Effects of dissolved oxygen supply level on phosphorus release from lake sediments

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Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 245–252

Effects of dissolved oxygen supply level on phosphorus release from lake sediments Shengrui Wang a , Xiangcan Jin a,∗ , Qinyun Bu a , Lixin Jiao a,b , Fengchang Wu c a

State Environmental Protection Key Laboratory for Lake Pollution Control, Research Center of Lake Eco-environment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China b Inner Mongolia Agriculture University, Huhhot 010018, China c State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China Received 17 May 2007; received in revised form 20 August 2007; accepted 4 September 2007 Available online 7 September 2007

Abstract Phosphorus (P) can be released from sediments into overlying water under certain environmental conditions, which may have a significant impact on water quality and result in continuing eutrophication. In this study, effects of oxygen supply level in the overlying water on the P release from sediments were investigated in controlled experiments. The results show that in anaerobic condition soluble reactive phosphorus (SRP) increased in the overlying water for the two studied sediments, and gradually reached equilibrium after 30 days. In anoxic condition, P was rapidly released within 1 day, SRP then decreased, and gradually reached equilibrium after 10 days. In aerobic and oxygen saturation conditions changes in oxygen supply levels did not significantly affect the P release. pH in the overlying water increased in anaerobic condition, and gradually remained constant after 30 days. In anoxic, aerobic and oxygen saturation conditions pH rapidly increased within 1 day in the overlying water, and then decreased slightly, and gradually remained stable after 10 days. P can only be released from the sediment of Lake Meiliang and Gonghu in anaerobic condition, and the released P was mainly from BD-P, HCl-P and NaOH-P fractions. The results also indicate that transformation between different P fractions occurred under different oxygen supply levels, biological activities may be the main reason. © 2007 Elsevier B.V. All rights reserved. Keywords: Oxygen supply levels; Phosphorus release; Phosphorus fractions; pH; Lake sediment

1. Introduction Phosphorus (P) was one of the most important nutrients for lake eutrophication [1]. It was introduced to lake ecosystems, and was accumulated in sediments. P can be released from sediments into the overlying water under some environmental conditions, which may have a significant impact on water quality and may result in continuing eutrophication [2,3]. Factors influencing the P release from lake sediments have been extensively reviewed by Bostrom [4]. Relevant environmental factors mainly include temperature, pH, redox potential and hydrological conditions, etc. [5,6]. Among them, dissolved oxygen (DO) was the main concern, and its effects on P release have been investigated in many previous studies [7–9]. The

∗

Corresponding author. Tel.: +86 10 84915185; fax: +86 10 84915190. E-mail address: [email protected] (X. Jin).

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.09.007

mechanism of the effects of DO in the overlying water on the P release from sediments was related to the redox and pH variations [8]. However, no quantitative data are available. In the filed water purification system, the increasing oxygen technique was used as the effective measure to improve water quality [10,11]. But the effects of the increasing DO content in the overlying water on total phosphorus (TP) and different P fractions in sediments are unknown. Since not all of the P fractions in lake sediments can be released and render to lake eutrophication [12], the understanding of the changes of TP and different P fractions in sediments under different oxygen supply levels is important to illustrate the P dynamics in shallow lake ecosystem and also useful to further improve the increasing oxygen technique. The role of the lake sediment P in promoting lake eutrophication can be more efficiently evaluated on the basis of both P fractions and TP [13]. In terms of potential bioavailability, the extracted P fractions in lake sediments may be characterized as the loosely sorbed P (NH4 Cl-P), the reductant soluble P (BD-P), the metal

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oxide bound P (NaOH-P) and the calcium bound P (HCl-P) [14]. Changes in oxygen content in the overlying water may result in changes in pH [15]. Therefore, pH in the overlying water should also be considered in the investigation of effects of oxygen supply levels on the P release. The objective of this study was to illustrate the effects of oxygen supply levels on the P release from sediments. The changes in pH, TP and P fractions were also examined during the controlled release experiments to understand the P dynamics in shallow lakes in different oxygen supply conditions. 2. Materials and methods 2.1. Study area Two sampling sites were chosen for this study in Lake Taihu (Fig. 1), position and brief description of the overlying water at the studied sites are listed in Table 1. Site 1 is in Lake Meiliang (31◦ 31 325 N, 120◦ 09 340 E), located in the northwest part of Lake Taihu. This Bay severs as a principal water resource for the industrial city, Wuxi City, and receives wastewater and sewage from the city, and has been identified to be one of the most polluted water bodies in China [16,17]. Site 2 was in the Gonghu Lake (31◦ 24 843 N, 120◦ 15 242 E), located in the northeast portion of Lake Taihu. Lake Gonghu has a vast water surface, good fluidity and a high capacity of oxygen restoration. A large submerged plant is found, this lake is in mesotrophication [18]. 2.2. Sample sampling Fifteen undisturbed sediment cores were collected from the two sampling sites by a core Plexiglas sampler with 30 cm

Table 1 Position and brief description of the overlying water at the two studied lakes Parameters

Sampling sites Lake Meiliang

Position Water depth (m) TP of the overlying water (mg L−1 ) DIP of the overlying water (mg L−1 ) SRP of the overlying water (mg L−1 ) TN of the overlying water (mg L−1 ) Transparence (m) pH DO (mg L−1 )

31◦ 31 325

N E 120◦ 09 340 1.87 0.23 0.11 0.07 3.74 0.34 6.99 1.29

Lake Gonghu N 31◦ 24 843 E 120◦ 15 242 1.34 0.04 0.03 0.01 1.66 0.73 7.34 2.59

length, 5 cm diameter cylinder tube in September 2003. After sampling, the overlying water was slowly siphoned off to avoid the sediment resuspension. The overlying water samples at the same site also were collected in 0.5 m depth, and were then filtered through 0.45 ␮m cellulose membranes in the filed. The undisturbed sediment cores and overlying water samples were in sealed plastic ice bags, and were taken to the laboratory. All samples were stored at 4 ◦ C before laboratory experiments. 2.3. Phosphorus release experiments P release experiments were performed in the sampling tubes, the sediment depth in the tube was 10 cm. The water sample was slowly added into the tube to the height of 300 mm to avoid sediment resuspension [19]. Four treatments were conducted: DO content of anaerobic treatment was lower than 1 mg L−1 , that of anoxic treatment was from 2 to 4 mg L−1 , that of aerobic treatment was from 5 to 7 mg L−1 and that of oxygen saturation treatments were higher than 8.6 mg L−1 . These studied sediment cores were all irradiated under the light intensity of 1500 lux with dark/light cycling of 12:12 h at 25 ± 2 ◦ C. Parts of the studied sediment cores were aerated by air, parts were aerated by oxygen (99% purity) and the others were aerated by nitrogen (99% purity) for 2 h every day after daily sampling. During the incubation, the tubes were sealed with plastic caps to prevent oxygen dissolution to the overlying water. Soluble reactive phosphorus (SRP) was monitored during the experimental period. Water samples (30 mL) were taken for SRP analysis at different time, and the overlying water samples were added to maintain the water depth. The pH value was measured every sampling time. All experiments lasted 40 days. 2.4. Analytical methods

Fig. 1. The map of Lake Taihu showing sampling site location.

Water samples were filtrated by 0.45 ␮m GF/C filter membrane before analyses. Samples from each site were analyzed in the field for pH, DO and transparence of the overlying water using the WPA CD7000 portable field pH meter, TOA portable field DO meter and transparence tray. The TP, total nitrogen (TN) and SRP concentrations of the overlying water samples were measured using autoanalyzer (Technician II).

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The sediment cores were freeze-dried, and ground for analysis. The sediment samples were analyzed for cationic exchange capacity (CEC), TN [20], TP was determined using the SMT protocol [34], organic matter were based on weight losses after drying and combustion at 105 and 550 ◦ C, respectively. The contents of major elements in sediments were measured by ICP-AES (P-E, USA, ICP/6500). The grain sizes were measured using a Mastersizer 2000 Laser Size Analyzer (Malvern Co., U.K.), and were classified into clay (<0.002 mm), silt (0.002–0.05 mm) and sand fractions (0.05–2 mm) [22]. The different P fractions were determined using the sequential extraction scheme for lake sediments [23]. 1 M NH4 Cl, 0.11 M NaHCO3 /Na2 S2 O4 , 1 M NaOH and 0.5 M HCl were used for sequential extraction. The extracts were centrifuged and the supernatants were filtered through 0.45 ␮m GF/C filter membranes, and SRP was determined by the molybdenum blue/ascorbic acid method [22]. This extraction procedure divided inorganic P fractions into: loosely sorbed P (NH4 Cl-P), reductant soluble P (BD-P), metal oxide bound P (NaOH-P) and calcium bound P (HCl-P). OP was calculated as the difference between TP and inorganic P fractions. 3. Results and discussion 3.1. Characteristics of the studied sediments and the overlying water TN, TP, SRP and dissolved inorganic phosphorus (DIP) concentrations of the overlying water used in the experiments from Lake Meiliang were 3.74, 0.23, 0.07 and 0.11 mg L−1 , respectively, and they were 1.66, 1.34, 0.01 and 0.04 mg L−1 , respectively, from Lake Gonghu (Table 1). The general characteristics of the sediments used in the experiments are shown in Table 2. The silt fraction (4–63 ␮m) was the predominant for the two studied sediment samples, the sample from Lake Meiliang contained less sand fractions (63–500 ␮m) (8.83%), more clay (<4 ␮m) and silt fraction (15.64% and 75.53%, respectively), while the sample from Lake Gonghu contained more sand fracTable 2 Grain size distribution, chemical composition, and concentration of organic matter and CEC of the two studied sediments Properties

Sample sites

(mg L−1 )

TP TN (mg L−1 ) Organic matter (%) CEC (mmol g−1 )

Lake Meiliang

Lake Gonghu

2345 4230 5.23 0.45

633 1542 1.65 0.21

Grain size (␮m)

Clay (<4) Silt (4–63) Sand (63–500)

15.64 75.53 8.83

11.23 64.17 24.6

Major element (%)

SiO2 Al2 O3 Fe2 O3 MnO CaO

65.64 14.57 7.45 0.18 1.09

72.32 11.31 4.56 0.21 1.12

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tion (24.6%), less clay and silt fractions (11.23% and 64.17%, respectively). Organic matter, TN and TP of the sediment from Lake Meiliang (5.23%, 4230 mg L−1 , 2345 mg L−1 ) were higher than those from Lake Gonghu (1.65%, 1542 mg L−1 , 633 mg L−1 ). The two studied sediments were of the siliceous type, and their proportions of SiO2 , Al2 O3 , Fe2 O3 , MnO and CaO were all higher than 80%. The sediment from Lake Meiliang contained higher Al2 O3 and Fe2 O3 (14.57% and 7.45%) than that from Lake Gonghu (11.31% and 4.56%). This may be caused by serious pollution from the watershed and the large input of Al and Fe. The proportions of MnO and CaO were similar in the two samples. According to the results of characteristic data of the two studied sediments and overlying water, the sediment from Lake Meiliang was more heavily polluted than that from Lake Gonghu. This is consistent with the previous reports [22]. 3.2. Effects of oxygen supply levels on SRP in the overlying water The results of the P release from the two studied sediments as a function of different oxygen supply levels are shown in Fig. 2. P release from sediments allowed sufficient time for SRP in the overlying water to equilibrate with that in the suspended sediment, which typically took 25–30 days depending on the sediments used [24]. This is similar to the previous reports (Fig. 2), for the two studied sediments, in anaerobic condition (DO < 1 mg L−1 ) SRP concentrations of the overlying water increased with time increasing, and gradually reached equilibrium after 30 days. Comparing the two studied sediments, the equilibrium SRP concentration from Lake Meiliang was higher than that from Lake Gonghu. This is due to the fact that the sediment from Lake Meiliang was more polluted and its TP was also higher than that Lake Gonghu (Table 2). At the condition of DO > 1 mg L−1 , SRP concentrations of the overlying water of the different oxygen supply levels were almost the same and low. At the anoxic condition, SRP rapidly increased with day from 0.066 and 0.010 mg L−1 to 0.43 and 0.019 mg L−1 , and then decreased with time increasing, and gradually reached equilibrium after 10 days. The equilibrium SRP concentration of the sediment from Lake Meiliang was higher than that from Lake Gonghu. At aerobic and oxygen saturation condition, the P release trend was the same for SRP concentrations of both sediments, their SRP concentrations varied within first 10 days, and then gradually reached stabilization. This may relate to the processes of P sorption and desorption at sediment and water interface [25]. The oxygen supply level was one of the important factors affecting the process of the P sorption and desorption on sediments [26], the process of P sorption or desorption can dominate sometimes [27]. The complicated reciprocity actions between P sorption and desorption on the sediments resulted in changes in their SRP concentrations [28]. Comparing between the two studied sediments, SRP concentrations of Lake Meiliang were higher than those from Lake Gonghu. This indicates that in aerobic and oxygen saturation conditions the changes in oxygen supply levels did not significantly affect the P release from the

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Fig. 2. The SRP release as a function of different oxygen levels for the two studied sediments.

studied sediments, and the difference of SRP concentrations in the overlying water was related to the pollution status (Table 2). Our previous report also conformed that SRP concentration of overlying water was remarkably related to the pollution level of the sediment, the higher pollution level of the sediment was, the higher SRP concentration in the overlying water was [29]. As discussed in earlier sections, in anaerobic condition P can be released rapidly from the two studied sediments, as DO concentrations increased their SRP concentrations decreased rapidly. In anoxic condition, SRP concentration increased rapidly with <1 day, and then decreased rapidly. In aerobic and oxygen saturation conditions, SRP concentrations varied within first 10 days, and then gradually remained stable. This suggests that the increase in DO concentration in the overlying water can result in P release from lake sediment, this technology was often used in many polluted water purification [11,30]. 3.3. Effects of oxygen supply levels on pH in the overlying water pH in the overlying water was significantly affected by oxygen supply levels, this was one of the most important mechanisms that the P releases were affected [31]. Effects of oxygen

supply levels on pH in the overlying water for the two studied sediments during the whole incubation period are shown in Fig. 3. According to Fig. 3, the rapid (<1 day) increase in pH in the overlying water occurred for the two studied sediments at different oxygen supply levels. Among them, pH increased with time increasing in anaerobic condition (M-1 and G-1), and gradually remained constant after 30 h. Comparing between the two studied sediments, the stable pH value in the sediment from Lake Meiliang was higher than that of the sediment from Lake Gonghu (Fig. 3). pH in the overlying water was affected by many factors, especially the biological factor [15]. At anaerobic condition, pH in the overlying water was elevated by the denitrification of microorganism and photosynthetic bacteria in the studied sediments [32,33], denitrification often occurred in the saturated sediments where oxygen was depleted [34]. During denitrification, pH increased as nitrate was consumed [33,35]. In addition, during the incubation period some algae such as Cyanobacteria may survive in low DO condition and may carry photosynthesis. In a previous study, algae were observed by electron microscope in the sediment surface in anaerobic condition [15], this indicates that biologic factor may play an important role in the pH increases in the overlying water in the anaerobic condition. This result is further supported by previous reports

Fig. 3. Changes in pH in the overlying water during the whole incubation period for the two studied sediments.

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that the released P from the sediments in the anoxic condition was much more available to the alga growth than that in the aerobic condition [36,37]. In addition, that fact that the final pH value of the sediment from Lake Meiliang was higher than that from Lake Gonghu may be related to the characteristics of their sediment composition. This aspect needs further investigation. At aerobic and oxygen saturation condition, pH in the overlying water for the two studied sediments increased with the time increasing (M-3, M-4, G-3, G-4), and gradually remained stable after 10 h, and its pH was lower than that in anaerobic condition. This may be also related to the processes of biological activities and similar results were also previously reported [3]. In anoxic condition, pH rapidly increased within 1 h, from 6.98, 7.43 to 7.73 and 7.70 for the sediments from Lake Meiliang and Gonghu, respectively, pH then decreased slightly, and gradually remained constant after 10 h (Fig. 3). This may be also related to the denitrification of microorganism and photosynthetic processes of algae and bacteria [15,38]. As a whole, according to the results of effects of pH on the P release [31], in this study, changes in pH in the overlying water are consistent with the SRP concentrations. This suggests that changes in pH in the overlying water resulted from oxygen supply levels were one of the most important mechanisms for P release processes in sediments. 3.4. Effects of oxygen supply levels on TP and P fractions in the sediments The initial P fractions in the two studied sediments are shown in Figs. 4–6. NH4 Cl-P, NaOH-P, BD-P and OP in sediments were considered as a potential P resource for the overlying water [21,23]), this indicates that those P fractions in sediments can be released in certain conditions. For the sediment from Lake Meiliang, NH4 Cl-P was the lowest, only 2.25 mg kg−1 . NaOHP was the main P fraction, accounting for 65% to TP, HCl-P, BD-P and OP were 414, 287 and 118 mg kg−1 , respectively. NaOH-P, BD-P, NH4 Cl-P and OP together accounted for 82% to TP. For the sediment from Lake Gonghu, NH4 Cl-P was also the lowest, only 0.47 mg kg−1 , HCl-P was the main P fraction, accounted for 54% to TP. NaOH-P, BD-P, NH4 Cl-P and OP together accounted for up to 46% of TP. Therefore, the content

Fig. 4. The NH4 Cl-P contents of the two studied sediments under different treatments.

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Fig. 5. Different P fractions from the sediment of Lake Meiliang under different treatments.

of potential P resource in the sediment from Lake Meiliang was higher than that from Lake Gonghu. The process of the P release from sediments was affected by oxygen supply levels in the overlying water [39]. In this study, TP and different P fractions in the two studied sediments in different oxygen supply levels after the release experiments are shown in Figs. 4–6, and their multiple comparisons are listed in Table 3. As shown in Figs. 5 and 6 and Table 3, after the release experiment, TP in the sediments from Lake Meiliang in anaerobic condition was significantly lower than its initial value. But there was no significant difference between HCl-P, OP and their initial values. Results show that P released was mainly from BD-P and NaOH-P, NH4 Cl-P increased significantly after the release experiments. In anoxic, aerobic and oxygen saturation conditions, TP, NaOH-P, HCl-P and OP in the sediment from Lake Meiliang before and after release experiments were almost the same, only its BD-P in anoxic condition was significant lower than the initial value. This suggests that P release only occurred in anaerobic condition for the sediment from Lake Mailiang, and the released P was mainly from the BD-P and NaOH-P fractions,

Fig. 6. Different P fractions in the sediment from Lake Gonghu under different treatment.

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Table 3 Multiple comparison of the different treatments for the studied sediment by LSD method P fractions

Lake Meiliang Treatments

Lake Guhu Means

Treatments

Means

NH4 Cl-P

M-1 M-3 M-2 M-4 M

2.62 2.55 2.44 2.31 2.24

± ± ± ± ±

0.28a 0.15a 0.20a 0.32ab 0.34b

G-1 G-2 G-4 G-3 G

0.79 0.49 0.48 0.47 0.47

± ± ± ± ±

0.13a 0.01a 0.15a 0.17ab 0.04b

BD-P

M-3 M-4 M M-2 M-1

306.22 280.27 230.87 228.75 167.85

± ± ± ± ±

22.85a 24.66a 33.28b 30.41bc 28.20c

G-2 G-3 G G-4 G-1

69.48 69.07 63.34 61.04 55.10

± ± ± ± ±

17.34a 16.00a 19.67a 13.75b 15.74b

NaOH-P

M M-3 M-4 M-2 M-1

1417.34 1313.24 1308.44 1256.48 1164.21

± ± ± ± ±

89.56a 67.40ab 71.79ab 58.10ab 59.94b

G-3 G-2 G-4 G G-1

162.37 144.68 126.74 122.84 108.08

± ± ± ± ±

18.16a 19.74ab 17.10bc 15.33bc 12.18c

HCl-P

M M-2 M-1 M-3 M-4

390.52 346.78 342.58 339.05 338.02

± ± ± ± ±

45.42a 47.61a 43.42a 54.32a 36.62

G G-4 G-3 G-2 G-1

339.10 280.00 268.48 253.67 209.29

± ± ± ± ±

32.89a 29.94b 26.27b 24.16b 24.42c

OP

M-1 M M-3 M-4 M-2

121.99 118.84 110.62 107.53 106.97

± ± ± ± ±

31.12a 33.84a 26.23a 24.66a 26.73a

G G-4 G-3 G-2 G-1

106.81 105.24 99.85 78.30 63.82

± ± ± ± ±

38.22a 37.74a 31.41a 22.04a 39.61b

TP

M M-4 M-3 M-2 M-1

2159.81 2036.57 2071.68 1941.42 1799.66

± ± ± ± ±

86.55a 105.73a 142.05a 169.34a 1483.26b

G G-3 G-4 G-2 G-1

632.55 600.24 573.50 546.62 437.08

± ± ± ± ±

58.65a 65.20a 57.41a 41.63a 55.11b

Note: (1) M and G are initial sediments, M-1and G-1 are the anaerobic treatment; M-2 and G-2 are anoxic treatment; M-3 and G-3 are aerobic treatment, M-4 and G-4 are oxygen saturation treatment. Following are the same. (2) The same letter indicates the difference is not significant, the small letter indicate P < 0.05.

similar results were also reported in previous studies [29]. As shown in Figs. 2, 5 and 6), although there was no P release for the sediment from Lake Meiliang in aerobic and oxygen saturation conditions, there were significant changes in BD-P and NH4 ClP fractions before and after release experiments. This indicates that the mutual transformation of different P fractions occurred, but their detailed processes need to be further investigated. For the sediment from Lake Gonghu, TP in anaerobic condition was significant lower than the initial value. Only OP did not change significantly before and after the release experiments, and NH4 Cl-P increased significantly after the release experiments. This shows that the released P was mainly from the BD-P, HCl-P and NaOH-P fractions. In anoxic, aerobic and oxygen saturation conditions, similar to the sediment from Lake Meiliang, although P in the sediment from Lake Gonghu was not released, BD-P, NaOH-P and HCl-P changed significantly before and after the release experiments. Therefore, the mutual transformation of different P fractions also occurred. NH4 Cl-P represents the loosely sorbed P in the sediments, and this fraction may include dissolved P in the pore water [13]. NH4 Cl-P in the two studied sediments increased significantly

in anaerobic condition, this is probably due to the fact that the sorption–desorption equilibrium was affected by anaerobic condition. BD-P represents the redox-sensitive P fraction, mainly including P bound to Fe-hydroxides and Mn compounds [40]. In occasional DO depletion environments, BD-P was released from the anaerobic sediments and acted an internal P source to the overlying water [41]. Therefore, BD-P in the two studied sediments was much lower that their initial value in anaerobic condition. NaOH-P is exchangeable including P bound to metal oxides, mainly of Al and Fe [13], and was once used for the estimation of available P in the sediments and was an indicator of algal available P [14]. This fraction can be released for the growth of phytoplankton when anoxic conditions were prevailed at the sediment–water interface [40]. HCl-P represents the P fraction that was assumed to mainly consist of apatite P (natural and detritus) including P bound to carbonates and traces of hydrolysable organic P [29]. This P fraction was deemed as a relatively stable fraction of inorganic P in the sediments [12]. Thus, under different oxygen supply levels HCl-P in the sediment from Lake Meiliang did not change significantly, while that from Lake Gonghu changed remarkably, this may be also

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relate to the organism processes. The mobility of HCl-P was considered to be low because the organism processes were seldom considered in previous experiments [1,9,22]. In oxygen supply condition, HCl-P can be released from the sediment as a function of biological activities. The biological activity of the slightly polluted sediment was higher than that of the heavily polluted [9,42]. At discussed, the pollution level of the sediment from Lake Meiliang was higher than that from Lake Gonghu, and so HCl-P in the sediment from Lake Gonghu changed remarkably. According to Ruban et al.’s studies [43], OP was the potentially releasable P fraction. But OP content in the sediment from Lake Meiliang did not change before and after the release experiments, suggesting the transformation between different P fractions. OP in the sediment from Lake Gonghu changed remarkably in anaerobic condition; this may be also related to the organism. Dissolved oxygen is one of the most important factors that affected many biogeochemical processes in lakes [15]. In order to describe the P release from sediments, ecological models have been developed which incorporate two-dimensional or threedimensional hydrodynamic treatments to predict the scale of this phenomenon, and quantitative information on nutrient and dissolved oxygen dynamics both at the sediment–water interface and in the water column is badly needed [44–47]. Many eutrophiction models have been employed [48,49]. Sediment–water interactions have been intensively investigated using sediment flux models, and the results are similar to many previous studies [50,51]. Therefore, the this would not only help the quantitative understanding of the influence of DO on the P release, and would also provide detailed process and scientific data for sediment P flux models. 4. Conclusions Oxygen supply level in the overlying water was one of the most important factors that affected P release from the sediment, and changes in pH in the overlying water and different P fractions in the sediment were one of the important reasons. In anaerobic condition SRP concentrations increased in the overlying water for the two studied sediments, and gradually reached the equilibrium after 30 days. The equilibrium SRP concentration in the sediment from Lake Meiliang was higher than that from Gonghu Lake. In anoxic condition, P was released rapidly within <1 day, its SRP concentration then decreased afterwards, and gradually reached equilibrium after 10 days. In aerobic and oxygen saturation condition changes in oxygen supply levels did not remarkably affect the P release. The pH value of the overlying water increased in anaerobic condition and gradually remained constant after 30 days. In anoxic, aerobic and oxygen saturation conditions, pH rapidly increased within 1 day in the overlying water, then decreased slightly, and finally remained stable after 10 days; pH was lower than that in anaerobic condition. The P release only occurred in the sediment from Lake Meiliang in anaerobic condition; the released P was mainly from BD-P and NaOH-P fractions. For the sediment from Lake Gonghu, P was also only released in anaerobic condition, and the released P was mainly from BD-P, HCl-P and NaOH-P fractions. The transfor-

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