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Estuarine, Coastal and Shelf Science 84 (2009) 147–153

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Estuarine, Coastal and Shelf Science

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Exudation and decomposition of chromophoric dissolved organic matter (CDOM)from some temperate macroalgae

Christopher J. Hulatt*, David N. Thomas, David G. Bowers, Louiza Norman, Chi ZhangSchool of Ocean Sciences, College of Natural Sciences, Bangor University, Menai Bridge, Anglesey, LL59 5AB, United Kingdom

a r t i c l e i n f o

Article history:Received 5 January 2009Accepted 13 June 2009Available online 21 June 2009

Keywords:chromophoric dissolved organic matterCDOMdissolved organic matterDOMmacroalgaephotochemical

* Corresponding author.E-mail address: osp418@bangor.ac.uk (C.J. Hulatt).

0272-7714/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.ecss.2009.06.014

a b s t r a c t

The quantity of chromophoric or coloured dissolved organic matter (CDOM) released by eleven species ofintertidal and sub-tidal macroalgae commonly found on UK shores was investigated. The subsequentbreakdown of CDOM was also measured by exposing collected CDOM samples to light and dark condi-tions for over two weeks. CDOM absorption properties were compared at a fixed wavelength of 440 nmand across two integrated wave - bands; UV-A (400–315 nm) and UV-B (315–280 nm). Absorptionspectra of macroalgal CDOM samples were typically characterized by peaks and shoulders in the UVbands, features which were species specific. The spectral slope, derived using the log-linear method,proved to be very specific to the species and to the effect of light. Slope measurements ranged from 0.010to 0.027 nm�1, in the range of normal seawater values. Significantly more CDOM was produced by algaewhich were illuminated, providing evidence for a light driven exudation mechanism. Averaged across allspecies, exudation in the dark accounted for 63.7% of that in the light in the UV-B band. Interspecificdifferences in exudation rate encompassed an order of magnitude, with the highest absorptionmeasurements attributable to brown algae. However, some brown algae produced considerably lessCDOM (e.g. Pelvetia canaliculata), which were more comparable to the green and red species. Over anexposure time of 16 days, significant photochemical degradation of CDOM was observed using a naturalsummer sunlight regime, showing that natural solar radiation could be an important removal mechanismfor newly produced algal CDOM. Though the most obvious effect was a decrease in absorption, photo-bleaching also caused a significant increase in the spectral slope parameter of 0.004 nm�1.

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1. Introduction

Chromophoric dissolved organic matter (CDOM) stronglyinfluences the optical characteristics of coastal seawater (Sieburthand Jensen, 1969; Carlson and Carlson, 1984; Bowers et al., 2004).CDOM sources and removal processes attract interest, due to theconsiderable effect on both the quantity and spectral quality of lightin aquatic systems (Søndergaard and Thomas, 2004). AlthoughCDOM has a multitude of sources (Chen et al., 2004), exudation byalgae has only really been considered important in the open seasand oceans, where the absence of riverine contributions points tophytoplankton as the major source of new organic material (Alu-wihare and Repeta, 1999). However, recent work proposes that incoastal areas, where macroalgal biomass far outweighs that ofphytoplankton, intertidal and subtidal macroalgae may act assignificant CDOM point sources (Swanson and Druehl, 2002).

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It has long been recognised that macroalgae release quantities ofdissolved organic matter (DOM, including CDOM) into seawaterduring normal, healthy growth. Most early work examinedexudation from a variety of species (Moebus and Johnson, 1974;Brylinsky, 1977), although more recent scientific investigation hasbeen directed toward species of brown algae (Jennings and Stein-berg, 1994; Wada et al., 2007). However, there is still little detailedinformation available to describe the optical characteristics of mostmacroalgal exudates. CDOM production can be attributed topassive diffusion from algal cells (Bjornsen, 1988), and also to activeover–production by photosynthesis (Fogg, 1966). Debate over theroles of passive and active release mechanisms is ongoing, althoughCarlson (2002) reasons that both pathways probably occursimultaneously.

The total quantity of dissolved organic matter (including thenon-chromophoric component) released by macroalgae can besurprisingly high; Khailov and Burlakova (1969) estimated thatliving macroalgae may exude up to 39% of gross production perannum. Much of the material released by living macroalgae consistsof biologically labile materials such as low molecular weight

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carbohydrates (Myklestad, 1995), although the production ofpolyphenolic materials is also widely reported (Sieburth, 1969;Shibata et al., 2006). Not all dissolved organic matter absorbsappreciable amounts of light though (Ferrari et al., 1996), and asa result the relationship between total dissolved organic matter(DOM) and the colored fraction is not well resolved (Ragan andCraigie, 1980; Nelson et al., 1998).

CDOM absorption spectra, at least for naturally occurringmaterial, are characterised by an exponential increase in lightattenuation with decreasing wavelength, so absorption is strongestthrough the blue and UV parts of the spectrum (Bricaud et al., 1981).Absorption in the blue part of the spectrum means that there is stillsignificant attenuation of short wavelength photosyntheticallyactive radiation (PAR), around the chlorophyll a absorptionmaximum at 440 nm�1 (Bowers et al., 2000), so the concentrationof CDOM is important for models of primary production (Jakobet al., 2005). CDOM light absorption can be described by itsextinction coefficient (m�1) at selected wavelengths, and also bythe slope of the absorption spectrum, modeled with an algorithmthat accommodates its exponential profile (D’Sa et al., 1999;Twardowski et al., 2004; Helms et al., 2008).

Although a handful of previous studies have examined CDOMproduction by different species of macroalgae, none have examinedits fate in seawater. The two recognised pathways for CDOMremoval are through bacterial metabolism and photochemicaloxidation (Gao and Zepp, 1998; Andrews et al., 2000; Rochelle-Newall and Fisher, 2002).

This work describes the optical properties of CDOM produced by11 species of macroalgae commonly found on North-West Euro-pean shores, and provides evidence for the photochemicaldegradability of CDOM produced by the brown alga, Fucus spiralis.

2. Methods

2.1. Species and irradiance

Macroalgae were collected from the shores of the Menai Strait,North Wales, during July and August 2007. Eleven species werecollected: Ascophyllum nodosum, Chorda filum, Fucus serratus, Fucusspiralis, Fucus vesiculosus, Laminaria digitata, Pelvetia canaliculata(brown algae), Ulva lactuca, Enteromorpha intestinalis, (green algae),Palmaria palmata, Chondrus crispus (red algae). Samples were placedin trays of running seawater, and were allowed to recover from thesampling disturbance and rehydrate fully overnight. Samples werethen transferred to 1 l acid–washed glass bottles containing 800 mlof 0.2 mm filtered seawater (salinity 34). Bottles were placed ina trough of running water at 17.5�C� 1.5�C, measured with a Tiny-Tag� datalogger, for the duration of the experiments. Lighting wasprovided by overhead cool-white fluorescent tubes (140 mmolphotons PAR m�2 s�1), though C. filum, F. serratus and L. digitata weremeasured separately using natural sunlight. Darkened conditions,used for half of the samples, were achieved by wrapping bottles inaluminium foil. Initial experiments showed that CDOM accumulatedlinearly over time up to at least 72 h from the start of incubations,and all subsequent samples presented here were run over 24 hperiods. For species under controlled light, a sample size of 5 wasused for each species� light treatment, and the three additionalspecies exposed to natural light consisted of 3 replicates per treat-ment. CDOM exudation was equivalent to net production, as it wasnot possible to measure any uptake of previously exuded material.

2.2. Degradation of CDOM

Material for CDOM degradation experiments was collected from15 samples of Fucus spiralis, each placed in a glass bottle containing

800 ml of seawater, and placed under the light bank. Followinga 24 h exudation period, samples were pooled together, mixedthoroughly, and 100 ml decanted into 2 sets (n¼ 8 per set) of150 ml glass bottles. A further 2 sets of bottles were each filled with100 ml of the seawater used to collect the algal CDOM. These wereused as controls for potential changes in the background level ofCDOM over the experiment duration. One of the sets of experi-mental algal CDOM bottles and one of the sets of controls were thenwrapped in foil, to control for the effect of light. The four sets ofbottles were then placed in a water bath, where the temperaturewas thermostatically controlled at 12�C (� 1�C). The samples wereshaken continuously over the duration of the experiment. Theexperiment was run over 16 days, and a sample was removed foranalysis every 2–3 days. The water bath was placed in full sunlight,behind a large, unobstructed, single-glazed window. Bottles werevented to the atmosphere daily to permit gas exchange. Irradiancewas recorded over the duration of the experiments using a Li-Cordatalogger and 2p PAR sensor (Li 190-SA), measuring irradiance at5 min intervals and integrating data into hourly values.

2.3. CDOM measurements

Samples were filtered into 10 cm path length quartz cells using0.45 mm nominal pore size Whatman� GDX glass fibre filters.Ultra-pure 0.2 mm filtered and UV treated Milli-Q� water was usedas a reference blank. CDOM measurements were made using a dualbeam Shimadzu 1601 UV-Vis spectrophotometer, which measuredthe sample absorbance from 750 nm (zero offset) to 230 nm(cell detection limit), at a resolution of 0.5 nm. Absorption datafrom the spectrophotometer was expressed by its absorptioncoefficient in m�1 using the equation derived from the Beer-Lambert law (Green and Blough, 1994):

aðlÞ ¼ ½2:303� ðAl � A750Þ�=l

where al¼ the absorption coefficient (m�1) at wavelength (l);2.303¼ conversion from Log10 to Log e; Al¼ the measured absor-bance of light at the same wavelength; A750¼ the absorbance oflight at a reference wavelength (750 nm); l¼ the light path lengthof the cuvette (m). In addition to the macroalgal samples, at least 5measurements were made of the stock seawater used in eachexperiment. Following each incubation and CDOM measurement,the macroalgal sample was dried at 60�C for 48 h, after which timethere was no further change in weight. Samples were weighedimmediately upon removal from the oven. Dry weights rangedfrom 0.14 g for the juvenile Laminaria digitata, to 6.77 g for samplesof Ascophyllum nodosum. Baseline readings for the stock seawaterwere subtracted from algal measurements, to correct for the effectof the background CDOM already present in the seawater. Data wasthen standardised, to express the CDOM concentration as a rate ofproduction per unit biomass.

ðal=tÞ=dwt

where a(l) is the absorption coefficient at wavelength (l) in m�1, t isthe elapsed time of the incubation in hours (h), and dwt is the dryweight of the macroalgal sample used (g). The final units were thusm�1 h�1 g�1.

2.4. Wavelength selection

In the screening experiment, where comparability betweenspecies was important, data was integrated across standard parts ofthe spectrum. The integration procedure had the advantage oversingle wavelength measurements in that peaks and shoulders inthe absorption spectra had lower wavelength–dependency,

Fig. 1. Representative absorption spectra of CDOM exudates from a) all members of thePhaeophyta (brown algae) and b) the Chlorophyta (green algae) and Rhodophyta (redalgae). Absorption is measured in m�1 h�1 g�1, and wavelength is presented in nm.

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improving comparability between different species. Integrationwas achieved by calculating the mean extinction coefficient forregions of the UV spectrum. The wavelength bands used were:UV-A (400–315 nm) and UV-B (315–280 nm). A third measurementcomprised only a single wavelength reading at 440 nm. Measure-ments of the spectral slope (nm�1), that describes the approxi-mately exponential increase in absorption with decreasingwavelength (Twardowski et al., 2004), were made using the log–linear transformation method (D’Sa et al., 1999). Data between 350and 500 nm was used, because there was insufficient resolution atlonger wavelengths and because of the abundant shoulders below350 nm. All fits were checked graphically for deviation from themodel, and in each case r2 was greater than 0.97. Since differentchemicals absorb light at different wavelengths, but overlap eachother, the spectral slope measurement is specific to the combinedproperties of all the chemicals and their relative concentrations inthe sample. Thus the slope might be used to identify opticaldifferences in CDOM composition between species of macroalgae.

3. Results

3.1. Absorption spectra

Representative absorption spectra for each species tested underlight are presented in Fig. 1. The spectra, whilst showing similaroverall properties to natural CDOM, exhibited distinct variations inprofile, characteristically displaying peaks and shoulders at variouswavelengths through the UV bands. These features were common toall species tested, in both light and dark conditions. However, theiroccurrence at different wavelengths showed that different speciesproduced optically unique types of CDOM. Of any group, the brownalgae produced spectra most similar to each other, with a singlecharacteristic peak centered in the far UV-B region, in the range 260–280 nm. Closer inspection though, showed that peaks produced bydifferent species of brown algae did not completely overlap, butwere shifted slightly with respect to wavelength, indicating thatspecies released slightly different chemicals. The red alga Palmariapalmata produced a most striking triple peak in the UV-B region,a feature that was clearly evident in three of the five replicates,though was less obvious in the remaining two. Ulva lactuca producedrelatively smooth absorption spectra, with evidence of a shoulder inthe UV-A part of the spectrum, and Enteromorpha intestinalisproduced a shoulder similar to, but less prominent than, the brownalgae. Though light attenuation was clearly strongest in the UVbands, measurements from all species showed that the presence ofexuded materials also caused absorption of PAR (above 400 nm).

Aside from changes in absolute absorption, most species did notexhibit changes in peak number or position between light and darktreatments, as exemplified by Fucus serratus in Fig. 2. However,Enteromorpha intestinalis produced two peaks at 280 and 395 nm inthe dark, but produced only a single broad shoulder between 295and 365 nm in the light. Although the spectral shape was similarbetween light and dark treated samples of Palmaria palmata, therewas no evidence of the UV-B triple peak in any of the dark samples.Fucus spiralis samples incubated in the light produced a moreexaggerated peak at 270 nm compared to dark samples.

3.2. Effect of species and light on CDOM production

Absorption measurements are presented for each species� lighttreatment in Table 1. In the UV-A and UV-B bands, Chorda filum andLaminaria digitata showed the greatest exudation rates of any of thespecies in the light, some 2.5 times greater than Fucus serratus. Inthe UV-B region, exudation by F. spiralis was similar to that of Fucusserratus, though the two species were less comparable in the UV-A

band and at 440 nm. Throughout the data set, the three highestCDOM producers in the light were each brown algae, whilst red andgreen algae both showed relatively low CDOM exudation rates.However, Ascophyllum nodosum, Pelvetia canaliculata and Fucusvesiculosus produced much lower values than the other brownalgae throughout the data set, and were more comparable to thered and green species. All species produced measurable quantitiesof CDOM which absorbed at 440 nm, and although C. filum stillproduced the greatest CDOM absorption measurements in thelight, Laminaria digitata measurements were exceeded slightly by F.spiralis and F. serratus.

The effect of species and light on CDOM exudation was testedusing a 2 factor ANOVA on data from the UV-B region, whereabsorption was strongest. After square-root transformation (thetreatment means were spread over an order of magnitude), thedata satisfied the test assumptions (approximately equal variancesand approximately normally distributed residuals). The F ratio forinteraction effects was reasonably high, but was not significant(F¼ 1.89, p¼ 0.076). There was, however, a significant main effectof species (F¼ 30.76, p¼<0.001), and also of light / dark treatment(F¼ 37.89, p¼<0.001), showing that a proportion of exudation wasdriven by exposure to light. As there was no data for dark condi-tions, Laminaria digitata was not included in the analysis.

Fig. 2. Representative CDOM absorption spectra obtained from samples of Palmaria palmata, Enteromorpha intestinalis, Fucus spiralis and Fucus serratus treated to light and darkconditions for 24 hours. Absorption is measured in m�1 h�1 g�1 and wavelength in nm.

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Measurements of the spectral slope were unsuitable for formalstatistical analysis due to unequal variances, though the datademonstrates the range and variability of S values derived fromalgal CDOM (Table 1). Median S values varied between treatmentsfrom 0.010 nm�1 (Fucus spiralis, dark) to 0.027 nm�1 (Enteromorphaintestinalis, light). Measurements of S were highly treatment-specific, and whilst in many cases there were clear differencesbetween combinations of species and light, there was no generaleffect of light on S. For example, Chorda filum produced very similarvalues in light and dark conditions (median 0.019 vs. 0.019respectively), whilst others (e.g. Fucus spiralis) produced verydifferent values (median 0.016 vs. 0.010 respectively). In contrast,Fucus vesiculosus produced greater slopes in the dark (median0.019), than in the light (0.011). The range of replicate S valueswithin treatments was also highly variable. For example, Fucusserratus had a range in S values of 0.002 nm�1 in light treatments,but a range of 0.012 nm�1 in the dark.

Table 1CDOM absorption and spectral slope data for all species� light treatments. For absorptionormalized extinction coefficient (m�1 h�1 g�1�10�2) with the standard error shown inthe range 350–500 nm, and the median is presented, with the minimum and maximum shLaminaria digitata, where n¼ 3.

Type Species UV-B UV-A

Light Dark Light Dar

Brown Ascophyllum nodosum 3.21 (0.45) 2.33 (0.89) 1.40 (0.21) 1.00Chorda filum 44.68 (10.36) 50.78 (8.69) 17.73 (4.94) 22.5Fucus serratus 18.35 (1.88) 13.08 (4.59) 7.85 (0.86) 5.96Fucus spiralis 15.26 (1.56) 12.65 (2.53) 2.73 (1.05) 2.48Fucus vesiculosus 6.51 (0.94) 3.43 (0.39) 2.53 (0.35) 1.26Laminaria digitata 48.28 (9.19) NA 17.21 (3.29) NAPelvetia canaliculata 3.76 (0.33) 3.08 (0.31) 1.29 (0.23) 0.89

Red Chondrus crispus 2.75 (0.45) 1.01 (0.31) 1.7 (0.15) 0.94Palmaria palmata 7.79 (1.76) 1.03 (0.21) 6.21 (1.49) 0.64

Green Enteromorpha intestinalis 6.25 (0.59) 3.89 (1.00) 2.13 (0.48) 2.06Ulva lactuca 4.76 (0.51) 2.41 (0.78) 5.60 (1.25) 5.20

3.3. Degradation of macroalgal CDOM

CDOM degradation was measured over the wavelength range600–320 nm. The initial concentration of background CDOM in alltreatments, measured at 440 nm, was 0.15 m�1. Linear regression(not illustrated) showed that there was no significant change inabsorption coefficient in control bottles for background CDOM(dark bottles: F¼ 4.3, p¼ 0.084, irradiated bottles: F¼ 0.01,p¼ 0.925). Natural CDOM therefore, seemed to be relatively resis-tant to decay over periods of more than 2 weeks, and allowed anydetectable effect on algal CDOM samples to be discounted. Themean value of background attenuation at 440 nm was thereforesubtracted from the respective algal CDOM samples. Fig. 3 showsthe changes in absorption at 440 nm over the experimental period,for both irradiated and dark treated algal samples. Samples con-taining the dark-treated macroalgal CDOM showed a significantreduction in absorption at 440 nm over the 16 day period, with

n measurements (UV-A, UV-B and 440 nm), data is presented as the mean biomassbrackets. The spectral slope (S, nm�1) data corresponds to measurements made overown in brackets. For each treatment, n¼ 5, except for Chorda filum, Fucus serratus and

440 nm S (350–500 nm)

k Light Dark Light Dark

(0.11) 0.24 (0.05) 0.26 (0.11) 0.019 (0.016–0.021) 0.015 (0.014–0.022)9 (3.93) 2.63 (1.46) 5.63 (0.79) 0.019 (0.018–0.019) 0.018 (0.018–0.019)(2.20) 1.78 (0.28) 2.01 (0.83) 0.016 (0.015–0.017) 0.013 (0.011–0.023)(0.63) 1.54 (0.23) 1.65 (0.28) 0.016 (0.014–0.017) 0.010 (0.010–0.013)(0.14) 0.74 (0.10) 0.34 (0.04) 0.014 (0.014–0.014) 0.019 (0.018–0.022)

1.51 (0.59) NA 0.016 (0.016–0.018) NA(0.26) 0.26 (0.05) 0.30 (0.10) 0.018 (0.009–0.026) 0.015 (0.011–0.022)

(0.26) 0.48 (0.13) 0.15 (0.11) 0.011 (0.011–0.019) 0.016 (0.016–0.024)(0.18) 0.63 (0.15) 0.11 (0.01) 0.020 (0.019–0.027) 0.025 (0.021–0.025)

(0.46) 0.25 (0.03) 0.70 (0.15) 0.027 (0.027–0.027) 0.019 (0.019–0.022)(0.99) 0.29 (0.15) 0.19 (0.10) 0.016 (0.016–0.017) 0.017 (0.011–0.018)

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absorption decreasing in a linear fashion, leaving 85% of the originalconcentration remaining at day 16 (F¼ 12.44, p¼ 0.012 for log-transformed data, linear least–squares fit). Irradiated bottlesshowed a clear trend of decreasing absorption, and when analysedin the same manner the relationship was highly significant(F¼ 788.6, p¼<0.001). In irradiated treatments the degradationrate was much greater, with just 37% of the original CDOMremaining at day 16.

A test for interaction using the general linear model showed thatthe log-transformed regression slope for irradiated bottles wassignificantly greater than that of dark bottles, demonstrating thathighly-significant photochemical degradation of macroalgal CDOMhad occurred (F¼ 222.68, p¼<0.001). Light-treated samplesdegraded some 6.3 times faster than their dark-treated counterparts;the log-transformed regression slope for light bottles was�0.082 m�1 d�1, whilst absorption losses in dark bottles proceededat just �0.013 m�1 d�1. Over the 16 day period, the light cycle was

Fig. 3. Degradation of macroalgal CDOM in light and dark bottles over 16 days,described by a) Changes in absorption (m�1) at 440 nm modeled using linearregression on log-transformed data (predicted values shown here are the exponent ofthe fitted regression values) and b) Changes in the spectral slope of macroalgal CDOMover 16 days in response to light and dark treatments. Measurements made over therange 350–500 nm�1 using the log-linear method. Open circles represent sampleskept in the light, whilst closed diamonds represent dark – treated bottles. For eachtreatment, n¼ 8.

regular, with irradiance reaching up to w2500 mmol photons PARm�2 s�1 around midday and an approximately linear increase incumulative light over the sampling period (data not shown). Sincemeasurements of irradiance were made using a PAR sensor, it wasassumed that UV light (<400 nm�1) was approximately proportionalto PAR throughout the experiment. Photochemical degradationoccurred throughout the UV (down to 325 nm) and short–wave-length visible part of the spectrum, and was not constrained to the440 nm region.

Changes in absorption over the spectrum were examined bymodeling the spectral slope over the range 350–500 nm using thelog–linear transformation method. Control bottles containing onlythe background CDOM material showed no significant changes inthe spectral slope (F¼ 5.73, p¼ 0.054; F¼ 0.56, p¼ 0.482 for darkand light bottles respectively), so mean absorption spectra for eachset of control bottles were then subtracted from their corre-sponding macroalgal CDOM samples. Macroalgal measurementswere then analysed using the same log-linear regression technique.Algal CDOM retained in the dark showed no significant change inthe spectral slope over the experimental period (F¼ 0.43,p¼ 0.537), so degradation in the absence of light did not seem toaffect the spectral characteristics of algal CDOM. However, irradi-ated bottles containing algal material did show significant changesin the spectral slope over time, with a gradual increase in S from0.016 to 0.020 nm�1 over 16 days (F¼ 38.28, p¼ 0.001). Thus,although absorption losses occurred throughout the measuredspectrum, the increase in the slope coefficient showed that, pro-portionally, photobleaching had a greater effect toward 500 nmcompared to 350 nm.

4. Discussion

4.1. CDOM production

All the species tested produced CDOM with curved absorptionspectra similar in appearance to those measured from naturalseawater samples. However, the presence of distinctive peaks inmost samples indicated that macroalgal CDOM was comprised ofa relatively small selection of compounds, each with overlappingabsorption bands. The presence of individual ‘fingerprint’ peaksand shoulders in absorption spectra implies that the magnitude ofthe macroalgal CDOM contribution to seawater could be investi-gated in natural samples using spectrophotometry; it might bepossible to identify contributions made by particular groups, suchas the fucoids, by examining absorption spectra from naturalsamples. Although the chemical nature of CDOM was not charac-terized here, the presence of peaks in the far UV (265–285 nm)strongly implies the release of phenolic compounds, especiallyfrom the brown algae. Shibata et al. (2006) found that livingspecimens of two brown algal species (Eisenia bicyclis and Eckloniakurome) tended to release three different types of monomericbromophenols, which had absorption maxima over a very similarrange (265–270 nm), suggesting that it is this family of compoundswhich could explain the differences in peak position. Furtherevidence for the production of similar compounds from red algaecomes from Phillips and Towers (1982), who investigated bromo-phenol production in Rhodomela laryx.

Although there were clear differences in spectral slope measure-ments between treatments, S was very sensitive to each set of treat-ment conditions, with little evidence of overall patterns. Further, allthe S values lay well within the normal range expected of seawatervalues (Twardowski et al., 2004), so there appears little potential touse the spectral slopes to distinguish between macroalgal CDOMsamples, or indeed from other sources. However, the specificityshown by the spectral slope measurements might have some use in

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making very detailed measurements of algal CDOM composition,since in some cases S can be related closely to the chemical compo-sition of CDOM (Helms et al., 2008). However to date, little detailedwork beyond total organic carbon has related chemical and opticalmeasures of algal exudates. Twardowski et al. (2004) illustrated thelack of comparability between S measurements made over differentwave bands from the same sample, so the constrained wavelengthrange available for the analysis of macroalgal samples might furtherlimit the usefulness of S in characterizing macroalgal CDOM.

The fact that exudation was significantly enhanced by exposureof samples to light and, in some species, was accompanied bychanges in absorption properties, shows that sunlight could controlthe exudation of CDOM from intertidal macroalgae, impacting thedelivery of dissolved organic matter into coastal waters on anhourly, daily and seasonal basis. However, significant exudationalso occurred through darkness, indicating that a proportion ofexudation was driven by passive diffusion alone, in agreement withBjornsen (1988). Based on the mean UV-B data across all species,‘passive’ processes in the dark accounted for, on average, 63.7 % ofCDOM exudation in the light. The remainder is attributable tolight–stimulated exudation, possibly through over–production ofphotosynthates (Fogg, 1966). This data is concordant with recentsummaries of DOM release (Carlson, 2002), and provides evidencefor the relative importance of the parallel exudation routes inintertidal macroalgae. Since the data presented here was obtainedduring June and July, when macroalgae were growing strongly, theabsolute and relative exudation rates may not necessarily reflectoverall annual release rates. For example, Abdullah and Fredriksen(2004) showed that both the quantity and quality of DOM releasedby Kelp varied annually in response to climate and physiologicalcondition. Intertidal macroalgae are thus likely to show changes inCDOM production seasonally in relation to their growth andreproductive strategies.

The data shows that different species of algae could make quitedifferent contributions to CDOM in seawater, with exudation ratesvarying over an order of magnitude. Since the overall CDOMcontributions from mixed intertidal communities will be a functionof the species-specific biomass at any one location, it is likely thatthe dominant brown algae, particularly Laminaria digitata, shouldcontribute by far the most CDOM to coastal seawater. Laminariadigitata samples were all juvenile specimens, so their high exuda-tion rate could in part be explained by a relatively high surface areato volume ratio, and possibly by differences in physiological status.This evidence does support the more recent focus on species of kelp(Wada et al., 2007), although it is clear that other species of brownalgae are also important CDOM producers. Two other brown algae,Fucus serratus and Fucus spiralis, both exhibited relatively high ratesof exudation throughout the different wavelengths, though Pelvetiacanaliculata showed lower exudation rates similar to those speciesfrom the red and green phyla. Ascophyllum nodosum and Fucusvesiculosus are interesting cases, because whilst they producedrelatively little CDOM they are typically abundant, so may makea significant CDOM contribution through their biomass. The redand green algae tended to produce relatively little CDOM per unitbiomass, and although they may reach high densities in intertidalzones, especially in estuaries, they are unlikely to have an impor-tant influence on seawater CDOM concentrations.

4.2. CDOM degradation

Compared to the CDOM present naturally in seawater and toaged algal material (Thomas and Lara, 1995), newly producedmacroalgal CDOM was apparently relatively labile. The decreasein absorption in dark treated bottles may have been the result ofone or more factors. Microbial activity, known to be a major

widespread removal mechanism in seawater, is likely to have hadan effect (Baines and Pace, 1991; Aluwihare and Repeta, 1999).However, it is conceivable that at least some of the observedabsorption losses could have been derived from aggregation andcolloid formation, possibly including adhesion to the bottlesurface and/or through re-filtration before measurement (Buffleet al., 1998; Wells, 1998). Such processes could have been exac-erbated by the relatively high CDOM starting concentrationsused, although this was necessary for measurement accuracy.Degradation experiments demonstrated that sunlight could bea highly significant CDOM removal mechanism in its own right,accountable for approximately 76% of the absorption lost at440 nm. This information must be applied carefully to naturalsystems though, due to the variability (both spatially andtemporally) of light in the water column. Since the light receivedby the bottles would have suffered some attenuation by thewindow, bottle and the sample of water itself, the data is notquite equivalent to that which would be experienced at thesurface of seawater. However, since degradation in the light wasstill some 6.3 fold greater than in the dark, the data clearly showsthe potential magnitude and importance of photochemicalactivity in removing newly produced macroalgal CDOM from thephotic zone. Our results should thus be slightly conservative inthis respect. The increase in spectral slope in response to irradi-ation, coupled with the overall decrease in absorption, impliesa reduction in the average molecular weight of exuded materials,coupled with the destruction of chromophores (Helms et al.,2008). Although there is some controversy over the effect ofphotochemical activity on the spectral slope of CDOM, which maybe brought about by methodological differences, the data heresupports an increase in the slope coefficient as a result of thephoto-bleaching of algal CDOM (350–500 nm), concordant withobservations by Moran et al. (2000) and Del Vecchio and Blough(2002).

5. Conclusions

Absorption spectra of CDOM from macroalgae were charac-terised by the presence of peaks and shoulders through the UVpart of the spectrum. Species tended to produce optically uniqueabsorption signatures, which might be used to distinguish theirpresence in natural samples. CDOM exudation, measured bybiomass-normalised absorption, varied between species overa single order of magnitude. Although the highest exudationrates were attributable to brown algal species, which tend toconstitute the greatest biomass in natural communities, somebrown algae such as Pelvetia canaliculata released much lowerquantities of CDOM, comparable to the four green and red algalspecies tested. Sunlight caused significant degradation of CDOM,and the decrease in absorption was accompanied by an increasein the spectral slope. Photo-bleaching is thus a potentiallysignificant removal mechanism for newly produced macroalgalCDOM.

Acknowledgements

We are grateful for the helpful comments of two anonymousreviewers.

References

Abdullah, M., Fredriksen, S., 2004. Production, respiration and exudation of dissolvedorganic matter by the kelp Laminaria hyperborea along the west coast of Norway.Journal of the Marine Biological Association of the United Kingdom 84, 887–894.

C.J. Hulatt et al. / Estuarine, Coastal and Shelf Science 84 (2009) 147–153 153

Aluwihare, L., Repeta, D., 1999. A comparison of the chemical characteristics ofoceanic DOM and extracellular DOM produced by marine algae. Marine EcologyProgress Series 186, 105–117.

Andrews, S., Caron, S., Zafirou, O., 2000. Photochemical oxygen consumption inmarine waters: a major sink for colored dissolved organic matter. Limnologyand Oceanography 45, 267–277.

Baines, S., Pace, M., 1991. The production of dissolved organic matter by phyto-plankton and its importance to bacteria: patterns across marine and freshwatersystems. Limnology and Oceanography 36, 1078–1090.

Bjornsen, P., 1988. Phytoplankton exudation of dissolved organic matter. Why dohealthy cells do it? Limnology and Oceanography 33, 151–154.

Bowers, D., Evans, D., Thomas, D., Ellis, K., Williams, P., 2004. Interpreting the colourof an estuary. Estuarine Coastal and Shelf Science 59, 13–20.

Bowers, D., Harker, G., Smith, P., Tett, P., 2000. Optical properties of a region offreshwater influence (The Clyde Sea). Estuarine Coastal and Shelf Science 50,717–726.

Bricaud, A., Morel, A., Prieur, L., 1981. Absorption by chromophoric dissolved organicmatter of the sea (yellow substance) in the UV and visible domains. Limnologyand Oceanography 26, 43–53.

Brylinsky, M., 1977. Release of dissolved organic matter by some marine macro-phytes. Marine Biology 39, 213–220.

Buffle, J., Wilkinson, K., Stoll, S., Filella, M., Zhang, J., 1998. A generalised descriptionof aquatic colloidal interactions: the three-colloidal component approach.Environmental Science and Technology 32, 2887–2899.

Carlson, C., 2002. Production and removal processes. In: Carlson, C., Hansell, D.(Eds.), Biogeochemistry of Marine Dissolved Organic Matter. Academic PressLondon, pp. 91–139.

Carlson, C., Carlson, M., 1984. Reassessment of exudation by fucoid macroalgae.Limnology and Oceanography 29, 1077–1087.

Chen, R., Bissett, P., Coble, P., Conmy, R., Gardner, B., Moran, M., Wang, X., Wells, M.,Whelan, P., Zepp, G., 2004. Chromophoric dissolved organic matter (CDOM)source characterisation in the Louisiana Bight. Marine Chemistry 89, 257–262.

DelVecchio, R., Blough, N., 2002. Photobleaching of chromophoric dissolved organicmatter in natural waters: kinetics and modeling. Marine Chemistry 78, 231–253.

D’Sa, E., Steward, R., Vodacek, A., Blough, N., Phinney, D., 1999. Determining theoptical absorption of colored dissolved organic matter in seawater with a liquidcapillary waveguide. Limnology and Oceanography 44, 1142–1148.

Ferrari, G., Dowell, M., Grossi, S., Targa, C., 1996. Relationship between the opticalproperties of chromophoric dissolved organic matter and the total concentra-tion of dissolved organic carbon in the southern Baltic Sea region. MarineChemistry 55, 299–316.

Fogg, G., 1966. The extracellular products of algae. Oceanography and MarineBiology Review 4, 195–212.

Gao, H., Zepp, R., 1998. Factors influencing photoreactions of dissolved organicmatter in a coastal river of the southeastern United States. EnvironmentalScience and Technology 32, 2940–2946.

Green, S., Blough, N., 1994. Optical absorption and fluorescence properties ofchromophoric dissolved organic matter in natural waters. Limnology andOceanography 39, 1903–1916.

Helms, J., Stubbins, A., Ritchie, J., Minor, E., Kieber, D., Mopper, K., 2008. Absorptionspectral slopes and slope ratios as indicators of molecular weight, source andphotobleaching of chromophoric dissolved organic matter. Limnology andOceanography 53, 955–969.

Jakob, T., Schreiber, U., Kirchesch, V., Langner, U., Wilhelm, C., 2005. Estimation ofchlorophyll content and daily primary production of the major algal groupsby means of multiwavelength-excitation PAM chlorophyll fluorometry:performance and methodological limits. Photosynthesis Research 83,343–361.

Jennings, J., Steinberg, P., 1994. In situ exudation of phlorotannins by the sublittoralkelp Ecklonia. Marine Biology 121, 349–354.

Khailov, K., Burlakova, Z., 1969. Release of dissolved organic matter by marineseaweeds and distribution of their total organic production to inshorecommunities. Limnology and Oceanography 14, 521–527.

Moebus, K., Johnson, K., 1974. Exudation of dissolved organic carbon by brownalgae. Marine Biology 26, 117–125.

Moran, M., Sheldon, W., Zepp, R., 2000. Carbon loss and optical property changesduring long term photochemical and biological degradation of estuarine dis-solved organic matter.

Myklestad, S., 1995. Release of extracellular products by phytoplankton with specialemphasis on polysaccharides. The Science of the Total Environment 165,155–164.

Nelson, N., Siegel, D., Michaels, A., 1998. Seasonal dynamics of colored dissolvedorganic material in the Sargasso Sea. Deep-Sea Research 45, 931–958.

Phillips, D., Towers, G., 1982. Chemical ecology of red algal Bromophenols II.Exudation of Bromophenols by Rhodomela laryx. Journal of ExperimentalMarine Biology and Ecology 58, 295–302.

Ragan, M., Craigie, J., 1980. Quantitative studies on brown algal phenols IV. Ultra-violet spectrophotometry of extracted polyphenols and implications formeasuring dissolved organic mater in seawater. Journal of Experimental MarineBiology and Ecology 46, 231–239.

Rochelle–Newall, E., Fisher, T., 2002. Chromophoric dissolved organic matter anddissolved organic carbon in Chesapeake Bay. Marine Chemistry 77, 23–41.

Shibata, T., Hama, Y., Miyasaki, T., Ito, M., Nakamura, T., 2006. Extracellular secretionof phenolic substances from living brown algae. Journal of Applied Phycology18, 787–794.

Sieburth, J., 1969. Studies on algal substances in the sea III. The production ofextracellular organic matter by littoral marine algae. Journal of ExperimentalMarine Biology and Ecology 3, 290–309.

Sieburth, J., Jensen, A., 1969. Studies on algal substances in the sea II. The formationof gelbstoff (humic material) by exudates of pheophyta. Journal of ExperimentalMarine Biology and Ecology 3, 275–289.

Søndergaard, M., Thomas, D.N., 2004. .. Dissolved Organic Matter (DOM) in AquaticEcosystems: A Study of European Catchments and Coastal Waters, The DomaineProject, pp. 72.

Swanson, A., Druehl, L., 2002. Induction, exudation and the UV protective role ofkelp phlorotannins. Aquatic Botany 73, 241–253.

Thomas, D., Lara, R., 1995. Photodegradation of algal derived dissolved organiccarbon. Marine Ecology Progress Series 116, 309–310.

Twardowski, M., Boss, E., Sullivan, J., Donaghay, P., 2004. Modeling the spectralshape of absorption by chromophoric dissolved organic matter. MarineChemistry 89, 69–88.

Wada, S., Aoki, M., Tsuchiya, Y., Sato, T., Shinagawa, H., Hama, T., 2007. Quantitativeand qualitative analyses of dissolved organic matter released by Ecklonia cavaKjellman, in Oura Bay, Shimoda, Izu Peninsula, Japan. Journal of ExperimentalMarine Biology and Ecology 349, 344–358.

Wells, M., 1998. Marine colloids: a neglected dimension. Nature 391, 530–531.