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.

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