Biogeochemistry(2009) 96:49-72 DOI 10.1007/sl0533-009-9344-6

Dynamicsof dissolvedand biogenicsilica in the freshwater reachesof a macrotidalestuary (The Scheldt,Belgium)

VincentCarbonnel • Marie Lionard • Koenraad M uylaert* Lei Chou

Received: 22 December2008 /Accepted: 1 July2009 /Publishedonline: 31 July2009 © SpringerScience+Business Media B.V. 2009

Abstract Temporal evolution of dissolved and Nevertheless,the detritalBSi was a significant biogenicsilica concentrationsalong the Scheldt tidal fractionof the total BSi pool, of whichless than riverand in its tributarieswas investigatedduring 1 10% couldbe attributedto phytoliths.The tidalriver yearin 2003. In thetributaries, dissolved silica (DSi) was dividedinto two zones forbudgeting purposes. concentrationsremained high and biogenic silica The highestproductivity was observedin the zone (BSi) concentrationswere low throughoutthe year. In thatreceived the highestwater discharge, as higher the tidalriver during summer, DSi was completely riverineDSi inputfluxes induced presumably a less consumedand BSi concentrationsincreased. Overall, restrictiveDSi limitation,but the dischargepattern mostof theBSi was associatedwith living diatoms could notexplain all by itselfthe variationsin DSi duringthe productiveperiod in the tidal river. consumption.Silica uptakeand retentionin thetidal riverwere important at theseasonal time-scale: from to 48% of the riverineDSi was V. Carbonnel(El) • L. Chou (El) May September, Laboratoired'Oceanographie Chimique et Geochimiedes consumed and 65% of the produced BSi was Eaux, Facultedes Sciences,Universite Libre de Bruxelles, deposited,leading to a silica (DSi + BSi) retention Campus de la Plaine, CP 208, Boulevarddu Triomphe, in the tidal riverof 30%. However,when annual 1050 Brussels,Belgium were DSi in thetidal river e-mail: [email protected] fluxes considered, uptake amountedto 14% of theDSi inputsand only6% of L. Chou silica was retainedin the e-mail: [email protected] the riverine (DSi + BSi) tidalriver. M. Lionard • K. Muylaert Protistologyand Aquatic Ecology, Biology Department, Keywords Biogenicsilica • Diatoms• GhentUniversity, Krijgslaan 281-S8, 9000 Ghent, Dissolvedsilica • Scheldtestuary • Belgium Silica budget• Tidal freshwater PresentAddress: M. Lionard Abbreviations des Sciences de la Mer de Universitedu Institut Rimouski, BSi silica Quebec a Rimouski,310 Allee des Ursulines, Biogenic CP 3300 Rimouski,QC G5L 3A1, Canada BSidet BSi notassociated with living diatoms BSinv BSi associatedwith living diatom PresentAddress: DiatChla Chlorophylla ascribedto diatoms K. Muylaert DSi Dissolvedsilica K.U. Leuven,Campus Kortrijk,Biology Department, E. Sabbelaan 53, 8500 Kortrijk,Belgium POC Paniculateorganic carbon

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POCnid POC notcorresponding to livingdiatoms et al. 2005). On its way to the coastal zone, DSi SPM Suspendedpaniculate matter participatesin aquatic biogeochemicalcycles in SPMnbid SPM notcorresponding to BSi norto riversand estuaries(Conley et al. 1993), involving livingdiatoms a numberof biological, chemical and physical processes.Diatom bloomsoccurring in spring/sum- mer in rivers and estuariesresult in significant decreasesin DSi and concomitantincreases in BSi concentrations,which can accountfor 50-70% of the Introduction total riverinesilica load (Admiraalet al. 1990; Conley1997). As particulatematerial, BSi can settle Dissolvedsilica (DSi) is a keynutrient for the aquatic and accumulatein thesediments, inducing retention ecosystemas diatomshave an essentialrequirement of silica in theecosystem. It can also be transported forDSi to build up theirfrustules, rigid outer cell downstreamas suspendedmaterial (Admiraal et al. walls made of amorphousbiogenic silica (BSi) 1990) and/orbe dissolvedat biologicaltimescales (Ragueneauet al. 2000). Diatoms oftendominate (Roubeix et al. 2008; Loucaides et al. 2008). BSi the primaryproduction in turbidaquatic environ- dissolutionis howevernot expectedto occurat the mentssuch as rivers,estuaries and coastal zones same ratealong the salinity gradient: BSi dissolution (Ragueneauet al. 2000), and accountfor 75% of the rate increaseswith salinity (Loucaides et al. 2008) primaryproduction in the world's coastal areas and the bacterial communitycan significantly (Nelson et al. 1995). They can supportan efficient enhance the remineralisation(Bidle and Azam aquaticfood chain (Turner et al. 1998) and mayplay 1999; Roubeixet al. 2008). In addition,phytoliths a significantrole in theexport of carbon from surface (BSi materialproduced by terrestrialhigher plants) waters(Treguer and Pondaven2000; Ploug et al. can add a significant,ifnot major, contribution to the 2008). The availabilityof DSi and its relative BSi pool carriedby rivers(Cary et al. 2005). They abundance compared to the other nutrientscan can originatefrom top soil erosion(Cary et al. 2005) influencethe compositionof the phytoplankton as well as directlyfrom the vegetationof the river community,which can subsequentlyaffect the eco- banksor tidalmarshes (Struyf et al. 2006). Marshes logical functioningof the ecosystem(Officer and can retain significantamounts of silica in the Ryther1980; Conley et al. 1993; Lancelot 1995; abovegroundbiomass and in sediments(Struyf Turneret al. 1998). However,riverine DSi fluxes, et al. 2005) and DSi and BSi exchangebetween the which are the main source of DSi to the oceans riverchannel and the marshes can affectthe estuarine (Wollast 1974; Tregueret al. 1995), have been silica cycle(Struyf et al. 2006). Thus,because of the alteredby humanactivities during the last decades. possiblyimportant riverine and estuarineBSi con- Land-useactivities, such as deforestation,may affect centrations,its specificbehaviour and its interaction theDSi inputsto rivers(Conley et al. 2008) and the withDSi, the studyof BSi dynamicsshould not be use of silica in washingpowders (Verbanck et al. omittedwhen silica fluxesand mass-balancesin 1994) and fertilizers(Datnoff et al. 2001) may also riversand estuariesare assessed. constitutean additionalDSi source to rivers.In In some estuaries,the tidal influencepropagates contrast,eutrophication results in enhancedsilica furtherinland than salt intrusion,leading to the retentionin lakes,rivers and estuaries(Conley et al. existenceof tidalfreshwater reaches which may host 1993). Decreased silica concentrationsare also importantchemical and biologicalprocesses (McLu- induced by river dammingand river regulation, sky 1993; Schuchardtet al. 1993; Muylaertet al. which cause particletrapping and reduce contact 2005). In particular,tidal freshwater reaches exhibit betweenthe waterand the riparianvegetated zone specificfeatures which can influencethe phytoplank- containingDSi-rich interstitialwaters (Humborg tondynamics (Schuchardt et al. 1993; Lionard2006) et al. 2006). and thusthe biogeochemical cycle of nutrients.They Mostof theDSi is originallyproduced during rock differfrom the adjacent riversby longer water weatheringon landwhere it generallygoes througha residencetimes and the presenceof tides, which terrestrialcycle beforereaching the rivers(Derry induceshigher turbulence and turbidity(Schuchardt

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This content downloaded from 193.191.134.1 on Thu, 16 Jul 2015 12:07:32 UTC All use subject to JSTOR Terms and Conditions Biogeochemistry(2009) 96:49-72 51 et al. 1993; McLusky1993). The resultinglow light shouldstill be performedand comparedto themodel conditionsmay limit phytoplankton growth but may results.Being a part of the suspendedpaniculate also favourdiatoms due to theirlower light require- matter(SPM), BSi in the Scheldttidal freshwater mentscompared to other algae (Reynolds 1988; reachesis expectedto followa complexbehaviour Cushing 1989; Lionard 2006). In contrastto the due to spatial and temporalvariability in SPM downstreamareas where freshwaterand seawater dynamics(Chen et al. 2005a). Due to diatom mix, thereis no phytoplanktonmortality due to mortality,resuspension of dead diatomfrustules and salinitystress in tidalfreshwater reaches (Schuchardt the possible presenceof phytoliths,a significant et al. 1993; Muylaertet al. 2000). Thus, if the fractionof theBSi maynot be associatedwith living residencetime is sufficientlylong, a maximumin the diatoms.In addition,marshes along the Scheldt tidal diatomproduction and theassociated DSi consump- freshwaterreaches may act as DSi recyclersor sinks tion may occur in the tidal freshwaterreaches forestuarine BSi, andmay play a rolein theestuarine (Anderson1986; Schuchardtand Schirmer1991; De silica cycle(Struyf et al. 2006, 2007b). As theyalso Seve 1993; Schuchardtet al. 1993; Muylaertet al. contain significantamounts of phytolithin the 2005). abovegroundbiomass (Struyfet al. 2005), BSi as In the presentstudy, we focus on the Scheldt phytolithsmay be providedto theestuarine waters by estuarywhich comprises an extensivetidal freshwater tidalexchange of thisbiogenic material between the area.The biogeochemistryof silica and thefluxes of riverchannel and marshes. thisnutrient in the continuumof the Scheldtare of In thisstudy, we presenthigh resolution DSi and primaryimportance as theycan affectthe ecosystem BSi temporalprofiles during one full annualcycle in the adjacentcoastal zone. Earlier works have alongthe tidal freshwater reaches of theScheldt and shownthat DSi drivesthe extent of theearly spring itstributaries. BSi concentrationsare comparedwith diatombloom in this zone, while the excess of those of chlorophylla and SPM to estimatethe dissolvedinorganic nitrogen stimulates a subsequent fractionof BSi associatedwith livingdiatoms. A massivedevelopment of flagellates(Phaeocystis sp.) budgetis performedfor DSi and the differentBSi whichalters both the food web and theenvironment fractions(associated or not withliving diatoms) for (Lancelot 1995). The highest(diatom-dominated) the periodfrom May to October2003 and annual phytoplanktonbiomass and production of theScheldt fluxesto thebrackish reaches of theScheldt estuary estuarywere observed in thetidal freshwater reaches, are estimated.The possiblecontribution of phytoliths resultingin strongseasonal patterns in DSi concen- to theestuarine BSi pool as well as theinfluence of trations,possibly consumed down to limitinglevels thesilica cyclingin marsheson theestimated silica in summer(Muylaert et al. 2001, 2005; Struyfet al. fluxesare discussed. 2004; Van Dammeet al. 2005; Soetaertet al. 2006). The long-termevolution of DSi concentrationsas well as the variationof DSi concentrationswith Materialsand methods increasingdischarge have been investigated(Muyla- ert et al. 2001; Struyfet al. 2004; Soetaertet al. Descriptionof thestudy area 2006). But untilnow, BSi dynamicshave not been studiedand littleis knownabout the silica biogeo- The 355 km long Scheldtriver and estuarytakes its chemical cycle in the Scheldt tidal freshwater sourcein NorthernFrance. It flowsthrough Belgium reaches.Low summerDSi concentrationsand high where it receives waters mainly from the Leie diatom-dominatedphytoplankton biomass (Muylaert (confluenceat Ghent),the Dender and the Rupel, et al. 2000; Van Dammeet al. 2005; Lionard2006) anddischarges into the North Sea via theNetherlands suggesthowever that BSi can be a majorconstituent (Fig. 1). The shallow,well-mixed and turbidmacro- of thesilica pool in thisenvironment. DSi and BSi tidal estuaryof the Scheldt has been extensively dynamicswere investigatedby Arndtet al. (2007) described(e.g. Chen et al. 2005a; Meireet al. 2005; and Arndtand Regnier(2007) usingmodel simula- Van Damme et al. 2005; Soetaertet al. 2006). The tionsof diatomdynamics calibrated on DSi concen- tidalfreshwater part of thissystem (called hereafter trations,but observations on measuredBSi dynamics the"tidal river") stretches from near Hemiksem (km

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Fig. 1 Map of the Scheldt estuary.City and river names are in regularand italic fonts,respectively. The samplingstations (in bold) are indicatedby black dots. On theDender, Zenne, Dijle, GroteNete, Kleine Nete and at Ghent,they correspondto the tidal limitsof the estuary.The Durme is nowadaysa dead arm withnegligible discharge.The dashed line followsthe borderbetween Belgium and the Netherlands.The areas (Zones la and lb, and Zone 2) consideredfor silica budgetcalculations are delimitedby greylines

91, i.e. 91 km from the mouth of the estuary at After decades of heavy pollution history, the Vlissingen) to Ghent (km 157). The tidal wave is Scheldt estuary is still stronglyaffected by human blocked by sluices at Ghent and at the mouthof the activities, and nitrogenand phosphorousconcentra- riverDender. The tidal riversystem also includes the tions are never limiting factors for phytoplankton river Rupel (confluence with the Scheldt at km 94) growth(Van Damme et al. 2005; Soetaertet al. 2006; and the downstreamparts of its fourtributaries where Van der Zee et al. 2007). In 2003, maximum the tidal influencedecreases naturally(Fig. 1). The dissolved inorganic nitrogenand phosphorous con- tidal river consists of a single ebb/flood channel centrationsreached 1,300 and 50 nmol L~\ respec- borderedby mudflatsand marshes,which account for tively(Van der Zee et al. 2007). In contrast,silica can 28% of the total surface of the tidal river system be depleted in summerin case of low waterdischarge (Meire et al. 2005). The channel is however almost (Muylaert et al. 2001). completely canalised upstream of Dendermonde The phytoplanktoncommunity in the tidal riveris (Meire et al. 2005). dominatedby diatoms throughoutthe year (Muylaert The Scheldt is a rain fed river exhibitingstrong et al. 2000; Lionard 2006; Lionard et al. 2008a). Two seasonal and inter-annualvariations in water dis- blooms have been identified:one in spring,originat- charge (Fig. 2; Soetaert et al. 2006). Before it enters ing from the upper Scheldt river and dominated by the Scheldt estuary,the water fromthe riverScheldt Stephanodiscus hanszchii; another one in summer, may be partiallyor even completelydeviated towards which develops in situ near Dendermonde and is the Ghent-Terneuzencanal to sustain industrialand dominated by Cyclotella scaldensis, a genus;glosely navigationneeds. The contributionof the Rupel to the related to C. meneghiniana(Muylaert et al. 2000). total freshwaterinput can thereforeincrease from about 46% in winterto more than 70% during dry Sampling stations summers.The increase in freshwaterresidence time duringthe summerperiod is thusmore pronouncedin The Scheldt tidal river and its tributarieswere the Scheldt between Ghent,Dendermonde and Temse sampled once a week from February 2003 to than in the Rupel and in the Nete (Table 1). February 2004, except in winter (once a month).

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125 -i At Dendermonde definedfor the budget calculations (Fig. 1). The two 100- zones have approximatelythe same volume(about 3 x 107 m3 at mid tide) and water surface area (about 7 x 106 m2) but differin total length(Table 1).

Chemical analyses Wj I J J A S O SPM, DSi and BSi

S 400 ^| Hemiksem N H| j Water samples (from 20 to 150 mL) were vacuum filteredon a pre-weighedpolycarbonate filter (What- man Nuclepore, 0 47 mm, 0.4 urn pore size). The filtratewas acidified(with 200 ^L of 2 mol L"1 HC1 per 50 mL of sample) and kept in the dark at 4°C FMAMij J AS O|ND J FM until it was measured colorimetricallyfor DSi on a Skalar Auto-analyser following a method adapted Fig. 2 Daily averageof theresidual freshwater discharge at fromKoroleff (1983). The SPM collected on the filter Hemiksem(from February 2003 to March 2004) and at was rinsedwith Milli-Q waterand dried at Dendermonde(from June to October2003) overnight 50°C. The filterwas weighed again for the SPM determinationby weightdifference and the filterwas The sampling was performedfrom a bridge or a kept forBSi analysis. landing stage. The upper Scheldt river and the five BSi was determinedby a wet-alkaline method. main tributaries(the Dender, Zenne, Dijle, Kleine Because high amountsof lithogenicsilica are present and Grote Nete) were sampled at the tidal limits in the SPM of the tidal river (Bouezmarni and (Fig. 1). The tidal riverwas sampled at threestations: Wollast 2005), the aluminium released during the Dendermonde (km 124), Temse (km 101) and digestions was used to correct for the concomitant Hemiksem (km 91). Hemiksem was considered as dissolution of Si from lithogenic material. The the downstreamlimit of the tidal river,as the salinity methodsfrom Kamatani and Oku (2000) and Rague- at thisstation was on average 1 and never exceed 2.9. neau et al. (2005) were combined and modified.The Two zones, delimitedby the sampling stations,were digestion was performedon a single SPM filterand,

Table 1 Characterizationof the Scheldt tidal river

River Average Residencetime (day)

section Depth(m) Length(m) Width(m) Summer Winter Scheldt fromGhent to Dendermonde 3.2 38 68 4.0 ± 1.7 1.3 ±0.9 Zone la fromDendermonde to Temse 4.5 25 190 7.4 ±2.8 2.3 ± 1.5 Zone I b fromTemse to Rupelmouth 4.8 6 394 3.8 ± 1.4 1.2 ±0.8 fromRupel mouth to Hemiksem 6.9 3 363 1.2 ±0.3 0.5 ±0.2

4.2 11 180 2.3 ±0.5 1.1 ±0.4 Rupel(entire) ^^ Nete(entire ) 2.8 16 49 1.9 ±0.6 0.8 ±0.3 Zenne(last 10km) 1.6 (10) 32 0.6 ±0.1 0.4 ±0.1 Dijle(last 8 km) 2.5 (8) 35 0.4 ±0.1 0.2 ±0.1

Averagedepths and widths were calculated from the volumes and water surface areas of the considered sections. Lengths, volumes andwater surface areas were estimated from WLHO (1966). No separatebathymetric data were available for the Grote and Kleine Nete.Residence times of water (with standard deviations) correspond to theperiod 1996-2005. They were computed as thequotient ofthe volume of the considered section by the average water discharge. The zones(rightmost column) correspond to thosein Fig. 1

This content downloaded from 193.191.134.1 on Thu, 16 Jul 2015 12:07:32 UTC All use subject to JSTOR Terms and Conditions 54 Biogeochemistry(2009) 96:49-72 compared to Ragueneau et al. (2005), fourdigestion steps were performed(instead of two) to improvethe Cn= (2) correction(Fig. 3). Furthermore,the SPM was not (n£-J rinsedbetween the steps to reduce the time necessary where BSi is the amount of BSi on the filter,VD and forthe digestion. VA the volumes of 0.2 mol L"1 NaOH additions The filterwith the SPM was placed at the bottom (respectively10 and 5 mL, Fig. 3) and d the dilution of a 15 mL polypropylenecentrifugation tube and factor of 1.25 due to the addition of HC1 in the covered with 10 mL (VD) of 0.2 mol L"1 NaOH aliquots of the supernatantsolutions. BSi (as well as (Fig. 3). The tube was incubated in a shaking water k9 not shown) is then calculated by a least-squares bath at 95°C for 1 h (step 1), and centrifugedat multipleregression on Sin:

- • ' (ZAlt) '(ZCn- Sin) (j: Cn-Aln) (£ SiHAln) = 1 \2=! / L BSi VD.d.\S=l >LJp] W2' (3) (Zcl)(zAlt)-(ZcnAln)\n=\ J \n=\ / \n=l /

5,000 rpm,5°C during 10 min. A 5 mL (VA) aliquot Among the 260 correlationcoefficients r2 obtained of the supernatantwas taken and acidified with fromthe multipleregressions performed for each BSi 1.25 mL of 1 mol L"1 HC1. This digestion step was measurementpresented in thisstudy, 7 1% were higher then repeated three times (steps 2-4): 5 mL of than0.999 and 95% higherthan 0.995. In addition,the 0.2 mol L"1 NaOH was added to the remaining method was tested on pure lithogenic and biogenic solution in the centrifugationtube, and the tube was silica suspensions, and known mixturesof the two placed again in the shaking water bath but for (data not shown). This supportsthe threeassumptions 30 min. indicatedabove and validates the use of Eq. 1. The fouraliquots of the supernatantsolutions were In the following,BSi concentrationsare expressed analysed forDSi and Al concentrations(Sin and Aln, in jxmolL"1 but the BSi contentof SPM is expressed • with n = 1-4), either by ICP-OES (Inductively as the mass contentof hydratedsilica SiO2 mH20, Coupled Plasma-Optic Emission Spectroscopy) or with m < 2 (Martin-Jezequel et al. 2000); we by colorimetry:using a Skalar Auto-analyserfor DSi assumed m = 1/3,a value close to the measurement (as describedpreviously) and manually fordissolved of Kamatani and Oku (2000). Al followingDougan and Wilson (1974). There were no significant differences between the results Paniculate organic carbon and contribution obtained by the two methods for DSi analyses. of diatoms to the chlorophylla concentrations Following Kamatani and Oku (2000) and Ragueneau et al. (2005), we assumed that: (1) all the BSi is SPM was collected on a precombusted(4 h, 500°C) dissolved duringthe firstdigestion step, (2) all the Al GF/F filter(0 47 mm, 0.7 |im pore size) for the measured originatesfrom the digestion of lithogenic determinationof paniculate organic carbon (POC), material and (3) this lithogenic material dissolves and on a GF/F filter(0 25 mm, 0.7 jam porosity)for witha constantSi/Al ratio (designatedhere k). Under the measurement of chlorophyll a and marker these assumptions, the DSi and Al concentrations pigments. Both filterswere kept frozen at -20°C measured in the aliquots follow the equation: untilanalysis. POC was measuredusing a Fisons NA- 1500 elemental analyser after carbonate removal = to acid Sin cn^- + k-Aln (1) fromthe filtersby overnightexposure strong fumes. Pigments concentrationswere measured by with

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Fig. 3 Scheme of the BSi digestionprotocol

- HPLC (High Pressure Liquid Chromatography)fol- DiatChki and SPM expressed in mg L ', and BSi lowing Wrightand Jeffrey(1997). The CHEMTAX in mmol L~\ - software(Mackey et al. 1996) was used to calculate the molar mass of BSi (as hydrated SiO2): the contributionof differentalgal groups to the total MBsiO2= 66gmol~l, - chlorophylla using ratios of markerpigments (spe- the chlorophyll a to POC mass ratio in the cific foralgal groups) to chlorophylla (Lionard 2006; (diatom-dominated)phytoplankton of the Scheldt Lionard et al. 2008a). The chlorophylla concentra- tidal river:j3poc/chi« = 30 (Muylaertet al. 2001), tions ascribed to diatoms (DiatChla) were estimated - the conversion factor from POC to organic using fucoxanthin,diatoxanthin and diadinoxanthinas matter: /?om/poc= 2.5. This value corresponds markerpigments for diatoms. The contributionof the to the ratio of the molar masses of CH2O and C, diatomsto thephytoplankton biomass was assessed as and was also observed in an eutrophicestuary by the ratio between DiatChla and chlorophylla con- Suzumura et al. (2004). centrations.Comparisons with microscopic observa- The distinction between BSiliv and BSidet was tions revealed thatthis methodcan indeed provide a made by performinga multiple regressionbetween good estimateof the phytoplanktonspeciation and of BSi and the two explicative variables DiatChla and the phytoplanktonand diatom biomasses in the SPMnbU1:the partial coefficients associated with Scheldt tidal river(Lionard et al. 2008a). DiatChki and SPMnbid(respectively kx and k2) were fittedto obtain the best correlationbetween measured Distinctionbetween fractionsof BSi associated BSi concentrations and those estimated by the or not with living diatoms multipleregression (BSi) calculated as:

BSi may not only be associated with living diatom B§i = kx - DiatChto + k2 • SPMnbid (5) (BSijiv) as a significant fraction may consist of BSiuv was estimated as the product of kx (called detritalbiogenic silica (BSidet) such as frustulesof hereafterBSi,iv/DiatChltf) and DiatChk*, and BSidet dead diatoms or phytoliths.It was assumed that the was calculated as the differencebetween BSi and dynamicsof BSi|iv and BSidet followed, respectively The well as correla- those of DiatChlfl and of the fractionof the SPM BSiiiv multiple regressions(as tions and statisticalStudent's /-tests)were performed which is not linked to BSi nor to living diatoms following Dagnelie (1973, 2006) but without any (SPMnbld),estimated as: independentconstant terms,as the latterwould not - • - SPMnbid= SPM BSi MBsio2 DiatChk* have been significantlydifferent from zero (at 95% * * ^POC/Chla 0OM/POC (4) confidence) if they were appended. with

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Dischargedata, silica fluxesand mass-balances Results calculations Distributionof DSi, BSi and DiatChtoat thetidal Daily dischargedata forthe upper Scheldt river and limitsof theScheldt and its tributaries thefive main tributaries as well as an estimationof theoverall lateral inputs for Zone 1 and Zone 2 were In general,diatoms accounted for less thanhalf of the providedby the FlemishAdministration (Afdeling phytoplanktonbiomass in thesix rivers(Table 2) and MaritiemeToegang). The lateralinputs (non-moni- the DiatChltfconcentrations were below 50 \xgL~l toredlateral sources of waterdischarging directly (Fig. 4a-f). DSi concentrationsremained high intothe tidalriver, such as run-offor small lateral throughoutthe year with mean annual concentrations streams)accounted for less than10% of the annual rangingfrom 154 |amolL"1 in the Kleine Nete to dischargein 2003; theywere redistributed among the 288 [j.molL"1 in theDijle. BSi concentrationswere tributariesof thezone, according to theircontribution lowerwith mean annual concentrations varying from to the total riverinewater input. Discharges at 2.9 nmolL"1 in the Dijle to 18.2 junolL"1 in the Dendermonde,Temse and Hemiksemwere calcu- upperScheldt river. Both the DiatChtoand theBSi latedas the sum of the dischargesof the upstream concentrationswere higherin the riverScheldt (at tributaries.However, as Dendermondeis situated Ghent)and in the Dender (Fig. 4a, b) thanin the roughlyhalf-way between Ghent and Temse,half of tributariesof the Rupel (Fig. 4c-f). Few BSi data thecontribution of the lateralinputs in Zone 1 was were available for the Zenne, as mostlynegative transferredfrom Zone la toZone lb in thewater (and valueswere retrieved from Eq. 3 (datanot shown, see thesilica) mass-balance(Fig. 1). "Discussion").At theend of Marchand thebegin- Silica fluxes(kmol day"1) were calculatedat a ning of April in the upper Scheldt river,higher weeklyresolution: daily discharge data wereweekly DiatChlflconcentrations indicated a springdiatom averagedand multipliedby the weekly measured bloom (Fig. 4a). Howeveronly a small increasein silica concentration.If no samplingwas carriedout BSi concentrationscould be observed.Other smaller duringa week,the concentrationwas estimatedby concomitantincreases in BSi and DiatChla patterns linearinterpolation between the two closest measure- wereobserved in springin all riversand in summer ments.In the tributariesof the Rupel,BSi was not in the upper Scheldt,the Dender and the Dijle measuredevery week, but weeklyBSi fluxeswere (Fig. 4a-f). DSi concentrationsalso decreasedduring calculatedwith BSi concentrationsderived from the bothseasons in theupper Scheldt and theDender. weeklyDiatChk concentrationsand the slopes and interceptsof the linear regressionsperformed betweenmeasured BSi and correspondingDiatChla to the concentrationsin each river.The interceptsof the Table 2 Averagecontributions of the diatoms phyto- planktonbiomass at the nine sampling stations (in percentages) linearregressions were assumedto provideestima- tionsfor the detrital BSi fractions.For theZenne, as Station Spring Summer Autumn-Winter thedataset did notallow such linearregression, the (March-May)(June- (October- September) February) slope and the interceptused were the averagesof thosefound for the Dijle, Groteand KleineNete. Ghent 60 40 39 Mass-balanceswere performed for DSi, BSiiivand Dender 30 23 30 BSidetin Zones la, lb and 2 duringthe productive Zenne 29 16 20 In each theloss of DSi was ascribedto period. zone, Dijle 36 24 23 DSi uptakeby diatomand thusto a productionof GroteNete 33 17 32 BSiiivThe mass-balancebetween this production and KleineNete 26 15 17 an the fluxesof BSiiiv was assumed to provide Dendermonde78 85 71 The mass-balance estimateof the diatommortality. Temse 72 69 60 and fluxesof was betweendiatom mortality BSidet Hemiksem 46 56 31 consideredas representativeof the BSi deposition(or Contributionsover 50% areindicated in bold resuspension)in each zone.

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Distributionof DSi, BSi and DiatChk Whilethe weather was summer-likein September alongthe tidal river 2003, the irradianceand the temperaturedropped suddenlyin Octoberand the rainfall increased (IRMB Diatoms accountedfor more than 60-70% of the 2003-2004). In Octoberat Dendermonde,the DSi phytoplanktonbiomass throughout the year2003 at concentrationincreased rapidly following the dis- Dendermondeand Temse (Table 2). Althoughthe charge peak (Figs. 2, 4g). DSi was no longer fractionof the phytoplanktonbiomass attributed to consumedand remained high at about200 umolL~~\ diatoms was significantlylower at Hemiksem while BSi and DiatChla startedto decrease. In (Table 2), diatomswere still the dominantphyto- parallel to what was observed at Dendermonde, planktonspecies (Lionard 2006). DiatChlflconcentrations at Temse werestill high in A springbloom could be observedat Dender- Septemberbut declinedfrom October onwards. At mondeat theend of March and the beginning of April Hemiksem,DiatChla startedto decline alreadyin (Fig. 4g) but was not noticeable at Temse and September.However, DSi concentrationsreached Hemiksem(Fig. 4h, i). Untilthe end of May, DSi winterlevels only at theend of Octoberat Temseand concentrationsremained high rangingfrom 150 to Hemiksem.Background levels of DiatChk lower 250 umolL"1 at the threestations and BSi stayed than10 jig L"1 wererecorded only in Februaryand low at around36, 20 and 13 umolL"1, respectively March 2004 at Dendermonde,whereas such low at Dendermonde,Temse and Hemiksem. concentrationswere already reached in Novemberat In summer,DiatChla and BSi concentrations Temse and Hemiksem(Fig. 4g-i). At these two increased concomitantlywhile DSi decreased locations,BSi did not decrease in parallel with (Fig. 4g-i), indicatingthe development of a summer DiatChla, high BSi concentrationsbeing even diatombloom (June to September)in thetidal river. recordedon 13 and 27 October2003 at Temse when At Dendermonde(Fig. 4g), DSi was consumedby theSPM concentrationreached its highestvalues of diatoms down to 2 umolL"1 and was entirely respectively249 and409 mgL"1. AtHemiksem, BSi transformedinto BSi fromthe beginning of Julyto remainedhigh in September,but declined from the end of September.The BSi concentrations Octoberonwards (Fig. 4i). fluctuatedat around200 umolL"1 and correlated well withthe DiatChk concentrations(except on 11 Fractionof the BSi associatedwith living diatoms and 25 August2003 whenDiatChla declinedwhile BSi remainedhigh). Increases in DSi observedat the To determineto whichextent BSi was associated beginningof July and Septemberoccurred a fewdays withliving diatoms and to estimatethe BSi contentin aftersharp increases in thefreshwater discharge, but livingdiatoms, correlations between BSi and Diat- thedischarge peaks at theend of Julyinduced only a Chla wereperformed on datasetscharacterizing the small DSi increase at the beginningof August productiveperiod (May to October)at Dendermonde, (Figs. 2, 4g). The dischargepeaks at the beginning Temse and Hemiksem. Concentrationsprofiles and at theend of Julyled to an increasein bothBSi (Fig. 4a, g) indeed suggesteda differentBSi to and DiatChla concentrations.However, the high DiatChlflratio in the springdiatom community. dischargepeak at the end of Augustresulted in an However,the correlationswere weak (r2 = 0.22, increasein BSi concentration(364 umolL"1) buta 0.14 and 0.40 at Dendermonde,Temse and Hemik- decreasein DiatChk concentration(92 ug L"1) on 8 sem, respectively,n = 25) due to the interfering September2003. At Temse and at Hemiksem presenceof BSidet.As SPMnbidwas notcorrelated to (Fig. 4h, i), DSi was also consumed down to DiatChla at any of the three estuarinestations 5 umolL"1 in summer.However, the uptake started {r2 < 0.01), multipleregressions between BSi and laterand lasted for a shorterperiod at Hemiksem thetwo explicativevariables DiatChla and SPMnbid comparedto Dendermondeand Temse.From June to (see "Materialsand methods")were thus performed August,BSi and DiatChla concentrationsincreased to distinguishBSiliv from BSidel. SPMnbld may have and correlatedwell at Temse and Hemiksem,but been furtherdivided in organicand inorganicfrac- never reached the high levels observed at tionsto investigatethe originof BSidetusing POC Dendermonde. andDiatChla data and the/? ratios defined above, but

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Fig. 4 DSi (leftaxis), BSi and DiatChla(right axis) concen- DiatChlaconcentrations for the tidal limits at Ghent,Dender, trationsat thenine sampling stations (a-i) fromFebruary 2003 Zenne,Dijle, Grote and Kleine Nete (a-f) to March2004. Note the differentscales forthe BSi and

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This content downloaded from 193.191.134.1 on Thu, 16 Jul 2015 12:07:32 UTC All use subject to JSTOR Terms and Conditions Biogeochemistry(2009) %:49-72 59 the correlationbetween organic and inorganic frac- tions (0.54 < r2 < 0.88, 23 < n < 25) ruled out the possibilityto use both of them in the same multiple regression. The BSiliv/DiatChlaratio at Dendermonde,Temse and Hemiksem was, respectively 0.58 ± 0.23 mol g"1 (±95% confidence interval), 0.68 ±0.15 mol g"1 and 0.71 ± 0.15 mol g"1. Although BSiUv/ DiatChla seemed to increase fromDendermonde to Hemiksem,it was not significantlydifferent from one station to another (Mest for equality: 0.56 < p < Fig. 5 BSi associatedwith living diatoms (BSi|iv) as a fraction 0.76). The average BSiUv/DiatChla ratio of 0.67 ± of thetotal BSi concentration(error bars: ±standarddeviation, 0.11 mol g"1 estimated when lumping togetherthe fourmeasurements per monthin general).The BSi|iv/DiatChla to data fromMay to October at the three stations was ratios used for "Spring" (frommid-March end-April)and forthe periodfrom May to November2003 were respectively not significantlydifferent from those found at each 0.20 mol g"1 and 0.67 mol g"1 (see textfor details) station either (Mest for equality: 0.34 < p < 0.99). There was a good correlation between the BSi measured and that estimated by the regression spring diatom community(r2 = 0.88, n = 15). At (r2 = 0.71, n = 75). Even, if the high-BSi data from Ghent, most of the BSi was associated with living 8 September 2003 at Dendermonde (Fig. 4g) was diatoms, whereas most of the BSi was present as excluded from the regressions, BSiiiv/DiatChla BSidctin the tidal river(Fig. 5). increased to 0.66 ±0.16 mol g"1 at Dendermonde (Mest forequality withBSi|iv/DiatChla at Temse and Silica budget duringthe productiveperiod Hemiksem: p = 0.90 and 0.75, respectively). A better correlation between the measured and esti- In order to assess the relative importanceof DSi, mated BSi was then obtained when data from the BSiiiv and BSidet fluxesduring the productiveperiod three stations were lumped together (r2 = 0.83, (May to October 2003, 184 days), a silica budgetwas n = 74) but the BSiUv/DiatChlaratio did not change established (Fig. 6). The transfersbetween DSi, significantly(0.68 ± 0.07 mol g"1, Mest forequality BSiuv and BSidet were also investigatedto quantify with BSiiiv/DiatChiaat Dendermonde:p = 0.73). diatom production and mortalityas well as BSi BSiiiv concentrationsat the threeestuarine stations settling. during the productive period were thereforedeter- The Scheldt tidal river received 260 Mmol of mined using a single BSiUv/DiatChla ratio of silica (DSi + BSi) during the productive period in 0.67 mol g"1. The BSiav fraction showed a high 2003, predominantlyin the dissolved form:total BSi variabilitywhen calculated on a weekly basis, but inputsto the tidal riveraccounted foronly 3% of this was higher at Dendermonde and Temse than at amount, mostly originatingfrom the upper Scheldt Hemiksem as a general pattern(except in August) river.The contributionof the Rupel tributariesto the (Fig. 5). About 75% of the measured BSi at Dend- total DSi inputwas 63%. ermonde and Temse from May to July could be In Zone la, there was a DSi uptake (40 Mmol) attributedto living diatoms, while only 50% at correspondingto nearly half of the DSi input flux Hemiksem. From August onwards,the BSi|iv fraction (Fig. 6). Most of it remained in the BSiliv pool and decreased at Dendermondeand Temse (but not lower only one-fourthwas transferredto the BSidetpool. No than 50% at Dendermonde), while it reached its settlingbut rathera resuspensionof a small amount maximum in August at Hemiksem and decreased of BSi occurredin thiszone. In Zone lb, the opposite later to a value as low as 16% in October. was observed: therewas littleDSi uptakebut mostof A similar multiple regression applied on lumped the BSi produced in Zone la settled in Zone lb data frommid-March to end-May at Ghent and from (37 Mmol). Our data did not allow us to distinguish mid-March to end-April at Dendermonde yielded a between BSi deposited as BSiuv or BSidet. Indeed, BSiUv/DiatChkiratio of 0.20 ± 0.06 mol g"1 forthe settlingof living diatoms could not be excluded. This

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Fig. 6 Silica mass-balance (in Mmol) for the Scheldt tidal difference,for each species, betweenthe inputsand outputsin river over one productive period (May to October 2003, each zone. BSi|iv fluxes at Ghent and in the Dender were 184 days). Arrowthickness is proportionalto theflux intensity. derivedfrom BSiUv concentrations estimated by multiplelinear Horizontalarrows indicate DSi or BSi fluxesat each station. regressions(see text). One should bear in mind that no Lateral inputsfor Zone la and 2 are notexplicitly represented discriminationwas possiblebetween BSi depositedas BSi|ivor but redistributedamong the tributaries(see text).For Zone lb, as BSidet,as indicated by the broken dashed line between thecontribution of thelateral inputs is indicatedin parentheses. "diatommortality" and "BSi deposition"(see text) Verticalarrows indicate productions or losses calculatedas the

is indicated by the dotted line between the "diatom deposited amounted to 61% of the BSi produced mortality"and the "BSi deposition" arrowsin Fig. 6. (estimated from DSi uptake). Despite a higher Despite the settlingof BSi in Zone lb, the proportion amount of BSi deposited, the retentionof silica in between BSiiiv and BSidet was similar at Dender- Zone 2 (20%) was lower thanin Zone 1 (33%) due to monde and at Temse. Overall, duringthe productive higherDSi inputsin Zone 2. At the outletof the tidal period in Zone 1, therewas a BSi loss corresponding river,BSi accounted for 28% of the total silica pool to 71% of the DSi uptake and a retentionof 33% of (DSi + BSi). the total amount of silica inputsto the zone. Overall in the entiretidal river,120 Mmol of DSi Zone 2 received an importantamount of DSi (206 were consumed, fromwhich 65% were deposited as Mmol at Temse) but ten times less BSi despite the BSi, leading to a 30% retentionof the totalamount of importantBSi production in Zone 1. Although the silica that entered the Scheldt tidal riverduring the water surface area is similar in both zones (about productive period. If compared to silica inputs,the 7 x 106 m2, Table 1), the DSi uptake, diatom mor- relativeDSi uptakeand silica retentionwere higherin talityand BSi deposition in Zone 2 were all higher Zone 1 than in Zone 2. However the opposite was than in Zone 1. DSi uptake in Zone 2 amounted to observed when consideringabsolute amountsof both 74 Mmol. Most of it was transferredto the BSidet DSi uptake and BSi loss: about 60% of the overall pool or deposited in this zone; the amount of BSi uptake and deposition occurred in Zone 2.

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Fig. 8 WeeklyDSi or BSi fluxesat Hemiksemfrom February 2003 to February2004. DSi input in the tidal riveris also indicated

Fig. 7 Three-week moving average of weekly DSi uptake fluxes(in kmol day"1) in Zone la and Zone 2 fromMarch to Table 3 DSi and BSi fluxes(in Mmol) fromthe tributaries November2003 (the 3-week movingaverages were performed and at Hemiksem,for the periodand at an annual to smoothout artefactspossibly induced by thewater residence productive timescale timebetween adjacent stations). Uptake fluxes are indicatedby positive values to allow the comparisonwith the DSi input Tributaries Hemiksem DSi Silica fluxes to the zone andrdashed The latter (dotted lines). consumption retention correspondto thepotential maximum for DSi uptakein thezone (%) (%)

Productiveperiod (1 May 2003-30 October2003, 184 days) DSi 252 132 48 30 Temporal evolution of the DSi uptake BSi 8.4 51 In both zones, DSi uptake startedin May and lasted Annual fluxes(7 February2003-6 February2004) untilOctober/November (Fig. 7). In May and begin- DSi 713 612 14 6 ningof June,DSi uptake increased identicallyin both BSi 21 80 in At zones reaching500 kmol day"1 mid-June. that DSi consumptionand total silica (DSi + BSi) retentionfor point,DSi uptakestarted to decrease in Zone la while both periodsare also given it continuedto increase in Zone 2 reachinga value of about 800 kmol at the end of June.DSi day"1 uptake duringthe productive period (6 months)was two times decreased in both zones from untilthe end of the July smallerthan that discharged during the rest of theyear. productiveperiod, more or less at the same rate,but it DSi concentrationsin the tributariesdid not exhibit in Zone 2 than in Zone la always stayed higher by importanttemporal variations (Fig. 4a-f), butfluxes of about 250 kmol day"1. riverine DSi delivered to the tidal river showed a If DSi is consumed, DSi completely uptake equals minimumin summerbecause of the lower discharge DSi fluxes.Thus, the DSi fluxcan be seen input input (Fig. 8). At theoutlet of thetidal river near Hemiksem, as a potentialmaximum for DSi uptake.This potential the annual variations in the DSi flux were further maximumwas in Zone 2 thanin Zone 1 and the higher enhancedby theeffect of diatomuptake superimposed observed DSi was from uptake complete mid-July to thatof the decreasing riverdischarge. Conversely, untilthe end of in Zone la, but at the * September only seasonal variations in discharge flattenedthe BSi end of and of in Zone 2. July beginning August fluxespattern, which ranged from100 kmol day"1 in spring/autumn/winterto 400 kmol day"1 in summer. Annual fluxesdischarged to the brackishestuary The annual DSi uptake and silica retentioncorre- spondedto 14 and 6% of totalinputs, respectively. This During our 1-year study period (7 February2003-6 is 3-5 times smaller thanthe estimatescharacterizing February 2004), the amount of water discharged the productiveperiod (Table 3).

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Discussion contributionof diatombiomass whichwas already representedin the multipleregression by the other Distinctionbetween BSinv and BSidet explicativevariable, DiatChk (Eqs. 4, 5). Due to the importantfraction of non-phytoplanktonorganic In orderto betterinvestigate the dynamics of BSi, a matterin theSPM of theScheldt tidal river (Hellings multiplelinear regression was performedwith the et al. 1999),these two factors could not be estimated explicativevariables DiatChltf and SPMnbid(defined fromour datasetand averageconstant ratios were inEq. 4) to distinguishbetween BSi associatedor not takenfrom the literature. In particular,jSpoc/chia may withliving diatoms. As thetwo explicative variables displayimportant variations (Geider 1987). However, werevery poorly correlated (r2 = 0.02, n = 75), the althougha variationin these ratioscan affectthe discriminationcould be regardedas robust.Admit- estimationof theBSiIiv/DiatChla ratio (kx in Eq. 5), tedly,it could not be excludedthat empty frustules theirinfluence was not expectedto be important. followedthe distribution of livingdiatoms, yielding UsingEq. 4, Eq. 5 can indeedbe rewrittenas: an overestimationof theBSiiiv/DiatChla ratio. How- B§i = DiatChla+ k2 ever,BSiuv/DiatChlfl was identicalat Dendermonde, (jfcj-k2-P)- •(SPM-BSi-MBSio2) (6) Temse and Hemiksem(see "Results"),despite the facts that amountsof BSi settledbetween = • large Withft ^poc/Chla0OM/POC Dendermondeand Temse (Fig. 6) and mortality In thisstudy, /? was equal to 75. The use ofanother increasedat Hemiksem(Fig. 5). p valuewould indeed induce a differentku butk2 and The use of the linearmultiple regression model thecorrelation between the measured BSi concentra- impliedthat BSiiiv was calculatedusing an average, tionsand thoseestimated by the regression would not constantBSiiiV/DiatChl# ratio. However, silica and/or be altered.Furthermore, as k2 was 0.48 mmolg"1 chlorophylla contentsin diatoms are known to whenn = 75 (k2 = 0.42 whenn = 74, cf. above), exhibitimportant variations depending on nutrient therelative variation would be 19 timessmaller for k\ availability,temperature and lightconditions (Geider thanfor /?. 1987; Ragueneauet al. 2000; Martin-Jezequelet al. In addition,using jSpoc/chim our BSinv/DiatChla 2000; Hildebrand2002). Due to thevariations in DSi ratiowould correspond to a Si/Cmolar ratio in living concentrations(Fig. 4g-i), light and temperature diatomsof 0.27 (± 0.04), whichwould fall in the duringthe productiveperiod, diatom cells in the rangeobserved in Cyclotellameneghiniana cultures Scheldtmay thus not have had a constantcontent of (0.38, Sicko-Goadet al. 1984; 0.12-0.30 forseveral silica and of chlorophylla. But in our study,the culturesof two Cyclotella sp. strainsisolated from the BSiiiv/DiatChk*ratio was not intendedto be esti- Scheldt,Carbonnel 2009). matedat the cellularscale (whichwould not have beenpossible from our dataset), but at thesystem and seasonalscales, where the variations of themeasured Phytolithcontribution to theBSi pool BSi concentrationswere expected to dependmore on thepresence of detritalBSi thanon the variationof BSi is notproduced by diatomsonly. Higher plants the chlorophylla and BSi contentsof the diatoms. can also take up DSi and formBSi particlescalled The multipleregression model, although very simple, phytoliths(Conley 2002), which can contribute reproducedindeed well the variationsin BSi con- significantlyto the BSi pool in rivers(Cary et al. centrationseven if thedata fromthe three estuarine 2005). The Scheldttidal river is connectedto 4.5 km2 stations were lumped together:the correlation ofmarshes, where vegetation and sedimentsrepresent betweenestimated and measuredBSi was good large reservoirsof BSi as phytoliths(Struyf et al. (r2 = 0.71 if n = 75, or r2= 0.83 if n = 74, see 2005). At each tide,significant amounts of DSi and "Results")and the 95% confidenceinterval associ- BSi are exchangedbetween the rivermain channel atedwith the BSiiiv/DiatChltf ratio was rathernarrow and these adjacent marshes(Struyf et al. 2006). (±16% if n = 75, or ±1 1% if n = 74). Phytolithsmay therefore be presentin watersamples The Ppoc/chiaand /?om/pocconversion factors takenin the mainriver. Most of the measuredBSi wereused to subtractthe SPM concentrationfrom the was attributedto living diatoms (Fig. 5) and

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This content downloaded from 193.191.134.1 on Thu, 16 Jul 2015 12:07:32 UTC All use subject to JSTOR Terms and Conditions Biogeochemistry(2009) 96:49-72 63 phytolithswould be included in the BSidet pool, as higherplants do not contain the pigmentsthat were used as markersfor diatoms (Lionard et al. 2008a). Unfortunately,no microscopic investigation was performedto partitionBSide, (diatom frustulesvs. phytoliths),but several indications suggest that the contributionof phytolithsis not significant. Among the vegetation borderingthe tidal river, reeds have by far the highest silica content (Struyf et al. 2005): 7% of Si per dry weight in dead shoots, but,due to the factthat phytoliths dissolve fasterthan plant tissues, the maximum silica content found in reed litter is 4% (Struyf et al. 2005, 2007a). Consideringa marsh surface area of 4.5 km2 (Struyf et al. 2005) occupied at 55% by reeds (Soetaert et al. 9 contributionsof and 2004), it would produce annually detritusamounting Fig. Averagemonthly BSidet phytoliths to theBSi pool at thethree estuarine stations in "Spring"(from to 134 Mmol of carbon (Soetaert et al. 2004). With a mid-Marchto end-April)and fromMay to November2003. 4% Si contentper dry weight,this would correspond The contributionof phytolithswas calculated assuminga Si to 6.1 Mmol of BSi as phytoliths.Given a 100% contentof 4% dryweight and a POC fromvegetation and soil to 23% of annual turnover of the aboveground vegetation corresponding POCn|d(see text) (Soetaert et al. 2004), this is in agreementwith the maximum estimate of BSi stock in aboveground amount of phytolithsestimated from such a POC biomass (Struyfet al. 2005). This amount is small fractionwould only contributeto about 14 ± 10% of compared to the BSi fluxes during the productive the BSide, or 5 ± 3% of the total BSi pool duringthe period (Fig. 6) and cannot account forthe increase of productiveperiod, even with a Si contentof 4% per BSidet (26 Mmol for the entire tidal river, Fig. 6). dryweight as foundin reed litter(Fig. 9). This content Furthermore,for all tidal cycles investigated by is indeed in the high range of the average values Struyfet al. (2006), the net transportof BSi was generally observed (1-3%; Conley 2002; Blecker always fromthe riverchannel to the marsh,and there et al. 2006). It is however acknowledged thatphyto- were six times more diatom frustulesthan phytoliths liths may become an important,if not the major, in marshsediments (Struyf et al. 2005). Marshes thus constituentof the BSi pool in the winterseason due to appear to be, at firstsight, rather a sink forestuarine lower diatom production but higher litter fall and BSi than a source of phytolithsto the main channel. precipitations(which enhance the soil erosion). Reed detritusare neverthelesscommon in the SPM of the Scheldt tidal river (Lionard et al. 2008a) and Accuracy and precisionof the DSi and BSi fluxes phytolithscould be broughtby soil erosion. Phyto- and mass-balance calculation liths distributionin soils is closely related to thatof soil organic matter(Alexandre et al. 1997; Blecker A source of imprecisionin our mass-balance calcu- et al. 2006) and organic matterdecomposes faster lation may arise fromthe factthat integrated seasonal thanthe phytolithsin immergedreed detritus(Struyf fluxes were estimatedfrom discrete weekly DSi and et al. 2007a). The organic matter from soil and BSi measurements.Selecting a methodfor evaluating vegetationwas thus considered as an indicatorof the annual riverinefluxes fromsuch concentrationdata- possible presence of phytoliths.Abril et al. (2002) sets is a controversialsubject (Kronvang and Pruhn estimated this organic matterfraction to amount to 1996; Moatar and Meybeck 2005, 2007). Kronvang 23% of the non-phytoplanktonPOC in the tributaries; and Bruhn (1996) and Moatar and Meybeck (2005) in the tidal river,a similar contentwas supposed for tested several methodswith differentsampling strat- the PCX! not corresponding to living diatoms egies on high frequencydatasets. They found that (POCnid, estimated by difference using POC and inaccuracy and imprecisioncould respectivelyreach DiatChla data and /Wr/chi*= 30). However, the 20 and 100%. Unfortunately,no high frequency

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This content downloaded from 193.191.134.1 on Thu, 16 Jul 2015 12:07:32 UTC All use subject to JSTOR Terms and Conditions 64 Biogeochemistry(2009) 96:49-72 dataset was available in the Scheldt to test the Anothersource of imprecisionin the budgetshown accuracy of our integrationmethod. Nevertheless, the in Fig. 6 is linked to the estimationof DSi and BSi values reported in Fig. 6 differ by less than 2% concentrationsin lateral inputs,which were assumed compared to the fluxes estimated when the often identicalto those foundin the adjacent rivers.Lateral recommended "linear integration"method (Kronv- inputswere negligible forBSi but significantfor DSi ang and Bruhn 1996; Moatar and Meybeck 2005) was (Fig. 6). This is in line withthe observationthat 80% applied to our data. Additionally,following Moatar of the lateral DSi input fluxes are throughprecipita- et al. (2006) and Moatar and Meybeck (2007), all DSi tionand run-offin the brackishestuary (Soetaert et al. and BSi fluxesprovided by this lattermethod would 2006) and that a similar origin may be assumed for be accurate (less than 0.3% deviation) but with a the tidal river. The rest of the lateral input fluxes precision of about 10-20% (flux weighted average: comes fromindustrial and domestic wastes, in which 12 and 8% for DSi and BSi, respectively).As both the DSi concentration is twice higher than that methods led to similar results, it could thus be measured in the tributaries(Soetaert et al. 2006). assumed thatthe values forDSi and BSi fluxesshown Therefore,with lateral inputs contributing 10% of the in Fig. 6 were calculated with similar precisions. total freshwaterdischarge and assuming that DSi However, a lower precision can be expected forBSi concentrationsin runoffwaters are similar to those due to analyticalerrors (10%, Ragueneau et al. 2005), measured in the tributaries,taking into account the and above all to the complex SPM dynamics at the higherDSi concentrationsin wastes would lead to an tidal scale (Chen et al. 2005a), which cannot be extra DSi input which would amount to only 1% of resolved by weekly sampling. Note thatthe partition the total DSi inputs to the tidal river.However, one between BSiiiv and BSidet,which was discussed in a could consider thatthe origin of the lateral inputsto previous section, influences the precision of the the tributarieswas similar to those of the tidal river, "diatom mortality"fluxes only, but not the "BSi so thatDSi concentrationsmeasured in the tributaries deposition" fluxes.As a result,with such an overall already reflect the contributionof industrial and precision,the BSi resuspensionin Zone la and the domestic wastes. DSi uptakein Zone lb do not appear to be significant Withrespect to thenature of thesoils in theScheldt as they could result from the imprecision in the basin, groundwaterinputs are not expected to play a determinationof the fluxes. significantrole either(Jacobs et al. 2008). Delstanche Despite low BSi concentrations and low BSi (2004) gathered206 values of DSi concentrationsin contentsin the SPM (annual averages rangingfrom groundwaterand firstorder streams in the Belgian 1.3 to 3.2%), the methodused forBSi determination watershedof the Scheldt. Althoughthe values ranged was consideredas applicable in the case of the Dijle from10 to 920 jimol L~\ 80% of the measurements and the Grote and Kleine Nete. BSi and DiatChla were between 100 and 400 |imol L~\ with an aver- concentrationsin these rivers displayed indeed con- age value of 270 |imol L"1, similar to the level comitantvariations (Fig. 4d-f) although correlation observed in the main tributariesof the Scheldt. coefficients were low (0.11 < r2 < 0.56, 13 < n < 15). On the contrary,Eq. 3 gave negative correla- Importanceof BSi dissolutionand DSi recycling tionfor the Zenne (data notshown) probablydue to the presenceof particulatematerial brought by the(at that The mass-balance estimationsshown in Fig. 6 give time still) untreated wastewater from the city of only an overall picture of the processes occurring Brussels. The Zenne howevercontributes to less than between the sampling stations. An internalcycle of 10% of the total water discharge. Also, BSi concen- silica comprising dissolution, uptake and settling trations in the tributariesof the Rupel were not cannot be excluded and would lead to a highergross measured weekly, but weekly fluxeswere calculated DSi uptake and BSi settling. Dissolution could fromlinear regressionswith DiatChla despite weak potentiallytake place in the sediments but also in correlations.Thus a higherlevel of uncertaintycan be the water column as it contains a significantamount expectedfor the BSi inputflux to theRupel. However, of BSidet(Figs. 5, 6). theimportance of thisflux is limitedwhen compared to Roubeix et al. (2008) measured a BSi specific theother fluxes in Fig. 6. dissolution rate of 0.084 day"1 in cultures of

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C. meneghinianaand bacteria originatingboth from Regnier 2007). Considering that this amount was the Scheldttidal river.However, BSi dissolutionrates calculated witha BSi dissolutionrate 25 timeshigher can be lowered by the incorporationof aluminiumin than what would be expected forBSi containing 1% the BSi matrix(Van Cappellen et al. 2002). Roubeix aluminium,it can be concluded thatBSi dissolutionis et al. (2008) did not investigate aluminium, but not expected to play a significantrole in the silica concentrationsof 1-2 umol L"1 were measured in dynamics within the tidal river during the selected February2003 in the tidal river(Carbonnel 2009). At studyperiod. aluminiumconcentrations higher than 200 nmol L"1, diatoms may build frustuleswith an Al/Si atomic DSi uptake in Zones 1 and 2 and influence ratio of 0.01 (Van Beusekom 1991, cited in Van of the freshwaterdischarge Cappellen et al. 2002). Such a ratio would resultin a BSi dissolutionrate as low as 0.001 day"1, about 70 The high DSi uptake in Zone 2 (Fig. 6) revealed an times lower than the rate observed for frustulesof importantdiatom growth, although this was not diatoms grown in aluminium-poor medium when expected because of short water residence times in normalised to specific surface area (Van Cappellen the Rupel and its tributaries(Table 1) and of possible et al. 2002). Even higherAl/Si ratiosmay be reached light limitationnear and downstreamof the conflu- by Al incorporationin the BSi matrixafter diatom ence with the Rupel. Indeed, the section of the death (Van Cappellen et al. 2002). As a result,the Scheldt comprised between Temse and Hemiksem specific dissolution rate can be expected to be, at contributesto roughlyhalf of the water surface area least, as low as 0.0012 day"1 in the Scheldt tidal and of the volume of Zone 2 (Fig. 1; Table 1). But river. Less than 1-2% of the BSi would then be near the mouthof the Rupel, the presence of a water dissolved consideringthe average residence times of energymaximum results in high SPM concentrations waterpresented in Table 1. and a longer residence time of riverbornematerial Struyfet al. (2006) foundthat marshes act as silica (Chen et al. 2005a), and water column deepens recyclersand thatthe fluxof DSi exported fromthe downstream of the confluence (Table 1; Muylaert 4.5 km2 of marshesbordering the Scheldt tidal river et al. 2005). However, diatom growth might have may exceed the fluxentering the river systemwhen occurredbetween Temse and the mouthof the Rupel, DSi concentrationand freshwaterdischarge are low. where the water depth is still shallow and where the Indeed, the average DSi fluxat Dendermondewas as diatom-richbut DSi-depleted Scheldt water receives low as 6 kmol day"1 at theend Julyand beginningof DSi from the Rupel due to tidal mixing. A diatom August 2003 (this study). However, this recyclingis bloom might have additionally occurred in the of minor importancecompared to the riverineDSi shallow Nete (Table 1) because of exceptionally inputs during the productive period (Fig. 6): by low water discharges during summer 2003 extrapolation, recycling would amount to (<30 m3 s"1). A local maximum in diatom produc- 8 ± 3 Mmol of DSi according to the rates measured tion in the lower part of the Nete was indeed by Struyfet al. (2006) duringthe productive period in predicted by the model simulation of Arndt et al. 2002 and 2003 in a marshlocated close to the mouth (2007) during summer 2003 (S. Arndt, personal of the Durme (Fig. 1). Using model simulations, communication).In August 1995, when discharge in Arndtand Regnier (2007) estimatedthat, during the the Scheldt tidal riverwas similarto thatin 2003, the productiveperiod in 2003, the highestrecycling rates diatom biomass in the Nete was also among the occurredindeed in Zone lb. They foundthat riverine highestof the tidal river(Muylaert et al. 1997). inputsand silica recyclingcould be of the same order The comparisonof the DSi consumptionin Zone 1 of magnitude,but only in mid-August 2003. Taking and 2 illustratedthe influenceof the dischargeon the spatial heterogeneityinto account, they however diatom production:a higherdischarge in Zone 2 than estimatedthat only 2 Mmol of DSi would be recycled in Zone 1 resulted in higher DSi inputs and also a between Ghent and the mouthof the Rupel between higherdiatom productionin Zone 2 (Fig. 6). This can Juneand November. As a result,less than 1% of the be explained by the fact that DSi was entirely overall diatom productionover the productiveperiod consumed fromJune to Septemberin Zone 1a (except could be sustained by recycled DSi (Arndt and however for periods following the discharge peaks,

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Fig. 4g), whereas this was the case only at the end stayed constant at about 230 nmol L '.At highest Julyand beginningof August in Zone 2 (Fig. 4i). In summer discharges, the DSi concentrationscould both zones, DSi concentrationscould drop below even reach winterDSi concentrations.Accordingly, 5 nmol L"1. Such low levels could limit the diatom Van Damme et al. (2005) reportedthat almost no DSi production, as half-saturationconstants for the was consumed in the tidal river duringthe wet and growthof two strainsof Cyclotella sp. isolated from mostlyovercast summer2000. Lionard et al. (2008b) the Scheldt were found to be 4 and 13 nmol L~l of found a negative correlation between the summer DSi (K. Muylaert, unpublished data). During these phytoplanktonbiomass and the summerdischarge for periods, DSi uptake corresponded thus to the DSi the period 1996-2004. The summerDSi concentra- inputsto the zones and, in particular,a minimumof tions also decreased with increasingsummer phyto- DSi uptake in Zone la in August was induced by a planktonbiomass, except however forthe years 1996 low water discharge (Fig. 7). The higherDSi inputs and 1999 (Lionard et al. 2008b). This highlightsthe in Zone 2 as compared with Zone la resulted in a fact thatparameters other than discharge may play a shorterperiod of potentialDSi limitationin Zone 2. role in regulating inter-annual variations of the Also, duringthe periods of potentialDSi limitation, summerDSi concentrations. the DSi uptake rate in Zone 2 was limitedat a higher At a seasonal timescale and for the entire tidal rate than in Zone la (Figs. 6, 7). A low water river,a decreasing trendcould be observed between discharge can thus induce a low rate of diatom DSi uptakeand discharge,but the relationshipdid not productiondue to DSi limitation.In contrast,it has appear stronglylinear and, above all, highlightedthe already been shown that a high discharge may fact thatDSi uptake dropped from 100 to 30% from hamper diatom growthby flushingthe diatoms out August to October even thoughthe 3-week moving of the zone (Muylaert et al. 2001, 2005; Arndtet al. average discharge stayed between 40 and 60 m3 s"1 2007). If all factorsinfluencing diatom growthother (Fig. 10). This suggests that discharge was not the than silica availability and discharge are kept con- only controllingparameter for DSi uptakein the tidal stant,an optimal dischargecan be foundat which the riverat this timescale. As in many estuarinesystems, highest diatom productionis reached, as shown by lightavailability is a major factorinfluencing phyto- model simulations carried out on Zone 1 during planktonproductivity in the Scheldt (Muylaertet al. summer2003 by Arndtet al. (2007). This phenom- 2005; Amdt et al. 2007). The seasonal variationof enon was also observed on an inter-annualtimescale light during the productive period in 2003 was by Petersonet al. (1985) in the San Francisco estuary. uncorrelatedto discharge (3-week moving average The summerof 2003 was exceptionallydry, warm data: r2 = 0.06, n = 26) and the drop in DSi uptake and sunny(IRMB 2003-2004) and duringthis period fromAugust to October could actually be attributed the Scheldt discharge was the lowest for the decade to the decrease of the incident light (IRMB 2003- 1996-2005 (56m3s~l, average 1996-2005: 2004). When the multipleregression was performed 77 m3 s"1). DSi uptake and silica retentionreached between DSi uptake and the explicative variables high values: 48 and 30%, respectively during the discharge and light,the correlationcoefficient (r2 = productive period (Table 3). This is comparable to 0.80, n = 26) was significantlyimproved compared to the DSi retentionestimated by Gamier et al. (2002) that of the regression shown in Fig. 10 and the for the Seine tidal riverduring the summerof a dry residuals of the regression did not exhibit any year (47% in 1993). Summer diatom productionand significantpattern. silica uptake are however expected to vary from 1 Parameters other than discharge, light and DSi year to another: the percentage of DSi consumed availabilitymight have influenceddiatom growth and should indeed decrease withincreasing discharge and thus DSi uptake. Limitationby nutrientsother than decreasing water residence times, as observed by DSi could be ruled out in the tidal riveras dissolved Gamier et al. (2002) in the Seine at an inter-annual nitrogenand phosphoruswere in high concentrations timescale. For the period 1996-2000, Struyfet al. duringthe studyperiod (Van der Zee et al. 2007). The (2004) also observed increasing summer DSi con- diatom productivityshould not have been controlled centrationswith increasing discharge in the Scheldt at by the zooplankton community,which is dominated Dendermonde, whereas winter DSi concentrations by rotifersin the Scheldt tidal river(Muylaert et al.

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Rupel than downstream,in the zone of maximumof turbidity(Chen et al. 2005a). These processes could not be considered in our study due to the sampling frequency and to the fact that only surface water samples were taken. The longitudinaldistribution of the BSi deposition over Zone 2 could not be assessed due to the absence of an intermediatesampling station.In Zone 1, DSi uptake and BSi deposition occurred in differentsub- zones (Fig. 6). Arndt and Regnier (2007) indeed predictedthat BSi depositionshould occur in Zone lb throughoutthe productiveperiod, which is in agree- mentwith our budget.These authorsalso foundthat a smaller amount of BSi would be deposited in Zone la, but only in August when diatom productionwas Fig. 10 DSi uptake (%) in the entire tidal river in 2003, shifted upstream of Dendermonde due to silica estimatedfrom DSi fluxesin the tributariesand at Hemiksem, limitation.Furthermore, Arndt et al. (2007) predicted versus discharge (m3 s~'). All values are 3-week moving that the maximum diatom production should be average located in Zone lb in June because of lower SPM concentrations.In contrast,our findingsrather sug- 2005 and reference therein). The phytoplankton gest that the diatoms bloomed at Dendermonde or biomass can have however a strong influence on furtherupstream (Zone la) duringthe whole produc- phytoplanktonproductivity (Muylaert et al. 2005). tive period and were subsequentlytransported down- The 3-week moving average of the biomass at stream.DiatChla was always higherat Dendermonde Dendermonde,Temse and Hemiksem correlatedwell thanat Temse. Except maybe duringthe firstweek of with the percentage of DSi consumed (r2 = 0.81, July because of a discharge peak, DSi was already n = 26). But, biomass also depends on light and completely consumed in June at Dendermonde discharge in the Scheldt tidal river (multiple linear (Fig. 4g, h). In October, the DSi uptake nevertheless regression:r2 = 0.74, n = 26) and its effectsshould occurred in Zone lb (Fig. 4g, h). thereforealready be implicitly included when the Deposition in Zone la cannot however be influenceof light is taken into account. excluded. SPM dynamics are driven by a complex hydrodynamic energy pattern determined by the Spatial distributionof the BSi deposition: convergence of the decreasing energies of riverine influenceof the SPM dynamics and marine origins,and by the channel morphology (Chen et al. 2005a; Arndtet al. 2007). Althoughthere The absence of correlation between DiatChla and is a net downstreamtransport of SPM in the upstream SPMnbld (? = 0.02, n = 75) tends to prove that part of the tidal river, a high fluvial hydrodynamic living diatoms do not follow the SPM dynamics. energyinduces a turbiditymaximum in Zone 1a, and However, the multiple regression between BSi and a local energy minimum is observed in Zone lb the two aforementionedindependent variables pro- (Chen et al. 2005a; Arndtet al. 2007). Despite a high vided a strongercorrelation (r2 = 0.71, n = 75) than temporal variability, the average measured SPM that between BSi and DiatChla only (r2 = 0.51, concentrations was indeed significantlyhigher at n = 75). It indicates thatBSi and SPM dynamics are Dendermonde (97 ± 61 mg L~\ mean ± SD)\han nevertheless linked to some extent. As a conse- at Temse (69 ± 71 mg L~\ r-test for equality: quence, BSi concentrationsin theestuary may exhibit p = 0.08) or in the rivers Scheldt (32 ± 9 mg L"1, vertical gradients, tidal deposition/resuspension p = 3x 10"8) and Dender (15 ± 8 mg L~\ p = cycles and tidal variations as it is the case for 10" H). The presence of a turbiditymaximum at or estuarineSPM, even if this SPM heterogeneityis less upstreamof Dendermondeindicates an increasedresi- pronounced upstream of the confluence with the dence time leading to the retentionor accumulationof

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This content downloaded from 193.191.134.1 on Thu, 16 Jul 2015 12:07:32 UTC All use subject to JSTOR Terms and Conditions 68 Biogeochemistry(2009) 96:49-72 suspendedparticles in thisarea. A consequence of the productiveperiod. This would suggest that most of BSi accumulationin Zone la is thatboth the BSi flux the 77 Mmol of BSi deposition would have taken at Dendermondeand the deposition in Zone lb may place in other areas such as shallow parts of the have been lower than what we estimated (Fig. 6). stream channel, mudflats or at the marsh edges. Nevertheless,this would have alteredonly the spatial Mudflats representabout 39% of the length of the repartitionof thedeposition of BSi withinZone 1, but tidal riverbanks, whereas marshes account for 32% not the estimationof its overall amount. (Meire et al. 2005). However, BSi deposited on The importantdeposition estimated for Zone lb is mudflatsor at marshedges mightbe re-suspendedin supported by the presence of the hydrodynamic winter by higher water currents and re-deposited energy minimum, which induces lower particle furtherinland in marshes. Together with higher concentrations(Chen et al. 2005a; Arndt et al. winter SPM deposition (Temmerman et al. 2003), 2007). Additionally,deposition in Zone lb may have winter BSi deposition rates in marshes are indeed been enhanced by the deepening and wideningof the higherthan the summerones (Struyfet al. 2007b). channel and by the increase of the residual current (Table 1). A decoupling between BSi and the rest of the SPM could however be observed at this stage as Silica fluxes at the annual timescale the average BSi contentduring the productiveperiod was significantlyhigher at Dendermonde (10.5 ± In 2003, almost 80% of the annual amount of DSi 7.0%, mean ± SD) than at Temse (6.0 ± 2.8%) was delivered to the brackish estuary outside the (Mest for equality: p = 0.004). This suggests a productive period (November-April, Table 3). preferentialsettling of BSi compared to non-BSi Despite high BSi concentrationsin the tidal river SPM in Zone lb. This may be due to the factthat the (Fig. 4), almost 90% of the silica dischargedannually SPM in the Scheldt tidal riveris composed of more to the brackish estuary was in the form of DSi thanhalf of clay material(<4 urn;Chen et al. 2005b), (Table 3). Diatom productionoccurred when silica which are finer than diatom frustules and thus inputfluxes to the estuarywere at theirlowest values expected to remain in suspension for a longer time (Fig. 8). As a result,annual silica retentionand DSi (cultures of C. meneghiniana isolated from the uptake were less than foreseenwhen only concentra- Scheldt exhibited cell sizes ranging from 10 to tions were examined (Fig. 4i) or when only the 25 |^m, V. Roubeix, personal communciation). In productiveperiod was investigated(Fig. 6; Table 3). addition, the presence of organic material around Outside the productive period, sampling was diatoms would enhance their aggregationinto floes performedat a monthlyfrequency. Following Moatar and increase theirsinking rate (Chen et al. 2005b). et al. (2006) and Moatar and Meybeck (2007), this Most of theBSi depositionshould occur in shallow would lead to an imprecision of about 30% on all areas, such as tidal mudflats, tidal marshes and annual DSi and BSi fluxes. Such an imprecision shallow parts of the river section, because of lower would preclude the comparison of the net annual currentsthan in the main streamchannel (Arndtand amountsof DSi consumed and BSi produced over the Regnier 2007). Struyfet al. (2006, 2007b) measured entire tidal river with the net seasonal values summerBSi deposition rates in two marshes along (Table 3). Nevertheless,DSi displayed a conservative the Scheldt tidal river. All together, individual behaviour outside the productive period, whereas measurementsvaried withintwo ordersof magnitude. some more BSi was produced (Fig. 9; Table 3). Most The high variabilitycould be explained by the fact of this production was attributedto late diatom thatSPM deposition in marshes varies exponentially activityin November and/orresuspension during the with the maximumtidal height,the marsh elevation very high discharge event around mid-January and the distance to the creek or to the marsh edge (Figs. 2, 4g-i and 8). The spring bloom had no (Temmermanet al. 2003). If the rates for the two significanteffect on the silica cycle in the tidal river. marsheswere averaged and extrapolatedto a 4.5 km2 A mass-balance constructedfor the period frommid- marsh surface area (Struyfet al. 2005), they would March to end-April suggested that DSi was not give an overall BSi deposition of 1.3 ± 1.1 and significantlyaffected along the estuary, with only 15.0 ± 14.6 Mmol forthe entire tidal riverduring the 2 Mmol of BSi produced in Zone la and settled in

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Zone lb: thespring bloom is importedfrom the upper References Scheldtriver and cannotdevelop in the tidal river (Muylaertet al. 2000, 2005). In our data, high Abril G, Nogueira M, EtcheberH, Cabec,adasG, Lemaire E, MJ Behaviourof DiatChk concentrationswere measured in at Brogueira (2002) organiccarbon in nine spring contrastingEuropean estuaries. Estuar Coast Shelf Sci Ghentand Dendermonde, but not further downstream 54:241-262 (Fig. 4a, g-i). Admiraal W, Breugem P, Jacobs DMLHA, de Ruytervan The brackishestuary received an almostconstant SteveninckED (1990) Fixationof dissolved silicate and sedimentationof silicatein thelower river Rhine BSi flux to theDSi fluxwhich could biogenic compared vary duringdiatom blooms. Biogeochemistry9:175-185 over morethan two ordersof magnitude(Fig. 8). AlexandreA, Meunier J-D, Colin F, Koud J-M (1997) Plant Thisseasonal asymmetry in theDSi fluxesmay be of impacton thebiogeochemical cycle of silicon and related significantimportance for the silica cycle in the weathering processes. Geochim Cosmochim Acta 61(3):677-682 brackish However, theresidence estuary. considering AndersonGF (1986) Silica, diatoms and a freshwaterpro- timeof thewater in thebrackish estuary (70 days in ductivitymaximum in Atlanticcoastal plain estuaries, summer,Soetaert and Herman1995), the summer Chesapeake Bay. EstuarCoast Shelf Sci 22:183-197 DSi in thetidal river would not have affected ArndtS, RegnierP (2007) A model for the benthic-pelagic uptake of silica in estuarine themouth of the and thecoastal zone before coupling ecosystems: sensitivity estuary analysis and system scale simulation.Biogeosciences Augustof thesame year. By latesummer, blooms of 4:331-352 diatomsand Phaeocystis sp. had terminated,DSi was ArndtS, VanderborghtJ-P, Regnier P (2007) Diatom growth no and its concentrationstarted to response to physical forcing in a macrotidalestuary: longerdepleted sediment and bio- increase in thecoastal zone nearthe mouth of coupling hydrodynamics, transport, again geochemistry.J GeophysRes 112:C05045 theestuary (Van der Zee and Chou 2005; Muylaert Bidle KD, Azam F (1999) Accelerateddissolution of diatom et al. 2006). As a result,the summer removal of DSi silica by marinebacterial assemblages. Nature 397:508- in theScheldt tidal river is notexpected to have had 512 Blecker Chadwick EF an effecton the ofDSi tothe coastal SW, McCulley RL, OA, Kelly (2006) important supply Biologic cyclingof silica across a grasslandbioclimose- zone duringcoastal phytoplanktonblooms. Instead, quence. Glob Biogeochem Cycle 20:GB3023 theextent of thesupply of DSi by theScheldt to the BouezmarniM, Wollast R (2005) Geochemicalcomposition of sedimentsin the Scheldt with on trace coastal zone in spring seems to be principally estuary emphasis metals. 540(1-3): 155-168 the winterriverine DSi flux,which Hydrobiologia supportedby Carbonnel V (2009) Silica dynamics and retentionin the is actuallydriven by the winterfreshwater dis- Scheldt tidal river and estuary (Belgium/TheNether- charge because of constantwinter riverine DSi lands). Ph. D. thesis, Universite Libre de Bruxelles, concentrations. Belgium Cary L, AlexandreA, Meunier J-D, Boeglin J-L, Braun J-J (2005) Contributionof phytolithsto thesuspended load of We are very grateful to Jean-Pierre Acknowledgments biogenic silica in the Nyong basin rivers (Cameroon). for constructivediscussions and commentson Vanderborght Biogeochemistry74:101-104 the Claar van der Zee and Nathalie Roevros manuscript. Chen MS, Wartel S, Van Eck B, Van Maldegem D (2005a) commentedon versionsof the manuscript.We would previous matterin the Scheldt estuary.Hydrobiologia Nicolas Renaat Suspended also like to thankNathalie Roevros, Canu, 540(1-3^:79-104 Christianede Marneffe,Michael and Dasseville, Tsagaris Stijn Chen MS, WartelS, TemmermanS (2005b) Seasonal variation Vannestefor their assistance in field and laboratory sampling of floe characteristicson tidal flats,the Scheldt estuary. Victor isolated the strains analyses. Chepurnov Cyclotellasp. Hydrobiologia540(l-3):181-195 forthe collectionof the GhentUniversity. Data phytoplankton ConleyDJ (1997) Riverinecontribution of biogenicsilica to the on water were by the Ministryof the discharge provided oceanic silica budget.Limnol Oceanogr42(4):774-777 Flemish (Afdeling Maritieme Toegang). We Community Conley DJ (2002) Terrestrialecosystems and the global bio- would also like to thankthe threeanonymous reviewers for geochemical silica cycle. Glob Biogeochem Cycle theirconstructive comments and This was suggestions. study 16(4):68(l)-68(7) financed the Federal Science Policy Office by Belgian Conley DJ, Schelske CL, StoermerEF (1993) Modificationof under contract number EV/11/17A (SISCO). (BELSPO) the biogeochemicalcycle of silica with eutrophication. Additional BELSPO from the TIMOTHY project funding Mar Ecol Prog Ser 101:179-192 AttractionPole, IAP, P6/13) is acknowledged. (Interuniversity Conley DJ, Likens GE, Buso DC, Saccone L, Bailey SW, This is also a contributionto the EU IP CarboOcean (contract JohnsonCE (2008) Deforestationcauses increaseddis- no. 51 1 We would like to dedicatethis paper to thelate 176-2). solved silicate losses in the HubbardBrook Experimental Roland Wollast who did the pioneeringwork on the silica Forest.Glob Change Biol 14(1 1):2548-2554 biogeochemistryin the Scheldtestuary.

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