Seasonal changes in nitrogen and phosphorus transport in the lower Changjiang River before the construction of the Three Gorges Dam

Estuarine, Coastal and Shelf Science 79 (2008) 239–250

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Seasonal changes in nitrogen and phosphorus transport in the lower Changjiang River before the construction of the Three Gorges Dam Shuiwang Duan a, *, Tao Liang b, Shen Zhang b, Lijun Wang b, Xiumei Zhang b, Xibao Chen b a b

Department of Marine Sciences, Texas A&M University at Galveston, 5007 Avenue U, Galveston, TX 77551, USA Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Datun Road 11A, Beijing 100101, P.R. China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 September 2007 Accepted 2 April 2008 Available online 10 April 2008

Water and sediment samples were collected at Datong from June 1998 to March 1999 to examine seasonal changes in the transports of nitrogen (N) and phosphorus (P) from the Changjiang River (Yangtze River) to the East China Sea (ECS). Dissolved inorganic nitrogen (DIN; dominated by nitrate) concentration exhibited small seasonality, and DIN flux was largely controlled by water discharge. Dissolved inorganic phosphorus (DIP) concentration was inversely correlated with water discharge, and DIP was evenly delivered throughout a year. The transports of DIN and DIP from the Changjiang River were consistent with seasonal changes in nutrient distributions and P limitation in the Changjiang Estuary and the adjacent ECS. Dissolved organic and particulate N (DON and PN) and P (DOP and PP) varied parallel to water discharge, and were dominantly transported during a summer flood. The fluxes of DOP and particulate bioavailable P (PBAP) were 2.5 and 4 times that of DIP during this period, respectively. PBAP accounted for 12–16% of total particulate P (PP), and was positively correlated with the summation of adsorbed P, Al–P and Fe–P. Ca–P, the major fraction of PP, increased with increasing percent of CaCO3. The remobilization of riverine DOP and PBAP likely accounted for the summer elevated primary production in DIP-depleted waters in the Changjiang Estuary and the adjacent ECS. The Changjiang River delivered approximately 6% of DIN (1459  106 kg), 1% of DIP (12  106 kg), and 2% of dissolved organic and particulate N and P to the totals of global rivers. The construction of the Three Gorges Dam might have substantially reduced the particulate nutrient loads, thereby augmenting P limitation in the Changjiang Estuary and ECS. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Changjiang River nitrogen phosphorus bioavailable phosphorus N/P ratio

1. Introduction The Changjiang River (Yangtze River) annually transports to the ocean 1430  106 kg of dissolved inorganic nitrogen (1280  106 kg as nitrate; Shen et al., 2003; Yan et al., 2003), twice that of the Mississippi River and higher than the Amazon (Meybeck and Ragu, 1995; Goolsby et al., 2000). This tremendous load of DIN, largely as a result of extensive use of chemical fertilizers (Duan et al., 2000; Yan et al., 2003), has caused elevated primary production, frequent harmful algal blooms and a large area of hypoxia in the Changjiang Estuary and the adjacent East China Sea (ECS; Li et al., 2002; Wang, 2006; Chen et al., 2007). Moreover, the eutrophication in this region exhibits strong seasonal variability, with the worst situation occurring in summer (Gong et al., 2003; Wang et al., 2003a,b). This seasonal variability might be coupled with the timing of riverine nutrient inputs as observed in other river-dominated ocean margins (e.g., northern Gulf of Mexico; Lohrenz et al., 1999). Prior

* Corresponding author. E-mail address: [email protected] (S. Duan). 0272-7714/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2008.04.002

studies on nutrient transports of the Changjiang River mainly focused on either downstream or long-term changes (e.g., Liu et al., 2003; Yan et al., 2003; Li et al., 2007), while their seasonal variations have not been well examined. Nitrogen (N) and phosphorus (P) in dissolved organic and particulate forms, which are more or equally important components to dissolved inorganic N and P in global rivers (Seitzinger et al., 2005), are seldom reported on the Changjiang. The current data on the concentrations of dissolved organic and particulate P (DOP and PP) differ in almost an order of magnitude for unknown reasons (Liu et al., 2003; Yan and Zhang, 2003), leading to large uncertainty in estimating annual P loads of the river. Dissolved organic and particulate nutrients from the Changjiang, might be important nutrient sources in the Changjiang Estuary and the adjacent ECS, based on the non-conservative behaviors of nitrate (NO 3 ) and DIP in the upper Changjiang estuary (Tian et al., 1993; Zhang, 1996), as well as much lower N contents in estuarine sediments than those in riverine suspended particles (Mayer et al., 1998). The remobilization of dissolved organic and particulate nutrients might be especially important to the DIP pool because P is considered as the limiting nutrient for the growth of marine phytoplankton in this region,

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according to nutrient ratios and nutrient addition experiments (Hu et al., 1989; Justic et al., 1995; Wong et al., 1998; Wang et al., 2003a,b). Currently, potential bioavailability and chemical species of particulate P (PP) of the Changjiang River have been examined (Yan and Zhang, 2003), but the influence of PP on the balance of reactive N and P is still unknown. As part of international collaboration through the Land-Ocean Interaction Study Program (LOIS), our project is to examine decadal, inter-annual, and seasonal changes in material transport (including water, sediments, nutrients, organic carbon and trace elements) from the Changjiang and Zhujiang Rivers to the China seas. This paper presents the results of seasonal investigations on the transports of N and P from the Changjiang River at five different river stages (pre-flood, summer flood, fall recession, winter base-flow and spring rising-flow), with the aim of estimating annual nutrient loads and examining seasonal coupling between riverine inputs and nutrient (phytoplankton production) distributions in the Changjiang Estuary and the adjacent ECS. To avoid cross-section heterogeneity in nutrient concentrations, water samples were collected in three water depths of a middle-river and two near bank sites in all five river stages. Historical nutrient data of the Changjiang River (1962–1979 and 1985–1990) at Datong Hydrological Station (DHS) and Global Environmental Monitoring Systems (GEMS) Wuhan site (total phosphorus data only) were included to compare our results with the records of these gauging stations. Some of our suspended sediment samples had been analyzed by Yan and Zhang (2003) to evaluate transport, composition and bioavailability of P during a flood in 1998. This study adds N and more P data so that annual nutrient loads of the Changjiang River can be estimated, and the balance between N and P can be examined as well; the data of river bed and flood plain sediments were also added, to study the effects of hydrological processes on modifying particulate N and P transports. The sampling times of this study (June, August, November and January of 1998–1999) approximately matched the cruise schedules in East China Sea for nutrient and primary production studies

(Gong et al., 2003; Wang et al., 2003a,b), providing an ideal chance for comparing how the nutrient transports from the Changjiang River affects the nutrient (phytoplankton production) distributions in the receiving water body. Furthermore, recent studies show that suspended sediment load of the Changjiang River has been substantially decreased after the closure of Three-Gorges Dam in 2004 (e.g., Xu et al., 2006) and phytoplankton production in the ECS has consequently declined (Gong et al., 2006). The decrease in phytoplankton production, though, is still arguable due to limited sampling frequency and area (Yuan et al., 2007). This study, which was performed just before the construction of Three-Gorges Dam, provides an excellent background to examine the effect of the dam construction on nutrient transports from the Changjiang River and nutrient biogeochemical cycles in the ECS. 2. Materials and methods 2.1. Study site description The Changjiang, the largest river in China and one of the largest in the world, delivers annually 900  109 m3 of water and 500  109 kg of suspended sediment to the ocean (Milliman et al., 1984). It is 6300 km long, drains 1/5 of China and flows eastwards in central China around the latitude of 35 N. Datong (117110 E and 30 460 N), approximately 400 km above the river mouth, was chosen as the study site for several reasons (Fig. 1). Firstly, this site is free from tidal effects and thus water and sediment discharge can be precisely measured (Chen and Shen, 1987). Actually, Datong is the gauging station for the Changjiang River’s total water and suspended sediment discharge; hydrographic and nutrient data are available for last few decades. Additionally, unlike the stations near the Changjiang Estuary, this site is away from large cities and has no near-bank pollution band. Thus, river water at Datong is well mixed and nutrient composition is more representative. The Changjiang River above Datong drains 94.3% of the total watershed and delivers more than 95% of water and sediment loads of the whole river

A B

A

B

Datong Hydrological Station 1

7 8

4

2

5

3

9 6

River bed

Beach

Floodplain

Fig. 1. Map of the Changjiang River and schematics of water and sediment sampling in the lower Changjiang River at Datong. Open and solid circles stand for water and sediment sampling locations, respectively.

S. Duan et al. / Estuarine, Coastal and Shelf Science 79 (2008) 239–250

(Shen et al., 2003), and thus nutrient fluxes at this site are likely close to the totals of the Changjiang River. The Changjiang River is nearly straight in the proximity of Datong. To the north of the river at Datong is the Lower Changjiang Floodplain, and man-made levees have been built for flood protection. To the south there are mostly hills and smaller separate floodplains with natural levees.

2.2. Sample collection and storage Water and suspended sediment were sampled from the Changjiang at Datong on five dates – June 15, August 19 and November 16 of 1998, January 21 and March 28 of 1999. The Changjiang River experienced an unusually severe flood for almost 100 days from late June through September 1998, which was the second largest event (second to 1954 flood) during the 20th Century (Wang and Wang, 1999; Yan and Zhang, 2003). The first sampling of this study was performed just before this flood, and the second (August 19, 1998) was done during the flood. When samples were collected for the third time (November 16, 1998), the flood had ended and the river was in recession stage. The river was in base-flow during the fourth sampling (January 21, 1999), and in the beginning of the rising-flow period for last time (March 28, 1999). At each time of sampling, water samples were collected on Hydrological Vessel 111 from three sites along the same crosssection where DHS staff collected their samples – 260 m, 1050 m and 1570 m from the north bank (Fig. 1). On each site, the upper, the middle and the lower layers [20%, 60% and 80% of water depths (see Table 1), respectively] were sampled using a Niskin bottle (10 L). Approximately 1.5 L of whole water was transferred directly from the Niskin bottle to two acid-washed polyethylene bottles (1 L and 0.5 L), and the rest of the 9 samples were mixed as a composite. Surface bed sediments were also collected at north and central sites in June and November 1998 and January 1999, using a sediment sampler. Water discharge, depth, water temperature and chemical oxygen demand (CODMn) were measured in situ by the DHS staff. Back in the DHS lab, the 9 water samples in 1 L bottles were immediately filtered through 0.45 mm pore size cellulose acetate membranes (Millipore) for dissolved nutrient measurements. The membranes were collected in June and August 1998 for measuring total suspended sediment (TSS) concentrations. The composite sample (approximately 75 L in total) was also filtered through the same type of membranes to collect suspended sediments for the analyses of particulate organic carbon (POC), particulate nitrogen and phosphorus (PN and PP), particulate bioavailable phosphorus (PBAP), CaCO3, and P sequential fractionation. The membranes and river bed sediments were air-dried at the DHS lab and stored at 4  C. Water samples were frozen until analyses. TSS concentrations were measured by reweighing the filters after drying. In November 1998, sediment samples from river beach and a small floodplain were also collected. The beach and floodplain are on the south bank near the fixed cross-section for water sampling. The floodplain is approximately 300 m in width but stretches out

Table 1 Instantaneous water discharge (Q), middle-river water depth, total suspended sediment concentration (TSS), water temperature (t), chemical oxygen demand (CODMn), DOC and POC of the Changjiang River at Datong 3

1

Q (m s 15-Jun-98 19-Aug-98 16-Nov-98 20-Jan-99 28-Mar-99

33,300 75,200 23,500 10,400 16,500

)



Depth (m)

TSS (mg l1)

t ( C)

CODMn (mg O2 l1)

DOC (mM)

POC (mM)

19.4 27.0 19.1 15.5 –

170 430 83.4 21.8 95.9

24.3 28.6 17.7 7.10 15.5

2.82 4.29 2.68 2.09 –

279 511 421 220 315

112 254 78 31 85

241

along the river for 5 km. It begins with a natural levee, declines with a gentle slope and then inclines till it meets a small red-shale hill (Fig. 1). The floodplain was totally submerged during the summer flood and covered with a layer of flood sediment (5–20 cm thick) after the flood. Surface sediments at 0 m, 100 m, 200 m and 250 m from the natural levee, and shale outcrop were collected. Below the natural levee, three beach sediments were also collected perpendicular to the river flow. All these sediment samples were air-dried and stored in the same way as river bed sediments. 2.3. Chemical analyses  þ NO 3 , nitrite (NO2 ), ammonium (NH4 ), and DIP in were analyzed using the UV spectrophotometric method, N-(I-naphthalin group) diaminoethane photometric method (Takeda and Fujiwara, 1993), Nash reagent colorimetric method (Solorzano, 1969) and molybdophosphoric acid method (Murphy and Riley, 1962), respectively.  The detection limit was 0.5 mM for NHþ 4 , 0.1 mM for NO2 , 2 mM for  NO3 , and 0.1 mM for SRP. The concentrations of total dissolved nitrogen (TDN) and P (TDP) were determined by peroxodisulphate digestion method in an autoclave followed by NO 3 and DIP determinations (Grasshoff et al., 1983). The concentrations of dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) were the difference between total dissolved nitrogen (TDN) and DIN, TDP and DIP, respectively. Dissolved organic carbon (DOC) was measured using high-temperature catalytic oxidation on a Shimazu TOC-5000 with precision of 5%. All sediment and rock samples, except the beach sands that were naturally <200 mm, were pulverized in an alumina mortar with a pestle, and then sieved through 100 mesh (<200 mm) before chemical analyses. Total organic carbon and total nitrogen were determined using a CHN analyzer. Total phosphorus and particulate bioavailable-P (PBAP) were determined using the method of Olsen and Sommers (1982) and iron oxide paper strips (Sharpley, 1993), respectively. Iron oxide paper strips were prepared by immersing Whatman 541 filter paper strips into a solution of 0.37 M FeCl3.6H2O for 1 h and dropping into a 2.7 M NH4OH solution for 1 min after dried. To measure PBAP, a dry iron oxide paper strip was shaken with 1 g of sediment (with 40 mL 0.01 M CaC12) for 16 h, and phosphorus absorbed on the strip was extracted with 40 mL of 0.1 M H2SO4 for 1 h and measured as DIP. Sediment P was sequentially fractionated by the method modified from Chang and Jackson (1957). Briefly, 50 mL 1 N NH4Cl (pH ¼ 7), 0.5 N NH4F (pH ¼ 8.2), 0.1 N NaOH, and 1 N HCl were subsequently added to sediment samples (or their residuals) with initial weights of 0.5 g, and the mixtures were shaken for 1 h and centrifuged, and the supernatants were decanted for determination of adsorbed P (Ads–P), aluminum P (Al–P), ferrous P (Fe–P) and calcium P (Ca–P), respectively. After heated with 40 mL 0.3 N sodium citrate (with addition of 10 mL 0.5 N NaOH and 1 g Na2S2O4) at 90  C for half an hour, the sediment residuals were digested with mixed acids (H2SO4:HClO4:HNO3 ¼ 1:4:7), and then centrifuged and decanted for determination of occluded ferrous P (O_Fe–P). Finally, the sediment residuals were extracted again with 0.5 N NH4F (pH ¼ 8.2) for occluded aluminum P (O_Al–P). P sequential fractionations were run in duplicate; and blanks were added per six samples.

2.4. Nutrient fluxes calculation Daily nutrient fluxes for the five sampling dates were obtained by multiplying nutrient concentrations with water discharge over a 24 h period. Total nutrient loads during the period from May 1998 to April 1999 were also estimated, by multiplying daily nutrient fluxes with assigned periods and then summing them up. The assumption was that nutrient concentrations were constant during

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an assigned period. Nutrient concentrations on June 15, 1998 were assigned for the period of May–June 1998 (pre-flood), and similarly August 19, 1998 for July–September 1998, November 16, 1998 for October–December 15, 1998, January 20, 1999 for December 16, 1998– February 1999, and March 28, 1999 for March–April 1999. Water discharge data of the Changjiang River at Datong were reported by Yan and Zhang (2003). Based on historical data, the error of overall NO 3 load estimation was around 13–15%, and larger  for other nutrients – NHþ 4 (60–90%), NO2 (66–95%) and TP (16%– 74%). 2.5. Historical data collection Historical nutrient concentration data, water discharge and TSS at Datong were collected from Changjiang Hydrological Yearbooks (1962–1990), in order to compare the changes in seasonal pattern of nutrient transport during two separate periods, i.e., 1962–1979 when chemical fertilizer use was low and 1985–1990 when chemical fertilizer had been extensively used. DIP concentrations have been not measured at Datong since 1985. Nutrient data after 1990 are still not open to the public. Because TP was not measured at Datong, TP concentration data at GEMS Wuhan site (Fig. 1) were taken instead. 3. Results and discussion 3.1. Water discharge, TSS and other accessory variables Water discharge (Q), water depth at the central site, total suspended sediment (TSS), water temperature (t), CODMn, DOC and POC of the Changjiang River at Datong are listed in Table 1. Water discharge, water depth, TSS and water temperature exhibited large seasonal variability, with the highest values observed during the flood in August 1998 and the lowest in January 1999. TSS was positively correlated with water discharge (R ¼ 0.99, n ¼ 5, p < 0.01). The same seasonal patterns of water discharge and TSS concentration were also observed in historical data at Datong during 1962–1990 (Fig. 3a,b). TSS concentration also displayed large cross-section variability. In June 1998, TSS increased with depth by 5% (from 140 mg L1 in the upper to 148 mg L1 in the lower layer) in the central site, by 25% (from 115 to 144 mg L1) in the south site and by 200% (138 –426 mg L1) in the north site. During the summer flood, SPM increased with depth by >40% in all the three sites (from 496 to 696 mg L1, 347 to 489 mg L1, and 473 to 798 mg L1 in the central, south and north sites, respectively). The spatial heterogeneity of TSS probably reflected divergent hydrodynamic conditions at different locations of the cross section. CODMn, DOC and POC were much higher during the summer flood than other seasons, suggesting an introduction of organic matter by the summer flood to the Changjiang river. 3.2. Dissolved inorganic nutrients NO 3 concentrations ranged from 70 mM during the summer flood (August 1998) to 97 mM during the pre-flood period (June 1998), displaying small seasonal and cross-section (three sites with three depths) variability (C.V. ¼ 13% and <3%; Fig. 2a). NO 3 was the main component of N for all the seasons, accounting for 48%–79% of  total N (TN; Fig. 4). Conversely, NHþ 4 and NO2 exhibited much larger seasonal variations (C.V. ¼ 90% and 109%), with the lowest values (1.8 and 0.2 mM) occurring during the flood or pre-flood periods and the highest values in winter (22.1 mM for NHþ 4 ) or spring þ (2.5 mM for NO 2 ; Fig. 2b, c). Larger cross-section variations in NH4  and NO (C.V.s ¼ 7% –12% and 2% –14%) than NO were also ob2 3  served. NHþ 4 and NO2 were minor forms of TN during the flood and pre-flood periods (1% and <0.2%, respectively), but NHþ 4 became an

important component during winter and spring (17–18% of TN; þ  Fig. 4). Because of the dominance of NO 3 over NH4 and NO2 , total  DIN displayed a similar seasonal pattern to NO3 (not shown). The þ NO 3 and NH4 concentrations of this study were in the same ranges of prior studies at or near Datong during the year 1997 (e.g., Yan et al., 2003; Liu et al., 2003). Historical data at Datong from 1985 to 1990 also showed relatively small seasonality of NO 3 , and slightly higher values during  pre-flood periods (May–June) were also observed; NHþ 4 and NO2 were also generally higher in winter and early spring, although elevated NHþ 4 concentrations were sometimes observed in summer þ  (Fig. 3h,j). However, NO 3 , NH4 and NO2 concentrations during 1962–1979 were lower and no apparent seasonal changes in NO 3 and NH4D were observed. The small seasonality of NO 3 concentrations of the Changjiang River since 1985 might be linked to multiple point and non-point sources (e.g., precipitation, soil loss, sewage, industrial waste, and urbane runoff) of NO 3 (Shen et al., 2003), while the slightly higher values during May–June matched chemical fertilizer use in spring (Duan et al., 2000). The enrichment of  NHþ 4 and NO2 in winter and early spring was likely attributed to temperature-driven low nitrification rate during these periods (Ward, 1996).  As a result of small seasonality of NO 3 concentration, NO3 daily flux was largely controlled by water discharge (R ¼ 0.98, n ¼ 5, p < 0.01), and the value during the summer flood was six times that  of winter low-flow period (Table 2). Although NHþ 4 and NO2 fluxes were higher in spring and winter as a result of their elevated concentrations during these periods, the daily flux of total DIN was still highest during the flood period, which was 4.5 times that of winter. Approximately 44% of DIN (647  106 kg) was transported during the three-month flood period of the year (Table 2). The seasonal pattern of DIN transport from the Changjiang River matched DIN distribution in the Changjiang estuary and the adjacent ECS. For example, during high-discharge periods (especially August 1998) when riverine DIN inputs were highest, waters with elevated DIN concentrations flowed to the northeast in a tonguelike shape and covered the whole outer continental shelf area of the northern ECS; in fall (November 1997) and winter (January 1999) when the Changjiang’s DIN fluxes were smaller, DIN concentrations in the Changjiang Estuary and the adjacent ECS were lower and the DIN-enriched waters were confined to a narrow band southward along the coast (Wang et al., 2003a,b). In another study (Gong et al., 2003) on seasonal changes in primary production in the ECS, very high NO 3 concentration and intensive primary production were observed in August 1998, and the average primary production in August 1998 was 3 times higher than those in other seasons. Thus, DIN fluxes from the Changjiang River and the distributions of DIN and primary production in the Changjiang Estuary and the adjacent ECS were highly coupled in seasonal scale. DIP concentrations exhibited small cross-section (C.V. ¼ 4%– 8%) but large seasonal variability (C.V. ¼ 70%). It increased from 0.2 mM during the summer flood to 1.0 mM during winter low-flow period (Fig. 2d). The contribution of DIP to TP increased from 3% during the summer flood to 60% during the winter low-flow period (Fig. 4b). Historical data of DIP concentrations before 1985 exhibited no apparent seasonal trend, but highest values were also observed in winter (Fig. 3k). DIP concentrations in June 1998 of this study (0.2 mM) were close to Liu et al.’s (2003) values (approximately 0.3 mM from figure) in May 1997 near Datong. The inversely relationship between DIP and water discharge suggests that DIP in the Changjiang River was largely derived from point sources (e.g., sewage from cities; Liu et al., 2003) that were ‘‘diluted’’ by heavy rainfalls during summer. Owing to this inverse relationship, the transport of DIP was more evenly distributed, with the highest value only twice that of the lowest one (Table 2). Correspondingly, the seasonal distributions of DIP in the

S. Duan et al. / Estuarine, Coastal and Shelf Science 79 (2008) 239–250

120

30

NO3-

a

90

243

NH4+

b

20

60 10 30 0

0

Concentration (µ µM)

3

1.5

NO2-

c

2

1.0

1

0.5

0

0.0 0.6

50

DON

e

40

DIP

d

f

DOP

0.4 30 20

0.2

10 0

0 40

8

PN

g

h

30

6

20

4

10

2

PP PBAP

0

0 J

J

A

1998

S

O

N

D

J

F

M

1999

J

J

A

S

1998

O

N

D

J

F

M

1999

þ  Fig. 2. Seasonal changes in NO 3 , NH4 , NO2 , DIP, DON, DOP, PN, PP and PBAP concentrations in the lower Changjiang River during the study period. The solid diamonds are mean values, and the ranges of dissolved nutrients stand for minimum and maximum of the 9 cross-section samples.

Changjiang Estuary and the adjacent ECS were also affected. In contrast to DIN, elevated DIP concentrations were only found along the coast for all seasons, and DIP concentrations during the summer flood were not significantly higher than other seasons (Gong et al., 2003; Wang et al., 2003a,b). Owing to the much larger seasonality of DIP concentrations than that of DIN, the seasonal variations in DIN/DIP ratios of the Changjiang River were largely controlled by DIP. DIN/DIP ratios decreased from over 360 during the flood and pre-flood periods when DIP was the lowest to 120 during the low-flow condition when DIP was the highest (Table 2). High DIN/DIP ratios (approximately 400–500) were also reported in prior downstream investigations in May 1997 (Liu et al., 2003). Hu et al. (1989) showed, by bioassay in laboratory experiments, that in the Changjiang Estuary N limits phytoplankton production if N/P <8, while P is the limiting nutrient if N/P >30. According to our study, the ratios DIN/ DIP in the Changjiang River were all above P limiting ratio (30) and ideal (Redfield) ratio for the growth of marine organisms (16), indicating that biological activities were strongly P limited for all seasons but much more severe during the summer flood and preflood periods. This seasonal pattern of DIN/DIP ratio in the Changjiang River at Datong also matched that of P limitation in the

Changjiang Estuary and the adjacent ECS, where the area and the intensity of the P limitation were largest during the summer flood (August 1998), followed by the pre-flood (May 1998), the recession (November 1997) and the base-flow periods (January 1999; Wang et al., 2003a,b). This coupling, once again, highlights the influence of the Changjiang River’s nutrient inputs on the stoichiometric balance of nutrients in the Changjiang Estuary and the adjacent ECS. The adjustment of water discharge of the Changjiang River by Three-Gorges Reservoir could have changed the seasonality and the magnitude of DIN and DIP fluxes, if the reservoir would store up water during the summer flood and discharge during low-flow periods. If so, DIN and DIP fluxes would be more evenly transported and the summer eutrophication in the Changjiang Estuary and the adjacent ECS would be alleviated to some degree. However, current data show the seasonal pattern of water discharge of the Changjiang River at Datong has not yet been altered (Gong et al., 2006). On the other hand, Zhang et al. (1999) estimated that as phytoplankton use DIN and DIP at Redfield ratio in the Three-Gorges Reservoir, the DIN/DIP ratio of the lower Changjiang River would increase, thereby further strengthening P limitation in the Changjiang Estuary and the adjacent ECS.

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1985-1990

1962-1979

a

60000

60000

40000

40000

20000

20000

0

0

600

500

c

400

400

200

200

100

50

0

90

20

60

10

30

0

0

20

f

120

30

25

80

g

h

60

15

40

10 20

5

0

1.5

3.0

1.0

2.0

0.5

1.0

0.0

0.0 25

15 10

Jan

Dec

Oct

Nov

Sep

Aug

Jun

July

Apr

May

0 Mar

5

0.0 Feb

0.5

Dec

1.0

Oct

TP (µM)

1.5

Jan

l

20

Nov

k

Sep

2.5 2.0

j

4.0

Aug

2.0

5.0

i

Jun

2.5

July

0

May

NH4 (µM)

150

e

Apr

NO3 (µM)

40

NO2 (µM)

d

300

0

DIP (µM)

b

Mar

TSS (mg L-1)

80000

Feb

Q (m3s-1)

80000

þ  Fig. 3. Historical data (1962–1979 and 1985–1990) on seasonal changes in water discharge, the concentrations of TSS, NO 3 , NH4 , NO2 at Datong. DIP concentration data during 1962–1979 at Datong and TP data during 1985–1989 at Global Environmental Monitoring System (GEMS) Wuhan site are also shown.

3.3. Dissolved organic and particulate nutrients In contrast to DIN and DIP, DON and DOP concentrations varied parallel to water discharge, ranging from 6.2 and 0.18 mM during the winter low-flow period to 42 and 0.49 mM during the summer flood (C.V. ¼ 71% and 40%), respectively (Fig. 3e,f). PN, PP and

particulate bioavailable P (PBAP) displayed similar seasonal patterns but with more pronounced variability (C.V. > 73%). They increased from 3.8, 0.62 and 0.10 mM during the winter low-flow period to 31, 6.5 and 0.79 mM during the summer flood, respectively (Fig. 2g,h). TP concentration data at GEMS Wuhan site during 1985 to 1989 showed similar seasonal pattern to that of PP in this study

S. Duan et al. / Estuarine, Coastal and Shelf Science 79 (2008) 239–250

a

245

100

80

Percent

PN DON

60

NO2 40

NH4 NO3

20

0

b

100

80

Percent

PUBAP 60

PBAP DOP

40

DIP 20

0 Jun98

Jul98

Aug98

Sep98

Oct98

Nov98

Dec98

Jan99

Feb99

Mar99

Fig. 4. Seasonal changes in the composition of total N and P of the lower Changjiang River during the study period. PBAP and PUBAP refer to particulate bioavailable P and particulate bio-unavailable P, respectively.

(Fig. 3l). PBAP accounted for 14% of PP on average, with the percentage varying from 12% during the summer flood to 16% during winter low-flow period. DON and DOP concentration values in June 1998 of this study (11 and 0.4 mM) was comparable to those (approximately 10 and 0.5 mM) in May 1997 near Datong that were measured by Liu et al. (2003), but our values of PN and PP concentrations (13.7 mM and 2.7 mM) were twice or more times higher than their values (roughly 6 mM and 0.5 mM). The reason for the difference will be discussed latter. As a result of their seasonal variations, the contribution of DON plus PN to the TN pool was

smallest in winter, but it increased to 50% and overtook that of NO 3 during the summer flood (Fig. 4). Similarly, PP was less important than DIP during the winter base-flow period, but dominated (>80%) the total P pool during the flood and pre-flood periods (Fig. 4). The increase in dissolved organic and particulate N and P concentrations with increasing water discharge suggests their nonpoint sources. Positive relationships between dissolved organic (or particulate) nutrients and water discharge have been reported for many other rivers (e.g., Spitzy and Ittekkot, 1991 and references

Table 2 Nutrient loads (in 106 kg) and atomic N/P ratios of the Changjiang River at Datong during the five sampling dates, the whole year (May 1998–April 1999) and the summer flood (July–September 1998)

NO3 NH4 NO2 DIN DON PN TN DIP DOP PP PBAP TP DIN/DIP DON/DOP PN/PP DIN/BAPa a

15-Jun-98

19-Aug-98

16-Nov-98

20-Jan-99

28-Mar-99

Whole year

Flood

Flood%

3.93 0.05 0.01 3.99 0.42 0.55 4.95 0.02 0.03 0.24 0.03 0.30 423 27 5.0 160

6.42 0.17 0.02 6.61 3.82 2.81 13.24 0.04 0.10 1.31 0.16 1.44 361 88 4.8 73

2.13 0.09 0.02 2.24 0.89 0.29 3.42 0.02 0.02 0.12 0.01 0.17 201 86 5.4 135

1.15 0.29 0.02 1.46 0.08 0.05 1.59 0.03 0.005 0.017 0.003 0.05 118 33 6.1 107

1.63 0.44 0.05 2.12 0.30 0.22 2.64 0.02 0.01 0.11 0.02 0.14 218 71 4.8 125

1354 93 12 1459 518 363 2341 12 15 166 21 193 259 76 4.9 97

629 16 2 647 374 275 1297 4 9 128 16 141 361 88 4.8 73

46 18 20 44 72 76 55 32 62 77 76 73

BAP: total bioavailable P, including particulate bioavailable P (PBAP) and DIP.

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therein; Russell et al., 1998). The possible reason is that heavy rainfalls leach and erode organic matter-enriched top soils during flooding seasons, while during base-flow conditions the rivers are supplied by groundwater from deep layers that are generally low in organic matter (Yano et al., 2004). In addition to soil organic matter, the extremely severe flood in summer 1998 probably also carried plenty of organic waste from the flooded area into the Changjiang River. By using NOAA High Resolution Radiometer datasets, Wang et al. (2003b) estimated that a total area of 358, 867 ha was inundated during the flood of 1998 in the middle and lower Changjiang Drainage Basin. The supporting evidence for the flood supply for organic nutrients was the elevated CODMn, DOC and POC levels, which increased by 52%, 83% and 130% from pre-flood to flood periods, respectively (Table 1). Additionally, the concentrations of DOC and POC during the flood period of 1998 were higher than those for a normal year at Datong (Wu et al., 2007). As a result of positive relationship between the concentrations of dissolved organic (particulate) nutrients and water discharge, daily fluxes of dissolved organic (particulate) N and P exhibited pronounced seasonal variability, with the values in summer 18–76 times higher than those in winter (Table 2). More than 60% of dissolved organic nutrients and 70% of particulate nutrients (including PBAP) were transport during the three-month flooding period. With PBAP counted in the total BAP, the ratio of DIN/BAP decreased to different extent in comparison to DIN/DIP, with the largest reduction occurring during the summer flood. The transports of tremendous dissolved organic (particulate) N and P from the Changjiang River by the summer flood explain the elevated primary production in the Changjiang Estuary and the adjacent ECS in summer, which occurred in an area with very high nitrate but low DIP concentrations (Gong et al., 2003). Gong et al. (2003) suggested that the demand for DIP in this region could be partly supported by DOP originating from the terrestrial input of the Changjiang River. Here, we found the DOP flux of the Changjiang River during the summer flood was 2.5 times that of DIP. Meanwhile, the flux of PBAP was four times that of DIP during the summer flood period. Although PP might largely settle down in the Changjiang Estuary, Watanabe (2007) found that the Changjiang River TSS could cross the ECS and reach as far as the coasts of the Japan Sea during this extreme flood. DON and PN might also be remobilized with P as well, but their influence to the DIN/DIP ratio might not match that of DOP and PP because dissolved organic and particulate nutrients were far more enriched in P than dissolved inorganic component (Table 2). Thus, P limitation in the ECS might be partially alleviated by the transports of DOP and PP from the Changjiang River, especially during the summer flood. However, this bonus might have been offset considerably by the construction of the Three-Gorges Dam, because nearly half of the Changjiang River’s sediment load (thus particulate nutrient loads) has been trapped after the closure of the Three-Gorges Dam (Xu et al., 2006). As a result, the primary production in the Changjiang Estuary and the adjacent ECS has substantially decreased and the phytoplankton assembly changed from the dominance of diatom to other species (Gong et al., 2006). Yuan et al. (2007) argued that the decreases in phytoplankton production in the ECS might be a result of a sampling bias by using data from only four short research cruises that were conducted within this highly variable system, but Satellite remote sensing method of Yuan et al. is also questionable when used in river dominated ocean margins such as the ECS. The larger cross-section variability of DON (C.V. ¼ 5%–19%) and DOP (C.V. ¼ 6%–20%), compared to DIN and DIP, was apparent (Fig. 2e,f), but no consistent cross-section patterns were observed (not shown). This larger variability might be partially derived from analytical errors, because DON (DOP) was calculated from total dissolved N (and P) and dissolved inorganic N (and P). Although PN and PP were not measured for individual site or layer of the cross-

section, they likely followed the cross-section pattern of TSS that exhibited large spatial variability (C.V. ¼ 26–55%), because PN and PP concentrations were positively correlated with TSS (R > 0.99, n ¼ 5, p < 0.01) when examining the seasonal data set. The large cross-section variability of TSS explains why our data of particulate nutrients were much higher than those of the Liu et al. (2003) while the values of dissolved nutrient concentrations almost matched, because Liu et al. took only surface water samples that were generally lower in TSS. Sutula et al. (2004) also found that PP and TSS concentrations were higher in bottom waters than surface layers of the lower Mississippi River, while their DOP and DIP concentrations between the two layers did not significantly differ. Thus, in order to obtain precise PN and PP fluxes of the large rivers such as the lower Changjiang and Mississippi, integrated water samples rather than surface waters are needed. 3.4. Annual N and P fluxes and composition Approximately 2342  106 kg of N and 192  106 kg of P were transported by the Changjiang River at Datong over the one-year period from May 1998 to April 1999 (Table 2). Approximately 58% of N load was in NO 3 , followed in a sequence of DON (22%), PN (15%),  NHþ 4 (4%) and NO2 (0.5%). P was dominantly transported in PN (87%), and DIP and DOP accounted for only 6% and 7% of the total flux, respectively. Approximately 21  106 kg of P (11%) was transported in PBAP. The DIN and DIP fluxes of this study were consistent with their trends of increases in the last few decades, as well as the increases in chemical fertilizer uses and human population in the drainage basin during the same period (Fig. 5). The value of NO 3 load of this study (1354  106 kg) was comparable to prior estimations by Shen et al. (2003) for 1998 (1438  106 kg) and Yan et al. (2003) for 1997 (1280  106 kg). However, our estimations of orþ ganic N (DON þ PN), NO 2 and NH4 loads were 20%, 42% and 68% lower than the values of Shen et al., respectively. This difference occurred likely due to: (1) higher water discharge (1318  106 m3) during the study period of Shen et al. than ours (1235  106 m3); (2) additional inputs between Datong and the river mouth; and (3) sample contamination by the pollution band along the river bank in the Changjiang Estuary (Shen et al., 2003). Annual P flux and composition of the Changjiang River have not been estimated before, but the consistency of our values of DIP and DOP concentration with prior research studies (e.g. Liu et al., 2003) suggests that our estimations of these fluxes are within reasonable ranges. The composition of N load of the Changjiang River at Datong was very similar to that of another large river that also drains a large area of croplands – the Mississippi (NO 3 : 61%, DON: 25%, PN: 13%, NHþ 4 : 2%; Goolsby et al., 2000), and differed from those of lessdisturbed large rivers (e.g., the Amazon and the Yukon) where N is largely bound with organic matter (Meybeck and Ragu, 1995; Guo et al., 2004). The enrichment in NO 3 over other N forms in the Changjiang and Mississippi Rivers was likely a result of several-fold increases in NO 3 load in the past few decades (Turner and Rabalais, 1991; Yan et al., 2003), and thus N compositions in these two rivers were likely not so different from more pristine rivers in the 1950s when chemical fertilizes were not extensively used in their drainage basins. According to our results, the annual DIN load of the Changjiang River (1.46  109 kg) accounted for 5.9% of global rivers’ total (24.8  106 kg), while the DON plus PN flux was 2.1% of global rivers (41.1  106 kg; Seitzinger et al., 2005). Unlike the composition of N load, the composition of P load of the Changjiang and Mississippi Rivers differed. The P load of the Changjiang River was dominated by PP especially the bio-unavailable fraction (PUBAP; Table 2), while DIP was a more important form in the Mississippi River (38% on average; Sutala et al., 2004). Annual load of DIP of the Changjiang River (0.012  109 kg) accounted for only 1.1% of global rivers’ total (1.09  109 kg; Seitzinger et al., 2005), lower than the

S. Duan et al. / Estuarine, Coastal and Shelf Science 79 (2008) 239–250

a

247

1500

8000 N Fertilizer

6000

1000 4000 500 2000

0

b

0

15

400 DIP P Fertilizer

350

Population

300 250

9

200 6

150 100

3

Population (106) P Fertilizer (107 kg)

12

DIP (106 kg)

N Fertilizer (106 kg)

NO3--N (106 kg)

NO3-N

50 0 1965

1970

1975

1980

1990

1985

1995

0 2000

Years NO 3

Fig. 5. Long-term changes in annual fluxes of and DIP from the Changjiang River, along with total chemical N and P fertilizer uses and population in the drainage basin. The NO 3 and DIP data of 1998 are from this study, and nutrient data during 1966–1997 are calculated from Annual Hydrologic Reports of China and from Yan et al. (2003). Data of chemical fertilizer uses and population are from Annual Reports of China (China State Statistical Bureau, 1978–1999).

contributions of DOP (2.1%) and PP (2.2%). Owing to the DIP depletion, PBAP load, which was almost twice that of DIP, was a more important BAP pool for the Changjiang River. 3.5. Sediment P fractionation P in TSS of the Changjiang River at Dating was dominated by Ca–P (66% on average), followed by occluded Fe–P (O Fe–P, 22%), Al–P (6%) and Fe–P (4%), occluded Al–P (O Al–P) and adsorbed P (<1%; Table 3). The contents of Ads–P, Al–P, Fe–P, O Fe–P and PBAP, together with total OC and N, were all the highest during the base-flow period and lowest during the summer flood, while Ca–P exhibited little seasonal variability except a slightly higher value in November 1998 (Fig. 6). The seasonal changes in P composition in TSS were likely a result of ‘‘hydrodynamic sorting’’, whereby finer particles, which were generally enriched in organic matter and aluminum/ ferrous (hydro)oxides (Spitzy and

Ittekkot, 1991), were transported during the low-flow period, while coarser particles were delivered during the summer flood due to large erosion and carrying capacity. The summation of Ads–P, Al–P and Fe–P was positively correlated with PBAP with a slope close to 1 (Fig. 7a), indicating that the majority of PBAP was either adsorbed onto the surfaces of suspended sediments or bonded loosely with Al and Fe hydroxides. Thus, when PBAP was transported to the Changjiang Estuary and the adjacent ECS, it might be remobilized due to ‘salting effect’ or redox of Fe (III) after buried, as observed in the Mississippi River Plume (Fox et al., 1985; Sutula et al., 2004). The minimal seasonality of Ca–P suggests that Ca–P was not affected by ‘‘hydrodynamic sorting’’ and did not vary with particle size. Actually, Ca–P was positively correlated with %CaCO3 (Fig. 7b), indicating that the majority of Ca–P was phosphorus minerals (e.g. apatite) that was in coexistence with calcite or aragonite. The uncommonly high Ca–P contents in the Changjiang River TSS, compared to the Zhujiang

Table 3 Comparison of TN, organic carbon (OC), CaCO3, TP, PBAP, inorganic P phases (in mg g1) between different types of sediments (S.) collected from the lower Changjiang River at Datong

Suspended S.a Bed S.a Beach S.a Floodplain Sa. Flood Suspended S.b Shalea Soilsc Zhujiang R.d a b c d

TN (&)

OC (&)

CaCO3 (%)

TP (&)

BAP

Ads–P

Al–P

Fe–P

Ca–P

O Fe–P

O Al–P

1.7 0.13 0.21 0.76 1.0 0.22 – 1.6

9.3 1.1 1.6 6.4 7.1 1.4 – 12.8

8.6 7.7 9.0 8.8 7.6 11.1 – 0.9

0.70 0.59 0.52 0.58 0.47 0.44 0.61 0.57

70 21 24 50 57 26 – 128

4.1 1.3 1.0 1.5 1.5 3.1 – 0.5

33 4 5 17 19 20 49 26

21 4 5 12 15 0 94 142

530 583 451 492 437 479 384 123

150 26 23 84 110 104 73 207

5.5 3.4 4.8 6.7 4.8 5.1 5.0 3.9

Sediments or rock collected in November 1998. River suspended sediment was collected in August 1998. Soils collected from the upper drainage basin (Sichuan province; Fu and Song, 1982). Average values of floodplain sediments collected from three principal tributaries (Xijiang, Beijiang and Dongjiang) and mainstream of the Zhujaing (or Pearl) River, China.

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Suspended Sediments

10

Ads-P

a

O Al-P

5

5 4

Bed Sediments

g Ads-P (M)

3

Ads-P (N)

2

O Al-P (M) O Al-P (N)

1 0

0

100 Fe-P

60

P Components (µg g-1)

b

Al-P

80

15

h

Al-P (M)

10

Al-P (N) h

40

Fe-P (M)

5

Fe-P (N)

20 0

0

600

c

400 Ca-P

800 600

i

Ca-P (M) Ca-P (N)

O Fe-P

400

200

O Fe-P (M) O Fe-P (N)

200

0

0

200

30

d

PBAP

150

j

20

PBAP (M)

100

PBAP (N)

10

50 0

0

15

e

15

10

10

5

5

k

OC (M) OC (N)

OC, TN (mg g-1)

%CaCO3 (M) OC

%CaCO3 (N)

%CaCO3

0

0

3

f

TN

0.2

l

0.15

2

TN (M) 0.1

1

TN (N)

0.05 0

0 J

J

A

S

1998

O

N

D

J

F

M

1999

J

J

A

1998

S

O

N

D

J

1999

Fig. 6. Seasonal changes in components of inorganic P, TN and organic C (OC), PBAP and CaCO3 contents in suspended sediments and river bed sediments at middle (M) and north (N) sites in the lower Changjiang River during study period. The O Fe–P and O Al–P data of bed sediment in November, 1998 in middle site were not available due to loss of sample during sediment P sequential exaction.

(China; Table 3), the Mississippi and other rivers (Sutula et al., 2004; Romero-Gonzalez et al., 2001), might be related to large area of carbonate rocks, purple calcite sandstone and shale in the upper drainage basin – the major source of TSS in the Changjiang River (Xu et al., 2006). This conclusion is supported by the characteristic high Ca–P contents in the shale outcrop, the soils (Table 3) and fluvial sediments (Xiangxi River; Fu et al., 2006) of the upper drainage basin. Thus, although PP concentrations in the Changjiang River were not low, the major fraction – Ca–P was

probably in bio-unavailable form and did not contribute much to P remobilization in the Changjiang Estuary and the adjacent ECS. Finally, PP samples collected during the first four river stages of this study (June 1998–January 1999) was also sequentially fractionated by Yan and Zhang (2003), but their method of fractionation was different. In their study, adsorbed P, O Fe–P and O Al–P were not measured, and the difference between total particulate P and the summation of Fe–P, Al–P and Ca–P was considered as particulate organic phosphorus.

S. Duan et al. / Estuarine, Coastal and Shelf Science 79 (2008) 239–250

BAP (µg g-1)

a

200

150

100 y = 1.047x + 13.764

50

R2 = 0.94

0 0

40

80

120

160

Ads-, Al- & Fe-P (µg g-1)

Ca-P (µg g-1)

b

800

600

400

y = 23.378x + 273 R2 = 0.75

200

249

The Changjiang River was enriched in DIN but depleted in DIP. DIN was largely transported during the summer flood and preflood periods while DIP was more evenly delivered throughout the year, leading to extreme high DIN/DIP ratio during the high-flow periods and the lowest value during base-flow conditions. The patterns of DIN and DIP transports were consistent with the seasonal variations in nutrient distributions and P limitation in the Changjiang Estuary and the adjacent ECS. Hydrograph of the Changjiang River at Datong has not yet been altered since the construction of the Three Gorges Dam, and the effects of the dam construction on DIN and DIP transports need further examination. Dissolved organic and particulate N and P were dominantly transported during the summer flood, and the fluxes of DOP and PBAP far exceeded that of DIP during this period. P limitation in the Changjiang Estuary and the ECS was partially alleviated by the inputs of DOP and PP from the Changjiang River, especially during the summer flood. Suspended sediments of the Changjiang River seasonally interacted with sediments of river bed and floodplains, and the influence of these interactions on particulate nutrient transports needs consideration in future studies. The construction of the Three-Gorges Dam has caused a reduction in sediment load of the Changjiang River by half, thereby decreasing PBAP load and augmenting P limitation in the Changjiang Estuary and the adjacent ECS.

0 0

5

10

15

20

Acknowledgments

CaCO3 (%) Fig. 7. Correlations of PBAP and adsorbed-P þ Al–P þ Fe–P (a), Ca–P and CaCO3 (b) for the sediments collected from the lower Changjiang River.

Compared to TSS, sediments from river-bed and beach were significantly lower in OC, TN, BAP, Al–P, Fe–P, O Fe–P and Ads–P (Table 3). Despite the difference, OC, TN, BAP and Fe–P in bed sediments, following those in TSS, were all highest in the winter low-flow period (January 1999) in both north and central sites; the highest values of %CaCO3 and Ca–P were found in November 1998 for both TSS and bed sediments (Fig. 6). This consistency between suspended and river-bed sediments suggests that a fraction of fine sediments was deposited to river bed during the recession and base-flow stages. On the other hand, the similarity in OC, TN and P composition between summer-flood TSS and surface floodplain sediments (Table 3) indicates that a proportion of TSS that was transported by the river during the summer flood was trapped in the small separate floodplain after the severe flood of 1998. If we assume that the surface layer of flood-plain sediment was exclusively from the Changjiang River during the 1998 summer flood, the trapped river sediments in this small floodplain were approximately 1.5  105 m3 (300 m in width  5 km in length  10 cm in thickness), amounting to 4.5  105 kg of PN, 3.5  105 kg of PP and 4.5  104 kg of PBAP (assuming sediment density ¼ 4 g cm3) or 0.2–0.3% of their annual loads. These proportions of particulate nutrients would be stored in the floodplain for a long run (years to decades) until being eroded by another severe flood. 4. Conclusion Approximately 2342  106 kg of N and 192  106 kg of P were transported by the Changjiang River over a year from May 1998 to April 1999. NO 3 was the main component of TN while particulate P was the dominant form of TP. Particulate P consisted mostly of Ca–P that was likely derived from rocks in the upper drainage basin; PBAP accounted for a small fraction (11%) of TP load. Prior nutrient measurements with surface waters underestimated particulate nutrient fluxes of the Changjiang River.

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