Estuarine, Coastal and Shelf Science 64 (2005) 591e600

www.elsevier.com/locate/ECSS

Dissolved organic carbon in the freshwater tidal reachesof the Schelde estuary

Koenraad Muylaert a,*, Renaat Dasseville a, Loreto De Brabandere b,Frank Dehairs b, Wim Vyverman a

a University Gent, Department Biology, Krijgslaan 281-S8, 9000 Gent, Belgiumb Vrije Universiteit Brussel, Laboratory of Analytical Chemistry, Pleinlaan 2, 1050 Brussel, Belgium

Received 27 September 2004; accepted 8 April 2005

Available online 21 June 2005

Abstract

To unravel the factors that regulate DOC dynamics in the freshwater tidal reaches of the Schelde estuary, DOC concentrationand biodegradability were monitored in the upper Schelde estuary and its major tributaries. Although the Schelde estuary possesses

a densely populated and industrialized catchment, our data suggest that the bulk of DOC in the freshwater tidal reaches is notderived from waste water. This was concluded from the low biodegradability of DOC (on average 9%), DOC concentrations thatare close to the mean for European rivers (4.61 mg l�1) and the absence of an inverse relationship between DOC and discharge.Most DOC originating from waste water being discharged in tributaries of the estuary appears to be remineralised before these

tributaries reach the main estuary. Although dense phytoplankton blooms were observed in the upper estuary during summer (up to700 mg chl a l�1), these blooms did not appear to produce large quantities of DOC in the freshwater tidal reaches as DOCconcentrations were low when phytoplankton biomass was high. The fact that DOC concentrations were high in winter and

decreased in summer suggests a predominantly terrestrial source of DOC in the freshwater tidal reaches of the Schelde estuary.� 2005 Elsevier Ltd. All rights reserved.

Keywords: dissolved organic carbon; DOC; freshwater tidal reaches; Schelde or Scheldt estuary; waste water; phytoplankton

1. Introduction

Oceanic dissolved organic carbon (DOC) comprisesone of the largest organic carbon pools in the biosphere.Recent studies suggest that a large part of this organicmatter is of terrestrial origin (Bauer and Druffel, 1998).This terrestrial DOC is mainly supplied to the oceansthrough estuaries. Processes influencing DOC dynamicsin estuaries may therefore influence the flux of terrestrialDOC to marine ecosystems. Recently, several studieshave dealt with DOC dynamics in estuaries. Thesestudies indicated that behaviour of DOC can vary

* Corresponding author.

E-mail address: koenraad.muylaert@ugent.be (K. Muylaert).

0272-7714/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ecss.2005.04.010

considerably between estuaries. Several studies haveobserved conservative behaviour of DOC in estuaries(e.g. Alvarez-Salgado and Miller, 1998; Abril et al.,2002). Other studies observed a significant degradationof DOC supplied by tributary rivers within the estuary(Raymond and Bauer, 2001). Other studies identifiedimportant internal sources of DOC within the estuary.These sources were linked to phytoplankton blooms(Fukushima et al., 2001) or to the vegetation fromsurrounding marshes (Raymond and Bauer, 2001).

Most of the studies mentioned above focused onDOC dynamics along the salinity gradient or dealt onlywith the marine part of the estuary. Few studiesconsidered the upper, freshwater tidal reaches (FTR)of estuaries. These FTR comprise the most upstreamestuarine zone, where freshwater supplied by the river is

592 K. Muylaert et al. / Estuarine, Coastal and Shelf Science 64 (2005) 591e600

subjected to a tidal regime but is not yet diluted withseawater. For several reasons, these FTR could play animportant role in the DOC dynamics of estuaries. First,terrestrial DOC enters estuaries via these FTR and theFTR are the first estuarine zone where riverine organicmatter can be processed. Second, as the FTR ofestuaries are often situated in a densely populated zoneof the estuarine catchment, they may receive high DOCinputs through waste water discharges. Third, comparedto more downstream situated estuarine reaches, they canbe very productive areas sustaining dense phytoplank-ton blooms which could be an important source of DOC(e.g. Schuchardt et al., 1993; Muylaert et al., 2000).

The Schelde estuary is situated in one of the mostdensely populated regions of Europe and is stronglyinfluenced by human activities. In contrast to manyother European estuaries, where sluices have beenconstructed at the freshwater seawater interface, theSchelde estuary still possesses extensive FTR andprovides a good opportunity to study these uniqueecosystems. While previous studies have dealt withparticulate organic carbon (POC) dynamics in the FTRof the Schelde estuary (Hellings et al., 1999; DeBrabandere et al., 2002), few studies have reported onDOC in this part of the estuary. Abril et al. (2002)presented DOC data from the freshwater tidal reachesof the Schelde estuary but did not discuss in detail thespatial and seasonal variability. The goal of this studywas to elucidate the processes that regulate DOCdynamics in the FTR of the Schelde estuary. ThereforeDOC and its biodegradability were monitored in theupper Schelde estuary and in its major tributaries. Giventhe strong human influences on the Schelde estuary ingeneral and its FTR in particular, it was predicted thatDOC would be derived mainly from waste water. Ourdata, however, suggest that most of the waste waterDOC discharged in the tributaries of the Schelde estuaryis respired before the tributaries join the main of theestuary.

2. Materials and methods

2.1. Study site

The Schelde estuary is a macrotidal estuary situatedin Western Europe (Belgium and The Netherlands)(Fig. 1). The estuary includes two major basins: theSchelde and the Rupel basin. This study focuses on thelonger of the two basins, the Schelde basin. The FTR inthis basin extend between Gent and the confluence withthe Rupel. Although the FTR of the Schelde basincomprise more than one-third of the total length of theestuary, their contribution to the estuarine volume issmall (!2%). The catchment of the Schelde estuary is

situated in a densely populated and highly industrialisedregion. The number of inhabitants per unit runoff ishigher than in most other European estuaries (about73!103 m3 s�1; Abril et al., 2002). Consequently, theestuary receives high inputs of organic matter throughmunicipal and industrial waste discharges (100e150!103 t C year�1; Wollast, 1989; Soetaert and Her-man, 1995). Much of this organic matter enters theestuary through the river Zenne, which discharges intothe Rupel basin. The river Zenne receives a large part ofBrussels’ waste water, most of which is at presentuntreated. Inorganic nutrient concentrations in the FTRof the Schelde estuary are high. Dissolved inorganic Nand P concentrations usually exceed 3000 mg N l�1 and160 mg P l�1 (Van Damme, unpublished data). Due tothe combination of a shallow water column and strongtidal currents, suspended matter concentrations are high(50e300 mg l�1). Despite this high turbidity, densephytoplankton blooms occur during spring and sum-mer, with chlorophyll a concentrations often exceeding100 mg l�1 (Muylaert et al., 2000). Due to the smallcatchment size (about 22 000 km2), discharge is relative-ly low (mean 120 m3 s�1) and responds rapidly toprecipitation events. Due to a high ratio of estuarinevolume to discharge, water residence time in the estuaryis long (30e90 days for the entire estuary and 10e15 days for the FTR) and the salinity gradient isrelatively gradual.

2.2. Sampling

Samples were collected monthly during the course of2003. Sampling focused on a transect of 16 sites in theupper reaches of the estuary. In addition to this transect,two riverine (Schelde and Dender) and two tidal (Durmeand the Rupel) tributaries were sampled. The 16estuarine sites were sampled from a boat. Samplingstarted at the most downstream sites and the entiretransect was covered in two consecutive days. Thetributaries were sampled from the shore. At all sites,subsurface water was collected by means of a bucketthat was thoroughly rinsed with sample water. A watersample was taken from this bucket in an acid-washedglass bottle that was also pre-rinsed with sample water.Salinity and temperature were measured in the field bymeans of a Datasonde 3 Multiprobe logger.

2.3. DOC determination

For determination of DOC concentration, water wasfiltered through a pre-combusted (450 �C, O3 h) GF/Fglass-fibre filter (nominal size cut-off: 0.7 mm) using anacid-washed glass syringe and a stainless steel filter unit.Filter, syringe and filter unit were rinsed with samplewater before filling three replicate, pre-combusted 5 mlborosilicate glass vials. These three subsamples were

593K. Muylaert et al. / Estuarine, Coastal and Shelf Science 64 (2005) 591e600

Gent

Antwerpen

10 km

N

North

Sea

Schelde

B

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km 140

km 147

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Dender

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Rupelkm 98

km 118km 128Leie

Zenne Dijle

Nete

Schelde basin Rupel basin

Fig. 1. Map of the Schelde estuary indicating the position of the sampling sites. Sites situated along the main estuarine axis are shown as white points

and marked with the distance from the mouth of the estuary. Sites located along tributaries are shown as grey points and marked with the name of

the tributary. The location of municipal waste water discharge points is indicated by black triangles (based on Deneudt et al., 2003).

immediately acidified to pH!2 with phosphoric acid,sealed with plastic caps lined with HCl-rinsed and oven-dried Teflon liners and stored refrigerated in the darkuntil analysis in the lab (within 1 month). DOC sampleswere analysed by means of high temperature catalyticoxidation using a Shimadzu TOC-5000 analyser equip-ped with a platinum catalyst on quartz wool. Hightemperature catalytic oxidation is generally consideredto be the most appropriate technique for determiningDOC concentrations in surface waters (Spyres et al.,2000). Before analysis, the TOC-5000 was conditionedby repeated injections with MilliQ water until the blankwas low and stable. The system blank was determinedby injecting condensed pyrolysed water in the furnace.This system blank was usually below 0.1 mg C l�1 andwas therefore not substracted from the DOC measure-ments. Before injection into the furnace, the acidifiedsamples were decarbonated with carbon-free oxygen ata flow of 150 ml min�1 during 4 min. Therefore, ourDOC measurements do not include volatile DOC. Threeto five replicate 50 ml subsamples were injected until thecoefficient of variation for these replicate injections wasbelow 2%. Calibration was carried out using potassiumphthalate dissolved in MilliQ water. A 5-point calibra-tion series in the range of 0e10 mg C l�1 was madeduring each run. The regression line for this calibrationalways had an r2O0.99. No significant differences wereobserved for slopes of calibration curves based on Milli-Q or artificial seawater. Glucose standard solutions of5 or 10 mg C l�1 were analysed every 20 samples.

Analyses of these standards usually varied less than 5%over time.

2.4. Biodegradability assays

The fraction of biodegradable DOC (BDOC) wasdetermined at every other station sampled along theestuary. No data are available for the samples collectedin the tributaries. To estimate the fraction of DOC thatis available to bacteria, bioassays were carried out inwhich DOC concentration was determined before andafter incubation of water in the presence of bacteria(Servais et al., 1989). Three replicate GF/F filteredsamples were incubated in pre-combusted 5 ml borosil-icate glass vials in the dark during a period of 1 month.Because GF/F filters are inefficient in retaining bacteria,it was assumed that these subsamples containeda sufficiently large bacterial inoculum (cf. Raymondand Bauer, 2000; Fukushima et al., 2001). As nutrientconcentrations in the upper Schelde estuary are highthroughout the year and are unlikely to be limiting forbacteria, no nutrients were added to the bioassays. After1 month, the subsamples were acidified with phosphoricacid and stored refrigerated in the dark until analysis forDOC. The fraction of BDOC was calculated bycomparing DOC concentration in incubated sampleswith samples that were immediately acidified. Thebioassays were carried out at a fixed temperature of 10�Cto exclude temperature effects on the biodegradabilityof DOC. Temperature, however, may have an important

594 K. Muylaert et al. / Estuarine, Coastal and Shelf Science 64 (2005) 591e600

influence on the biodegradability of DOC in estuaries(Raymond and Bauer, 2000). Therefore, care should betaken when extrapolating the results of the BDOCassays to the field situation.

2.5. Other analyses

For chlorophyll a determination, a known volume ofwater was filtered onto a GF/F filter. The filter wasstored frozen at �80 �C until pigments were extracted in90% acetone and chlorophyll a concentration wasdetermined by means of HPLC according to the methodof Wright et al. (1991). Suspended particulate matterwas measured gravimetrically after filtration of a knownvolume of water onto a pre-weighed GF/C filter. ForPOC analysis, a known volume of water was filteredonto a pre-combusted GF/F filter which was stored at�20 �C in the dark until analysis. Before analysis, filterswere exposed to HCl vapours in vacuum to removecarbonates. The filters were analysed for carbon contentusing a Carlo Erba NA 1500 analyser. Calibration wascarried out using pre-weighed amounts of acetanilide.

2.6. Statistical tests

Simple t-tests were used to compare DOC concen-trations at the start and at the end of the incubation inthe BDOC assays. For each month, all estuarinesampling sites were assigned to a salinity class basedon the Venice system (FTR: salinity !0.5; oligohaline:salinity 0.5e5; and mesohaline: salinity O 5; Anony-mous, 1959). Mean concentrations of DOC, chlorophylla, POC and SPM in the oligo- and mesohaline zones andin the tributaries were compared with mean concen-trations in the FTR by means of paired t-tests withmonths as the pairing factor. A p-level of 0.05 wasassumed to be significant in all statistical tests.

3. Results

Water temperature was highest in August (24.3 �C)and lowest in January (2.2 �C) and usually did not varymore than 2 �C between the sampling sites. Meandischarge of the Schelde estuary downstream theconfluence of the Schelde and Rupel basins in 2003was 113 m3 s�1, which is close to the mean discharge ofthe past 50 years. Discharge was highest in the periodJanuaryeMarch, decreased gradually afterwards andincreased again slightly in December (Fig. 2). Salinity inthe Schelde estuary was inversely related to dischargeand was lowest in the period JanuaryeMarch (Fig. 3).Salinity was below 0.5 in the tributary rivers Schelde andDender and in the most upstream stations of theestuary. Salinity started to increase only below 75 km

when discharge was high. The increase in salinity wassituated more upstream when discharge was low.Salinity at the most downstream station sampled variedfrom only 1.2 in January to 16.3 in September andNovember. Salinity of the tidal tributaries Durme andRupel was comparable to that of the adjacent stations inthe main estuary and was generally around 0.5. The sitesin the main estuary where salinity was below 0.5 wereconsidered as the FTR. Sites where salinity was between0.5 and 5 were classified as oligohaline and sites witha salinity above 5 as mesohaline. No mesohaline siteswere sampled in January to March due to the highdischarge and resulting low salinity.

SPM concentration was inversely correlated withdischarge (Pearson correlation coefficient rZ�0.80,pZ0.002) and was lowest in February and March(Fig. 4). The mean SPM concentration was 106 mg l�1

in the FTR and was similar in the oligohaline zone(Table 1). SPM concentration was significantly higher inthe mesohaline zone when compared to the FTR. SPMconcentrations in the tributary rivers Schelde andDender and in the tidal tributaries Rupel and Durmewere significantly lower than in the FTR.

Mean chlorophyll a concentration was highest in theFTR and was significantly lower in the oligo- andmesohaline zones (Table 1). Chlorophyll a concentra-tion in the FTR in 2003 peaked at 714 mg l�1, which ishigher than maxima measured during previous years(unpublished data). When compared to the FTR,chlorophyll a concentration was similar in the Durmebut was significantly lower in the Rupel. Chlorophylla concentration was also significantly lower in thetributary rivers Schelde and Dender when compared tothe FTR. Chlorophyll a concentration in the mainestuary as well as in the tidal tributaries Durme andRupel was lowest in January and February andincreased in March to June (Fig. 4). Chlorophylla peaked in the period JulyeOctober and decreasedagain in November and December. In the tributaryrivers Schelde and Dender, on the contrary, chloro-phyll a concentration peaked in April with a secondarypeak in August or September.

POC concentration was significantly correlated withchlorophyll a concentration (Pearson correlation co-efficient rZ0.67, p!0.001) and displayed a similar spatialand temporal pattern as chlorophyll a. Differencesbetween seasons and sites, however, were less pronouncedfor POC than for chlorophyll a (Fig. 4). Along theestuarine gradient of the Schelde, POC concentration washighest in the FTR and was significantly lower in theoligo- andmesohaline zone (Table 1). POC concentrationdid not differ significantly between the tidal tributariesand the FTR. Like chlorophyll a, POC concentration waslower in the Schelde and Dender tributary rivers,although the difference was only marginally significantfor the Schelde river.

595K. Muylaert et al. / Estuarine, Coastal and Shelf Science 64 (2005) 591e600

0

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JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

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e (m

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-1)

Fig. 2. Seasonal variation in discharge of the estuary downstream of the confluence of the Schelde and Rupel sub-basins during the course of 2003.

DOC concentrations were measured in 220 watersamples. The standard deviation of the three replicatesubsamples from each water sample was on average11% of the mean. The annual mean DOC concentrationin the FTR was 4.6 mg l�1 and was similar in theoligohaline zone (Table 1). DOC concentration wassignificantly lower in the mesohaline zone whencompared to the FTR. DOC concentrations did notincrease or decrease with distance in the FTR (Fig. 3). Alinear regression of DOC concentration versus distancein the FTR revealed a slope that was significantlydifferent from 0 only in February, when a significantincrease in DOC concentration in the downstreamdirection was observed ( p!0.001). This increase inDOC concentration with distance may have been relatedto a major discharge peak just prior to sampling(O400 m3 s�1 during 2 days). Freshwater suppliedduring this discharge peak apparently had a lowerDOC concentration (4e6 mg l�1) than the water thatwas already present in the upper estuary (6e8 mg l�1).Due to this sudden increase in discharge, the water thatwas present in the FTR and that was rich in DOC wasmoved downstream from 100 km and replaced withwater supplied by the tributaries that had a lower DOCconcentration. DOC concentrations at a given station inthe FTR sometimes deviated markedly from adjacentstations (Fig. 3). DOC concentrations in the tidaltributaries Rupel and the Durme and in the riverinetributaries Schelde and Dender did not differ signifi-cantly from those in the FTR (Table 1). DOCconcentration displayed a similar seasonal trend in thedifferent salinity zones of the main estuary and in itstidal and riverine tributaries, being highest in JanuaryeFebruary and NovembereDecember (Fig. 4). Monthlymean DOC concentrations in the oligo- and mesohalinezones were positively correlated with concentrations inthe FTR (Pearson correlation coefficients O0.82,p!0.01). Monthly DOC concentrations in the tidaltributaries Rupel and Durme were also significantlypositively correlated with concentrations in the FTR

(Pearson correlation coefficients O0.58, p!0.05). DOCconcentrations in the rivers Schelde and Dender werepositively correlated with concentrations in the FTR butthe correlations were only marginally significant (Pear-son correlation coefficients O0.49, 0.05!p!0.1).

The fraction of BDOC was determined in 89 samplescollected along the main estuarine axis. In all but fiveanalyses, a decrease in DOC concentration was observedduring incubation. In 49 cases, this decrease wassignificant according to a simple t-test ( p!0.05). Inthe FTR, on average 9% of total DOC was respiredduring incubation (Table 1). This fraction was slightlyhigher in the oligohaline zone and slightly lower in themesohaline zone, but these differences were not signif-icant. The fraction of DOC that was respired variedstrongly from month to month and ranged froma minimum mean of 3% in February to a maximummean of 24% in July (Fig. 5). The highest fraction ofDOC that was respired was 32%, which was measuredin a sample from the FTR in July. The BDOC fractiontended to be higher in summer than in winter. June wasan exception to this seasonal pattern as the fraction ofBDOC in June was exceptionally low compared to othersummer months. The data from June, however, shouldbe interpreted with caution as only the most down-stream estuarine sites were sampled in that month and,as a result, relatively few BDOC measurements wereavailable.

4. Discussion

As the Schelde estuary possesses a densely populatedand heavily industrialized catchment (Wollast, 1989), itwas expected that most DOC in the FTR was derivedfrom anthropogenic waste water. Our observations,however, do not support this hypothesis. Waste waterDOC tends to be highly biodegradable. Servais et al.(1995) analysed the biodegradability of DOC in wastewater outflows from Brussels and found that bacteria

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Distance from estuary mouth (km) Distance from estuary mouth (km)

Fig. 3. Monthly profiles of salinity (triangles) and DOC concentration (circles) in the upper reaches of the Schelde estuary and in the tributaries

sampled. DOC data shown are means and standard error of three replicate subsamples.

597K. Muylaert et al. / Estuarine, Coastal and Shelf Science 64 (2005) 591e600

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Fig. 4. Seasonal variation in mean concentrations of SPM, chlorophyll a, POC and DOC in the different salinity zones in the upper estuary (left) and

in the tributaries (right). For each salinity zone, the mean for all stations situated in that zone is presented. Error bars correspond to the standard

error of the mean.

could respire more than 50% of DOC within 1 week.DOC in the Schelde estuary, on the contrary, wasrelatively refractory. On average only 9% of DOC in theFTR was found to be biodegradable in a 1 monthperiod. This is comparable to less polluted estuaries(Moran et al., 1999; Raymond and Bauer, 2000;Wiegner and Seitzinger, 2001). As the Schelde estuaryhas a higher ratio of inhabitants in the catchment overdischarge than most other European estuaries (Abrilet al., 2002), the mean DOC concentration in theSchelde estuary was expected to be higher than the meanDOC concentration in European rivers. But the mean

DOC concentration in the FTR of the Schelde estuary(4.6 mg l�1) was very close to the mean for Europeanrivers, which is 4.2 mg l�1 according to Meybeck (1982)or 5.0 mg l�1 according to Ludwig et al. (1996). Finally,in estuaries where DOC is mainly derived from wastewater, DOC concentration tends to be inversely relatedto discharge because a relatively constant input of wastewater becomes progressively diluted as discharge in-creases (e.g. Tipping et al., 1997). This was clearly notthe case in the FTR of the Schelde estuary, where thehighest mean DOC concentration was measured duringJanuary, the high discharge month.

598 K. Muylaert et al. / Estuarine, Coastal and Shelf Science 64 (2005) 591e600

Table 1

Mean values and number of observations (n) of some variables in the different zones of the upper Schelde basin and significance level of a paired t-test

comparing monthly concentrations in the estuarine salinity zones and the tributaries with monthly concentrations in the FTR

Variable Freshwater

tidal reaches

Oligohaline

zone

Mesohaline

zone

Rupel

(tidal)

Durme

(tidal)

Schelde

(river)

Dender

(river)

DOC (mg l�1) Mean 4.6 4.8 3.8 5.0 5.0 4.2 4.9

n 118 33 25 12 12 12 11

p-level 0.500 0.009 0.233 0.220 0.136 0.482

Fraction BDOC (%) Mean 8.9 10.0 5.5 e e e en 54 17 14 e e e e

p-level 0.561 0.758 e e e e

POC (mg l�1) Mean 5.6 4.2 2.7 4.5 5.5 4.1 2.7

n 93 31 21 10 9 10 10

p-level 0.003 0.006 0.145 0.759 0.054 !0.001

SPM (mg l�1) Mean 106 100 178 48 65 62 29

n 115 35 25 12 12 12 12

p-level e 0.623 0.001 0.003 0.035 0.006 !0.001

Chlorophyll a (mg l�1) Mean 129 38 10 42 92 42 35

n 114 35 25 12 12 12 12

p-level 0.008 0.011 0.013 0.158 0.044 0.021

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mesohaline (salinity > 5)

%B

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BD

OC

Fig. 5. Seasonal variation in the fraction of BDOC in the different

salinity zones in the upper reaches of the Schelde estuary. For each

salinity zone, the mean for all stations situated in that zone is

presented. Error bars correspond to the standard error of the mean.

White points indicate that only one measurement was available for

that month and no standard error could be calculated.

Although our data suggest that waste water is not animportant source of DOC in the FTR of the Scheldeestuary, it is nevertheless without doubt that the Scheldecatchment receives high inputs of organically rich wastewater (Wollast, 1989; Abril et al., 2002). Most ofBrussels’ waste water is currently discharged into theZenne tributary river without treatment. This wastewater enters the main estuary via the Rupel basin. HighDOC concentrations have indeed been measured in theRupel basin and its tributary Zenne (Abril et al., 2002).In our study, sampling focused on the Schelde basin andour sampling station in the Rupel basin was situated notfar upstream of the confluence of the Rupel with theSchelde basin. DOC concentrations at the Rupel stationwere not significantly higher than in the adjacent FTRand did not increase downstream from the confluence ofthe Schelde basin with the Rupel. This seems to indicatethat most DOC supplied by waste water discharges intothe Zenne river is respired before it reaches the mainestuary.

Phytoplankton can exude a large fraction of itsprimary production in the form of DOC (e.g. Bainesand Pace, 1991). In the Hiroshima Bay estuary,Fukushima et al. (2001) observed a positive relation-ship between chlorophyll a and DOC concentration,pointing to a significant contribution of phytoplanktonexudates to DOC. Dense phytoplankton bloomsoccurred in the FTR of the Schelde estuary in thesummer of 2003, with chlorophyll a concentrations upto 714 mg l�1. DOC concentrations, however, werelowest during the summer period. This suggests thatphytoplankton blooms in the FTR of the Scheldeestuary do not produce large amounts of DOC. Whenthe bloom collapses, however, large quantities of DOCmay be released. But due to the low sampling

599K. Muylaert et al. / Estuarine, Coastal and Shelf Science 64 (2005) 591e600

frequency used in this study such an event may havebeen missed. Dynamics of DOC are in contrast withPOC dynamics. POC was closely correlated withchlorophyll a concentration, indicating that phyto-plankton blooms contribute to total POC in theSchelde estuary. The importance of phytoplankton asa source of POC in the Schelde estuary was previouslydemonstrated by means of isotope signatures of POC(Hellings et al., 1999; De Brabandere et al., 2002).

DOC concentrations in the FTR of the Scheldeestuary were highest in the winter period (JanuaryeFebruary and OctobereDecember) and decreasedduring summer (MarcheSeptember). Such a seasonalpattern in DOC concentration is indicative of a pre-dominantly terrestrial origin of DOC. In terrestrialecosystems, the supply of DOC to surface waters isclosely linked to the flow path of the water (Boyeret al., 1997; Findlay and Sinsabaugh, 1999). Duringwinter, terrestrial sediments are saturated with waterand DOC enters surface waters mainly through lateraltransport over the sediment surface, leading to minoradsorptive losses of DOC to the sediment. In summer,DOC enters surface waters after percolation throughthe sediment, resulting in adsorption of a fraction ofDOC to sediment particles. Differences in DOCconcentrations between winter and summer may alsobe related to increased degradation of terrestrial DOCduring transport to the estuary in summer. Degrada-tion of DOC may be stimulated in summer due tohigher temperatures (e.g. Raymond and Bauer, 2000).DOC leaching from terrestrial ecosystems can beexpected to be more refractory in winter than insummer, as it is mainly derived from dead plant litterin winter in contrast to fresh biomass in summer.Results from our DOC biodegradability assays suggestthat the biodegradable fraction of DOC measured wasindeed higher in summer than in winter. In therelatively pristine tributaries of the Humber estuary,lower DOC concentrations in summer than in winterled Tipping et al. (1997) also to conclude that DOCwas mainly derived from natural, terrestrial sources.Analyses of the isotope composition of DOC indicatedthat terrestrial sources were also the major source ofDOC in two North American estuaries (Raymond andBauer, 2001; Goni et al., 2003).

DOC concentrations in the FTR did not differsignificantly from those in the major tributary rivers ofthe FTR, the Schelde and Dender rivers. Within theFTR, DOC concentrations rarely increased or de-creased significantly along the longitudinal axis. Thissuggests that no major sources or sinks of DOC werepresent within the FTR. It should be noted, howeverthat the standard deviation of replicate DOC analysesfrom a single sample was high (11%), higher than thebiodegradable fraction of DOC (9%). Therefore,degradation of DOC during downstream transport in

the FTR or sources of DOC that do not alter DOCconcentrations by O11% would have been difficult todetect. Variation in DOC concentrations betweenadjacent sampling sites in the upper estuary was oftenrelatively large. In most months, one or a few sites hada DOC concentration that was O 2 mg l�1 higher thanthat of adjacent sampling sites. This suggests thatminor, local sources of DOC may be present in theupper estuary. These sources may be small tributariesentering the estuary or municipal waste water dis-charge points, which are numerous in the FTR of theSchelde estuary (see Fig. 1; Deneudt et al., 2003). Thefact that these DOC peaks were usually local suggeststhat they had no major impact on the mean DOCconcentration in the upper Schelde estuary. Near thesalinity gradient, a decline in DOC concentrationcould often be observed. Moreover, DOC concentra-tions were significantly lower in the mesohaline zonecompared to the FTR. This may be the result ofmixing of DOC rich freshwater with brackish waterthat has a lower DOC concentration. Declines in DOCconcentration with increasing salinity have beenobserved in many estuaries (e.g. Mantoura andWoodward, 1983; Alvarez-Salgado and Miller, 1998;Miller, 1999), including the Schelde estuary (Abrilet al., 2002).

In conclusion, our data suggest that DOC from wastewater discharges in the tributaries of the Schelde estuarydoes not make up the bulk of DOC in the freshwatertidal reaches of the Schelde estuary. Most waste waterDOC discharged into the tributaries appears to berespired before these tributaries enter the main estuary.Peaks in DOC concentration suggest that minortributaries or waste discharges may be locally importantbut do not affect the mean DOC concentration in thefreshwater tidal reaches. Despite the fact that phyto-plankton attained high biomass in the freshwater tidalreaches in 2003, these phytoplankton blooms did notresult in increased DOC concentrations. Like in otherless polluted estuaries, DOC in the freshwater tidalreaches of the Schelde estuary appears to be mainlyderived from terrestrial sources.

Acknowledgements

This study was financially supported by the FlemishFund for Scientific Research through the project‘Bacterial use of dissolved organic carbon’. Samplingwas done in the framework of the OMES project‘Environmental impacts of the Sigma-plan’, which aimsat evaluating the impacts of water management on thefunctioning of the Schelde ecosystem. Prof. P. Meire andS. Van Damme are thanked for organising the monthlysampling cruises and providing data on temperature andsalinity. M. Lionard frequently assisted in field work.

600 K. Muylaert et al. / Estuarine, Coastal and Shelf Science 64 (2005) 591e600

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