Historical accumulation and environmental risk of nitrogen and phosphorus in sediments of Erhai Lake, Southwest China

Ecological Engineering 79 (2015) 42–53

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Historical accumulation and environmental risk of nitrogen and phosphorus in sediments of Erhai Lake, Southwest China Zhaokui Ni a,b,c , Shengrui Wang a,b,c, * a

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China State Environmental Protection Key Laboratory For Lake Pollution Control, Research Center of Lake Environment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China c Dongting Lake Ecological Observation and Research Station,Yueyang 414000, China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 July 2014 Received in revised form 15 January 2015 Accepted 8 March 2015 Available online xxx

The release of endogenous nutrients from sediments significantly affects water quality and accelerates eutrophication. In general, lakes from the Yungui Plateau of China are characterized by high sediment nutrient contents and relatively good water quality. Thus, the risk of nutrient release from sediments may be enhanced under certain environmental conditions, and this enhanced risk would become a general concern during eutrophication. Knowledge about historical nutrient accumulation and effect of environmental parameters on nutrient dynamics at the sediment–water interface in the lakes from this region is important to understand the eutrophication processes for these lakes. Hence, this study reconstructs the historical accumulation of nutrient and the effects of environmental parameters on possible release risk of nutrient at the sediment interface for Erhai Lake in the Yungui Plateau region. This study also analyzes historical changes in environmental conditions to predict the future release of nutrient from sediment. In the past decades, the burial fluxes (BFs) of nitrogen (N) in the sediments have continuously increased, whereas those of phosphorus (P) have not or only slightly increased, except in highly polluted areas. These situations may be attributed to the long-term fertilization practices of farmers in the watershed, where the application of N fertilizers is much higher than that of P fertilizers. The mentioned phenomena can also be ascribed to the different biogeochemical behaviors of N and P. Nutrient pollution started in the 1970s and worsened from the 1990s. Before the 1970s, nutrients were relatively low and stable. Thereafter, N in the entire area and P in seriously polluted areas (northern area) dramatically increased because of natural and anthropogenic processes, such as excessive artificial N and P fertilization. After the early 1990s, the BFs of nutrients were steady but high because of the degradation of aquatic vegetation and the implementation of pollution control policies. The burial efficiencies (BEs) of TN range between 44% and 85%, with a mean value of 71%. Combined with the high increase rate of BFs during the past decade, this relative low BE indicates that the release of sediment N into overlying water has increased yearly. The BEs of TP range between 98% and 102%, with a mean value of 99%, and the increase rate of historical BFs is small. This result suggests that the sediment generally serves as a sink for P. In addition, the diffusion fluxes of nutrients is an important factor because their forms and environmental conditions (dissolved oxygen (DO), pH, etc.) influence water quality. However, the risk of nutrient release from sediment source might increase if these environmental conditions change (i.e., decreased DO and increased pH). ã 2015 Elsevier B.V. All rights reserved.

Keywords: Nutrients Historical accumulation Environmental parameters Risk Sediment

1. Introduction Lake eutrophication is a major threat to the health of aquatic ecosystems worldwide (Conley et al., 2009; Liu et al., 2014).

* Corresponding author at: State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China. Tel.: +86 1084915277; fax: +86 1084915277. E-mail address: [email protected] (S. Wang). http://dx.doi.org/10.1016/j.ecoleng.2015.03.005 0925-8574/ ã 2015 Elsevier B.V. All rights reserved.

Nitrogen (N) and phosphorus (P) are primary limiting nutrients in freshwater systems; excess concentrations of these nutrients negatively affect the ecosystem and cause eutrophication (Hecky and Kilham, 1988; Banaszuk and Wysocka-Czubaszek, 2005). Nutrients that enter the lake ecosystem may be adsorbed to sediments and thereby accumulate on the lake bottom. Sediments with stored N and P can act as a new pollutant source for the overlying water (Jin et al., 2005; Schenau and De Lange, 2001). Hence, N and P release from sediments can dramatically affect

Z. Ni, S. Wang / Ecological Engineering 79 (2015) 42–53

water quality and accelerate eutrophication when external nutrient sources are under control (Jin et al., 2006). The deposition and burial of sedimentary N and P are closely related to sedimentary environmental conditions, such as hydrodynamic forces, particle size, and density, which principally influence the geochemical characteristics of N and P. Thus, a record of the temporal variation in N and P in sediments can be used as an effective proxy to reflect the historical natural and anthropogenic processes (Jiang and liu, 2013; Gao et al., 2008). In general, the historical reconstruction of N and P can be achieved by determining the vertical distribution of the burial fluxes (BFs) of TN and TP in sediments and by using other dating methods (ÁlvarezIglesias et al., 2007; Duan et al., 2013). However, the deposition of nutrients does not necessarily result in a stable environment. A dynamic equilibrium of adsorption and release exists in the sediment–water interface. Changes in environmental conditions may trigger the release of nutrients into the water through diffusion, convection, and sediment resuspension, ultimately causing secondary pollution (Kuwabara et al., 2003). Therefore, studying the diffusion fluxes (DFs) of TN and TP at the water– sediment interface and the impact mechanism of environmental parameters on N and P exchange is important to quantify the release risk of sedimentary N and P. Erhai Lake in the Yunnan Guizhou Plateau region is one of the largest fault lakes in China, and its region has a high population density; given its large area, Erhai Lake plays a significant role in local socio-economic development issues, including drinking water sources, irrigation, fisheries, and tourism (Guo et al., 2011). However, the lake underwent anthropogenic eutrophication after the application of large amounts of artificial fertilizers to support rapid agricultural intensification development around the basin in the last few years (Zhang et al., 2014). Since the opening of agriculture jobs in the late 1970s, a high load of excessive external nutrients has been directly input in the lake (Zhao et al., 2013a). Consequently, Erhai Lake is suffering from serious deterioration of the ecosystem and from a critical transition of water quality from mesotrophic to eutrophic (Wang et al., 2012a). Monitoring data showed that the water quality of Erhai Lake is graded as class II to III in accordance with the overlying water environment quality standard (GB3838-2002) in China. However, the sediment of Erhai Lake is seriously contaminated, containing higher total nitrogen (TN) and total phosphorus (TP) than most eutrophicated lakes in China (Zhang et al., 2011). Monitoring data showed continuous changes in environmental conditions (including pH and DO) during the past decades, which could increase the risk of nutrient (TN and TP) release from the sediments (He et al., 2011). Extensive effort has been exerted in the past decade to control exogenous pollution in Erhai Lake. Hence, the effects of the release of N and P from sediments have received increasing attention from the public and the local government; such effects include algal bloom and declined water quality. Accordingly, the present study aims to (1) reconstruct the historical accumulation processes of N and P in the sediments, (2) to examine the effects of environmental parameters on N and P release at the sediment–water interface of Erhai Lake, and (3) to predict the future potential release of N and P from sediments on the basis of historical changes in environmental conditions (DO, pH, etc.) and water quality. 2. Materials and method 2.1. Study area Erhai Lake (25
360 –25
580 N, 100
060 –100
18E) is located in Dali City, Yunnan Province, with a water surface area of approximately 249 km2 and a watershed area of 2565 km2 (Fig. 1). The lake has a mean water depth of about 10.5 m and a volume of about 2.8

43

Fig. 1. Map of Erhai Lake and distribution of sampling sites.

 109 m3. The residence time of the water is long, typically at an average time of three years. The area has an annual average climate temperature of 15
C, an annual average precipitation of 1100 mm, and an annual evaporation capacity of 1970 mm. The contamination of the lake is mainly caused by agricultural nonpoint source (NPS) pollution rather than industrialization and urbanization (Zhang et al., 2014). 2.2. Sample collection 2.2.1. Surface sediments Seventeen surface sediments (0–10 cm) from Erhai Lake were collected using a self-made core sampler device in January, April, July, and November 2010 (Fig. 1, Table 1). Samples were selected in accordance with different environmental conditions (topography, aquatic plant coverage, and water depth, among others) from the northern (E1, E2, E3, E4, E5), central (E6, E7, E8, E9, E10, E11), and southern (E12, E13, E14, E15, E16, E17) areas of the lake (Zhao et al., 2013). Meanwhile, the overlying water (10 cm) was collected so that the samples consisted of each sediment site. The DO concentration, pH value, Eh value, and temperature of overlying water were also measured in each collection session of sediment samples. 2.2.2. Core sediments Three sediment cores (E1, E9, and E13) were collected at different areas of the Erhai Lake by using a core sampler (HL-CN, Hengling technology Ltd., Corp., China) in November 2010. Core E1 (northern site) is located in the lower edge of the alluvial fan of the

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Table 1 Sampling locations of surface sediment along Erhai Lake and summary of analytical results in 2010 (DO concentration, pH value, Eh value, and temperature of overlying water are the average of four sampling period). Area

Site

Latitude

Longitude

Water level m

DO mg L1

pH

Eh mV




Temperature C

E1 E2

25
540 0000 25
560 0800

100
090 1300 100
050 5700

11.9 3.9

7.13 7.92

8.6 8.2

384 378

18.0 17.3

Northern

E3 E4 E5 E6 E7

25
5501000 25
530 4500 25
530 1300 25
5103600 25
500 0700

100
070 3100 100
100 4700 100
080 3800 100
100 4600 100
1103000

7.5 11.6 7.5 11.5 13.5

7.47 6.74 6.18 6.20 5.76

8.6 8.6 7.9 8.7 8.7

392 352 330 360 363

17.9 18.1 17.8 17.8 17.7

Central

E8 E9 E10 E11 E12 E13

25
480 4700 25
470 5900 25
450 4300 25
450 0200 25
430 1100 25
410 2400

100
090 5700 100
1104900 100
090 5900 100
120 0700 100
120 5100 100
140 0500

15.8 21.1 13.5 10.5 7.9 7.5

5.72 5.31 6.11 6.35 7.08 7.01

8.6 8.7 8.7 8.7 8.7 8.7

341 382 345 401 388 391

17.8 17.5 17.4 17.3 17.1 16.8

Southern

E14 E15 E16 E17

25
390 3000 25
390 3200 25
380 0300 25
360 2500

100
130 2300 100
150 5600 100
150 0200 100
140 0400

7.4 11.3 9.6 4.9

6.02 6.73 7.64 7.66

8.7 8.8 8.7 8.7

370 374 379 369

17.2 17.1 17.0 17.0

Yongan River, a major inflow river. The area has no plant growth because of the seriously polluted condition (excess N and P concentration) of the sediment. Core E8 barely had aquatic plants, and its water level was the deepest, with a low margin of circulation. Core E13 is located in the underwater platform of south Erhai. Submerged macrophytes were abundant in this core before 2000 AD. However, the submerged macrophyte communities in this area have mostly disappeared probably because of the increase in water level from 2003 to 2006. Immediately after collection, the sediment columns were divided into 2 cm slices in sequence, stored in precleaned polyethylene bags, sealed, and then refrigerated until laboratory analysis. 2.3. Experiment and chemical analyses 2.3.1. BFs Seventeen surface sediments and three sediment cores were freeze dried to reduce the amount of water. Then, samples were mixed using the quartering method, with a portion used for density and porosity measurements; additional samples were passed through a 100-mesh sieve after grinding and used for TN, TP, and total organic carbon measurements. TN content was measured by using the semimicro Kjeldahl method (Jin et al., 1990). TP content was measured in accordance with the SMT protocol (Ruban et al., 1999). Organic matter (OM) content was measured by using the dichromate external heating method (Nanjing institute of soil, Chinese Academy of Science, 1978). The BFs of the biogenic elements (TN, TP, and OM) were calculated as follows (Schenau et al., 2005): BF = c  v  rd = c  S  (1 – f)  rd, where BF is the BFs of the biogenic elements, g (yr cm2)1; c is the content of the biogenic elements, mg kg1; v is the mass deposition rate of the sediments, g (yr cm2)1; S is the deposition rate, cm yr1; f is the porosity; and rd is the dry density of the sediments, g cm3. The deposition rate S was obtained through sediment 137Cs and 210 Pb dating; the deposition rates in the northern, central, and southern parts were 0.24, 0.16, and 0.20 cm yr1, respectively (Zhang et al., 1993; Ni, 2011).

The surface samples were performed seasonally in each position, and then the arithmetic mean  discrete degree values were utilized in this study. 2.3.2. DFs The DFs of N and P in the surface sediments were measured by using the cylindrical core sample simulation method (Boers and Van llese, 1988). This method can be employed under controlled conditions (DO, pH, light, disturbance, etc.) without destroying the sediments. The surface sediments together with the core organic glass tube (as far as possible kept in the original state) were taken to the laboratory. The overlying water was immediately removed by siphoning and placed in the sampling tube for the analysis. The sediments were set up at the same depth of about 10 cm. A rubber plug was inserted in the bottom of the cores, and then 2 L of the lake water from which the algae and suspended particles were filtered was slowly added. The artificial disturbance on the sediment–water interface was minimized. After the addition of the lake water, all core samples were vertically placed in the glass incubator that contains the lake water. The incubation was performed away from light without the lid. The experiment was performed for a cycle of 3 days. During each cycle, 200 mL of overlying water in the middle section was collected. After sampling, an equal amount of lake water filtered by a 0.45 mm Whatman GF/F membrane was slowly replenished along the wall of the tube. The sampling times in each cycle were 6, 12, 24, 48, and 72 h after the experiment, yielding a total of five samplings. The analyzed water samples included the in situ overlying water of the sediment, the lake water filtered with the membrane, the water samples collected in the experiment, and the replenished lake water. Dissolved total nitrogen (DTN) and dissolved total phosphorus (DTP) were measured by using alkaline persulfate oxidation assay and molybdate colorimetric assay, respectively. The TN and TP release rates of the sediments r were calculated by using the following formula (Steinman et al., 2004): ½VðC n  C 0 þ rj ¼

n P n1

St

V j1  C a Þ ;

Z. Ni, S. Wang / Ecological Engineering 79 (2015) 42–53

where r is the diffusion rate, mg (m2 d)1; V is the volume of the overlying water, L; Cn, C0, and Cj1 are the DTN and DTP contents during the n-th, 0-th, and j1-th sampling, respectively, mg L1; Ca is the TN and TP concentration of the added water samples, mg L1; Vj1 is the volume at the j1-th sampling, L; S is the column-like water–sediment contact area, m2; and t is the experiment time, d. The annual DFs of sediments TN and TP in the whole lake can be calculated as follows (Fan et al., 2002): W¼

n X j



t X

rij  Aj  T i  103

j

where W is the diffusion flux, kg yr1; rij is the release rate under temperature i (representing four seasons) of the sediments in region j, mg (m2 d)1; Aj is the area of region j, km2; and Ti is the time period under temperature (yr). 2.4. Data quality control Method blank, field duplication samples, spiked samples, and standard reference materials were used to control data quality. The relative percent difference for different parameters in duplicate samples was <10%. Precision was assured by determining all samples in triplicate with relative standard deviation higher than 10%. A confidence level of 95% was used to establish significance (p < 0.05). 3. Results and discussion 3.1. Spatial distribution of BFs The spatial distribution of BFs was calculated by the concentration of target parameters and then combined the age of the sediment at this depth in different positions. The spatial distribution of the BFs of TN, TP, and OM in the surface sediments (0–10 cm) of Erhai Lake is presented in Fig. 2. The figure shows considerable variability because of the different natural processes and anthropogenic activities in different areas. The BFs of TN ranged from 353  28 mg (m2 yr)1 to 1796  145 mg (m2 yr)1, with an average of 915  76 mg (m2 yr)1. The overall spatial distribution shows the following variation pattern: northern >

45

southern > central, the averages of which were 1319  65, 783  19, and 710  43 mg (m2 yr)1, respectively. The high values were concentrated mainly in the northern part. The BFs of OM ranged from 7889  161 mg (m2 yr)1 to 34,088  428 mg (m2 yr)1, with an average of 15,536  278 mg (m2 yr)1. The spatial distribution is consistent with the variation of TN, with average values of 22,031  348, 15,560  87, and 10,100  197 mg (m2 yr)1 in the northern, southern, and central parts, respectively. The BFs of TP were within 175  9 and 614  36 mg (m2 yr)1, with an average value of 318  23 mg (m2 yr)1. The spatial distribution shows the following variation pattern: northern > central > southern, the averages of which were 410  27, 314  19, and 242  12 mg (m2 yr)1, respectively. The distribution of the high values is consistent with that of TN in the northern part of the lake. 3.2. Temporal distribution of BFs The BFs of TN, TP, and OM versus the sample age (sediment profiles were established by using the sediment rate in different site) of the three selected cores (Fig. 3) were calculated to reconstruct the historical environmental evolution and reflect the impact of natural and anthropogenic activities on the different lake regions in Erhai Lake during the past decades. On the basis of the BFs of TN, TP, and OM, their temporal changes were divided into three stages that are most closely linked with the effect of natural processes and human activities. Stage A refers to the period before the 1970s, when the BFs of TN, TP, and OM in the different cores of the lake showed no obvious trend and had mean values of 705  57, 214  11, and 8416  185 mg (m2 yr)1, respectively. Stage B refers to the period between the 1970s and 1990s, when the BFs of TN, TP, and OM in the sediments began to rise and exhibited an augmented increase rate. Significant differences were found in different parts of the lake. The increase rate of TN in different parts of the lake showed the following variation pattern: northern > southern > central, which was consistent with the spatial distribution of the BFs of TN. TP had a larger increase rate in the northern part than in the central and southern parts. The increase rate of the BFs of OM was the lowest in the sediments in the northern part but the highest in the southern and central parts.

Fig. 2. Spatial distribution of burial fluxes (BFs) of TN, TP, and OM in the surface sediment (0–10 cm) of Erhai Lake.

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Z. Ni, S. Wang / Ecological Engineering 79 (2015) 42–53

BFs of TN mg· (m2·year)

Year

0

500

1000 1500 2000

0

100 0 10

200

300

-1

BFs of OM mg· (m2·year)

400

0

2010

2010

1990

1990

1990

1970

1970

1970

1950

1950

1950

1930

1930

1930

1910

1910

1910

1890

1890

1890

1870

1870

1870

1850

1850

1850

0

1000

2000

-1

BFs of TP mg· (m2·year) -1

3000

100

0

200

0

300

2010

2010

1990

1990

1990

1970

1970

1970

1950

1950

1950

1930

1930

1930

1910

1910

1910

1890

1890

1890

1870

1870

1870

1850

1850

1850

1830

1830

1830

1810

1810

1810

1790

1790

1790

0

1000

2000

-1

BFs of TPmg· (m2·year)

3000

0

100

200

300

-1

10000 20000 30000 4000 40000 0

BFs of OM mg· (m2·year)

2010

BFs of TN mg· (m2·year)

Year

BFs of TP mg· (m2·year)

2010

BFs of TN mg· (m2·year)

Year

-1

10000

20000

-1

30000

BFs of OM mg· (m2·year)

-1

0

400

2010

2010

2010

1990

1990

1990

1970

1970

1970

1950

1950

1950

1930

1930

1930

1910

1910

1910

1890

1890

1890

1870

1870

1870

1850

1850

1850

1830

1830

1830

20000

40000

Fig. 3. BFs of TN, TP, and OM in sediment cores E1 (northern region), E9 (central region), and E13 (southern region) of Erhai Lake.

-1

60000

Z. Ni, S. Wang / Ecological Engineering 79 (2015) 42–53

Stage C refers to the period from the 1990s to the present, when the BFs of TN and TP in the sediments throughout the entire area gradually stabilized. The BFs of TN has shown stability in the northern and central parts, whereas that in the southern part has continued to increase. The BFs of TP have remained steady in the three parts, and those of OM have remained steady in the central part but increased in the northern and southern parts. 3.3. Spatial distribution of the DFs The spatial distribution of the DFs of TN and TP in the surface sediments of Erhai Lake is shown in Fig. 4. The DFs of TN were between 300  27 and 331  43 mg (m2 yr)1, with an average value of 315  35 mg (m2 yr)1. Different parts showed minimal differences, with average values of 314  42, 311  27, and 317  31 mg (m2 yr)1 in the northern, central, and southern parts, respectively. The DFs of TP ranged from 3  1.1 mg (m2 yr)1 to 18  2.8 mg (m2 yr)1, with an average of 10  2.1 mg (m2 yr)1. The high values mainly concentrated in the northern part, with an average value of 7.4  1.4 mg (m2 yr)1, whereas the low values concentrated in the central and southern parts, with mean values of 4.4  1.6 and 4.6  1.1 mg (m2 yr)1, respectively. 4. Discussion 4.1. Relationships between socioeconomic progress and records of sediment BFs The burial processes of sediment TN and TP are significantly affected by several factors, such as source of nutrients, sedimentary rate, hydrodynamic conditions, bioturbation, and redox environment (Lu et al., 2005). The sources that contain natural and anthropogenic inputs are important factors that influence nutrient distribution. The present study found a strong relationship between socioeconomic progress and historical sediment BFs of TN and TP in Erhai Lake. Before the 1970s, the BFs of nutrients and OM in the sediments were small and showed no obvious trend, thus suggesting that their source was constantly natural inputs; monitoring data showed that Erhai Lake was naturally developing and showed good water quality, high transparency, and low nutrient

47

concentrations during this period (Pan et al., 1999). The BFs of TN, TP and OM in the sediments dramatically increased after the 1970s. On the one hand, the distribution and biomass of submerged macrophyte communities rapidly increased because of the decline in water level, which resulted from the operation of the Xi'erhe hydropower plant (Ni et al., 2011). This phenomenon led to nutrient accumulation in the sediment. On the other hand, the increased BFs could be attributed to the rapid intensification of agricultural activities that resulted from China’s opening up in 1978, during which large amounts of anthropogenic N, P and OM were introduced into the sediments (Pan et al., 1999). This situation rapidly increased the accumulation of N and OM in the entire lake area and P in the sediments in the seriously polluted lake area. The increase rate of the BFs of TN and TP showed high values in the northern part but low values in the southern part because the three major rivers in the northern part of the lake flow from north to south, and they are the main input sources of nutrient pollutants in Erhai Lake. Exogenous pollutants were also deposited in the northern area, which increased the loading of nutrients into the sediments. The sediment OM showed contrasting distributions, which can be attributed to the distribution of the submerged plants during this period. In particular, the distribution of submerged plants (e.g., Potamogeton maackianus, Myriophyllum spicatum, and Ceratophyllum demersum) significantly expanded in the southern and central parts because of the increased nutrients in water and gradually declined water level at this period. However, the distribution of sediment OM varied only slightly in the northern part because of the effect of hydrodynamic forces (Li and Shang, 1989). After the 1990s, the steady or even reduction in TN and TP BFs occurred in almost all three cores despite the increase in economic activities and population. The steady or even reduction in TN and TP BFs in the sediment cores from the 1990s probably resulted from the degradation of submerged macrophytes. With the rise in water level and the decline in water quality, the numbers of submerged macrophyte communities gradually declined at this stage. Specifically, the decline in water transparency from 3.63 m to 1.47 m (Fig. 9d) is also an important factor because of the influence of high-intensity agricultural activities in the upper area of the watershed, which accelerated the degradation of submerged plants. From 1990 to 2010, the biomass of aquatic plants decreased

Fig. 4. Spatial distribution of DFs of TN and TP in the surface sediment of Erhai Lake.

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from 3.9 km m2 to only 0.7 km m2 (a 450% decrease), the coverage decreased from 40% to 5% of the area of Erhai Lake (a 700% decrease), and almost no aquatic plants inhabited the part of the lake deeper than 6 m, which has fallen to historic lows (Ni et al., 2011). Serious degradation mainly occurred in the northern and southern parts. OM accumulated in the sediments at the northern and southern parts (Zhao et al., 2013a), which increased the porosity and decreased the dry density in the sediments. This situation finally reduced the BFs of TN and TP, which explained the continued increase in TN and TP contents in the sediments in the southern and northern parts and the slight fluctuation in the BFs of TN and TP of the lake. Faced with the challenge of accelerated lake eutrophication and severe ecological degradation, the Chinese central and local governments issued a series of plans and measures. Targeted activities have included lake peripheral pollution interception, in-lake ecological restoration, rural point source control, and inflow river pollution control. A total of 1.5 billion yuan (approximately 1.02 billion USD) has been invested to improve watershed load reduction and lake ecological restoration. These works tightened the restrictions on the external input of nutrient loading, which partly restricted the rapid increase in the TN and TP contents in sediments. Although the BFs of TN and TP at the top layers of the cores were stable or even lower than those at the deeper layers, they were still much higher than the background values in natural processes before the 1970s. The large sediment N and P reservoirs could act as sources of N and P to the overlying water in given circumstances. Thus, the future risk of sedimentary N and P in Erhai Lake must be given considerable attention. Agricultural NPS pollution is an important source that accounted for 70% of the total pollution in the watershed (Li and Dong, 2011). The land use of Erhai Lake watershed has remained quite constant over the past decades, unlike in some lakes where substantial land use change by human activities is the main cause of water environmental changes. Agricultural activities are conducted in a limited area with large inputs of chemical fertilizers and other resources. Fig. 5 shows an overview of the application of N and P fertilizers, with the corresponding TN and TP concentrations in dated water in the Erhai Lake watershed in the past decades. N fertilizer application is significantly positively correlated with TN (r = 0.823, n = 23, p < 0.01) in water; P fertilizer application is also positively correlated with TP (r = 0.510, n = 23, p < 0.05) in the overlying water. This finding implies that N and P loading from chemical fertilizer was an important source of nutrient input in Erhai Lake. The BFs of TN in the sediments showed a higher increase rate than those of TP. This result could be related to the long-term N fertilization practices of the farmers in the watershed. Excessive artificial N and P fertilizer applications have resulted in serious environmental problems, but the overuse of artificial N and P

fertilizers is still common in local farming; specifically, the excessive application of N fertilizers is much higher than that of P fertilizer (Ju et al., 2009). In addition, the soil loss rate of N fertilizers to the environment in the watershed was 68%, which is higher than the 21% of P fertilizers (Tang et al., 2011). As a result, more N than P was transported from the agricultural NPS sources into the lake. The extensive use of artificial fertilizers has changed the traditional pattern of agricultural production since the 1970s, which has threatened to the environment within the watershed. Before the 1970s, almost no artificial P fertilizers were used, and only N fertilizers were used at an amount of 20 kg ha2. By 2010, the average amount of P fertilizer used had reached 73 kg ha2, which was about 13 times that of the dosage used 40 years before. The use of N fertilizers (nitrate, ammonium) rapidly increased from 1970 to 2010, at which point usage had reached 230 kg ha2, which was about 12 times that of the dosage used 40 years before. At present, the N fertilizer dosage in the watershed is 3 times that of the P fertilizer dosage, and the increase rate of the application dosage of N fertilizers is significantly larger than that of P fertilizers. This pattern may explain why more N than P has accumulated in the sediments of Erhai Lake in the past four decades. The different biogeochemical behaviors of N and P could also affect the BFs of TN and TP. Hence, we examined the migration and transformation of TN and TP at the water–sediment interface to reveal difference in nutrient accumulation (Fig. 6). The results showed that the BE of TP (average of 99%) in the sediments was higher than that of TN (average of 71%), which might be attributed to the much higher contents and proportion of labile N than P (Zhao et al., 2013c). This result indicates that the sediment N was more easily released to the overlying water than P, which partly reduced the N/P ratio of the sediment. 4.2. Effects of environmental parameters in the overlying water on the release of N and P in sediments A number of studies reported that the conditions of DO, pH, Eh, microbial activities, organic content, and hydrology may affect the release of nutrients from sediments (Geurts et al., 2010; Penn et al., 2000; Jin et al., 2006; Gomez et al., 1998) and then accelerate lake eutrophication. In recent years, the allochthonous source of pollution input to Erhai Lake has been gradually controlled. Thus, near-term management of Erhai Lake is important to reveal the exact mechanism for the endogenous release of TN and TP. However, the environmental conditions of overlying water are beginning to change (Fig. 9). Hence, studies must determine the effect of environmental parameters in overlying water on the release of N and P from sediments in Erhai Lake.

Fig. 5. Historical usage of artificial N (nitrate, ammonium) and P fertilizer from the Erhai Lake watershed and corresponding TN and TP concentrations in water (data was provided by the Dali City Agriculture Bureau and Environmental Protection Bureau, respectively).

Z. Ni, S. Wang / Ecological Engineering 79 (2015) 42–53

49

Fig. 6. Spatial distribution of BE of TN and TP in surface sediments of Erhai Lake.

The relationship between the BFs and DFs of TN and TP in surface sediments and the influencing factors in overlying water and sediments was analyzed by using Pearson’s correlation (Table 2). The BFs of TN are significantly positively correlated with the BFs of OM at a correlation coefficient of 0.503, which indicates that sediment TN mainly existed in the organic residues and lightweight particles of Erhai Lake. The BFs of TN positively correlated with DO (r = 0.443, P < 0.05), which indicates that high DO concentration in overlying water would reduce N release from sediment. This promotion can be ascribed to the fact that less NH4+-N and TN are released from the sediments under aerobic than under anaerobic conditions (Ye et al., 2006; Qiu et al., 2011). The results implied that the aerobic condition of overlying water in Erhai Lake (DO concentration range between 5.3 and 7.9 mg L1) is not propitious to the release of N from sediments to the overlying water. The BFs of TN significantly positively correlated with deposition rate (r = 0.610, P < 0.01). Therefore, deposition rate is another major factor that determines the BFs of TN. Results showed that the BFs of TN are negatively correlated with water depth (r = – 0.349, P < 0.05) because deeper water corresponded to lower DO concentration, which reduced the contents and BFs of N in the sediments. The pH in overlying water is another important factor that affects the release of N from sediments. Zhang et al. (2014) and Ye et al. (2006)’s studies showed that pH <9 minimally influences the release of N in sediment, but pH >10 enhances the release of nitrate from sediment in Erhai Lake under aerobic conditions. The

current pH of overlying water ranges between 7.9 and 8.8, which suggests that the release amount of N from sediments may be relatively small. However, the pH in the overlying water of Erhai Lake increased (Fig. 9b), which indicates that the release risk of N would increase with increasing pH. The BFs of TP are positively correlated with DO (r = 0.369, P < 0.05), and the DFs of TP are significantly negatively correlated with DO (r = –0.480, P < 0.05). This result indicates that the low DO concentration in the overlying water might enhance P release from the sediment. The result is consistent with the report of Wang et al. (2008), who demonstrated that soluble reactive P increases in the overlying water from sediments under anaerobic conditions. Erhai Lake is currently at an aerobic state (average DO concentration of 8.4 mg L1), which is conducive to inhibit the release of P. Water pH is another important factor that may affect the release of P from sediments. For this study, the BFs of TP showed a poor linear correlation with pH but showed a “U”-shaped curvilinear correlation. This result suggests that the release of P in sediments is at its lowest when pH is close to 7, whereas an increase in acid or alkaline levels might increase the release of P. This result can be due to the fact that P is mainly released by dissolution at low pH and by ion exchange at high pH, which can enhance the release of P in sediments (Wang et al., 1996). Hence, the gradual increase in pH (Fig. 9b) in overlying water may increase the release of P in Erhai Lake.

Table 2 Pearson’s correlation coefficients for the relationship between the BFs and DFs of TN and TP in the surface sediments and the impact factors in overlying water and sediment (n = 17). Item

BF of TN

BF of TP

BF of OM

RF of TN

RF of TP

BF of TN BF of TP BF of OM RF of TN RF of TP Water depth pH DO Eh Particle size Rate

1 0.515b 0.503b 0.382a 0.135 0.349a 0.232 0.443a 0.050 0.202 0.610b

1 0.002 0.759b 0.036 0.164 0.045 0.369a 0.014 0.228 0.254

1 0.120 0.393a 0.757b 0.493b 0.764b 0.279 0.120 0.709b

1 0.171 0.290 0.331a 0.181 0.117 0.022 0.02

1 0.213 0.114 0.480b 0.351 0.254 0.121

a b

Correlation is significant at the 0.01 level (2-tailed). Correlation is significant at the 0.05 level (2-tailed).

Water depth

1 0.343 0.376a 0.219 0.209 0.559b

pH

DO

Eh

1 0.425a 0.428a 0.019 /

1 0.269 0.039 /

1 0.115 /

Particle size

1 /

Rate

1

50

Z. Ni, S. Wang / Ecological Engineering 79 (2015) 42–53

4.3. Possibility of environmental effects Burial efficiency (BE) is the ratio of BFs of biogenic elements to their DFs and is plotted as a function of total sediment mass accumulation rate (Ingall and Jahnke, 1994). The spatial distribution of the BE of TN and TP in the surface sediments in Erhai Lake is shown in Fig. 6. The BE of P in the sediments reached a relatively high value (between 98% and 102%), with a mean value of 99%. This result indicates that the total amount of P released from the sediments was low; thus, sediment was a sink for P. This finding was associated with the characteristic of P forms in Erhai Lake. The content of labile P ranges from 1.7 mg kg1 to 8.6 mg kg1, which accounts for only 0.2–2% of TP, although the content of TP is high in the sediment (Zhao et al., 2013). This finding explains why the P content in the sediments was high but that in the water was low. The BE of TN in the sediments ranged from 44% to 85%, with an average of 71%. The mean content of TN in the sediments in the northern part was the highest at 80%  6%, whereas those in the southern and central parts were the lowest, with average values of 69%  5% and 68%  8%, respectively. This result is also attributed to the effect of N forms. The concentration of labile N ranged from 66 mg kg1 to 130 mg kg1, which accounted for 3.8–5.1% of the TN in the sediments. To further understand the environmental effects of N in the sediments of Erhai Lake, this study compared the DFs of TN at the sediment–water interface from the Yunnan Plateau lakes (Erhai Lake) in China, the Eastern Plain lakes (Tai Lake, Chao Lake, and Poyang Lake) in China, and the Laurentian Great Lakes (Superior Lake, Huron Lake, and Erie) in the USA (Fig. 7). The results showed that the DFs of TN in Erhai Lake were generally lower than those of the other selected lakes, but the contents in sediments were significantly larger than those of the other lakes. This result is attributed to the effect of water depth and N forms. Unlike the shallow lakes in the Eastern Plain lakes of China, Erhai Lake is a

semi-deep lake whose sediment experiences minimal disturbances from hydrodynamic force and wind; hence, less N is released from the sediment of Erhai Lake than from the sediments of the Eastern Plain lakes. In addition, the content of labile N forms (primarily exchangeable N) ranges from 207 mg kg1 to 1107 mg kg1, which accounts for 29.8–57.3% of TN in sediments from the Eastern Plain lakes (Wang, 2007). This value is considerably higher than the labile N content and percentage in Erhai Lake (Zhao et al., 2013a,b,c). Thus, the amount of TN released from the sediment is lower in Erhai Lake than in the Eastern Plain lakes. Compared with the deep lakes in the Laurentian Great Lakes, the sediment from Erhai Lake removed a significant amount of water column NO3 through denitrification because of the anaerobic conditions at the sediment–water interface in the lakes (Gaston et al., 2014). However, the high concentration of DO in the sediment is not conducive for the release of N in Erhai Lake. The TP content of the sediments in Erhai Lake was generally higher than that of the sediments in the Eastern Plain lakes in China, but the DFs of TP were much smaller in Erhai Lake than in the Eastern Plain lakes (Fig. 8). This result agrees with our findings on the DFs of TN and can also be attributed to the effects of different P forms, water depths, and pollution statuses. In Erhai Lake, nonlabile P was the dominant P form (Zhao et al., 2013b) whereas labile P was the dominant P form in the Eastern Plain lakes (Jin et al., 2008). Meanwhile, the sediment experienced fewer disturbances by hydrodynamic force and wind in Erhai Lake than in the Eastern Plain lakes. Consequently, less TP was released in Erhai Lake than in the Eastern Plain lakes. In summary, the release flux of TN and TP in the sediments from the Erhai Lake was relatively small. This pattern explains why Erhai Lake has good water quality but seriously contaminated sediment. From the perspective of historical evolution, the BFs of TP in the sediments of the lake have only slightly changed since the 1970s. However, the BE of TP ranged from 98% to 101%, with a mean value

4000

160

3000

140 120

TN content

2500

100 Water depth

2000

80

1500

60

1000

40

500

20

0

Water depth (m)

TN contnet (mg·kg-1)

Great Lake, USA

Eastern Plain, China

3500

0 Erhai Lake

Tai Lake

Poyang Lake

Chao Lake

Superior Lake Huron Lake

Erie Lake

Lake 30 Great Lake, USA

DFs of TN (mg·m-2·d-1)

Eastern Plain, China

20

10

0 Erhai Lake

Tai Lake

Poyang Lake

Chao Lake

Superior Lake Huron Lake

Erie Lake

Lake Fig. 7. Results of TN content and corresponding DFs of TN in surface sediments from different lakes. Data source of content and DFs of TN: Erhai Lake – present study; Taihu Lake – Zhang et al. (2006); Poyang Lake – Wang et al. (2012b); Chao Lake – Pan et al. (2007); Great Lake (Superior Lake, Huron Lake, Erie Lake) – Gaston et al. (2014).

Z. Ni, S. Wang / Ecological Engineering 79 (2015) 42–53

51

4000

12

3000

9 TP content

2000

6

Water depth 3

1000

0

water depth (m)

TP contnet (mg·kg-1)

Eastern Plain, China

0 Erhai Lake

Tai Lake

Poyang Lake

Chao Lake Lake

Hong Lake

Liangzi Lake

6

DFs of TP (mg·m-2·d-1)

Eastern Plain, China

3

0 Erhai Lake

Tai Lake

Poyang Lake

Chao Lake Lake

Hong Lake

Liangzi Lake

Fig. 8. Results of TP content and corresponding DFs of TP in surface sediments from different lakes. Data source of content and DFs of TP: Erhai Lake – present study; Taihu Lake – Fan et al. (2006); other lakes – Zhang et al. (2008).

decades. Denitrification is possibly the main factor for N attenuation (Zhang et al., 2014) and always occurs under anaerobic conditions (Jing et al., 2013). In Erhai Lake, the average annual DO concentration in the overlying water gradually decreased (Fig. 9a), which caused the release of sediment TN to the overlying water in some anaerobic areas. The difference in TN and TP release could explain the increased N/P ratio (from 11.8 to 20.8) of the overlying water in Erhai Lake during the past decades when the

of 99%, which indicates that the sediment served as a sink for P during the past four decades. In addition, only a small amount of P was released from these massive reservoirs into the overlying water. The historical BFs of TN in the sediments significantly increased, but the BEs were between 44% and 85%, with a mean value of 71%. This result indicates that an increasing amount of TN in the sediments of the lake was released yearly into the overlying water and that the increase rate was augmented in the past

75

9 b 8.8

70 pH

DO saturation (%)

a

65

8.6

60 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 Year

8.4 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 Year

30

4

c

d Transparency (m)

25 N/P

20 15 10 5 0 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 Year

3 2 1 0 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012

Year

Fig. 9. Annual changes of the DO concentration (overlying water), pH value (mixture from surface water to water–sediment interface), N/P ratio and transparency in the water between 1992 and 2012 in Erhai Lake. (Every annual data of DO, pH, N/P and transparency were the results of average of 12 months in whole lake).

52

Z. Ni, S. Wang / Ecological Engineering 79 (2015) 42–53

allochthonous source of N and P was gradually controlled (Fig. 9c). Overall, these results imply that the N risk control of the sediments should be prioritized in Erhai Lake. The present environmental conditions (DO and pH, among others) of the Erhai Lake ecosystem are not propitious to the release of N and P from lake sediments. However, the pH increased from 8.48 to 8.87 during the 20-year monitoring period because of increased input of carbonate from the upper area of the watershed. DO concentration declined from 7.4 mg L1 to 6.6 mg L1 because of the sustained death of aquatic plant and increasing population size of phytoplankton, which consume large amounts of DO in water. The increase in pH or decrease in DO would enhance the release of N and P from the sediment. Accordingly, the concentrations of TN and TP increased from 0.32 mg L1 to 0.57 mg L1 and 0.025 mg L1 to 0.031 mg L1, respectively (Fig. 5). Hence, continuous ecosystem deterioration could lead to changes in environmental conditions. The release risk of N and P from sediments would increase when DO concentration decreases to an anoxic state or pH exceeds 10. In addition, the species and amount of microorganisms in the sediments from Erhai Lake showed an increasing trend (Hu et al., 2005). The decomposition of OM in the sediments is mostly accelerated by microorganisms (Zhang et al., 2013; Compton and BooNE, 2002), which consequently increases the release risk of N and P from the sediments (Wang et al., 2005; Zhao et al., 2013c). Therefore, future studies should focus on controlling exogenous pollution to reduce the sediment endogenous risk of N and P and protect the ecological environment of Erhai Lake. Meanwhile, the implementation of ecological restoration, optimization and adjustment of key environmental factors in ecosystems, and the maintenance of water habitats will be the key points for the protection of Erhai Lake. 5. Conclusion This study reconstructed the historical accumulation of nutrients and the effects of environmental parameters on nutrient release at the sediment interface of Erhai Lake in the Yungui Plateau region. Results showed that the BFs of TN in the sediments continuously increased and that TP did not or only slightly increased, except in highly polluted areas, in the past decades. These situations may be attributed to the long-term fertilization practices of farmers in the watershed, where the application of N fertilizers is much higher than that of P fertilizers. These situations also depend on the difference in the biogeochemical behavior of N and P. Nutrient pollution started in the 1970s and worsened from the 1990s. Before the 1970s, nutrients were relatively low and stable. Thereafter, N content in the entire area and P content in the seriously polluted area (northern area) dramatically increased because of natural and anthropogenic processes (e.g., excessive artificial N and P fertilization). After the early 1990s, the BFs of nutrients were steady but high because of the degradation of aquatic vegetation and the implementation of policies on pollution control. The BEs of N ranged between 44% and 85%, with a mean value of 71%, and the BFs showed a high increase rate during the past 4 decades. This result indicates that the release of sediment N into the overlying water increased yearly. The BE of P ranged between 98% and 102%, with a mean value of 99%, and the historical BFs showed a low increase rate. This finding suggests that the sediment generally served as a sink for P. The fewer DFs of nutrients are crucial because their form, water depth, and environmental conditions (DO and pH, among others) affect water quality in Erhai Lake. However, the risk of nutrient release from sediments might increase if environmental conditions change (decreased DO or increased pH). Therefore, ecological restoration, optimization and adjustment of ecosystems

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