering. University of New South Wales; 2007. [188 pages].
Murphy K, Ruiz G, Sytsma M. Standardized sampling protocol forverifying mid-ocean ballast water exchange. US Coast GuardResearch and Development Center, Groton, CT; 2003.
Murphy K, Boehme J, Coble P, Cullen J, Field P, Moore W, et al.Verification of mid-ocean ballast water exchange usingnaturally occurring coastal tracers. Mar Pollut Bull2004a;48:711–30.
MurphyK,RuizG,Coble P,BoehmeJ, Field P,Cullen J, etal.Mid-oceanballast water exchange: approach &methods for verification. US
25S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 1 – 2 6
Coast Guard Research and Development Center Report No.CG-D-02-04, Groton, CT; 2004b.
Murphy KR, Ruiz GM, Dunsmuir WTM, Waite TD. Optimizedparameters for fluorescence-based verification of ballast waterexchange by ships. Environ Sci Technol 2006;40:2357–62.
Murphy KR, Stedmon CA, Waite TD, Ruiz GM. Distinguishingbetween terrestrial and autochthonous organic matter sourcesin marine environments using fluorescence spectroscopy. MarChem 2008;108:40–58.
Nakatsuka S, Okamura K, Norisuye K, Sohrin Y. Simultaneousdetermination of suspended particulate trace metals (Co, Ni,Cu, Zn, Cd and Pb) in seawater with small volume filtrationassisted bymicrowave digestion and flow injection inductivelycoupled plasma mass spectrometer. Anal Chim Acta2007;594:52–60.
Newell RC, Lucas MI, Linley EAS. Rate of degradation andefficiency of conversion of phytoplankton debris by marinemicroorganisms. Mar Ecol, Prog Ser 1981;6:123–36.
Nozaki Y, Yamamoto Y, Manaka T, Amakawa H, Snidvongs A.Dissolved barium and radium isotopes in the Chao PhrayaRiver estuarine mixing zone in Thailand. Cont Shelf Res2001;21:1435–48.
Ogura N. Further studies on decomposition of dissolvedorganic-matter in coastal seawater. Mar Biol 1975;31:101–11.
Okamura K, Gamo T, Obata H, Nakayama E, Karatani H, Nozaki Y.Selective and sensitive determination of trace manganese insea water by flow through technique using luminol hydrogenperoxide chemiluminescence detection. Anal Chim Acta1998;377:125–31.
Palmer MR, Edmond JM. Uranium in river water. GeochimCosmochim Acta 1993;57:4947–55.
Pan G, Krom MD, Herut B. Adsorption–desorption of phosphate onairborne dust and riverborne particulates in east Mediterraneanseawater. Environ Sci Technol 2002;36:3519–24.
Roberts BJ, Howarth RW. Nutrient and light availability regulatethe relative contribution of autotrophs and heterotrophs torespiration in freshwater pelagic ecosystems. Limnol Oceanogr2006;51:288–98.
Rose AL, Waite TD. Role of superoxide in the photochemicalreduction of iron in seawater. Geochim Cosmochim Acta2006;70:3869–82.
Ruiz GM, Carlton JT. Invasive species: vectors and managementstrategies. Washington, D.C.: Island Press; 2003. p. 600.
Ruiz GM, Smith G. Biological study of container ships arriving tothe Port of Oakland. Final report to the Port of Oakland, March22 2005; 2005.
Ruiz GM, Carlton JT, Grosholz ED, Hines AH. Global invasions ofmarine and estuarine habitats by non-indigenous species:mechanisms, extent, and consequences. Am Zool1997;37:621–32.
Ruiz GM,Murphy KR, Verling E, Smith G, Chaves S, Hines AH. Ballastwater exchange: efficacyof treatingships' ballastwater to reducemarine species transfer and invasion success. Final report to theUS Fish & Wildlife Service, American Petroleum Institute andPrince William Sound Regional Citizens' Advisory Council.Smithsonian Environmental Research Center, Edgewater; 2005.p. 17.
Schlitzer R. Ocean Data View. http://odv.awi.de2007.Shammon TM, Hartnoll G. The winter and summer partitioning
of dissolved nitrogen and phosphorus. Observations acrossthe Irish Sea during 1997 and 1998. Hydrobiologia2002;475:173–84.
Shaw TJ, MooreWS, Kloepfer J, Sochaski MA. The flux of barium tothe coastal waters of the southeastern USA: the importance ofsubmarine groundwater discharge. Geochim Cosmochim Acta1998;62:3047–54.
Shiller AM. Manganese in surface waters of the Atlantic ocean.Geophys Res Lett 1997;24:1495–8.
Sohm JA, Capone DG. Phosphorus dynamics of the tropical andsubtropical North Atlantic: Trichodesmium spp. versus bulkplankton. Mar Ecol, Prog Ser 2006;317:21–8.
Sohrin Y, Isshiki K, Kuwamoto T, Nakayama E. Tungsten in NorthPacific waters. Mar Chem 1987;22:95–103.
SohrinY, FujishimaY,UedaK,AkiyamaS,Mori K,HasegawaH, et al.Dissolved niobium and tantalum in the North Pacific. GeophysRes Lett 1998;25:999–1002.
Statham PJ, Yeats PA, Landing WM. Manganese in the easternAtlantic Ocean: processes influencing deep and surface waterdistributions. Mar Chem 1998;61:55–68.
Statham P, Leclercq S, Hart V, Batté M, Auger Y, Wartel M, et al.Dissolved and particulate trace metal fluxes through thecentral English Channel, and the influence of coastal gyres.Cont Shelf Res 1999;19:2019–40.
Stumm W, Morgan JJ. Aquatic Chemistry: An introductionemphasizing chemical equilibria in natural waters. 2nd ed.New York: Wiley-Interscience Publication; 1981.
Sunda WG, Huntsman SA. Microbial oxidation of manganese in aNorth-Carolina estuary. Limnol Oceanogr 1987;32:552–64.
Sunda WG, Huntsman SA. Effect of sunlight on redox cycles ofmanganese in the southwestern Sargasso Sea. Deep-Sea Res,Part A, Oceanogr Res Pap 1988;35:1297–317.
Sunda WG, Huntsman SA. Diel cycles in microbial manganeseoxidation andmanganese redox speciation in coastal waters ofthe Bahama-islands. Limnol Oceanogr 1990;35:325–38.
SundaWG,HuntsmanSA,HarveyGR. Photoreductionofmanganeseoxides in seawater and its geochemical and biologicalimplications. Nature 1983;301:234–6.
Suzumura M, Ingall ED. Distribution and dynamics of variousforms of phosphorus in seawater: insights from fieldobservations in the Pacific Ocean and a laboratoryexperiment. Deep-Sea Res, Part I, Oceanogr Res Pap2004;51:1113–30.
Szymczak R, Waite TD. Photochemical activity in waters of theGreat Barrier Reef. Estuar, Coast Shelf Sci 1989;33:605–22.
Taylor JR, Falkner KK, Schauer U, Meredith M. Quantitativeconsiderations of dissolved barium as a tracer in the ArcticOcean. J Geophys Res-Oceans 2003;108(C12):3374.
Tercier-Waeber ML, Belmont-Hebert C, Buffle J. Real-timecontinuous Mn(II) monitoring in lakes using a novelvoltammetric in situ profiling system. Environ Sci Technol1998;32:1515–21.
Tomczak M, Godfrey JS. Regional Oceanography: an Introduction.Delhi: Daya Publishing House; 2003.
USCG. (United States Coast Guard). Mandatory Ballast WaterManagement Program for U.S. Waters: Final Rule, 33 CFR 151,Subpart D. 69. 144, 2004, pp. 44952–44961.
VanCappellen P, Wang YF. Cycling of iron and manganese insurface sediments: a general theory for the coupled transportand reaction of carbon, oxygen, nitrogen, sulfur, iron, andmanganese. Am J Sci 1996;296:197–243.
Verling E, Ruiz GM, Smith LD, Galil B, Miller AW, Murphy KR.Supply-side invasion ecology: characterizing propagulepressure in coastal ecosystems. Proc Royal Soc B2005;272:1249–56.
Wells ML, Smith GJ, Bruland KW. The distribution of colloidal andparticulate bioactive metals in Narragansett Bay, RI. Mar Chem2000;71:143–63.
WilsonW, Chang P, Verosto S, Atsavapranee P, Reid DF, Jenkins PT.Computational and experimental analysis of ballast waterexchange. Nav Eng 2006;118:25–36.
Wonham MJ, Carlton JT. Trends in marine biological invasions atlocal and regional scales: the Northeast Pacific Ocean as amodel system. Biological Invasions 2005;7:369–92.
26 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 3 ( 2 0 0 8 ) 1 1 – 2 6
WonhamMJ,WaltonWC, Ruiz GM, Frese AM, Galil BS. Going to thesource: role of the invasion pathway in determining potentialinvaders. Mar Ecol, Prog Ser 2001;215:1–12.
Wu JF, Sunda W, Boyle EA, Karl DM. Phosphate depletion in thewestern North Atlantic Ocean. Science 2000;289:759–62.
Yaqoob M, Nabi A, Worsfold PJ. Determination of nanomolarconcentrations of phosphate in freshwaters using flow injection
with luminol chemiluminescence detection. Anal Chim Acta2004;510:213–8.
Yoshimura T, Nishioka J, Saito H, Takeda S, Tsuda A, Wells ML.Distributions of particulate anddissolvedorganic and inorganicphosphorus in North Pacific surface waters. Mar Chem2007;103:112–21.
