Impacts of human activities on nutrient transports in the Huanghe (Yellow River) estuary

Journal of Hydrology 430–431 (2012) 103–110

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Impacts of human activities on nutrient transports in the Huanghe (Yellow River) estuary Su Mei Liu a,⇑, Ling Wei Li a, Gui Ling Zhang a, Zhe Liu b, Zhigang Yu a, Jing Ling Ren a a b

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, PR China College of Environmental Sciences and Engineering, Ocean University of China, Qingdao 266100, PR China

a r t i c l e

i n f o

Article history: Received 31 May 2011 Received in revised form 28 December 2011 Accepted 2 February 2012 Available online 10 February 2012 This manuscript was handled by Laurent Charlet, Editor-in-Chief, with the assistance of Eddy Y. Zeng, Associate Editor Keywords: Nutrients Transport Water–sediment regulation event Huanghe estuary

s u m m a r y The impacts of anthropogenic activities on nutrient transport in the Huanghe/estuary were investigated using biogeochemical observations carried out during 2008–2009 to examine how nutrient change during a water–sediment regulation event. Nutrient concentrations in the Huanghe are characterized by relative high concentrations of nitrate and dissolved silicate but low phosphate and DOP levels and shows seasonal variations with very high DIN/PO3 4 ratios. The water–sediment regulation event resulted in high monthly average water discharge and sediment load occurring at least 2 months prior to before the event. The nutrient transport fluxes increased 8–30 times during that period. The regulation event has shifted the seasonal patterns of water and nutrient transport, which cannot only increase nutrient inputs to the coastal ecosystem but can also result in nutrient imbalance, affecting phytoplankton production and composition. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The Huanghe is regarded as the second largest river of the world in terms of sediment load over the last several thousand years and alone represents 6% of the estimated global river sediment flux to the ocean. However, in recent years the Huanghe sediment load has decreased, reverting to its pristine levels of the middle Holocene, prior to human intervention (Saito et al., 2001; Wang et al., 2007). More than 3147 reservoirs have been built in the watershed of the Huanghe, with a total storage capacity of 57.4  109 m3, which is a major source of freshwater for about 107 million people within the river basin, about 8.7% of the total population in China, and water resource consumption by industrial and agricultural activities increasing three times in the early 1990s relative to the 1950s (Chen et al., 2005; Wang et al., 2007). The freshwater discharge shows clear inter-annual signatures during 1950–2009 with a reduction rate of 7.83  108 m3 per year (Fig. 1). The period without water flow at the Huanghe mouth increased from 20 days in 1972 to 200 days in 1997 and the river course without water flow increased two times from 310 km to 683 km

⇑ Corresponding author. Tel./fax: +86 532 66782100. E-mail addresses: [email protected] (S.M. Liu), [email protected] (L.W. Li), [email protected] (G.L. Zhang), [email protected] (Z. Liu), zhigangyu@ ouc.edu.cn (Z. Yu), [email protected] (J.L. Ren). 0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2012.02.005

(cf. Zhang et al., 2001). Although zero water flow day has never happened since the 2000s, the decline of the Huanghe sediment load, as well as synchronous decrease in water discharge has had profound physical, ecological, and geomorphological effects on the lower reaches of the river and the adjacent sea (Wang et al., 2007; Fan and Huang, 2008; Liu et al., 2008). Since 2002, the Yellow River Conservancy Commission has carried out water–sediment regulation scheme at the beginning of every flood season after three field experiments in 2002, 2003 and 2004 to set up this scheme, in order to avoid the situation that there is no water flow and to improve the proportion relationship between water and sediment transport by flushing the reservoirs and reducing sediment deposition in the lower reaches of the river. This management could results in the Huanghe water discharge to attain very high value within a short period, and is expected to influence substance transports and the ecosystem of the Huanghe estuary and the adjacent sea (Li et al., 2009; Yang and Liu, 2007), but it has not been well documented except water chemistry records in the Huanghe before the management practices (Chen et al., 2005; Yu et al., 2010). This paper reports the results of biogeochemical observations in the Huanghe/estuary during 2008–2009. Dissolved inorganic and organic nutrients were measured for samples collected from monthly observations in the Huanghe from November 2008 to December 2009, daily observations in the river and three cruises in the estuary before, during and after the water–sediment regula-

S.M. Liu et al. / Journal of Hydrology 430–431 (2012) 103–110

Discharge

a

40

Sediment

8

3

Discharge (10 m )

30

30 20 20 10 10

0 Sep/08

Dec/08

Mar/09

Jul/09

Sediment load (Mt)

104

0 Jan/10

Oct/09

Date

3

Discharge (m /s)

4000

b

3000

After

Before

2000

During

1000 0 6-Jun

11-Jun 16-Jun 21-Jun 26-Ju n 1-Jul

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16-Jul

21-Jul

26-Jul

Date

Discharge

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Sediment

600

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800

5.0

c

8

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1000

0.0 1960

1970

1980

1990

2000

2010

Time (yr) 6

8

3

Fig. 1. Sediment load (Mt = 10 tons) and freshwater discharge (10 m ) in the Huanghe in 2008–2009 (a), and daily freshwater discharge (m3 s1) during the regulation event with arrows indicating before, during and after the water–sediment regulation event (b). Annual water discharge and sediment load (Gt = 109 tons) (c) in the lower reaches of the river since 1950s are also provided for comparison. Data are from National Compilation Committee of River and Sediment Communiqué (2000) and http:// www.yellowriver.gov.cn/nishagonggao.

Fig. 2. Location of the study area and map of the Huanghe estuary, which shows the sampling sites for three cruises on 15 June, 1 July, and 19 July as before (s), during (), and after (4) the water–sediment regulation event.

tion event in June–July 2009. The purpose of the investigation was to determine the effects of anthropogenic activities (such as the sediment–water regulation events) on nutrient transports in the Huanghe/estuary and the ecosystem of the Bohai Sea.

2. Materials and methods River water samples were collected monthly at Station Lijin, the nearest hydrological station to the river mouth (100 km

105

6.0 Ammonium

5.0

Nitrite

4.0 3.0 2.0 1.0 0.0

350 300 250 200 150 100 50 0

Spring

Summer

0.6

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Spring

Summer

Fall

Winter

Spring

Summer

Fall

Phosphate

DOP

0.5 0.4 0.3 0.2 0.1 0.0

Fall

Si(OH)4 (µM)

NO 3 (µM)

Winter

Concentration (µM )

Concentration (µM )

S.M. Liu et al. / Journal of Hydrology 430–431 (2012) 103–110

Winter

Spring

Summer

Fall

Winter

Spring

Summer

Fall

130 125 120 115 110 105 100 95

DON (µM)

50 40 30 20 10 0

Fig. 3. Monthly nutrient concentrations (lmol L1) in the Huanghe in 2008–2009.

Ammonium

Content (µM)

6.0

Nitrite

4.0 2.0

Concentration (µM)

0.0 Nitrate

300

Dissolved silicate

200 100 0

0.9

Content (µM)

DON

Phosphate

DOP

0.6 0.3 0.0

1200

DIN/DIP

Si/DIN

0.8

900

0.6

600

0.4

300

0.2

0

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Si/DIN ratio

Si/DIP

DIN/P or Si/DIP ratio 19 2 1 J un 2 3 J un 25 J un -J 27 un 29 Jun -J 1 - un J 3- ul Ju 5- l J 7- ul Ju 9- l 11 J ul 1 3 Jul 1 5 Ju -J l ul

upstream), in the lower reaches of the Huanghe from November 2008 to December 2009. During this period, the Huanghe freshwater discharge and sediment contents show higher values in June and July, which is just during water–sediment regulation event, than in the other months (Fig. 1). During a water–sediment regulation event in the summer of 2009, daily observations were conducted at Lijin in the river from 19 June to 13 July, water discharge recorded at Station Lijin rapidly increased from ca. 200 m3 s1 on 21 June to 3200 m3 s1 on 24 June. After maintaining a water discharge larger than 3200 m3 s1 for 9 days, water discharge decreased from 3135 m3 s1 on 3 July to less than 800 m3 s1 on 8 July (Fig. 1). Moreover, three cruises were carried out in the estuary on 19 June, 1 July and 19 July, respectively, which were before, during and after the regulation event. In each cruise in the estuary, three fishing boats were employed, covering six sections (Fig. 2). Data of water discharge during observation period are from http://www.yellowriver.gov.cn/nishagonggao based on specification on water flow monitoring. In the field observations, water samples were collected with polyethylene bucket at three sites across the river on a bridge. The bucket was pre-cleaned with hydrochloric acid (pH = 2) and anhydrous ethanol, and rinsed with Milli-Q water before use. So concentrations of nutrients at each observation in the river were averages of three samples. In the estuary, water samples were collected with polyethylene bottles at surface (0–0.5 m), and a plexiglas bottle was used to collect near-bottom waters. After collection, water samples were stored in a cooler, transported back to the lab as soon as possible, and then filtered immediately through pre-cleaned, 0.45-lm pore-size, acetate cellulose filters by soaked in diluted hydrochloric acid (pH = 2) overnight, then rinsed with Milli-Q water, and the filtrates were poisoned by addition of saturated HgCl2 (ca. 1.5  103 v/v). After filtration, the filters were dried at 50 °C and weighed to determine suspended particulate matter. Nutrient concentrations were quantified using an autoanalyzer (Skalar SANplus) and manual methods (Liu et al., 2009). The  3 þ analytical precision of NO 2 , NO3 , NH4 , PO4 , and Si(OH)4 were 0.01 lmol L1, 0.06 lmol L1, 0.09 lmol L1, 0.03 lmol L1, and

Fig. 4. Daily nutrient concentrations (lmol L1) and nutrient ratios in the Huanghe during the water–sediment regulation event in 2009.

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3

Monthly freshwater discharge (×10 m /month)

Discharge

40 30 20 10 0 800

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DON

Dissolved silicate

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2009-06

2009-05

2009-04

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2009-02

2009-01

2008-12

2008-11

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Fig. 5. Monthly average freshwater discharge (108 m3 month1) and nutrient transport fluxes (108 mol month1) from the Huanghe to the Bohai Sea.

0.15 lmol L1, respectively. The dissolved inorganic nitrogen  þ concentration (DIN) is the sum of NO 3 , NO2 , and NH4 . Total dissolved nitrogen (TDN) and phosphorus (TDP) were measured with boracic acid–persulfate oxidation method (Grasshoff et al., 1999). The analytical precision of TDN and TDP were 0.68 lmol L1 and 0.02 lmol L1, respectively. Dissolved organic nitrogen (DON) and phosphorus (DOP) concentrations are the respective differences between TDN and DIN or TDP and PO3 4 . 3. Results 3.1. Nutrients in the river Nutrient levels in the Huanghe are characterized by high concentrations of nitrate and dissolved silicate but low phosphate and DOP levels. Concentrations of ammonium and nitrite were higher in the winter and spring than in the summer and fall. Nitrate and TDN levels were lower in the summer than in the other seasons, while concentrations of DON were higher in the summer than in the other seasons. Concentrations of phosphate and DOP show seasonally stable levels. Dissolved silicate concentrations were higher in the summer than in the other seasons (Fig. 3). At the beginning of water–sediment regulation event, concentrations of phosphate and dissolved silicate increased up to three times and concentrations of nitrate and DON slightly increased with freshwater discharge increase. While concentrations of ammonium and nitrite slightly decreased and DOP concentration remained variable (Fig. 4). Monthly nutrient fluxes are estimated by the product of the nutrient concentrations and monthly average freshwater discharges of the Huanghe and show high values in June and July when freshwater discharge is the highest, which represents 40% of the total annual freshwater discharge (Fig. 5).

Monthly average nutrient fluxes in June and July, representing 35–60% of annual transports, are 3–8 times the other monthly averages (Fig. 5). 3.2. Nutrients in the estuary In the Huanghe estuary, compared to the case before the water– sediment regulation event, average salinity decreased 7 units in the surface layer and 1 unit in the near-bottom layer during the regulation event in 2009 and decreased 2 units in the surface and maintaining the similar in the near-bottom layer after the event. Before the regulation event, the river plume front defined by the 26-isohaline was hardly identifiable. During the event, the river plume front can easily be traced as far as 24 km offshore from the river mouth in the surface layer while less than 10 km in the near-bottom layer (Wang et al., 2011). Afterwards, one can trace the plume front ca. 15 km in the surface layer and only ca. 3 km in the near-bottom layer (Fig. 6). Both the observation and simulation results also show that the water–sediment management has played an important role in the salinity of the Bohai Sea, especially in Laizhou Bay where salinity during the regulation event was distinctly lower than that before (Mao et al., 2008; Wang et al., 2011). It is expected that the water–sediment regulation event affects the ecosystem of the Huanghe estuary and the Bohai Sea. Concentrations of nitrite and ammonium kept similar levels during and before the event but increased 2–4 times after the event. Concentrations of nitrate, DIN and TDN during the event were 2–3 times higher than before the event, and after the event they maintained similar levels or slightly increased compared to before the event. Concentrations of phosphate and dissolved silicate increased 2–6 times during the event and 2–2.6 times after the event compared to levels measured before. Concentrations of DON and DOP, however, only slightly increased during the event (Fig. 6).

S.M. Liu et al. / Journal of Hydrology 430–431 (2012) 103–110

107

Fig. 6. Horizontal distributions of nutrient concentrations (lmol L1) in the Huanghe estuary before, during and after the water–sediment regulation event.

4. Discussion 4.1. Nutrient transport in the lower reaches of the Huanghe Nutrient levels in the Huanghe show 2–30 times variations depending on the element. DIN concentrations are relatively higher compared to other Chinese rivers and the global river data (Liu et al., 2009; Smith et al., 2003). The excessive riverine DIN is derived from increase in population, cropland use, fertilizer application, and municipal sewage discharge over the drainage basin. Although both ammonium and nitrite concentrations are also higher than other Chinese rivers, they only represent less than 2% of DIN. This may be related to the accelerated nitrification process under the presence of high suspended solids (Xia et al., 2004).

While phosphate concentrations are comparable to other Chinese rivers and they are at the pristine level of the global river data (Smith et al., 2003; Liu et al., 2009) related to less phosphorus export off the soil and adsorption of phosphorus onto particulates as suspended particulate matters are abundant in river waters. Therefore, DIN/PO3 4 ratios were very high ranging from 439 to 818 likely resulting from very low phosphate values. Moreover, DON/DOP ratios were also high ranging from 43 to 310. Concentrations of dissolved silicate in the Huanghe are also higher than in other Chinese rivers in the cold and dry northern China and are similar to those in the Changjiang and Zhujiang. Si(OH)4 in the Huanghe largely originated from mechanical denudation, chemical weathering, and a much higher evaporation over precipitation rate in a large part of its river basin (Li and Zhang, 2003; Chen et al., 2005; Cai et al.,

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1969-1985

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1950-1968

1969-1985

1986-1996

2002-2009

Fig. 7. Monthly average freshwater discharge (a) and sediment load (Mt = 106 tons) (b) at Lijin gauge in three periods related to the operation of the Liujiaxia (1968) and Longyangxia (1986) reservoirs and comparison with since the beginning of the Huanghe water–sediment regulation event in 2002. The right y-axis is just for values since the beginning of the regulation event. Data are from Ding and Pan (2008), Chen et al. (2005) and http://www.yellowriver.gov.cn/nishagonggao.

Table 1 Historical freshwater discharge (108 m3) and sediment load (108 metric tons) during water–sediment regulation events and their percent (%) of annual freshwater discharge and sediment load, respectively. These data are from government reports and documents (http://www.yellowriver.gov.cn/nishagonggao) and Zhang (2008).

a

Year

Period

Days

Discharge (108 m3)

Percent of annual discharge

Sediment load (108 mt)

Percent of annual load

2002 2003 2004 2005 2006 2007 2008 2009

4–15 July 6–18 September 19 June–13 July 16–30 June 10–29 June 19 June–7 July; 29 July–7 August 19 June–3 July 19 June–8 July

11 12 19a 15 20 27 14 20

22.94 27.19 48.01 42.04 48.13 61.76 46.05 34.88

55 14 24 20 25 30 32 25

0.32 1.207 0.697 0.613 0.648 0.9733 0.2007 0.3429

59 33 27 32 43 66 26 61

It is the real days, which does not include 5 days at low freshwater discharge due to boat sinking.

2008; Liu et al., 2009) which enhances Si(OH)4 concentrations. Si/ PO3 4 ratios were also high varying from 167 to 343. While Si/DIN ratios were 0.2–0.5 due to very high concentrations of nitrate. Nutrient transport from the Huanghe can result in phosphorus limitation for phytoplankton growth in the Huanghe estuary and the adjacent Bohai Sea (Turner et al., 1990; Zhang et al., 2004). Although freshwater discharge in the lower reaches of the Huanghe significantly decreased due to operations of reservoirs and decrease of precipitation related to climate change (Wang et al., 2007; Huang et al., 2009), even with the period without water flow at the river mouth increased 10-fold from 20 days in 1972 to 200 days in 1997 (cf. Zhang et al., 2001), the water discharge regime was similar with the highest values in August to October (Fig. 7). Since 2002, the Yellow River Conservancy Commission has carried out water–sediment regulation scheme at the beginning of every flood season. For the last eight water–sediment regulation events (2002–2009), freshwater discharge during water–sediment regula-

tion event represented 14–55% of annual river water discharge with an average of 28%; the sediment load during water–sediment regulation event accounted for 26–66% of annual sediment load with an average of 41% (Table 1). This is significantly different from the previous long-term monthly average freshwater discharge, which shows high values during August to October (Fig. 7). This results in earlier and more discharge of Huanghe water into the Bohai Sea and extends the period of higher discharge for several months. The highest monthly average water discharge and sediment load advance to as early as June, i.e. at least 60 days earlier than before this management was implemented (Fig. 7). Based on limited monitoring data, the transported nitrogen and phosphorus loads in the lower Huanghe decreased in the later 1990s due to desiccation in the lower reaches (Yu et al., 2010). Nutrient fluxes from the Huanghe to the Bohai Sea are closely related to the river runoff at p < 0.001 and concentrate within short periods (1–2 months) when water discharge is high. Thus, nutrient

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3

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4000 3000 2000 1000 0 0

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-2000

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Date (dd-mm-yr) Fig. 8. Daily variations of freshwater discharge (m3 s1) and P- and N-ICEP (105 mol C day1) at Lijin gauge in 2008–2009.

transport fluxes from the Huanghe to the coastal ocean have been significantly affected by the water–sediment regulation events. 4.2. The effects of Huanghe water–sediment regulation event on the ecosystem of the Bohai Sea During the last 40 years of the 20th century, seawater salinity, temperature, DIN concentration, and the N:P ratio all increased, while concentrations of dissolved oxygen, phosphate, and dissolved silicate, and the Si:N ratio all decreased in the Bohai Sea (Zhang et al., 2004; Liu et al., 2008; Ning et al., 2010). This maybe caused by a combination of reduced freshwater inflow, climate change, and anthropogenic activities. These changes have also had an influence on phytoplankton production. The ratio of diatoms to dinoflagellates has been dramatically reduced by eight times from 57 to 7 over the last four decades (Kang, 1991; Wang and Kang, 1998; Sun et al., 2002). As nutrient transports are closely related to freshwater discharge as discussed above, one might expect to observe ecosystem changes. Indeed, chlorophyll a, phytoplankton biomass, and primary production were observed to decrease by 42%, 55% and 31%, respectively, from 1982–1983 to 1992–1993 while the freshwater discharge decreased by 59% (Fei et al., 1991; Wang and Kang, 1998; Lü et al., 1999; Tang and Jin, 2002). If average water discharge and nutrient transport fluxes in the lower reaches of the Huanghe before 23 June were taken as the case prior to water–sediment regulation and the average values between 19 June and 8 July were taken as that during the event, water discharge increased nine times, DIN, DON and TDN fluxes all increased 10 times, phosphate, DOP and TDP fluxes increased 8–18 times, while dissolved silicate fluxes increased 30 times during the event. Water–sediment regulation event, therefore, is more able to influence offshore water than normal freshwater dis-

charge in summer, which tends to be confined to the coastal shelf. The coastal environment off the Huanghe estuary should be given more attention. Moreover, molar ratios of DIN/PO3 4 decreased, but molar ratios of Si/PO3 4 and Si/DIN increased during the water–sediment regulation event (Fig. 4). However, it is hardly identifiable changes of monthly average molar ratios of nutrients resulted from the water–sediment regulation event (not shown). With the existing data, it is difficult to understand how the water–sediment regulation event affects the nutrient ratios. Based on investigations during 1959–1999, the annual cycle of phytoplankton biomass in the Bohai Sea has two peaks, one in March or April, and the other in July to October (Fei et al., 1991; Lü et al., 1999; Sun et al., 2003). Nutrient transports from the Huanghe may compensate for the nutrients consumed during the spring bloom and may lead to the second bloom being initiated earlier than before and with a different phytoplankton composition. Observations on phytoplankton species composition and biomass in May and August of 2004– 2006 in the Huanghe estuary indicated that the dominant species composition changed greatly compared to in 1990s, and the biomass in August was 1–2 orders higher than in May (Zhang et al., 2010). Both the increased delivery of nutrients and their imbalance are known to be major threats to coastal zone ecosystems, causing severe eutrophication problems, such as harmful algal blooms and hypoxia (Turner and Rabalais, 1994; Liu et al., 2009). The indicator of coastal eutrophication potential (ICEP) of terrigenous nutrient inputs is addressed based on the flux of nitrogen (N-ICEP) or phosphorus (P-ICEP) delivered in excess over silica and is expressed in terms of carbon equivalence (Garnier et al., 2010 and references therein). Based on calculations from the DIN, DIP, TDN, TDP and DSi fluxes, the Huanghe transports show negative or very low P-ICEP values but positive N-ICEP values, especially during water– sediment regulation event (Fig. 8), indicating that P limitation

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should prevent the growth of phytoplankton after the spring bloom despite the presence of excess nitrogen. The water–sediment regulation event, therefore, cannot only increase nutrient inputs to the coastal ecosystem but can also further result in nutrient imbalance, affecting phytoplankton production and composition. Further observations are needed to examine the response of the Bohai Sea ecosystem to the water–sediment regulation event.

5. Conclusions The biogeochemical observations indicated that the Huanghe are enriched with nitrate and dissolved silicate and depleted in phosphate and DOP levels with very high DIN:PO3 4 ratios ranging from 439 to 818 likely resulting from very low phosphate values. The excessive riverine DIN is derived from increase in population, cropland use, fertilizer application, and municipal sewage discharge over the drainage basin. High concentrations of Si(OH)4 largely originated from weathering, and a much higher evaporation over precipitation rate in a large part of its river basin. And low levels of phosphate and DOP are related to less phosphorus export off the soil and adsorption of phosphorus onto particulates as suspended particulate matters are abundant in river waters. Nutrient concentrations show seasonal variations with high values in dry season and low values in flood season. Nutrient transport from the Huanghe can result in phosphorus limitation for phytoplankton growth in the Huanghe estuary and the adjacent Bohai Sea. Since 2002, water–sediment regulation scheme has been carried out at the beginning of every flood season, resulting in high monthly average water discharge and sediment load advance to as early as 2 months earlier than before the event with the highest in August to October. Nutrient transport fluxes from the Huanghe to the coastal ocean increased 8–30 times during the water–sediment regulation events. The regulation event has shifted the seasonal patterns of water and nutrient transport, cannot only increase nutrient inputs to the coastal ecosystem but can also result in nutrient imbalance, affecting phytoplankton production and composition. Further observations are needed to examine the response of primary production and species composition in the Bohai ecosystem to the water–sediment regulation events.

Acknowledgements The authors are very grateful to colleagues from the College of Environmental Sciences and Engineering, and the Laboratory of Marine Biogeochemistry, Ocean University of China for their help in field work; thank Prof. Zuosheng Yang for discussion on water discharge observations. We are also thankful for the constructive comments by associate editor and reviewers. This study was funded by Special Funds from National Key Basic Research Program of PR China (No. 2011CB409802), Chinese Natural Sciences Foundation (Nos. 40925017, 40876054 and 40706007). The authors thank the captains and crews of the fishing boats for their assistance on board.

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