The freshwater dissolved organic matter fluorescence–total organic carbon relationship

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  • HYDROLOGICAL PROCESSESHydrol. Process. 21, 20932099 (2007)Published online 4 June 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.6371

    The freshwater dissolved organic matter fluorescencetotalorganic carbon relationship

    Susan A. Cumberland and Andy Baker*School of Geography, Earth and Environmental Sciences, The University of Birmingham, Birmingham B15 2TT, UK

    Abstract:The fluorescent properties of dissolved organic matter (DOM) enable comparisons of humic-like (H-L) and fulvic-like (F-L)fluorescence intensities with dissolved organic carbon (DOC) in aquatic systems. The fluorescence-DOC relationship differedin gradient, i.e. the fluorescence per gram of carbon, and in the strength of the correlation coefficient. We compare thefluorescence intensity of the F-L and H-L fractions and DOC of freshwater DOM in north Shropshire, England, featuring ariver, wetland, spring, pond and sewage DOM sources. Correlations between fluorescence and DOC varied between samplesites. Wetland water samples for the F-L peak gave the best correlation, r D 0756; the lowest correlation was from finaltreated sewage effluent, r D 0167. The relationship between fluorescence and DOC of commercially available InternationalHumic Substances Society standards were also examined and they generally showed a lower fluorescence per gram of carbonfor the F-L peak than the natural samples, whereas peat wetland DOM gave a greater fluorescence per gram of carbon thanriver DOM. Here, we propose the strength of the fluorescenceDOC correlation to be a useful tool when discriminatingsources of DOM in fresh water. Copyright 2007 John Wiley & Sons, Ltd.

    KEY WORDS dissolved organic carbon; dissolved organic matter; fluorescence; wetlands; rivers

    Received 20 December 2005; Accepted 22 May 2006

    INTRODUCTIONHumic substances occur in every natural water samplethat has ever been analysed for them (Malcolm, 1985).Dissolved humic substances can be defined as passingthrough a 045 m membrane, and are made up of humicand fulvic substances, the latter being present in largerproportion and characterized as being soluble under allpH conditions (Aiken, 1985). Freshwater dissolved humicsubstances are derived from dead and decaying detritus,aquatic plants, animals and debris from overhanging veg-etation, and tend to be of a younger state than that ofsoil humic substances (Hongve, 1999). River aquatic ful-vic substances are derived from plant and tree residues,which contain more phenolic and lignin-derived organiccompounds than those found in soil (Chen et al., 2003).Historically, much work has gone into the determinationand separation of these humic and fulvic substances, andyet still relatively little is understood about the molec-ular distribution and fluorescent characteristics of thesemolecules (Alberts and Takacs, 2004). Analysis of freeze-dried fulvic acid using 13C nuclear magnetic resonancespectroscopy demonstrates that it contains a number ofgroups, such as aliphatic carbon atoms, carbohydrates,olefinic carbon atoms, aromatic carbons atom, carboxylester amide carbon atoms and ketonic and aldehydeiccarbon atoms (Klapper et al., 2002; Sierra et al., 2005).They are responsible for water colour (Hongve, 1999) and

    * Correspondence to: Andy Baker, School of Geography, Earth andEnvironmental Sciences, The University of Birmingham, BirminghamB15 2TT, UK. E-mail: a.baker.2@bham.ac.uk

    sometimes pH due to the carboxylic acid groups attachedto them (Klapper et al., 2002). The aromatic and car-boxylic fractions provide the fluorescence of humic andfulvic substances (Senesi et al., 1989); fulvic substancesare more highly fluorescent than humic substances andhumic fluorescence intensity is more pH dependent(Goslan et al., 2004; Sierra et al., 2005). Owing to theambiguous characteristics of these substances, we feelthat it is more appropriate to refer to them, hereafter, ashumic-like (H-L) and fulvic-like (F-L) substances.

    In freshwaters, it has been suggested that dissolvedorganic carbon (DOC) is usually dominated by the H-Land F-L fractions, making up 5075% of the total (ofwhich colloidal matter comprises up to 20% of the totalDOC; Hope et al., 1994). However, the proportion of H-Land F-L substances in the DOC is variable. For example,about 50% of uncoloured (USA) stream water consistsmostly of humic substances and 90% of these humicsubstances found in dissolved organic matter (DOM)are the F-L fraction (Malcolm, 1985). In the Amazonbasin, humic substances are attributed to as much as60% of the aquatic total organic carbon (TOC; Patel-Sorrentino et al., 2002). In stream/aquatic systems, DOClosses are possible due to biotic processes such as biofilmrespiration and absorption onto algae and abiotic uptakeonto mineral surfaces (Dawson et al., 2001). Losses ofCO2 from volatile DOC fractions by degassing to theatmosphere, as well as variable DOM inputs throughoutand between catchments, can also lead to this variability.

    The presence of fluorescent DOM enables the compar-ison of H-L and F-L fluorescence intensity with DOC.

    Copyright 2007 John Wiley & Sons, Ltd.

  • 2094 S. A. CUMBERLAND AND A. BAKER

    There are many potential benefits of using fluorescence asa tool for determining and measuring DOC in aquatic sys-tems. It is a relatively quick process, with a typical scantaking approximately 1 min; this is valuable, as labileDOM can degrade over time. The method is non-intrusiveand thus does not interfere with the molecular structure(Klabitz et al., 2000; Baker, 2002). However, given thevariable fraction of fluorescent DOM as part of the totalDOC pool, it is essential to investigate the fluorescenceintensityDOC relationship. Therefore, we compare thefluorescence intensity of the F-L fraction and DOC offreshwater DOM. The study site that this paper focuseson, Norton in Hales, in north Shropshire, England, is asite that features river, wetland, spring, pond and sewageDOM sources all within a small location (approximately500 m), covering a wide range of variation in aquatic andsoil-derived water samples.

    SITE DESCRIPTION AND METHODOLOGY

    Norton in Hales [UK National Grid Reference SJ706 384], north Shropshire, UK, is situated along theRiver Tern, a tributary of the River Severn, 16 kmfrom its source. The River Tern is a lowland catchment,primarily groundwater fed from red sandstone overly-ing the Permo-Triassic Sherwood sandstone. Its catch-ment boundary, thus, is not defined by its topography(described by Clay et al. (2004)). This amounts to avery stable base flow in the River Tern, showing min-imal response to normal rainfall events. Local land useis generally under semi-intense mixed cereal/dairy/beeffarming.

    Figure 1 illustrates the diversity of the sample siteswithin a small area. In-stream water samples werecollected (river sites A, C, E and H), as a transect downthe river starting above the wetland (A) and finishingbelow the sewage treatment outflow pipes (H). Betweenthese points, samples were also taken from the sewage

    treatment works (STW) outlet itself (G), a groundwater-fed pond overflow, POND (B), a wetland dischargesite (D) (not always connected to the main river) anda groundwater spring diverted through small concreteconduit (F). Additionally, three samples were taken fromthe wetland: surface water (I), 045 cm depth (J), and50100 cm depth (K). To obtain samples from the deeperpeat, two perforated drain pipes, about 50 cm apart, weresunk (to 50 cm deep and 1 m deep, with the latter beingperforated from 50 to 100 cm) and sealed at both ends, toreduce oxidation and contamination from other externalsources. The sample was obtained by unscrewing a lidand dipping for the sample, throwing away the first fewdips so that the sample was taken from the middle of theperforated column depth. The wells were then completelyemptied before resealing. Thus, water in tubes was notallowed to stagnate, but perforation allowed flow ofthe wetland and was representative of (actual) bog water.These samples (D, I, J and K) were grouped together toform BOG.

    Water samples were collected fortnightly from Nor-ton in Hales from March to October 2005. Plastic bot-tles (500 ml), pre acid washed (10% HCl, distilled-waterrinsed), were used to ensure carbon- and fluorescence-free blanks (Bolton, 2004). Bottles were also pre-rinsedwith sample before filling. On return to the laboratory(same day) the samples were then filtered through pre-rinsed Whatman GF/C filter papers under vacuum. Sam-ples were kept in the refrigerator overnight between 4 and6 C they were then analysed for fluorescence and TOCthe next day, with DOC analysis completed with 36 h ofcollection (on two occasions this was a day later due tomachine downtime); this was critical, as the fluorescenceproperties of a sample can alter over time (Baker, 2002).

    DOC was measured using a Shimadzu 5050 carbonanalyser from filtered and unfiltered (not reported here)samples. DOC was obtained by measuring the total car-bon and subtracting the measured inorganic carbon andcarbonates (dissolved CO2 and CO23 ). As other studies

    H GF

    B

    E

    J K

    I

    C

    A

    Ponddrain

    D

    N

    Sewage Works

    Key

    Telford BirminghamSJ 707 385RiverTern

    NORTON INHALES

    Norton-in-Hales

    0 50 100 metres

    BogWater

    Figure 1. Location of sample sites

    Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 21, 20932099 (2007)DOI: 10.1002/hyp

  • FRESHWATER DOM FLUORESCENCETOC RELATIONSHIP 2095

    have demonstrated, some methodologies suggest purgingwith acid to remove the inorganic fraction (Clark et al.,2002). This was not executed in this case because thetotal carbon and inorganic carbon concentrations are sig-nificantly different from each other, giving differencingerrors of

  • 2096 S. A. CUMBERLAND AND A. BAKER

    Tabl

    eI.

    Fluo

    resc

    ence

    ,org

    anic

    and

    inor

    gani

    cca

    rbon

    ,nu

    trie

    ntan

    dpH

    data

    fort

    hefiv

    esa

    mpl

    egr

    ou

    ps

    F-L

    H-L

    DO

    Cm

    gl

    1 In

    t./D

    OC

    Org

    anic

    mg

    l1

    C/N

    pH

    Ex.(

    nm

    )Em

    .(nm

    )In

    t.(a.

    u.)Ex

    .(n

    m)

    Em.

    (nm)

    Int.

    (a.u.)

    TCIC

    DO

    CTO

    Na

    NO

    2-N

    NH

    4-N

    PO4-

    PN

    P

    Alld

    ata

    Min

    .30

    040

    626

    219

    408

    7412

    9603

    002

    900

    000

    000

    000

    004

    90

    0163

    1M

    ax.

    344

    450

    951

    255

    461

    1558

    1223

    086

    7341

    6228

    9203

    256

    546

    748

    907

    181

    5M

    ean

    331

    425

    220

    237

    428

    366

    4847

    3753

    1095

    2014

    805

    005

    051

    052

    210

    009

    128

    730

    SD8

    816

    47

    722

    517

    5713

    4686

    474

    700

    710

    912

    112

    901

    704

    6N

    o.29

    229

    229

    229

    129

    129

    114

    814

    814

    829

    2929

    2929

    2948

    Rive

    rM

    in.

    315

    417

    113

    235

    414

    217

    2383

    1801

    274

    760

    002

    000

    013

    079

    001

    682

    Max

    .34

    044

    042

    425

    444

    460

    059

    0142

    9239

    8788

    300

    715

    803

    342

    302

    578

    1M

    ean

    330

    427

    172

    239

    428

    309

    4294

    3404

    889

    1938

    764

    004

    041

    018

    207

    007

    111

    754

    SD7

    569

    56

    8862

    355

    175

    423

    300

    206

    500

    914

    201

    002

    7N

    o.

    6464

    6464

    6464

    6464

    6412

    1212

    1212

    1215

    BOG M

    in.

    321

    416

    7721

    941

    615

    512

    9603

    013

    700

    000

    000

    000

    006

    400

    163

    1M

    ax.

    344

    450

    951

    250

    461

    1558

    1223

    8673

    4162

    559

    006

    565

    029

    261

    009

    777

    Mea

    n33

    442

    740

    923

    142

    964

    563

    1646

    5616

    6024

    6611

    700

    208

    700

    812

    800

    381

    570

    0SD

    58

    208

    59

    286

    2445

    1989

    990

    275

    003

    188

    011

    124

    008

    046

    No

    .49

    4949

    4848

    4949

    4949

    99

    99

    99

    20ST

    W Min

    .32

    040

    617

    724

    042

    028

    422

    4215

    9147

    914

    8701

    801

    722

    137

    202

    570

    1M

    ax.

    340

    426

    311

    250

    436

    407

    4697

    3533

    1795

    2892

    032

    074

    467

    489

    071

    750

    Mea

    n33

    341

    924

    924

    542

    634

    038

    9926

    4412

    5519

    8717

    3001

    903

    428

    830

    903

    707

    173

    0SD

    54

    423

    437

    709

    564

    340

    1299

    014

    032

    223

    213

    031

    023

    No

    .16

    1616

    1616

    1616

    1616

    33

    33

    33

    5B

    Min

    .30

    041

    668

    230

    416

    159

    2916

    2500

    384

    205

    000

    000

    000

    064

    001

    761

    Max

    .33

    043

    611

    224

    543

    425

    551

    0840

    6814

    6520

    600

    800

    400

    108

    900

    081

    5M

    ean

    318

    426

    8923

    442

    420

    041

    7634

    2175

    511

    7720

    600

    400

    200

    107

    700

    236

    479

    0SD

    105

    144

    534

    544

    414

    318

    119

    004

    002

    000

    045

    001

    020

    No

    .14

    1414

    1414

    1414

    1414

    22

    22

    22

    5F

    Min

    .31

    541

    240

    235

    412

    7735

    3029

    7502

    912

    7900

    200

    201

    420

    60

    0168

    2M

    ax.

    340

    428

    160

    251

    438

    260

    5911

    4690

    1696

    1893

    004

    007

    016

    303

    006

    773

    Mea

    n33

    142

    065

    244

    424

    112

    4147

    3662

    485

    1345

    1679

    003

    004

    015

    260

    002

    029

    721

    SD6

    528

    58

    4762

    051

    443

    188

    600

    200

    300

    713

    600

    303

    8N

    o.

    1616

    1616

    1616

    1616

    163

    33

    33

    34

    aTo

    talo

    xid

    ized

    nitr

    ogen

    (nitra

    tean

    dnitr

    ite).

    Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 21, 20932099 (2007)DOI: 10.1002/hyp

  • FRESHWATER DOM FLUORESCENCETOC RELATIONSHIP 2097

    0 10 20 30 40 50 60

    Inte

    nsity

    Inte

    nsity

    Inte

    nsity

    0

    200

    400

    600

    800

    1000

    0 10 20 30 40 50 600 10 20 30 40 50 600

    200

    400

    600

    800

    1000

    mgL1 TOC0 10 20 30 40 50 60

    0

    200

    400

    600

    800

    1000

    0 10 20 30 40 50 60

    r = 0.409 r = 0.756

    r = 0.631 r = 0.657

    r = 0.167

    mgL1 TOC0 10 20 30 40 50 60

    NORDIC r = 0.992SRFA r = 0.997SRHA r = 0.994SRNOM r = 0.998

    (a) (b)

    (c) (d)

    (e) (f)Figure 2. Comparisons of the fluorescence of F-L peak versus DOCfor: (a) RIVER, y D 792x C 10315; (b) BOG, y D 1587x C 12103;(c) SPRG, y D 415x C 458; (d) POND, y D 289x C 6777; (e) STW,

    y D 209x C 21982; (f) IHSS samples

    (Ex/Em 230/460 nm). As for the F-L fluorescence, theNorton in Hales samples showed a higher fluorescenceper milligram of carbon for the H-L peak than the IHSSstandards.

    DISCUSSION

    Figures 2 and 3 demonstrate that H-L and F-L fluo-rophores have a correlation that varies with sample site.The strongest correlations are in the BOG and PONDsamples, where we hypothesize that natural H-L and F-Lmaterial dominates the DOM pool. In contrast, the weak-est relationship occurs in the final treated effluent, a watersource where H-L and F-L materials make up a relativelysmall component of DOM. These results match thoseof Baker (2002), investigating a small urban catchment.There, the F-LDOC correlation for the whole catchmentwas r D 068, again with stronger correlations in the trib-utary containing greater proportions of natural DOM andpoorer correlations in the subcatchments where anthro-pogenic influences were stronger (Table II). We suggestthat the strength of the F-LDOC correlation could beused as an indicator of anthropogenic influence on DOM.However, within-stream processing of DOM and mixingof DOM from different sources also has to be taken intoconsideration. For example, Clark et al. (2002) observedremarkable correlations (0999) with DOM fluorescence

    0 10 20 30 40 50 60 0 10 20 30 40 50 60

    0 10 20 30 40 50 60 0 10 20 30 40 50 60

    0 10 20 30 40 50 60 0 10 20 30 40 50 60

    Inte

    nsity

    Inte

    nsity

    Inte

    nsity

    0

    200

    400

    600

    800

    1000

    0

    200

    400

    600

    800

    1000

    mgL1 TOC

    0

    200

    400

    600

    800

    1000

    mgL1 TOC

    NORDIC r = 0.985SRFA r = 0.961SRHA r = 0.984SRNOM r = 0.956

    r = 0.609

    r = 0.463

    r = 0.630

    r = 0.504

    r = 0.277

    (a) (b)

    (c) (d)

    (e) (f)Figure 3. Comparison of the fluorescence of H-L peak versus DOCfor: (a) RIVER, y D 942x C 22462; (b) BOG, y D 1442x C 3773 (onedata point is off scale, DOC D 165, Int. D 1558, not shown tofacilitate equal scaling); (c) SPRG, y D 68621x 8069; (d) POND,y D 666x C 15088; (e) STW, y D 2919x C 29969; (f) IHSS samples

    and DOC when they went from a fresh water to a salinewater (Shark Head River to Florida Bay). Burdige et al.(2004) also reported good correlations from contrastingsites in the Chesapeake Bay, USA, with r D 092. Incontrast, the scatter observed in the BOG, SPRG andRIVER TOCfluorescence intensity relationships indi-cates the vari...

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