Developing numeric nutrient criteria for the Yangtze River Estuary and adjacent waters in China

Journal of Hydrology 579 (2019) 124188

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Research papers

Developing numeric nutrient criteria for the Yangtze River Estuary and adjacent waters in China Fuxia Yanga, Tiezhu Mib,c, Hongtao Chena,b, Qingzhen Yaoa,b,

T

⁎

a

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, 238 Songling Road, Qingdao 266100, China Laboratory of Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China c College of Environmental Science and Engineering, Ocean University of China, 238 Songling Road, Qingdao 266100, China b

A R T I C LE I N FO

A B S T R A C T

This manuscript was handled by Huaming Guo, Editor-in-Chief, with the assistance of Bibhash Nath, Associate Editor

Developing numeric nutrient criteria for estuaries and adjacent waters has been exceedingly complex due to the uniqueness of this ecosystem and its dynamic properties. To control and improve water quality, based on data from 1984 to 2014, a population distribution approach was applied to establish the numeric nutrient criteria for the Yangtze River Estuary and adjacent waters. Based on the natural geographical characteristics, currents effects and biological community features, the Yangtze River Estuary and adjacent waters are divided into four sea subareas: the transitional zone (segment I), Hangzhou Bay (segment II), the coastal zone outside of the estuary (segment III) and Zhoushan adjacent area (segment IV). The respective recommended criteria for dissolved inorganic nitrogen (DIN) and phosphate (PO4-P) were 1.05–1.24 mg/L and 0.03–0.036 mg/L for estuary segments I and II and approximately 0.2 mg/L and 0.01 mg/L for the adjacent water segments III and IV. The dissolved oxygen (DO) criteria are similar for the four segments, i.e., 6–7 mg/L. The chemical oxygen demand (CODMn) criteria for segments III and IV (0.56–0.59 mg/L) are much lower than those for segments I and II (1.04–1.15 mg/L). The total nitrogen (TN) and total phosphorus (TP) criteria for segments I and II (1.56–1.85 mg/L and 0.060–0.072 mg/L, respectively) are much higher than those for segments III and IV (0.27–0.29 mg/L and 0.023–0.028 mg/L, respectively). The proposed criteria were reasonable and reliable based on validation using nutrient-algal bloom sensitivity and comparison with current sea water quality standards and previous studies.

Keywords: Nutrient criteria Population distribution approach Nutrient-algal bloom sensitivity Yangtze River Estuary

1. Introduction Eutrophication, an enrichment of coastal waters by nutrients such as N and P, is becoming increasingly serious worldwide, leading to the frequent occurrence of undesirable harmful algae blooms (HABs) and seriously impacting aquatic ecosystems and water uses (Anderson et al., 2008; Davidson et al., 2014; Glibert et al., 2005; Gowen et al., 2012; Howarth et al., 2011; Lewitus et al., 2012). The Chinese coastal waters, which consist of mostly closed and semiclosed shelf sea, are easily vulnerable to the effects of anthropogenic perturbations, resulting in serious cultural eutrophication, especially in estuaries and bays (Liu et al., 2013; Qian et al., 2018; Strokal et al., 2014; Wang, 2006; Wang et al., 2016; Yin et al., 2014). There are no available and suitable standard for estuaries in China, which are located in a mixed zone of freshwater and seawater, because Sea Water Quality Standards (GB 3097-1997) and Environment Quality Standards for Surface Water (GB

3838-2002) are set for sea water and inland surface freshwater, respectively. Nutrient criteria are defined as the maximum acceptable concentrations that could not threaten a particular designated beneficial use of the waterbody (US EPA, 1998). The formulation and application of nutrient criteria are not only measures to effectively prevent water eutrophication but also scientific bases for the comprehensive monitoring, evaluation and management of estuarine nutrients (Meng et al., 2008). Thus, it is necessary and urgent to develop estuarine nutrient criteria, providing scientific theories and methods for the establishment of the corresponding standards (Wu et al., 2010). For developing numeric nutrient criteria, the United States has released a series of peer-reviewed technical guidance documents to address nitrogen/phosphorus pollution in different water body types (e.g., lakes and reservoirs, rivers and streams, estuarine and coastal marine waters, and wetlands) (US EPA, 2000a,b, 2001, 2008). Three types of

⁎ Corresponding author at: Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, 238 Songling Road, Qingdao 266100, China. E-mail address: [email protected] (Q. Yao).

https://doi.org/10.1016/j.jhydrol.2019.124188 Received 27 May 2019; Received in revised form 20 September 2019; Accepted 27 September 2019 Available online 27 September 2019 0022-1694/ © 2019 Elsevier B.V. All rights reserved.

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2.2. Data sources

scientifically defensible approaches, the reference condition approach, mechanistic modeling and the stressor-response analysis, were recommended for setting regional numeric nutrient criteria (Hausmann et al., 2016; Heatherly, 2014; US EPA, 2010). To control water eutrophication, the European Union also developed nutrient criteria strategies (Bouleau and Pont, 2015; Poikāne et al., 2010; Solheim, 2005). Based on the defensible empirical nutrient criteria approaches, some progress has been made in establishing region-specific nutrient criteria for lakes in China (Huo et al., 2012; Huo et al., 2015a,b; Huo et al., 2017; Huo et al., 2019; Zhang et al., 2014). However, with regard to developing estuarine nutrient criteria, few systematic studies have been conducted. A population distribution approach was applied to build a reference state and establish nutrient criteria in estuaries and adjacent waters (Hu et al., 2011; Su et al., 2016; Zheng et al., 2013a,b). Because they are considered the most vital, estuary classification and delineation as well as reference condition determination are specifically addressed (Meng et al., 2008). On the basis of the segmentation of the Jiulong River Estuary, Liu et al. (2018a) used reference conditions, statistical model analysis and cumulative frequency distributions to determine nutrient criteria. According to the stress-response relationship model of diatom biomass and nutrients, Yang et al. (2016) developed the nutrient criteria for the Daliaohe River Estuary. The Yangtze River Estuary and adjacent waters have suffered from serious eutrophication for the past decades due to elevated riverine nutrients (Dai et al., 2011; Zhang et al., 2007; Zhou et al., 2008). From 1963 to 2004, the nitrate and phosphate concentrations increased from 0.15 to 1.36 mg/L and from 0.012 to 0.029 mg/L, respectively (Chai et al., 2006). The frequency and area of red tide outbreaks are increasing significantly (Li et al., 2014; Liu et al., 2013; Wang, 2006). For example, red tides occurred 174 times from 1972 to 2009, 25 instances of which were larger than 1000 km2. Therefore, to control and improve water quality and provide effective management measures, it is urgent and crucial to establish effective nutrient criteria. Based on the data of the Yangtze River Estuary and adjacent waters from 1984 to 2014, the aims of this study were (1) to establish nutrient criteria by the population distribution approach and (2) to verify the nutrient criteria on the basis of nutrient algal bloom sensitivity. This study forms a preliminary contribution to the nutrient monitoring, evaluation and management of estuaries and coastal marine waters.

The recent data used in this study are derived from observations taken during 4 cruises (August 2011, August 2013, February/July 2014) from 2011 to 2014 in the Yangtze River Estuary and adjacent waters (Fig. 1b). Historical data were collected from unpublished survey data from 1984 to 2007. Most surveys were carried out in the spring (April), summer (July/August), and autumn (November) from 1984 to 2000 and during every season for each year after 2000. The sampling and analytical methods are strictly based on standard methods (GB 17378.4-2007; Grasshoff et al., 1983, 1999), which ensured that the data were comparable throughout the study period. The survey parameters included phosphate (PO4-P), nitrate (NO3-N), nitrite (NO2-N), ammonia (NH4-N), dissolved oxygen (DO), chlorophyll a (Chla), and chemical oxygen demand (CODMn). PO4-P, NO3-N, NO2-N and NH4-N were measured using phosphate-molybdenum, the cadmium-copper reduction method, the standard pink azo dye method and the indophenol blue method after the water was filtered through a 0.45 µm cellulose acetate membrane. The detection limits for PO4-P, NO3-N, NO2-N, and NH4-N were 3.1 × 10−4, 2.8 × 10−4, 2.8 × 10−4 and 4.2 × 10−4 mg/L, respectively. The dissolved inorganic nitrogen (DIN) concentration was calculated as the sum of NO3-N, NO2-N and NH4-N. Total nitrogen (TN) and total phosphorus (TP) were measured by alkaline potassium peroxodisulfate oxidation-colorimetry (Grasshoff et al., 1999). Chla was extracted with acetone and analyzed with a fluorometric method, and the detection limit was 0.01 µg/L after the water was filtered through GF/F filters (which were precombusted at 450 °C for 4 h). DO and CODMn were measured using the Winkler titration method and the potassium permanganate alkaline method, respectively. In particular, the Hangzhou bay data, CODMn data and DO data of the transition zone were only from 1984 to 2007. The mean values of each nutrient measured at the stations in each of the four seasons were calculated and used to represent the annual mean nutrient concentrations. To make the data appear symmetrically distributed, the ln (log base “e” (approximately 2.71828)), transformation was applied to the values of DIN and Chla of the Yangtze River Estuary adjacent waters. The drawing software Origin 8.5 was used for data analysis and figure creation. 2.3. Analytical approach

2. Materials and methods The reference condition method is suitable for watersheds with minimal or unimpaired sites, and reference sites in the upper 25th percentile are commonly used to develop nutrient criteria. If reference sites are lacking, the population distribution method can serve as an alternative to the reference distribution method. The population distribution method does not involve the identification of reference sites and uses the entire population of a region to determine nutrient criteria. One percentile from the lower 5th to 25th percentiles of the population sites has been commonly suggested as a threshold for criteria determination (US EPA, 2000a). Moreover, a criterion based on the lower 25th percentile, which was the 25th percentile of the accumulative frequency, of all the sampled streams was proposed for regions without minimally impacted systems and was expected to be a reasonably close approximation of the reference 75th percentile (US EPA, 1998). Due to the heavy input of waste loads from watersheds and human activities, the Yangtze River Estuary and adjacent waters have been seriously polluted (Liu et al., 2013; Wang, 2006). Therefore, the lower 25th percentile of the population frequency distribution was used to develop the nutrient criteria.

2.1. Study area The Yangtze River Estuary and adjacent waters are located at the junction of the East China Sea and the Yellow Sea, on the continental shelf at the western rim of the Pacific Ocean (Fig. 1a), receiving 890 km3 of fresh water and 3.97 × 1011 kg of sediment annually from the Yangtze River. It is a mesotidal estuary with an average tidal amplitude of 2.8 m and a maximum of more than 5 m (Chai et al., 2006). The Yangtze River Estuary and its adjacent sea area is a highly dynamic system, mainly comprised of the Yangtze River diluted water (CDW) as well as coastal currents (Fan and Song, 2014). Morphology, tidal amplitude, salinity, and circulation are natural factors that strongly influence the overall nutrient status of estuaries and coastal water systems. Based on the natural geographical characteristics, the effects of currents and the features of different biological communities, the Yangtze River Estuary is divided into four sea subareas: the transitional zone (segment I), Hangzhou Bay (segment II), the coastal zone outside of the estuary (segment III) and Zhoushan adjacent area (segment IV) (Fig. 1a) (Liu et al., 2011; Zhou et al., 2003). Given that the red tides occurred the most frequently in segments III and IV (29–32°N, 122–124°E) (Liu et al., 2013; Tang, 2009), this area was called the whole sea area. The available numbers of data in different segments are shown in Tables 1 and 2.

2.4. Causal and response indicator variables In general, the primary variables include the causal variables, such as TN, TP, DIN and PO4-P, and the response variables, including a 2

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Fig. 1. (a) Study area and distribution of red tides. This figure was adapted from Liu et al. (2013). (b) The sampling sites of the Yangtze River Estuary and adjacent areas.

Marine Waters (2001) recommended that TN and TP were the principal causative agents, while Chla, SD and DO were the response variables. TN, TP, Chla, etc. were used in setting estuarine and coastal water nutrient criteria, e.g., in Florida (US EPA, 2012). According to the monitoring indexes of estuaries and coastal waters in China, DIN and PO4-P are routine monitoring data. Furthermore, DIN and PO4-P are required for water quality monitoring in the Sea Water Quality

measure of algal biomass (e.g., Chla for phytoplankton or macroalgal biomass (ash free dry weight, AFDW)) and water clarity (Secchi depth, SD), with the addition of DO and CODMn, as appropriate (Hu et al., 2011; Liu et al., 2018a; US EPA, 2001; Zheng et al., 2013a,b). Increases in nutrient concentrations lead to the proliferation of phytoplankton, which increases phytoplankton density and decreases SD and DO. The Nutrient Criteria Technical Guidance Manual: Estuarine and Coastal

Table 1 Frequency distribution results of variables in the Yangtze River Estuary (mg/L). Index

DIN PO4-P DO CODMn

I

II

N

5%

25%

50%

75%

N

5%

25%

50%

75%

750 770 637 682

0.78 0.017 6.06 0.87

1.24 0.030 6.90 1.15

1.56 0.037 7.60 1.35

1.84 0.047 8.30 1.68

539 521 549 569

0.09 0.025 6.60 0.56

1.05 0.036 7.25 1.04

1.37 0.045 8.02 1.36

1.63 0.053 8.78 1.65

The bold values show the recommended criteria values of the variables in each segment of the Yangtze River Estuary and the adjacent waters. 3

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Table 2 Frequency distribution results of variables in the adjacent waters of the Yangtze River Estuary (mg/L) (Chla, μg/L). Index

Whole sea area

DIN PO4-P DO CODMn Chla

III

IV

N

5%

25%

50%

75%

N

5%

25%

50%

75%

N

5%

25%

50%

75%

2154 2224 2108 1535 257

0.055 0.001 2.72 0.25 0.39

0.20 0.011 6.00 0.56 1.20

0.43 0.022 7.12 1.01 2.94

0.98 0.03 8.34 1.37 6.15

1553 1604 1554 1085 –

0.06 0.001 2.45 0.25 –

0.20 0.012 5.78 0.59 –

0.42 0.02 7.04 1.09 –

1.06 0.03 8.34 1.43 –

601 620 554 450 –

0.048 0.001 3.53 0.27 –

0.19 0.010 6.38 0.56 –

0.45 0.02 7.33 0.84 –

0.84 0.03 8.34 1.21 –

The bold values show the recommended criteria values of the variables in each segment of the Yangtze River Estuary and the adjacent waters. 150

150

segment

250

segment

segment

125

120

75 50

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25 0

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0.00

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30

0

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1

2

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60 40 20

0.00

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0

PO4-P/(mg/L)

DIN/(mg/L)

160

5

6

7

8

9

10

11

12

13

DO/(mg/L)

250 segment

segment

200

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120

Frequency

12

segment

100

60

10

120

segment

40

8

DO/(mg/L)

180

segment

60

6

PO4-P/(mg/L)

DIN/(mg/L)

Frequency

90

80

150

100

40 50

0

0.5

1.0

1.5

2.0

2.5

0

3.0

COD/(mg/L)

012

3

4

5

COD/(mg/L)

Fig. 2. Frequency distributions of variables in the Yangtze River Estuary.

0.20 and 0.19 mg/L for DIN, 0.030, 0.036, 0.012 and 0.010 mg/L for PO4-P, 6.90, 7.25, 5.78 and 6.38 mg/L for DO, and 1.15, 1.04, 0.59 and 0.56 mg/L for CODMn (Figs. 2 and 3, Table 3). Similarly, the recommended criteria values of the whole sea area for DIN, PO4-P, DO, CODMn and Chla were 0.20, 0.011, 6.00, 0.56 mg/L and 1.20 μg/L, respectively (Fig. 3, Table 3). The TP and TN criteria have been widely used in China and in other countries (Meng et al., 2008; Heatherly, 2014; Hu et al., 2011; US EPA, 2000a,b, 2010). In segments I and III and the whole sea area, the DIN/TN values were approximately 0.67, 0.68 and 0.58, respectively, and the PO4-P/TP values were

Standard (GB 3097-1997). Based on the historical data of the Yangtze River Estuary and adjacent waters, DIN, PO4-P, TP and TN were selected as the causal variables, and Chla, DO and CODMn were selected as the response variables. 3. Results The population distribution curves and corresponding percentile values are shown in Figs. 2 and 3 and Tables 1 and 2. In segments I, II, III and IV, the respective recommended criteria values were 1.24, 1.05, 4

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350

150

400

300 120

Frequency

300 250 200 150

250

Frequency

Frequency

350

200 150

60

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100

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0 -5

0 -5

-4

-3

-2

-1

0

1

-4

-3

-2

-1

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0 -4

1

-3

-2

LN(DIN/(mg/L))

LN(DIN/(mg/L))

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320

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250

240

-1

0

1

LN(DIN/(mg/L))

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PO4-P/(mg/L)

350

400

Frequency

0.04

PO4-P/(mg/L)

PO4-P/(mg/L)

250 200 150

120 90 60

100

100

30

50

0 4

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10

DO/(mg/L)

12

14

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Frequency

40

30

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-2

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LN(CKOD/(ȝg/L))

Fig. 3. Frequency distributions of variables in the adjacent waters of the Yangtze River Estuary.

China's environmental quality standards for sea water quality (GB 3097-1997), the water quality is divided into four categories. Class I and Class II (Table 3) apply to marine fisheries, marine nature reserves, rare and endangered marine life reserves, and aquaculture areas, bathing beaches, marine sports or recreational areas and industrial water areas. Compared with the current sea water quality standard in

approximately 0.50, 0.42 and 0.35, respectively (Table 3). The respective DIN/TN and PO4-P/TP values were similar for segments I and II, and the ratios for segments III and IV were also similar (Table 3). Thus, the criteria in segments I, II, III, IV and the whole sea area were approximately 1.85, 1.56, 0.29, 0.27 and 0.34 mg/L for TN and 0.06, 0.072, 0.028, 0.023 and 0.031 mg/L for TP (Table 3). According to 5

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in the East China Sea (ECS) was 0.013 mg/L. Moreover, the Japan Fisheries Society (JFS) (1973) indicated critical values for seawater eutrophication, in which DIN, DIP and CODMn were 0.2–0.3, 0.045 and 1–3 mg/L, respectively. Subsequently, these values were substantiated by the nutrient data recently collected prior to the onset of the red tide outbreaks in the China Sea (Li, 2009; Yang et al., 1990; Zhong et al., 2002). The recommended criteria for DIN, PO4-P and CODMn in the whole sea area were 0.20, 0.011 and 0.56 mg/L (Table 3), respectively, which were significantly smaller than the corresponding thresholds. Furthermore, DIN and PO4-P criteria values were also lower than the corresponding concentrations in the spring before the red tide occurrence in the high red tide area (Table 4). These accurately explained the comparative feasibility and reliability of the recommended criteria in the Yangtze River Estuary and adjacent waters.

Table 3 Recommended criteria of variables in the Yangtze River Estuary and adjacent waters (mg/L) (Chla, μg/L). I

DIN DIN/TN TN PO4-P PO4-P/TP TP DO CODMn Chla

1.24 0.67a 1.85 0.030 0.50a 0.060 6.90 1.15 –

II

1.05 0.67 1.56 0.036 0.50 0.072 7.25 1.04 –

III

0.20 0.68b 0.29 0.012 0.42b 0.028 5.78 0.59 –

IV

0.19 0.68 0.27 0.010 0.42 0.023 6.38 0.56 –

Whole sea area

0.20 0.58c 0.34 0.011 0.35c 0.031 6.00 0.56 1.20

Sea water quality standard Class I

Class II

0.20 – – 0.015 – – 6.00 2.00 –

0.30 – – 0.030 – – 5.00 3.00 –

4.2. Comparison of numeric nutrient criteria

The bold values show the criteria values of the Yangtze River Estuary and adjacent waters are not within Class I or Class II. Data were from aChai et al, 2009; bWang et al, 2016a,b; cLi et al., 2009.

The numeric nutrient criteria among different estuaries varied greatly (Table 5). This may be attributed to the difference in natural characteristics, such as dilute water, water residence time and vertical stratification, of different estuaries (US EPA, 2001). Even for the same estuary, due to the salinity, circulation or runoff difference, the nutrient criteria were also different in each subarea (Liu et al., 2018a). The TN criterion was closely related to hydrologic features and disturbances, which resulted in an increase in the TN criterion (Zhang et al., 2014). Yang et al. (2016) showed that salinity had a strong influence on the DIN criterion in the Daliaohe River Estuary. As for the adjacent waters of the Yangtze River Estuary, Zheng et al. (2013a,b) determined reference condition values for eutrophication indicators and showed that there were obvious differences among different seasons. The proposed nutrient criteria of the study were in the range of the reference condition values, except for the DIN and PO4-P criteria in the Zhoushan adjacent area, which were lower than the corresponding reference condition values (Table 5). This suggests that the DIN and PO4-P criteria in the Zhoushan adjacent area are stricter. Due to the wider range of the study and higher primary productivity, the Chla criterion in the whole sea was slightly higher than 1.02 μg/L (Zheng et al., 2013b). In general, the proposed nutrient criteria of the Yangtze River Estuary and the adjacent waters are reasonable and reliable.

China, all of the criteria values of the Yangtze River Estuary and adjacent waters were within Class I or Class II except for the DIN in segments I and II and the PO4-P in segment II (Table 3). The respective recommended criteria for DIN, PO4-P and CODMn in the in-shore areas (segments I and II) are much higher than those in the offshore areas (segments III and IV) (Table 3). The increase in the loads of land-based pollutants (e.g., nutrients, sediment, and pesticides) caused by human-based changes in land-use during urbanization can result in a decrease in water quality (Álvarez-Romero et al., 2013; Liu et al., 2018b). Dodds and Oakes (2004) applied the influence of anthropogenic land uses on nutrient concentrations to extrapolate nutrient criteria. The DIN and PO4-P concentrations in the in-shore areas were clearly higher than those in the offshore areas after 1986 (Fig. 4a, b), which was mainly attributed to the increased human population density, the industrial and municipal wastewater and consumed fertilizers in the Changjiang River basin (Dai et al., 2011; Li et al., 2007), the convergence of high nutrient tributaries (Liang and Xian, 2018), and the Huangpu River and four sewage outlets from Shanghai City which discharge into the Yangtze River Estuary (Chai et al., 2006; Chen et al., 2012). Additionally, high turbidity, strong tidal mixing and short residence time in the in-shore areas strongly constrained phytoplankton growth (Chai et al., 2006). Due to the input of land source pollutants, CODMn concentrations in the in-shore areas with low salinity are higher than those in the offshore areas with high salinity (Fig. 4c) (Deng et al., 2010; Tang, 2009).

4.3. Developments in the estuarine nutrient criteria determination in China To effectively control eutrophication of estuaries and the adjacent water and protect aquatic ecosystems, nutrient criteria have recently attracted wider attention. Nutrient criteria belong to ecological criteria, and their theories and methods need to be further improved. Based on the estuary classification and partition, it is necessary to systematically carry out ecological investigation. The national database of coastal estuary ecological environment characteristics is gradually established by integrating historical data. Then, the formulations of nutrient criteria for different types of estuaries are vigorously promoted. The nutrient criteria are a theoretical basis for determining ecosystem health-based estuarine nutrient standards and a scientific basis for estuarine nutrient management. Finally, the control standards and management strategies of nutrients in different estuaries are proposed.

4. Discussion 4.1. Verification of nutrient criteria by nutrient-algal bloom sensitivity Nutrient critical thresholds for cyanobacterial blooms have been used to define nutrient criteria in some lakes (Carvalho et al., 2013; Yuan et al., 2014; Yuan and Pollard, 2015). Therefore, correlations between the HABs occurrence numbers and interannual nutrient concentrations were analyzed to verify the recommended nutrient criteria. A linear correlation was observed between the DIN concentrations and HABs occurrence numbers, implying that the DIN concentration strongly contributed to the HABs occurrence. Furthermore, when the DIN value was higher than 0.30 mg/L, the HABs occurrence numbers increased significantly in the study area, implying that threshold relationships occurred in the DIN concentration and HABs occurrence (Fig. 5a). Similarly, as for PO4-P, the HABs occurrence increased obviously when the PO4-P level was higher than 0.02 mg/L (Fig. 5b). The thresholds of the DIN and PO4-P concentration and HABs occurrence in the study area were 0.30 and 0.02 mg/L, respectively. Li et al. (2014) showed that the threshold of PO4-P concentration and HABs occurrence

5. Conclusion Nutrient criteria of different segments in the Yangtze River Estuary and adjacent waters were set by population frequency distribution based on data from 1984 to 2014. Due to the land-based pollutants caused by human-based changes in land-use and high turbidity, the recommended DIN (1.05–1.24 mg/L), PO4-P (0.030–0.036 mg/L) and CODMn (1.04–1.15 mg/L) criteria in segments I and II were much higher than those in segments III and IV (0.19–0.20 mg/L, 0.010–0.012 mg/L 6

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3.0

segment segment segment segment

a

segment segment segment segment

b

0.08

PO4-P/(mg/L)

DIN/(mg/L)

2.5

0.10

2.0 1.5

0.06

0.04

1.0

0.02 0.5

0.00

0.0 198 4

198 8

199 2

199 6

200 0

200 4

200 8

201 2

1984

2016

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1992

1996

6

c

2004

2008

2012

2016

segment segment segment segment

4

COD/(mg/L)

2000

Year

Year

2

0

198 4

198 8

19 92

199 6

200 0

200 4

2008

Year Fig. 4. Interannual variation in the concentrations in the study areas: (a) annual mean DIN concentration, (b) annual mean PO4-P concentration, (c) annual mean COD concentration. Table 4 DIN and PO4-P concentrations in spring before the occurrence of red tide in the high red tide area in the Yangtze River Estuary and the adjacent waters (mg/L).

DIN PO4-P

20

a 16 12

2

R =0.47 P=0.050

8 4 0 0.0

0.8

0.4

2002

2003

2004

2005

0.37 ± 0.08 0.017 ± 0.001

0.26 ± 0.09 0.019 ± 0.006

0.33 ± 0.17 0.017 ± 0.005

0.29 ± 0.04 0.019 ± 0.004

Data were from Zhang, 2008.

Number of HABs occuurence per year

Number of HABs occuurence per year

and 0.59–0.60 mg/L, respectively). The respective TN and TP criteria in segments I and II (1.56–1.85 mg/L and 0.060–0.072 mg/L) were approximately five and two times those in segments III and IV (0.27–0.29 mg/L and 0.023–0.028 mg/L). Nutrient-algal bloom sensitivity, comparison with DIN and PO4-P concentrations in the high red tide area and previous studies verified the reliability and validity of the criteria values. Numeric nutrient criteria differences between different estuaries may be attributed to the different natural characteristics. The set of estuarine nutrient criteria should be further adapted at a national scale, and then the guidelines for estuarine water quality management

1.2

DIN/(mg/L)

20

b 16 12 2

R =0.49 P=0.040

8 4 0 0.000

0.008

0.016

0.024

0.032

0.040

PO4-P/(mg/L)

Fig. 5. Correlations between the HABs occurrence number and the annual average concentrations of DIN and PO4-P in the Yangtze River Estuary and the adjacent waters. 7

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Table 5 Reference condition or criteria in different Chinese estuaries (mg/L) (Chla, μg/L). Liaohe River Estuary DIN PO4-P DO CODMn Chla

a

0.77 0.005a 5.73a – –

Coastal area of Liaohe River Estuary b

0.11 0.006b 6.14b – 0.90b

Jiulong River Estuary c

0.196 ~ 0.896 0.024 ~ 0.028c – – –

outside Yangtze River Estuary d

0.211 ~ 0.317 0.009 ~ 0.018d 4.22 ~ 8.36e 0.42 ~ 0.56e 0.84 ~ 1.88e

f

0.20 0.012f 5.78f 0.59f –

Zhoushan adjacent area 0.273 ~ 0.441d 0.018 ~ 0.029d 5.94 ~ 8.75e 0.37 ~ 0.55e 0.73 ~ 1.00e

0.19f 0.010f 6.38f 0.56f –

Data were from aSu et al., 2016; bHu et al., 2011; cLiu et al., 2018a; dZheng et al., 2013a; eZheng et al., 2013b; fThe study.

are expected to derive in China in the near future.

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