The Netherlands) from the freshwater tidal limits to the North Sea

Marine Chemistry 106 (2007) 76 – 91 www.elsevier.com/locate/marchem

Phosphorus speciation, transformation and retention in the Scheldt estuary (Belgium/The Netherlands) from the freshwater tidal limits to the North Sea Claar van der Zee ⁎, Nathalie Roevros 1 , Lei Chou 1 Laboratoire d'Océanographie Chimique et Géochimie des Eaux, Université Libre de Bruxelles, Campus de la Plaine, CP 208, Boulevard du Triomphe, B-1050 Brussels, Belgium Received 22 February 2006; received in revised form 5 January 2007; accepted 5 January 2007 Available online 21 January 2007

Abstract Nutrient delivery to the Belgian coastal zone is to a great extent through the Scheldt estuary. We have measured the phosphate (PO4), poly-phosphate (poly-P), dissolved organic phosphorus (DOP), particulate inorganic phosphorus (PIP) and particulate organic phosphorus (POP) concentrations at the freshwater tidal limits of the Scheldt estuary, along the salinity gradient and in the coastal zone during one year. In addition, nitrate (NO3), nitrite (NO2), ammonium (NH4), dissolved organic nitrogen (DON) and particulate nitrogen (PN) were determined in the freshwater Scheldt sub-basin and NO3, NO2 and NH4 in the Rupel subbasin, the Scheldt estuary and the coastal zone. The behaviour of each P species along the continuum from the freshwater tidal limits to the coastal zone is discussed. Phosphorus (P) and nitrogen (N) budgets were made for the Scheldt sub-basin during the productive period (May–September). The main P and N transformation processes were identified as PO4 sorption, algal uptake and nitrification. Retention of P and N in the entire freshwater tidal area was estimated. In the brackish part of the estuary, there was no net source or sink of dissolved inorganic nitrogen (DIN). Desorption of PO4 was the most important transformation process for P along the salinity gradient. The PO4 pool increased and the PIP pool decreased, while total P behaved conservatively. Mass balance calculations show that the transformation of particulate P to PO4 enhances the PO4 flux from the Scheldt to the coastal zone. © 2007 Elsevier B.V. All rights reserved. Keywords: Phosphorus; P budget; Nitrogen; Scheldt River; Estuary

1. Introduction Estuaries are key sites where biogeochemical processes modify nutrient fluxes from land to the sea. Not only dissolved, but also particulate matter undergoes changes within the freshwater-estuarine continuum. ⁎ Corresponding author. Tel.: +32 2 6505218; fax: +32 2 6505228. E-mail address: [email protected] (C. van der Zee). 1 Tel.: +32 2 6505218; fax: +32 2 6505228. 0304-4203/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2007.01.003

Hydrological processes influence the suspended particulate matter (SPM) distribution, which in turn affects the light availability and thus primary production and other microbial and geochemical processes. Sewage is a major source of phosphorus (P) to rivers (Jickells, 2005). Agricultural land run-off is less significant, because phosphate (PO4) is very particle reactive (Froelich et al., 1982) and some of the PO4 is retained within the soils (Hessen, 1999). Sewage treatment plants generally remove ammonium (NH4) by oxidation to

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nitrate (NO3). Therefore, both sewage and agricultural practice are sources of NO3 to the estuary. Important estuarine processes for PO4 include adsorption on or desorption from Fe oxy-hydroxide bearing suspended matter or underlying sediments (Froelich, 1988; Van Raaphorst and Kloosterhuis, 1994). When the Fe oxyhydroxides are reductively dissolved under anoxic conditions, the adsorbed PO4 is liberated. The sorption capacity of Fe oxy-hydroxides decreases with increasing pH and salinity. Thus, when heavily PO4-loaded Fe particles reach seawater with a PO4 concentration below the equilibrium concentration, PO4 will be desorbed and released to the surrounding water. This is called estuarine buffering of phosphate (Froelich, 1988). Dissolved inorganic nitrogen (DIN) is not very particle reactive, but its transformations are often related to the oxygen saturation level of the water column. For example, bacterial denitrification, the complete reduction of nitrate to dinitrogen gas, occurs in the suboxic zone in the water column or in the sediment, whereas bacterial nitrification, the oxidation of ammonium to nitrate, occurs under oxic conditions. Consumption of both the PO4 and DIN takes place during their assimilation into bacterial and phytoplankton biomass and both are released again upon the degradation of organic matter. As the Scheldt estuary is net heterotrophic (Frankignoulle et al., 1998), i.e. organic matter respiration is more important than the primary production, it can be a source of nutrients. The Scheldt estuary is amongst those estuaries in the world with the highest nitrogen and organic carbon concentrations. Consequently many investigations have been devoted to the quantification of the N budget (Billen et al., 1985; Soetaert and Herman, 1995; Regnier and Steefel, 1999). Wollast (1983) reported that the N and P concentrations in the Scheldt River had increased by one to two orders of magnitude compared to pristine situations. In the 1980s, the implementation and upgrading of wastewater treatment was started in an effort to improve the surface water quality, which was indeed achieved (Van Damme et al., 2005). Countries surrounding the North Sea agreed on reducing the anthropogenic input of nutrients by 50% between 1985 and 1995 for areas where nutrients cause pollution and eutrophication. Since then, riverine P inputs have declined steadily due to the active reduction of P in wastewaters, whereas the 50% target was not reached for N (OSPARCOM, 2000). Annually averaged nutrient concentrations increased until the mid-1970s in the tidal Scheldt estuary and declined linearly afterwards, upon the restoration of the water column oxic conditions (Soetaert et al., 2006). The still large excess in riverine

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delivery of N and P over Si required by diatoms has led to coastal eutrophication in the Southern Bight of the North Sea and particularly in the Belgian coastal waters characterised by massive nuisance blooms of Phaeocystis colonies (Lancelot et al., 1987). Some investigations on the distribution of P species were conducted in the 1970s (e.g. Wollast, 1982), while the only study on seasonal variability and biogeochemistry of P species in the Scheldt was conducted in the late 1980s (Zwolsman, 1994). Van Damme et al. (2005) published the results of 7 years of monitoring (1995– 2001) of PO4 and the total phosphorus (TP) and other parameters in the Scheldt estuary. The authors concluded that P was consumed by phytoplankton downstream and that physico-chemical processes dominated more upstream and referred to the study of Zwolsman (1994) to discuss trends. The nutrient situation has changed drastically in the Scheldt between 1965 and 2002; for example, the estuary switched from a net sink to a net source of PO4 after 1995 (Soetaert et al., 2006). The poly-P concentration has decreased from 22–45 μM in the summer of 1975 (Wollast, 1982) to 0.6–3.6 μM in 1987–1988 (and one measurement of 30.3 μM; Zwolsman, 1994) and from 0.7 μM to below detection limit in 2002–2004 at Hemiksem situated at about 90 km from the mouth of the estuary (present study). The Scheldt estuary represents an important input source of PO4 to the Belgian coastal zone, where this nutrient can be limiting for phytoplankton growth in spring (Van der Zee and Chou, 2005) Previous studies did not include the simultaneous water sampling for dissolved, particulate, organic and inorganic P species in the freshwater tidal reaches of the Scheldt, in its estuary along the salinity gradient and in the coastal plume zone. In this study, we aim at identifying the sources of P to the Scheldt estuary and the P species involved, and establishing their seasonal variation and transformations going from the freshwater tidal limits towards the coastal zone. 2. Materials and methods 2.1. Study area and sampling strategy The Scheldt River and its tributaries drain an area of 21,580 km2 in northern France, western Belgium and the southwestern Netherlands and flows into the North Sea. The river Scheldt and its branches are rain-fed and discharge varies with minimum values in summer and autumn and maximum values in winter and spring. The tidal Scheldt extends from the mouth of the estuary at Vlissingen to Ghent, 170 km upstream, where sluices

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block the tidal action. Only about 39% of the discharge coming from the Upper Scheldt enters the estuary in Ghent and the remainder flows into channels, notably the canal of Ghent–Terneuzen, which empties into the estuary further downstream near Terneuzen (Meire et al., 2005). The hydrographic basin is situated in one of the most densely populated regions of Europe where highly diversified industrial activity has developed. As a result, the Scheldt estuary, a well-mixed tidal system, receives huge anthropogenic nutrient inputs. The Scheldt estuary is located in the north of Belgium and the southwest of The Netherlands (Fig. 1). The freshwater tidal area is situated between Ghent and Temse (stations 1 and 9, respectively). The major tributaries are the rivers Dender and Rupel, the latter receiving untreated wastewater from the city of Brussels via the river Zenne (station 3). Weekly water sampling has been carried out in the Rupel basin (stations 3, 4, 5 and 6), in the Scheldt subbasin (stations 1, 2, 8 and 9) and the Scheldt basin (stations 10 and 11) (Fig. 1). Stations 1–6 correspond to the tidal limits of the Scheldt estuary and represent the rivers Scheldt, Dender, Zenne, Dijle, Grote Nete and Kleine Nete, respectively. Between Dendermonde (station 8) and Temse (station 9), the river Durme enters the Scheldt, but at present-day this river does not

contribute any water to the Scheldt. Station 11 near Antwerp was sampled starting in August 2002 until September 2004, whereas the stations 1–9 were sampled from March 2003 to February 2004 and station 10 from February 2003 to June 2004. Station 10 is located near Hemiksem just after the confluence of the Scheldt with the Rupel and before the Scheldt flows through Antwerp. In the Scheldt estuary and plume, water samples were taken during cruises with the RV Belgica in March and July 2003, and in February, May, July, September and December 2004. 2.2. Phosphorus speciation Dissolved phosphorus species were determined after filtration through precombusted (4 h, 500 °C) GF/F filters, which were used for the particulate phosphorus speciation. Soluble reactive phosphorus, hereafter phosphate (PO4), was measured according to the method of Grasshoff et al. (1983). Dissolved inorganic phosphorus (DIP) was determined as phosphate after digestion with 9 N H2SO4 (120 °C, 30 min) (Grasshoff et al., 1983). DIP comprises phosphate and polyphosphates, although hydrolysis of acid-labile organic phosphorus compounds may also contribute to this parameter. Polyphosphate (poly-P) was determined as

Fig. 1. Study area of the Scheldt continuum river-estuary-coastal zone. The names of the rivers and tributaries are indicated in italics.

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the difference between DIP and PO4. Total dissolved phosphorus (TDP) was measured by wet oxidation in acid persulphate (120 °C, 30 min) (Grasshoff et al., 1983). Dissolved organic phosphorus (DOP) was subsequently calculated as the difference between TDP and DIP. Filters for the total particulate phosphorus (TPP) determination were combusted at 500 °C for 1.5 h with MgSO4 and subsequently extracted in 1 N HCl for 24 h (Solorzano and Sharp, 1980). Filters for particulate inorganic phosphorus (PIP) were extracted in 1 N HCl for 24 h. Particulate organic phosphorus (POP) was calculated as the difference between TPP and PIP. The unfiltered water sample was in addition digested by wet oxidation in acid persulphate, which gives the total phosphorus (TP).

3. Results

2.3. Nitrogen speciation

3.1.1. Phosphate The most abundant P species in the Scheldt sub-basin was PO4 (stations Ghent, Dender, Dendermonde and Temse) and in the River Zenne (Fig. 2). The river Zenne is anoxic during most of the year, due to the discharge of untreated waste that it receives from the city of Brussels. The mean PO4 concentration declined in the Scheldt sub-basin from 17 μM in the Scheldt at Ghent and 15 μM in the Dender to 12 μM at Dendermonde and further to 6 μM at Temse. The PO4 concentrations in the Rupel basin were very low in the Dijle, the Grote Nete and the Kleine Nete, but very high in the anoxic river Zenne, with mean concentrations of 3.4 μM, 0.5 μM, 1.5 μM and 30.5 μM, respectively. After the confluence of the Scheldt and the Rupel, the mean PO4 concentration amounted to 4.4–4.6 μM at Hemiksem and Antwerp as well as in waters between salinities 0 to 15. It declined seaward to 0.9 μM at salinities higher than 30. The trends in PO4 concentrations along the salinity gradient are given in Fig. 3. The PO4 concentration ranged between 3.3 μM and 6.0 μM at near zero salinity, which is comparable to the range of concentrations measured at Hemiksem. It declined seaward and ranged from 0.04 μM to 1.32 μM in the coastal zone. The longitudinal profile of PO4 showed a nearly conservative mixing behaviour during the campaign in March 2003. The highest PO4 concentrations were observed in early July 2003, exhibiting a maximum value around salinity 10. The positive deviation from the mixing line indicates a source of PO4 within the estuary. In 2004, the variation in the PO4 concentration with salinity was very similar in the months February, July, September and December. The PO4 concentration was more or less constant in the upper estuary, which was followed by a decline in the lower estuary. Biological uptake of PO4

Dissolved inorganic nitrogen (DIN) species were determined colorimetrically. Nitrate (NO3) and nitrite (NO2) were analysed with a Technicon Autoanalyzer system following Grasshoff et al. (1983). Ammonium (NH4) was measured with the indophenol blue technique according to Koroleff (1969). Total dissolved nitrogen (TDN) was measured as NO3 plus NO2 after wet oxidation in alkaline persulphate (120 °C, 30 min) (Grasshoff et al., 1983). The dissolved organic nitrogen (DON) was determined as the difference between TDN and DIN (the sum of NO3, NO2 and NH4). Particulate nitrogen (PN) was determined on particulate matter collected by filtration of seawater on precombusted (4 h, 500 °C) GF/F filters, using a Fisions NA-1500 elemental analyser. 2.4. Water discharge data and flux calculations Daily averaged water discharged data were obtained from the Ministry of the Flemish Community (Afdeling Maritieme Toegang) for the five tributaries and the Scheldt at Melle (near Ghent). These data include lateral inputs. Phosphorus and nitrogen fluxes were then calculated by the direct method. The concentration of the P or N species (in mmol m− 3) measured on a given day was multiplied with the daily average flow (m3 s− 1) of that day and are finally expressed as fluxes of P or N in mol d− 1. The dynamics of the Scheldt estuary are dominated by tidal forcing and the residence time of freshwater in the mixing zone is fairly long (2–3 months). Therefore we limit our simple flux calculations to the freshwater stations from the tidal limits to station Hemiksem after the confluence of the major tributaries.

3.1. Distribution of phosphorus along the Scheldt continuum The data in the freshwater tidal part of the estuary are compiled in Fig. 2. Generally, the box plot represents 36 measurements in the case of the stations in the Scheldt sub-basin, 41 measurements for the stations in the Rupel basin, 51 measurements at Hemiksem and 91 measurements at Antwerp (Fig. 1). The data from the salinity gradient were grouped per five salinity units in order to compare them with those of the freshwater tidal part (Fig. 2). The complete P speciation along the salinity gradient is given in Fig. 3.

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was only evident during the May campaign: the spring phytoplankton bloom lowered the PO4 concentration between salinity 20 and 31, where oxygen saturation exceeded 100% and Chl a values were elevated (data not shown). 3.1.2. Poly-phosphate Poly-P was generally the least abundant P species (Fig. 2). The mean poly-P concentration decreased from 0.55 μM in the Scheldt at Ghent and from 0.39 μM in the Dender to 0.33 μM and 0.34 at Dendermonde and Temse, respectively. The mean poly-P concentration was the highest, 0.95 μM, in the River Zenne and rather low

in the other Rupel tributaries: 0.21 μM, 0.08 μM and 0.10 μM in the Dijle, Grote Nete and Kleine Nete rivers, respectively. After the confluence of the major rivers, the mean poly-P concentration was 0.21 μM at Hemiksem and 0.17 μM at Antwerp. In general, the mean poly-P concentration decreased over the subsequent salinity intervals: from 0.17 μM (salinity 0–5), to 0.14 μM (salinity 5–10), 0.13 μM (salinity 10–15), 0.14 μM (salinity 15–20), 0.11 μM (salinity 20–25), 0.09 μM (salinity 25–30) and finally 0.08 μM at salinity N30. The longitudinal distributions of poly-P concentrations along the salinity gradient during the different campaigns are shown in Fig. 3. The concentrations

Fig. 2. The PO4, poly-P, DOP, POP, PIP and TP concentrations in the Scheldt sub-basin (Ghent, Dender, Dendermonde (DM) and Temse), the Rupel basin (Zenne, Dijle, Grote Nete and Kleine Nete), in the Scheldt at Hemiksem, Antwerp and along the salinity gradient. The box-plots were made with Sigma Plot and indicated are the outliers (dots), minimum and maximum values (horizontal bars), the quartiles (boxes) and the median values (horizontal bar in the boxes).

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Fig. 2 (continued ).

decline between salinity 0 and 5 in March 2003 and remain constant at a very low levels thereafter (Fig. 3). A decreasing trend with salinity was observed in July, September and December 2004, whereas no trend was found in July 2003 and May 2004. 3.1.3. Dissolved organic phosphorus The second least abundant P species was DOP (Fig. 2). The mean DOP concentration decreased downstream as well from 1.02 μM in the Scheldt at Ghent and 0.87 μM in the Dender to 0.77 μM and 0.63 μM at Dendermonde and Temse, respectively. The mean DOP concentration in the Rupel tributaries was 0.34 μM in the Kleine Nete, 0.35 μM in the Grote Nete,

0.76 μM in the Dijle and 1.81 μM in the Zenne. It was 0.59 μM at Hemiksem and 0.50 μM at Antwerp. The mean DOP concentrations along the successive salinity intervals exhibit a mid-estuarine minimum, but are the lowest at sea: 0.52 μM (salinity 0–5), 0.41 μM (salinity 5–10), 0.34 μM (salinity 10–15), 0.37 μM (salinity 15–20), 0.36 μM (salinity 20–25), 0.40 μM (salinity 25–30) and 0.29 μM at salinities higher than 30. The DOP concentrations along the salinity transect are shown in Fig. 3. They were invariable with salinity in March and July 2003 and in May and September 2004. In July and December 2004, however, the DOP values decreased when going seawards. The DOP concentration was typically below 1 μM, but higher

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Fig. 3. Longitudinal distributions of PO4, poly-P, DOP, POP, PIP and TP concentrations, and POP and PIP contents in the Scheldt estuary during cruises conducted in March and July 2003 in the left panels and in February (only PO4 and DOP data), May, July, September and December 2004 in the right panels. The linear regression lines and equations are given for the TP concentrations in May, July, September and December 2004.

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Fig. 3 (continued ).

values were measured in February 2004 in the lower salinity range. 3.1.4. Particulate organic phosphorus The POP concentrations were the highest in the River Zenne, which receives domestic wastewater (Fig. 2). In the Scheldt sub-basin, the highest POP concentrations were measured at Dendermonde where massive diatom blooms were observed in summer. The mean POP concentration was 2.8 μM, 2.3 μM, 4.8 μM and 3.0 μM at Ghent, in the Dender, at Dendermonde and at Temse, respectively. In the Rupel basin, it was 16.7 μM, 2.2 μM, 2.0 μM and 1.3 μM in the rivers Zenne, Dijle, Grote Nete and Kleine Nete, respectively. After the confluence of the major rivers it was 4.5 μM at Hemiksem and 4.0 μM at Antwerp. The mean POP concentration declined progressively along the salinity intervals: from 2.2 μM (salinity 0–5), 2.1 μM (salinity 5–10), 1.1 μM (salinity 10–15), 0.61 μM (salinity 15– 20) and 0.60 μM (salinity 20–25) to 0.09 μM (salinity 25–30). It was higher in the coastal plume zone, 0.50 μM at salinity N 30.

The POP concentrations exhibit a maximum around salinity 1 or 5 and an exponential decline seawards during all cruises (Fig. 3). In July and September 2004, a secondary POP maximum was observed close to salinity 15 and 9, respectively. In the Scheldt sub-basin, the mean POP content was 92 μmol g− 1 at Ghent, 160 μmol g− 1 in the Dender, 55 μmol g− 1 at Dendermonde and 68 μmol g− 1 at Temse. In the Rupel basin, it was 303 μmol g− 1 in the Zenne, 62 μmol g− 1 in the Dijle, 107 μmol g− 1 in the Grote Nete and 102 μmol g− 1 in Kleine Nete. The POP content was higher in the freshwater tidal part than in the brackish estuary, but no clear trend was observed with salinity (Fig. 3). The values ranged from 4 μmol g− 1 to 145 μmol g− 1 and was on average 34 μmol g− 1. 3.1.5. Particulate inorganic phosphorus The highest PIP concentrations were encountered at Antwerp (Fig. 2), which is located in the dynamic area where bottom currents upstream and downstream converge resulting in the maximum turbidity zone (Baeyens et al., 1998). The mean PIP concentrations

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were generally higher than the mean POP concentrations. The mean PIP concentration was 5.5 μM at Ghent, 3.4 μM in the Dender, and increased to 12.1 μM at Dendermonde, after which it declined again to 8.6 μM at Temse. In the Rupel basin, PIP was the most abundant P species in the Dijle, Grote Nete and Kleine Nete. The mean PIP concentration was 13.7 μM, 8.0 μM, 8.5 μM and 3.9 μM in the rivers Zenne, Dijle, Grote Nete and Kleine Nete, respectively. After the confluence it was 8.7 μM at Hemiksem and increased to 16.4 μM at Antwerp. The mean PIP concentration declined along the salinity intervals: 7.4 μM (salinity 0–5), 5.0 μM (salinity 5–10), 3.5 μM (salinity 10–15), 1.2 μM (salinity 15–20), 0.86 μM (salinity 20–25), 0.51 μM (salinity 25–30) and 0.61 μM (salinity N30). The PIP concentrations along the salinity gradient show a strong decline in the upper estuary and a constant and low level in the lower estuary (Fig. 3). The highest PIP concentrations are found around salinity 1 or 5. In general, the PIP content of the suspended matter decreased going seaward from ∼ 140 μmol g− 1 to ∼ 25 μmol g− 1, although there are some exceptions in the salinity range 25 to 35 (Fig. 3). The mean PIP content in the Scheldt sub-basin was 183 μmol g− 1 in the Scheldt at Ghent, 260 μmol g− 1 in the Dender, 133 μmol g− 1 at Dendermonde and 149 μmol g− 1 at Temse. The mean PIP content in the Rupel tributaries was higher: 243 μmol g− 1 in the Zenne, 266 μmol g− 1 in the Dijle, 442 μmol g− 1 in the Grote Nete, 366 μmol g− 1 in the Kleine Nete. The higher PIP contents in the freshwater tidal part compared to those in estuary suggest that desorption of PO4 from the suspended matter is important or that the freshwater particles have mixed with relatively PO4-poor estuarine/marine suspended matter. 3.1.6. Total phosphorus The TP concentrations were high in the River Zenne and at Antwerp (Fig. 2). The mean TP concentration increased from 27.8 μM in the Scheldt at Ghent and 21.7 μM in the Dender to 30.0 μM at Dendermonde after which it decreased again to 19.3 μM at Temse. The mean TP concentration in the Rupel basin was 63.5 μM, 14.7 μM, 11.5 μM and 7.2 μM in the rivers Zenne, Dijle, Grote Nete and Kleine Nete, respectively. After the confluence it was 18.8 μM at Hemiksem and 24.9 μM at Antwerp. The mean TP concentration decreased over the successive salinity intervals: 13.8 μM (salinity 0–5), 11.1 μM (salinity 5–10), 9.1 μM (salinity 10–15), 6.2 μM (salinity 15–20), 4.7 μM (salinity 20–25), 3.4 μM (salinity 25–30) and 1.7 μM at salinities N30.

The TP concentration showed high values between salinity 0 and 15, the area of high turbidity, and a decreasing trend downstream in March and July 2003 (Fig. 3). In 2004, the TP concentration declined linearly along the estuarine transects. 3.2. Distribution of nitrogen along the Scheldt continuum The NO3 concentration was generally higher than the NO2, NH4 and DON concentrations (Fig. 4). The DIN (NO3 + NO2 + NH4) concentrations were lower in summer than in winter (data not shown). No clear seasonal trend in the DON distributions was observed (data not shown). The PN concentrations were the highest in summer and followed the same seasonal trend as the chlorophyll a concentrations (data not shown) at the stations Ghent, Dender and Dendermonde. The total DIN concentrations decreased linearly with salinity. 3.2.1. Nitrate The most abundant DIN species was generally NO3, except in the predominantly anoxic River Zenne (Fig. 4). The mean NO3 concentration was 185 μM, 127 μM, 251 μM and 214 μM at Ghent, in the Dender, at Dendermonde and at Temse, respectively. The NO3 concentration was lowest in summer in the Scheldt at Ghent and Dender. The mean NO3 concentration was only 38 μM in the Zenne and 204 μM, 116 μM and 76 μM in the rivers Dijle, Grote Nete and Kleine Nete, respectively. NO3 was nearly absent during the low discharge period in summer in the Zenne. After the confluence the mean NO3 concentration was 198 μM at Hemiksem and 226 μM at Antwerp. The highest mean NO3 concentration was observed at the 0–5 salinity interval and it decreased strongly seaward: 341 μM (salinity 0–5), 307 μM (salinity 5–10), 250 μM (salinity 10–15), 186 μM (salinity 15–20), 131 μM (salinity 20– 25), 69 μM (salinity 25–30) and finally 17 μM at salinity N30. The longitudinal NO3 profiles were very similar during the different months, except for a small variation in the concentrations upstream. 3.2.2. Nitrite The highest NO2 concentrations were encountered at Ghent and at Hemiksem (Fig. 4). The mean NO2 concentration was 31.4 μM, 12.0 μM, 11.8 μM and 17.0 μM in the Scheldt at Ghent, in the Dender, at Dendermonde and at Temse, respectively. In the Rupel basin, it was 12.3 μM, 15.2 μM, 8.2 μM and 5.2 μM in the Zenne, Dijle, Grote Nete and Kleine Nete rivers, respectively. After the confluence it was higher,

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Fig. 4. The NO3, NO2 and NH4 concentrations in the Scheldt sub-basin (Ghent, Dender, Dendermonde (DM) and Temse), the Rupel basin (Zenne, Dijle, Grote Nete and Kleine Nete), in the Scheldt at Hemiksem, Antwerp and along the salinity gradient. The box-plots were made with Sigma Plot and indicated are the outliers (dots), minimum and maximum values (horizontal bars), the quartiles (boxes) and the median values (horizontal bar in the boxes). The high NH4 concentrations for the Zenne (323–1347 μM, median value 683 μM) are not presented in the figure, as they would obscure all the other values.

25.6 μM at Hemiksem, but it decreased rapidly to 10.7 μM at Antwerp. The mean NO2 concentration decreased seaward: from 8.9 μM (salinity 0–5), to 3.6 μM (salinity 5–10), 2.7 μM (salinity 10–15), 2.5 μM (salinity 15–20), 2.2 μM (salinity 20–25), 1.7 μM (salinity 25–30) and 0.8 μM at salinity N30. 3.2.3. Ammonium The NH4 concentrations are presented in Fig. 4 except for those of the Zenne River. Very high NH4 concentrations were measured in the anoxic Zenne:

from 323 μM to 1347 μM and a median value of 683 μM. The NH4 level was generally lower in summer than in winter. The mean NH4 concentration decreased from 148 μM at Ghent and 88 μM in the Dender to 47 μM at Dendermonde and 52 μM at Temse. In the Rupel basin, it was 697 μM, 67 μM, 47 μM and 23 μM in the rivers Zenne, Dijle, Grote Nete and Kleine Nete, respectively. In the River Zenne, the NH4 concentration was the lowest when the NO3 was the highest and vice versa. After the confluence the mean NH4 concentration was 86 μM at Hemiksem and decreased to

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Fig. 5. Seasonal trends in the (a) PO4, (b) NH4 and (c) NOx concentrations, and (d) DIN to PO4 molar ratio in the Scheldt at Hemiksem and Antwerp after the confluence of major tributaries.

49 μM at Antwerp. The NH4 concentration, like the NO2 concentration, declined rapidly in the low salinity region of the estuary due to nitrification. The oxygen saturation levels were low in this region (oxygen data are not shown). The mean NH4 concentrations were

55.8 μM (salinity 0–5), 18.6 μM (salinity 5–10), 11.3 μM (salinity 10–15), 7.7 μM (salinity 15–20), 6.0 μM (salinity 20–25), 5.3 μM (salinity 25–30) and 4.3 μM (salinity N30) for the different salinity intervals.

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3.2.4. Dissolved organic nitrogen and particulate nitrogen In the Scheldt at Ghent, the DON ranged from 0 to 229 μM (mean 56 μM) and the PN ranged from 8 μM to 97 μM (mean 34 μM). In the Dender, the DON concentration ranged from 8 μM to 223 μM (mean 41 μM) and the PN concentration ranged from 8 μM to 185 μM (mean 36 μM). At Dendermonde, the DON concentration ranged from 13 μM to 240 μM (mean 50 μM) and the PN concentration ranged from 17 μM to 148 μM (mean 64 μM) with the highest values observed in summer. At Temse, the DON concentration ranged from 7 μM to 346 μM (mean 66 μM) and the PN concentration ranged from 10 μM to 81 μM (mean 35 μM). PN and DON were not determined in the Rupel basin, at Hemiksem and Antwerp or along the salinity gradient. 3.3. Phosphorus speciation and dissolved inorganic nitrogen at the confluence of the major tributaries At the six tidal limit stations, PO4 accounted for 43%, PIP for 34%, POP for 18%, DOP for 4% and poly-P for 2% of the total P flux in 2003. After the confluence of the major rivers, at Hemiksem, the relative contributions of the P species to the total P load had changed towards a higher contribution of the particulate P and a relatively lower contribution of PO4 to the total P: PIP 48%, POP 25%, PO4 23%, DOP 3% and poly-P 1%. The PO4 concentrations in the Scheldt at Hemiksem and Antwerp followed the same seasonal trend (Fig. 5a): they were low in February and March and high in August and September. The NH4 concentration at Antwerp was very low between the beginning of June and November in 2003, and was elevated during the period from early November to mid-May (Fig. 5b). The NOx concentration showed a less pronounced but similar seasonal trend to NH4 (Fig. 5c). The seasonal DIN trend was the mirror image of the seasonal PO4 trend, resulting in a strong temporal fluctuation of the

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molar DIN:PO4 ratio (14–322) that reached its highest values in March (Fig. 5d). Poly-P, DOP, POP and PIP did not exhibit any seasonal variation at Hemiksem and Antwerp. 4. Discussion 4.1. Phosphorus and Nitrogen budgets in the freshwater Scheldt sub-basin In 2003, an intense summer bloom of diatoms occurred between Ghent and Dendermonde, which resulted in the exhaustion of dissolved silicate at Dendermonde (V. Carbonnel, pers. com.). However, the PO4 and DIN concentrations were not depleted and remained well above any limiting levels. The major processes occurring in the P and N cycles during the diatom bloom can be assessed by comparison of the net time-integrated fluxes of all P and N species during the productive period (see Table 1), which started in May and lasted until the end of September (V. Carbonnel, pers. com.). At the tidal limits in the Scheldt (at Ghent) and in the Dender, 66% of the total P flux was in the form of PO4, which amounts to 6.6 Mmoles of PO4 during the productive period (Table 1). Of the 6.6 Mmoles of PO4, 2.1 Mmoles were consumed before the water reached Dendermonde, while the PIP and POP pools increased. One-third of the PO4 lost between the tidal limits and Dendermonde (i.e. 0.7 Mmoles), may have been assimilated and converted into biomass resulting in the increase of 0.7 Mmoles of POP. The other two-thirds, 1.4 Mmoles PO4, may have been removed by sorptive uptake into the PIP pool, which increased by 2.4 Mmoles between the tidal limits and Dendermonde in the productive period. The losses of PO4, poly-P and DOP do not account for the increases of POP and PIP: there is thus an additional source of total P, probably from resuspended sediment (or PO4 fluxes from the sediment

Table 1 Time-integrated fluxes over the productive period (May–September) and the P and N retention in the Scheldt sub-basin

Ghent (G) Dender (D) Dendermonde (DM) Temse G + D − DM DM − Temse Retention Retention in %

PO4 (kmoles)

poly-P (kmoles)

DOP (kmoles)

PIP (kmoles)

POP (kmoles)

NOx (Mmoles)

NH4 (Mmoles)

DON (Mmoles)

PN (Mmoles)

5555 1018 4493 2288 2080 2205 4285 65

167 34 117 181 84 −64 20 10

291 72 275 284 88 −9 79 22

1562 246 4211 2655 − 2403 1556 − 847 − 47

786 205 1686 1049 − 695 637 − 59 −6

66 9 106 87 − 31 19 − 12 − 16

48 7 21 25 34 −3 30 55

11 3 15 19 −1 −4 −5 − 38

8 3 24 11 − 13 13 0 2

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and subsequent uptake or adsorption). Likewise, most of the 155 Mmoles of N supplied at the tidal limits of the Scheldt sub-basin were in the dissolved inorganic form (Table 1). Between the tidal limits and Dendermonde 34 Mmoles of NH4 were consumed or lost. Presumably, this NH4 was nitrified since the NOx increased by 31 Mmoles. In addition, it may have been assimilated into biomass, as the PN increased by 13 Mmoles, or converted into DON, which increased by 1 Mmol. The total N pool increased with 11 Mmoles. This N may have come from the river sediment or intertidal areas. Both PN and POP increased between the tidal limits and Dendermonde: the ratio of these amounts of PN to POP is approximately 18 suggesting the net production of fresh algal biomass (Redfield N:P is 16). Thus, the main processes involving N and P cycling before Dendermonde were nitrification, PO4 sorption and algal growth. Between Dendermonde and Temse, a further loss of 2.2 Mmoles of PO4 occurred in addition to a loss of 0.6 Mmoles of POP and 1.6 Mmoles of PIP (Table 1). The very small increases in poly-P and DOP, 0.06 and 0.01 Mmoles, respectively, do not explain the overall loss of 4.3 Mmoles of P. Thus, a large amount of P was lost from the water column. In this area, the river and tidal energy is at a minimum (Chen et al., 2005) allowing the settling of suspended matter and thus POP and PIP. Most of the biogenic silica produced upstream was also lost between Dendermonde and Temse (V. Carbonnel, pers. com.). Possibly, the loss of PO4 was due to scavenging by settling dead diatoms and inorganic suspended material as PO4 is very particle reactive. The intertidal sediments of the upper estuary could subsequently act as a sink for phosphorus in the form of Fe-bound P at depth in the sedimentary column (Hyacinthe and Van Cappellen, 2004). Similar to P and Si, a large net loss of 24 Mmoles N from the system occurred between Dendermonde and Temse: 19 Mmoles of NOx were lost and 13 Mmoles of PN, whereas only 3 Mmoles of NH4 and 4 Mmoles of DON accumulated (Table 1). Apparently, mineralization processes dominated the N transformations here: PN was degraded or settled out and denitrification can account for the loss of NOx. Both NH4 and DON are liberated upon the degradation of PN. Billen et al. (2005) also observed a dominant role of denitrification in the N budget in the late 1990s in the Scheldt basin. 4.2. Seasonal variability at the confluence of the major rivers The seasonality in the PO4 concentration (Fig. 5) observed at the confluence of the Scheldt and the Rupel

is due to the input from the Rupel basin and mostly from the river Zenne. This river is anoxic throughout most of the year because of the high load of wastewater it receives from the city of Brussels. Dissolved oxygen is present only during high freshwater discharges in winter. Previously, the higher summer PO4 concentration after the confluence was explained by the additional release of sorbed PO4, due to summer oxygen depletion in the fluvial sediments or SPM (Hoenig, 1976; Zwolsman, 1994). In addition, it is likely that the PO4 concentration was diluted during high freshwater discharges in winter. The PO4 concentration would be the highest in summer during low flow conditions as this nutrient comes mostly from constant point sources. In contrast, nitrogen has a diffuse origin, with fluxes depending on discharge (Billen et al., 2005). Ammonium in the Scheldt originates mainly from domestic wastewater, intensive stock farming and in situ degradation of organic matter (Wollast, 2003). This idea is supported by the negative linear correlation (R2 = 0.57; n = 58) between PO4 and the discharge (excluding the eight highest discharge data) and the lack of a correlation between the NOx concentration and the discharge. The positive linear correlation (R2 = 0.69; n = 58) between the NH4 concentration and the discharge can be explained by more intensive degradation of organic matter in summer when the temperatures are higher and the water discharge is lower. 4.3. Estuarine phosphate source The longitudinal profiles of PO4 revealed a source of this nutrient around salinity 10 in the estuary (Fig. 3). This source does not contribute to DIN, which behaves nearly conservatively after the confluence of the major rivers, as previously observed (Soetaert et al., 2006; Regnier and Steefel, 1999). Given a steady state and a conservative salinity distribution, the export of a given constituent from the estuary can be estimated by multiplying the zero-salinity value, obtained by regression of the linear part of the property-salinity plot (Fig. 3), by the freshwater discharge (Officer, 1979). The import to the estuary of this constituent can be evaluated by multiplying the freshwater end-member concentration by the freshwater discharge, in our case this corresponds to the flux of PO4 at Hemiksem. Then, the loss or gain of PO4 in the estuary can be estimated. Our calculations show that in addition to the PO4 from the River Scheldt, the brackish part of the Scheldt estuary was also a significant source of PO4 for the coastal zone. In March 2003, the estuary was a small source for PO4 (Table 2). In July 2003, February and

C. van der Zee et al. / Marine Chemistry 106 (2007) 76–91

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Table 2 PO4 and TP import and export fluxes and the estuarine PO4 source as a percentage of the export flux

March 2003 July 2003 February 2004 May 2004

PO4 import flux (kmol d− 1)

PO4 export flux (kmol d− 1)

Estuarine PO4 source (%)

TP import flux (kmol d− 1)

TP export flux (kmol d− 1)

42 27 48 22

47 51 114 45

11 40 53 51

115 90

118 77

May 2004, the estuarine source accounted for about half of the PO4 export flux to the coastal zone (Table 2). We estimated the import and export fluxes of TP in 2003 in the same way as we did for the PO4 (Table 2). Even though particles are transported differently than dissolved species and we have to assume steady state in the estuary, which is not the case (Regnier et al., 1998). The flux calculations indicate that the Scheldt estuary is not a source of total phosphorus to the coastal zone, although it is for phosphate (Table 2). The estuarine source of PO4 could be provided through desorption from suspended matter and sediments containing PIP (Froelich, 1988; Lebo, 1991; Van Raaphorst and Kloosterhuis, 1994). Generally, oxygen saturation increases from ∼ 20% to ∼ 70% between the lowest salinities sampled during a campaign and salinity 10. Thus, in this oxygen-deficient zone of the estuary, desorption of PO4 from the Fe-bearing suspended matter may occur upon reduction of the Fe oxy-hydroxide. The sorption capacity for PO4 declines as the salinity and pH increase (Froelich, 1988). Therefore, even if Fe reduction did not take place, an instantaneous desorption of PO4 would occur as freshwater mixes with seawater. The PIP concentration decreased going seaward (Fig. 3). This

did not only result from a decrease in SPM concentration, since the PIP content also decreased with salinity (Fig. 3). It was neither due to mixing with marine particles that are relatively poor in PIP. The PIP content declined exponentially with salinity, resulting in a negative deviation from the mixing line. Our results point towards a consumption of PIP around salinity 10 to 15, exactly where the estuarine source of PO4 is. Thus, PO4 is probably desorbed from the SPM, i.e. the PIP pool, and released into the water column. An estuarine source of PO4 has been observed in the Ems estuary, where about one third of the imported particulate phosphorus is released and exported as dissolved phosphorus (Van Beusekom and de Jonge, 1998). The Elbe estuary also functions as a PO4 source (Van Beusekom and Brockmann, 1998). 4.4. Phosphorus budget During the sampling period of almost one year, 14.6 Mmoles of PO4, 53% of the PO4 that enters the aquatic system at the tidal limits, was retained in the freshwater tidal area, due to sorptive removal and algal uptake from the water column (Table 3). The retention

Table 3 Time-integrated fluxes from March 2003 to February 2004 and the P retention PO4 (kmoles) Ghent Dender Zenne Dijle Grote Nete Kleine Nete Freshwater tidal area Tidal limits Hemiksem Retention Retention in % Retention Scheldt sub-basin a Retention Rupel basin b a

poly-P (kmoles)

DOP (kmoles)

PIP (kmoles)

POP (kmoles)

TP (kmoles)

13141 4288 7016 2779 102 519

601 99 290 142 18 47

1167 327 400 678 63 127

4995 1798 6994 9076 2412 2816

2762 882 5869 2692 412 765

22666 7395 20569 15368 3006 4274

27846 13225 14621 53 8576 6045

1197 592 605 51 179 425

2762 1824 939 34 577 361

28090 29419 − 1328 −5 − 7281 5952

13382 9786 3595 27 − 704 4300

73278 54846 18432 25 1348 17084

Retention in the Scheldt sub-basin was calculated as the difference in the time-integrated fluxes at Temse (not shown) and the sum of the tidal limits Ghent + Dender. b Retention in the Rupel basin was calculated as the difference in the time-integrated fluxes at Hemiksem minus Temse (not shown) and the sum of the tidal limits Zenne + Dijle + Grote Nete + Kleine Nete.

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of PO4 was significant both in the Scheldt sub-basin and the Rupel basin. The mean estuarine PO4 source was 29.5 kmol d − 1 , yielding an annual PO4 flux of 10.8 Mmoles. Thus, there was a net loss of 3.8 Mmoles of PO4. A total of 0.6 Mmoles of poly-P were retained between the tidal limits and Hemiksem and 0.9 Mmoles of DOP. These compounds may have been hydrolysed to PO4. The freshwater tidal area was a net sink for POP of 3.6 Mmoles. In the Rupel basin 4.3 Mmoles were retained while 0.7 Mmoles of POP were transferred from the Scheldt sub-basin to the estuary. There was a net transfer of 1.3 Mmoles of PIP from the freshwater tidal area to the estuary. This is the result of the retention of 6 Mmoles of PIP in the Rupel basin and the transfer of 7.3 Mmoles of PIP from the Scheldt sub-basin to the estuary mostly during winter. The particulate P species behaved differently in the Scheldt sub-basin compared to the Rupel basin, in contrast to the dissolved P species. The Scheldt sub-basin is a source for PIP and POP, and the Rupel basin is a sink for PIP and POP. As a consequence, the TP retention is one order of magnitude larger in the Rupel basin than in the Scheldt sub-basin. During the sampling period, 25% of the TP delivered by the rivers at the tidal limits was retained in the freshwater tidal area. In contrast, no TP retention was observed in the estuary. 5. Conclusions This study showed the temporal and spatial trends in the dissolved, particulate, organic and inorganic P species and dissolved inorganic N species along the Scheldt continuum. Both the rivers Scheldt and the Zenne were important suppliers of P and N to the Scheldt estuary. The inorganic P fraction was more abundant than the organic fraction. Mass-balances of N and P were made for the productive period (May–September) in the Scheldt sub-basin. This area was a sink for PO4, poly-P, DOP, NH4 and PN, but a source of PIP, POP, NOx and DON to the brackish Scheldt estuary. The main processes involving N and P transformations in the freshwater Scheldt sub-basin were nitrification, PO4 sorption and biological uptake during the productive period. Denitrification took place between Dendermonde and Temse. A mass-balance of P was made for the period March 2003 to February 2004 in the entire freshwater tidal area, including the Scheldt and the Rupel basins. The freshwater tidal area was a net sink for PO4, poly-P, DOP, POP and TP, but not for PIP. After the confluence of the major rivers, DIN behaved more or less as a conservative quantity. Reduced PO4

concentrations related to biological activity in the brackish part were only observed in May 2004. Desorption appeared to be the main process affecting P transformation in the upper Scheldt after the confluence with the Rupel. The Scheldt estuary acts currently a source of PO4 to the coastal zone in addition to the riverine PO4 source (this study), whereas it was a sink for PO4 before the mid-1990s (Soetaert et al., 2006). A new wastewater treatment plant in the North of Brussels is scheduled to be operational in 2007. Treatment of the sewage from the city of Brussels, that enters the Zenne, could therefore lead to a significant reduction of the PO4 flux to the Scheldt estuary. Further reduction of the PO4 load to the estuary may, however, not lead to a proportional decrease in the PO4 flux to the coastal zone, as the sorption equilibrium will shift further towards desorption (Prastka et al., 1998) and thus releasing additional PO4. Acknowledgements The authors wish to thank N. Canu, V. Carbonnel, C. de Marneffe, P. De Weireld and M. Tsagaris (ULB-LOCGE) for their work in the field, onboard the RV Belgica and in the laboratory. R. Dasseville, M. Lionard, and S. Vanneste (RUG) are acknowledged for sampling stations 1, 2, 8 and 9 and L.S. Schiettecatte (ULg) for sampling station 11. We are grateful for the logistic support provided by the officers and crew of the RV Belgica, especially the boatswain E. De Witte. The Ministry of the Flemish Community (Afdeling Maritieme Toegang) is acknowledged for the daily averaged water discharged data. The reviews of F.T. Mackenzie and J.E.E. van Beusekom greatly improved the manuscript. This study was supported by the Belgian Federal Science Policy Office under contract numbers EV/11/17A (SISCO) and EV/11/20B (CANOPY). The Belgian French Community provided partial funding (FRFC, convention no. 2.4545.02). This is also a contribution to CarboOcean (contract no. 511176-2). Finally, we would like to dedicate this paper to our dear colleague, the late Roland Wollast, for his pioneering work in the biogeochemistry of carbon, nutrients and heavy metals in the Scheldt estuary. References Baeyens, W., van Eck, B., Lambert, C., Wollast, R., Goeyens, L., 1998. General description of the Scheldt estuary. Hydrobiology 366, 1–14. Billen, G., Somville, M., De Becker, E., Servais, P., 1985. A nitrogen budget of the Scheldt hydrographical basin. Neth. J. Sea Res. 19, 223–230. Billen, G., Garnier, J., Rousseau, V., 2005. Nutrient fluxes and water quality in the drainage network of the Scheldt basin over the last 50 years. Hydrobiology 540, 47–67.

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