Forms of Nitrogen and Phosphorus in Suspended Solids: a Case Study of Lihu Lake, China

Article Forms of Nitrogen and Phosphorus in Suspended Solids: A Case Study of Lihu Lake, China

Jialu Li 1,2 and Qiting Zuo 1,2,*

1 School of Water Conservancy Engineering, Zhengzhou University, Zhengzhou 450001, China; [email protected] 2 Zhengzhou Key Laboratory of Water Resource and Water Environment, Zhengzhou 450001, China * Correspondence: [email protected]; Tel.: +86-136-5381-7257

Received: 9 April 2020; Accepted: 4 June 2020; Published: 19 June 2020

Abstract: Suspended solids are an important part of lake ecosystems, and their nitrogen and phosphorus contents have a significant effect on water quality. However, information on nitrogen and phosphorus forms in suspended solids remains limited. Therefore, a case study was conducted in Lihu Lake (China), a lake with characteristically high amounts of suspended solids. Nitrogen and phosphorus speciation in suspended solids was analyzed through a sequential extraction method. We also evaluated the sources of various forms of nitrogen and phosphorus and their different effects on eutrophication. The total nitrogen (TN) content was 758.9–3098.1 mg/kg. Moreover, the proportions of various N forms in the suspended solids of the study areas were ranked as follows: Hydrolyzable nitrogen (HN) > residual nitrogen (RN) > exchangeable nitrogen (EN). Total phosphorus (TP) ranged from 294.8 to 1066.4 mg/kg, and 58.6% of this TP was inorganic phosphorus (IP). In turn, calcium (Ca)-bound inorganic phosphorus (Ca-Pi) was the main component of IP. The correlation between various nitrogen and phosphorus forms showed that there were different sources of suspended nitrogen and phosphorus throughout Lihu Lake. Correlation analysis of water quality indices and comparative analysis with surface sediments showed that in Lihu Lake, the dissolved nitrogen and phosphorus contents in water were influenced by sediment through diffusion, while particle phosphorus content in water was influenced by suspended solids through adsorption; however, due to the higher phosphorus contents in suspended solids, we should pay more attention to the impact of suspended solids.

Keywords: suspended solids; nitrogen; phosphorus; form analysis; Lihu Lake

1. Introduction Eutrophication is among the most important threats to global surface water quality. Currently, more than 75% of closed water bodies worldwide present some degree of eutrophication [1–3]. According to the Organization for Economic Cooperation and Development (OECD), “eutrophication” is defined as an undesirable degradation in water quality associated with increases in the productivity of algae and aquatic plants, which is caused by increases in nutrient concentrations and affects the beneficial uses of water. As socioeconomic development continues to grow, the term “eutrophication” is commonly used to refer to events linked to anthropogenic activities, typically within a short time scale (i.e., hours, days, months, years) [4,5]. Eutrophication can cause great damage to aquatic ecosystems and has adverse effects on aquatic products, water services, human health, and economic activities in surrounding areas [6–8]. The mechanisms of eutrophic formation and diffusion have been studied since the 1970s. Nitrogen (N) and phosphorus (P) are the main growth-limiting nutrients for algae, bacteria, protozoa, and other aquatic organisms that inhabit freshwater ecosystems [9–11]. However, the excessive input of these nutrients may accelerate the eutrophication process in the affected water

Sustainability 2020, 12, 5026; doi:10.3390/su12125026 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 5026 2 of 27 bodies. Therefore, the effects of nitrogen and phosphorus on eutrophication have been studied both in water and sediments. Although external inputs are the main factor leading to the eutrophication of lakes, the biochemical reaction of nitrogen and phosphorus at the sediment/suspended solid water interface has an impact on the nutrient contents in water. Different forms of nitrogen and phosphorus play different roles in biogeochemical cycles and, therefore, contribute differently to eutrophication [12,13]. In general, nitrogen and phosphorus are converted into different molecular forms at the sediment/suspended solid water interface. Nitrogen in sediments and suspended solids can be divided into convertible and non-convertible forms. Among them, convertible nitrogen participates actively in the nitrogen cycle [14]. On the other hand, phosphorus in sediments/suspended solids mainly exists in adsorbed, organic, iron-/aluminum-bound, and calcium-bound forms. However, only bioavailable nitrogen and phosphorus can promote algae growth and lead to eutrophication [15]. In recent years, most studies have focused on the forms of nitrogen and phosphorus that are most common in sediments. However, few studies have characterized these elements in suspended solids. Suspended solids (SS) are among the most important elements of freshwater ecosystems, as they represent a substrate for nutrients to bind to [16,17]. The re-release of nitrogen and phosphorus in suspended solids is also one of the most important factors affecting the eutrophication of lakes. Suspended solids can easily adsorb contaminants (e.g., nutrients, heavy metals, organic contaminants) [18–21]. Moreover, suspended solids can exchange substances with water bodies during re-suspension, or reincorporate into the sediments after flocculation and sedimentation. On the other hand, the degradation of nutrients in suspended solids during suspension may lead to an accelerated migration of biogenic elements into the water bodies. These migrated elements can ultimately supplement the nutrients that are often depleted in summer due to blue-green algae blooms, and thus perpetuate algae bloom cycles. Therefore, studying the forms of nitrogen and phosphorus in suspended particulate matter is critical in establishing a theoretical basis for lake eutrophication control. Taihu Lake, located in Jiangsu and Zhejiang province (i.e., the most economically developed areas of China), is among the five largest freshwater lakes in China. It represents the main drinking water source for Wuxi and Suzhou cities in Jiangsu province, and is one of the main water supplies for Shanghai and East Zhejiang province. However, with rapid urbanization, Taihu Lake has experienced numerous water pollution problems since the 1980s, particularly eutrophication and subsequent cyanobacterial blooms. Lihu Lake, a bay located in Wuxi City, is a typical shallow lake, with an average water depth of only 2.25 m [22,23]. Wind and wave disturbances are prevalent and affect the water dynamics of the lake. Therefore, although water quality has been greatly improved due to the implementation of aquatic environment treatment projects, the concentration of suspended solids in the lake remains high. Large amounts of SS not only reduce the transparency and affect the underwater light conditions, but are also a great challenge for the drinking water of Wuxi City. However, previous studies on Lihu Lake mainly focused on the water quality [24], nitrogen and phosphorus forms in the sediment [25,26], etc. Although Lihu Lake has been subject to a series of abatement actions to restore the ecological environment, the content of suspended solids in this lake is still high, and the eutrophication still exists. Therefore, the study of nitrogen and phosphorus forms in suspended solids is of great significance for the further ecological restoration and environmental management of Lihu Lake. However, there are few studies on the forms of nitrogen and phosphorus in suspended solids of Lihu Lake. In this study, a nitrogen and phosphorus fractionation extraction approach based on improvements to previously reported methods was used to study the forms and spatial distributions of nitrogen and phosphorus in suspended solids in Lihu Lake. Particularly, this study sought to characterize the migration and transformation of nutrients between the water–SS and SS–sediment interfaces based on the following aspects: (1) Contents and spatial distribution of nitrogen and phosphorus forms in suspended solids, (2) correlations between all of the nitrogen and phosphorus forms in overlying water, suspended solids, and surface sediments, and (3) comparison between the nitrogen and phosphorus contents in suspended solids and surface sediments. Sustainability 2020, 12, 5026 3 of 27

2. Materials and Methods

2.1. Study Area

Lihu Lake (120.22◦ E–120.29◦ E, 31.48◦ N–31.55◦ N) is located north of Taihu Lake, with a 6 km length from east to west and a 0.3–1.8 km width, covering an 8.6 km2 surface. The lake is connected to the Meiliang Bay of Taihu Lake, the Beijing-Hangzhou Grand Canal, and Gonghu Bay, which is relatively independent but still a part of Taihu Lake. In general, the overall flow rate of the study area is low. In the 1950s, Lihu Lake exhibited medium nutrient levels and was rich in aquatic plants and fisheries resources. Since then, the lake has provided drinking water for the entire city of Wuxi [27]. However, since the 1970s, the locals reclaimed land from the lake, and large-scale aquaculture farms were constructed. Additionally, vast amounts of domestic and industrial sewage from the surrounding areas began to be discharged into the lake. As a result, the total nitrogen (TN) and total phosphorus (TP) concentrations increased, and the water quality of the lake and its inflow rivers deteriorated rapidly, which has led to severe eutrophication. In 2001, the annual mean permanganate, total nitrogen (TN), total phosphorus (TP), and chlorophyll-a (Chl.a) index values were 7.80, 6.38, 0.20, and 0.08 mg/L, respectively, which were 1.39, 1.93, 2.44, and 2.61 times higher than the overall historical indices of Taihu Lake [28]. Therefore, starting in 2003, the Wuxi City government banned aquaculture, relocated the residents around the lake, and eliminated pollution sources to prevent further eutrophication of Lihu Lake. The central and local governments jointly implemented the “environmental dredging and ecological reconstruction project for heavily polluted water” in Lihu Lake, which entailed environmental dredging, vegetation reconstruction, and gate control or blockage of contaminated rivers. The water quality of Lihu Lake has since been maintained. However, water transparency and suspended solids have not improved significantly, and thus, nutrients in suspended solids may still aggravate the eutrophication process. Nitrogen and phosphorus are the main pollutants affecting the water quality of Lihu Lake, and these contaminants originate from many external sources, as well as from endogenous release. Given that Lihu Lake is virtually isolated from the surrounding rivers by gates and dams, there is no water exchange between the lake and the surrounding rivers. The main sources of nitrogen and phosphorus in Lihu Lake are mainly internal and external sources through dry and wet deposition. In this study, Lihu Lake was divided into four zones (A, B, C, and D), based on the physical boundaries within the study area (Lihu Dike, Baojie Bridge, and Lihu Great Bridge; Figure1). Zone A used to contain a large number of fish ponds, which seriously polluted the lake, after which the surface sediments were removed by draining the lake water. Zone B was known as the “western Lihu Lake” before the implementation of comprehensive remediation measures via mechanical dredging. Moreover, aquatic vegetation reconstruction was implemented offshore. Zone C, bordered by the Baojie Bridge to the west and Lihu Great Bridge to the east (including Changguangxi Wetland Park), and also implemented the comprehensive coastal improvement project. Zone D, with more residential areas offshore, exhibits the worst water quality in Lihu Lake. Sustainability 2020, 12, 5026 4 of 27 Sustainability 2020, 12, x FOR PEER REVIEW 4 of 28

FigureFigure 1. 1. LocationLocation and and sampling sampling sit siteses in in L Lihuihu Lake. Lake. 2.2. Sampling and Sample Preparation 2.2. Sampling and Sample Preparation Suspended solids and surface sediment samples from 28 sites (Figure1) in Lihu Lake were Suspended solids and surface sediment samples from 28 sites (Figure 1) in Lihu Lake were collected in October 2019. A total of 50 L of water at a 0.5 m depth were collected from each sampling collected in October 2019. A total of 50 L of water at a 0.5 m depth were collected from each sampling site into a clean and light-proof plastic sampling bottle which had been previously rinsed with Milli-Q. site into a clean and light-proof plastic sampling bottle which had been previously rinsed with Water samples were filtered with glass fiber filters (GF/F) (Whatman, Maidstone, Kent County, U.K.) Milli-Q. Water samples were filtered with glass fiber filters (GF/F) (Whatman, Maidstone, Kent (these filters were pre-soaked in 1:1000 (v:v) HCl for 24 h, then washed to neutral pH with Milli-Q County, U.K.) (these filters were pre-soaked in 1:1000 (v:v) HCl for 24 h, then washed to neutral pH water, and finally dried at 400 C for 5 h) in situ to obtain wet suspended solid samples. These samples with Milli-Q water, and finally◦ dried at 400 °C for 5 h) in situ to obtain wet suspended solid samples. were then sent to the laboratory within 24 h at 4 C and vacuum freeze-dried at 50 C for 72 h, then These samples were then sent to the laboratory− within◦ 24 h at −4 °C and vacuum− freeze◦ -dried at −50 stored in dark, sealed bottles at 4 C until they were analyzed. At each site, the surface sediment °C for 72 h, then stored in dark, −sealed◦ bottles at −4 °C until they were analyzed. At each site, the samples were collected using a Petersen dredge. These samples were stored in clean black polythene surface sediment samples were collected using a Petersen dredge. These samples were stored in bags at 4 C before being sent to the laboratory, vacuum freeze-dried at 50 C, and ground through clean black− ◦ polythene bags at −4 °C before being sent to the laboratory, −vacuum◦ freeze-dried at −50 a 100 mesh sieve. The prepared samples were also stored in light-proof glass sealing bottles at 4 C. °C, and ground through a 100 mesh sieve. The prepared samples were also stored in light-−proof◦ In the field, water samples were filtered through pre-weighted filters (GF/F) (Whatman, Maidstone, glass sealing bottles at −4 °C. In the field, water samples were filtered through pre-weighted filters Kent County, U.K.) to measure the suspended solid contents. (GF/F) (Whatman, Maidstone, Kent County, U.K.) to measure the suspended solid contents. 2.3. Sample Analysis 2.3. Sample Analysis 2.3.1. Nitrogen Forms 2.3.1. Nitrogen Forms Based on sequential extraction, the nitrogen forms in suspended solids were divided intoBased exchangeable on sequential nitrogen extraction, (EN), acid-hydrolyzable the nitrogen forms nitrogen in suspended (HN), and solids residual were nitrogen divided (RN). into exchangeableThe sequential nitrogen extraction (EN), steps acid for- nitrogenhydrolyzable forms nitrogen and the measurement (HN), and residual methods nitrogen for each (R indexN). The are sequentialillustrated inextraction Figure A1 steps (Appendix for nitrogenA). forms and the measurement methods for each index are illustrated in Figure A1 (Appendix A). 2.3.2. Phosphorus Forms 2.3.2. Phosphorus Forms According to the absorption and desorption characteristics of each phosphorus form in suspended solids,According as well asto theirthe absorption stability once and combined desorption with chara thecteristics suspended of each solids, phosphorus and based form on our in suspimprovementsended solids, of as previously well as their published stability sequential once combined extraction with methodsthe suspended for phosphorus, solids, and ourbased study on ourdivided improvements inorganic phosphorus of previously into published weakly adsorbedsequential inorganic extraction phosphorus methods for (WA-P phosphorus,i), potentially our study active dividedinorganic inorganic phosphorus phos (PA-Pphorusi), iron into (Fe)- weakly or aluminum adsorbed (Al)-bound inorganic phos inorganicphorus phosphorus (WA-Pi), potentially (Fe/Al-Pi), activeand calcium inorganic (Ca)-bound phosphorus inorganic (PA phosphorus-Pi), iron (Fe) (Ca-P- ori ). aluminum Moreover, (Al) the- organicbound phosphorus inorganic phosphorus forms were (Fe/Aldivided-Pi), into and weakly calcium absorbed (Ca) organic-bound phosphorus inorganic phosphorus(WA-Po), potentially (Ca-Pi). active Moreover, organic the phosphorus organic phospho(PA-Po),rus medium forms active were organic divided phosphorus into weakly (MA-P absorbeo), andd organic non-active phosphorus organic phosphorus(WA-Po), potentially (NA-Po). activeAmong organic these, phosphorusMA-Po includes (PA- mediumPo), medium active active organic organic phosphorus, phosphorus which (MA can-Po be), and extracted non-active with organica sodium phosphorus hydroxide (NaOH) (NA-Po). solution Among(MA these,NaOH-Po MA-),Po as includes well as non-active medium active organic organic phosphorus, phosphorus, which whichcan be can extracted be extracted with a with sodium a so hydroxidedium hydroxide (NaOH) (NaOH) solution solution (NANaOH-Po (MANaOH), and-Po), as residual well as phosphorus non-active organic phosphorus, which can be extracted with a sodium hydroxide (NaOH) solution (NANaOH-Po), Sustainability 2020, 12, x FOR PEER REVIEW 5 of 28 Sustainability 2020, 12, 5026 5 of 27 and residual phosphorus (R-Po). The sequential phosphorus extraction steps and the measurement methods for each index are shown in Figure A2 (Appendix A). (R-Po). The sequential phosphorus extraction steps and the measurement methods for each index are shownNitrogen in Figure and A2 phosphorus (Appendix Aforms). in the surface sediments were also extracted according to the aforementionedNitrogen and method. phosphorus The measurement forms in the method surface of each sediments index in were the alsooverlying extracted water, according suspended to solids,the aforementioned and surface sediments method. The was measurement performed as method described of each by indexWang in[25] the and overlying Wang water,[26]. suspended solids, and surface sediments was performed as described by Wang [25] and Wang [26]. 2.4. Quality Assurance and Quality Control (QA/QC) 2.4. QualityThe analytical Assurance data and quality Quality was Control assessed (QA by/QC) implementing quality assurance and quality control (QA/QC)The analytical procedures, data including quality was an assessedalysis of by reagent implementing blanks, quality duplicate assurance samples, and quality and standard control refe(QArence/QC) procedures,materials (GSD including-3a) for analysis each sample of reagent batch. blanks, The analytical duplicate samples,precision and for standardreplicate referencesamples wasmaterials within (GSD-3a) ±10% and for the each measurement sample batch. errors The analyticalbetween determined precision for and replicate certified samples values was were within less than10% 5%. and the measurement errors between determined and certified values were less than 5%. ± Statistical analyses of mean values and standard standard deviations were performed using the SPSS 22.0 software (IBM,(IBM, Armonk, Armonk, USA). U.S. The). The Kriging Kriging method method was employed was employed using ArcGIS using 10.2ArcGIS (ESRI, 10.2 Redlands, (ESRI, Redlands,USA) and Surfer U.S.) and 11 (Golden Surfer 11 Software, (Golden Golden, Software, USA) Golden, to map U.S.) the spatialto map distribution the spatial ofdistribution nitrogen and of nitrogenphosphorus and forms. phosphorus Other experimental forms. Other data experimental graphs were data created graphs with Excel were 2010 created (Microsoft, with Excel Redmond, 2010 (Microsoft,USA) and Origin Redmond, 2020 U.S.) (OriginLab, and Origin Northampton, 2020 (OriginLab, USA). Northampton, U.S.).

3. Results Results and and Discussion Discussion

3.1. Spatial Spatial Distribution Distribution of of Total Total SuspendedSuspended Solids The term term “suspended “suspended solids” solids” (SS) (SS) generally generally refers refersto plankton to plankton and inanimate and inanimate particles suspended particles suspendedin water. Notably, in water. the total Notably, suspended the total solid (TSS) suspended content solid is not (TSS) only positively content is correlated not only with positively nutrient correlatedlevels, but with is also nutrient positively levels correlated, but is also with positively total phosphorus correlated (TP) with contents total phosphorus in water. Therefore, (TP) contents TSS inis oftenwater. used Therefore, as a reference TSS is often index used to reﬂect as a reference the level index of nutrients to reflect in water.the level In of 65 nutrients oligotrophic in water. lakes In in 65the oligotrophic USA and Argentina, lakes in the the USA average and Argentina, annual mean the TSS average value annual was only mean 1.00 TSS mg /valueL[29], was whereas only 1.00 this mg/valueL has[29], been whereas reported this tovalue be as has high been as 7.3 reported mg/L in to 86 be eutrophic as high shallow as 7.3 mg/L lakes in Europe86 eutrophic [30]. However, shallow lakesthe average in Europe TSS [30]. value However, in highly the eutrophic average shallow TSS value lakes in inhighly Denmark eutrophic was asshallow high as lakes 22 mg in/ L[Denmark31]. As wasshown as high in Figure as 222 ,mg/L in this [31]. study, As sh theown average in Figure TSS value2, in this in Lihu study, Lake the was average 27 mg TSS/L, value which in is Lihu similar Lake to wasthat of27 15mg/L, highly which eutrophic is similar shallow to that lakes of in15 Denmarkhighly eutrophic and West shallow Lake in lakes Hangzhou in Denmark (25.02 mgand/L) West [32]. LakeNonetheless, in Hangzhou the aforementioned (25.02 mg/L) [32]. value Nonetheless, was lower the than aforementioned those of Taihu value Lake was (32.32 lower mg than/L) [those33] and of TaihuChaohu Lake Lake (32.32 (43.61 mg/L) mg/L) [33] [34 and], which Chaohu are Lake more (43.61 eutrophic. mg/L) [34], which are more eutrophic.

Figure 2. SpatialSpatial distribution distribution of of total suspended suspended solid solidss ( (TSS)TSS) in Lihu Lake.

Since Lihu Lake is isolated from the surrounding rivers by gate and dams, the particle solids fromfrom the surroundingsurrounding rivers can be ignored.ignored. There Therefore,fore, planktonic algae proliferation, residues residues of Sustainability 2020, 12, 5026 6 of 27

aquaticSustainability plants, 2020 and, 12, x sediment FOR PEER REVIEW resuspension are the main factors inﬂuencing the TSS concentration6 of 28 in water. Due to an ecological reconstruction project, a considerable area of submerged plants has been plantedaquatic in plants Zone, A, and and sediment the contents resuspension of SS in are these the main areas factors are lower influencing than those the inTSS other concentration areas. The in main reasonwater. is thatDue to the an submerged ecological reconstruction plants can not project, only adsorb a considerable the suspended area of submerged solids, but plants also competehas been with phytoplanktonplanted in Zone for A, nutrients and the contents and light, of SS thus in inhibitingthese areas theare lower growth than of those algae in further. other areas. The main reason is that the submerged plants can not only adsorb the suspended solids, but also compete with As summarized in Table A1 (AppendixA), the particulate nitrogen (PN) and particulate phytoplankton for nutrients and light, thus inhibiting the growth of algae further. phosphorus (PP) fractions were 0.06–0.71 and 0.04–0.21 mg/L, respectively, with average values As summarized in Table A1 (Appendix A), the particulate nitrogen (PN) and particulate of 0.35 and 0.11 mg/L, accounting for 65.03% and 82.63% of the total nitrogen and total phosphorus phosphorus (PP) fractions were 0.06–0.71 and 0.04–0.21 mg/L, respectively, with average values of contents,0.35 and respectively. 0.11 mg/L, accounting Moreover, for the 65.03% TSS and value 82.63% was of found the total to nitrogen have a positiveand total correlation phosphorus with thecontents, concentrations respectively. of PP Moreover, (r = 0.260, the n = TSS28) value and TP was (r =found0.331, to nhave= 28) a (Figurepositive A3correlation). with the concentrations of PP (r = 0.260, n = 28) and TP (r = 0.331, n = 28) (Figure A3). 3.2. Spatial Distribution of Nitrogen Forms in Suspended Solids 3.2. Spatial Distribution of Nitrogen Forms in Suspended Solids 3.2.1. Spatial Distribution of Exchangeable Nitrogen (EN) in Suspended Solids 3.2.1.Figure Spatial3 illustrates Distribution the of spatial Exchan distributiongeable Nitrogen characteristics (EN) in Suspended of the Solids exchangeable nitrogen forms in suspendedFigure 3 solids illustrates of Lihuthe spatial Lake. distribution Exchangeable characteristics nitrogen of in the suspended exchangeable solids nitrogen mainly forms includes in + + exchangeablesuspended solids NH4 of-N Lihu (E-NH Lake.4 -N), Exchangeable NO3−-N (E-NO nitrogen3− -N), in suspended and exchangeable solids mainly soluble includes organic nitrogenexchangeable (SON). NH Exchangeable4+-N (E-NH4+-N), nitrogen NO3−-N in (E suspended-NO3−-N), and solids exchangeable can maintain soluble dynamic organic nitrogen equilibrium with(SON). the overlying Exchangeable water nitrogen and surface in suspended sediments solids through can maintain adsorption dynamic/desorption, equilibrium ion exchange, with the and biologicaloverlying disturbances water and surface [35]. Importantly, sediments through it represents adsorption/desorption, a nutrient source ion that exchange, can be directlyand biological utilized by primarydisturban producersces [35]. in Importantly, lakes [36]. Therefore, it represents the aamounts nutrient of source EN in that SS play can an be extremely directly utilized important by role primary producers in lakes [36]. Therefore, the amounts of EN in SS play an extremely important in the nitrogen cycle of suspended solids. role in the nitrogen cycle of suspended solids.

Figure 3. Spatial distribution of exchangeable nitrogen (EN) in suspended solids of Lihu Lake. Figure 3. Spatial distribution of exchangeable nitrogen (EN) in suspended solids of Lihu Lake. Sustainability 2020, 12, 5026 7 of 27

As shown in Figure3, the spatial distribution characteristics of exchangeable nitrogen in suspended solids of Lihu Lake were largely uniform. Overall, the contents in the estuary were higher than in the central lake. The estuary is constantly subjected to water disturbances, which are conducive to + the adsorption of NH4 -N and NO3−-N onto suspended solids, resulting in high EN contents in the bay, with only slight differences between the studied zones. Nonetheless, the spatial distribution was ranked as follows: Zone D > C > B > A. Currently, a large number of submerged plants has been identified in Zone A, including Myriophyllum verticillatum, Hydrilla verticillata, and Vallisineria spiralis. However, other floating plants, such as Hydrocharis dubia, water hyacinth, and water chestnut, have been identified in other zones. Compared with floating plants, submerged plants have a relatively longer growth cycle, and their resulting detritus exists as organic nitrogen, which can only be released through mineralization. Furthermore, even after being released, it can be easily absorbed by aquatic plants to synthesize the substances that they need, which results in low nitrogen contents in the suspended solids of Zone A. Moreover, environmental dredging projects were carried out in Zones A and B to remove a large amount of pollutants from the sediment, which led to a significant reduction in nitrogen and phosphorus loads in sediments, as well as a reduction in re-suspension. Sediment dredging has also been carried out in some parts of Zone C. Additionally, Changguangxi is rich in aquatic plants, and the resulting macrophyte decay has increased the nitrogen loads in this area. Zone D is greatly affected by urban sewage, which causes a large amount of nutrients to be imported into the lake, and also increases the re-suspension of sediments. Zone D has not yet been subjected to large-scale sediment dredging, and, therefore, the nitrogen content in this zone is relatively high, resulting in a significantly higher EN content. The EN concentration of the suspended solids of Lihu Lake ranged from 63.5 to 354.4 mg/kg (dry weight, the concentration for the solid fraction in this study is all dry weight), with a mean value of 174.4 mg/kg. The most heavily polluted were the northeastern regions of Zones A and D, whereas the areas with less pollution were mainly concentrated in the western regions of Zones A and B. Compared with other studies [37], the concentration of EN in suspended solids was higher. This could be because, after the submerged plant restoration project and fish regulation project, massive residues produced by the death of plants and fish accumulated on the surfaces of the suspended solids, resulting in high SON contents in the suspended solids. The main EN form is SON, which ranges from 17.6 to 187.9 mg/kg, with a mean value of 94.7 mg/kg, accounting for 54.3% of the EN. SON is mainly composed of free amino acids, amino sugars, urea, and low-molecular-weight organic acids, and is an important part of EN. Additionally, it plays a significant role in the mineralization, immobilization, migration, and transformation of nitrogen [38]. The organic matter (OM) in suspended solids forms a large amount + of NH4 -N via mineralization, which is then distributed between the overlying water-suspended + solids and the surface sediments. Therefore, E-NH4 -N is the most “active” component of the various + forms of nitrogen in suspended solids. The maximum value of E-NH4 -N was 218.1 mg/kg, whereas the minimum value was 10.86 mg/kg; the mean value was 62.9 mg/kg, accounting for 36.1%. Compared with other forms of nitrogen, the E-NO3−-N content was relatively low. Specifically, its mean value was 16.8 mg/kg, accounting for only 9.6% of the EN.

3.2.2. Spatial Distribution of Acid-Hydrolyzable Nitrogen (HN) in Suspended Solids The acid-hydrolyzable nitrogen (HN) in the suspended solids is a kind of nitrogen form that can be transformed after mineralization and then released into the overlying water. HN mainly exists in the form of organic nitrogen. The most identiﬁable organic compounds in HN are amino acid nitrogen (AAN), amino sugar nitrogen (ASN), ammonium nitrogen (AN), and acid-hydrolyzable unknown nitrogen-containing compounds (HUN) [39]. Figure4 illustrates the spatial distribution of all forms of HN in suspended solids of Lake Lihu. Sustainability 2020, 12, x FOR PEER REVIEW 8 of 28 indicates that HUN can also become bioavailable through biodegradation [40]. HUN had the highest proportion of HN (72.1%), followed by AN and AAN (19.6% and 6.5%, respectively). The suspended solids produce a certain amount of AN during the process of acid hydrolysis. The proportion of AN in the sampling sites of the eastern Zone A, the western Zone B, and the northern Zone D was relatively high, exceeding 28% in all cases. According to Wang et al. [41], AN is positively correlated with fixed ammonium nitrogen and organic matters. Therefore, we can infer that the organic matter and fixed ammonium nitrogen contents in suspended solids are important factors affecting AN. AAN (i.e., one of the most identifiable nitrogen-containing organic compounds), which is a kind of mineralizable nitrogen, and also a kind of stable SON [42]. Amino acids are mainly derived from proteins and polypeptides in organic matters. Thus, the OM content and its compounds will likely affect AAN contents directly. Suspended solid ASN mainly originates from the cell contents of microorganisms and aquatic insects. Its main component is glucosamine, followed by lactosamine; in fact, the sum of these two amines is close to the amount of nitrogen in the amino sugar fraction [43]. The results show that amino sugars are rarely found in higher plants [44,45], and are instead mainly concentrated in the chitin of microorganisms and insects. ASN represented the smallest Sustainability 2020, 12, 5026 8 of 27 fraction, with only 1.84%. The mean values of HN in Zones A, B, C, and D were 681.9, 697.3, 814.9, and 903.5 mg/kg, respectively. Moreover, the spatial distribution was ranked as Zone D > C > B > A.

Figure 4. SpatialSpatial d distributionistribution of of acid acid-hydrolyzable-hydrolyzable nitrogen ( (HN)HN) in suspended solids of Lihu Lake.Lake.

3.2.3.The Spatial spatial Distribution distribution of Residual of various Nitrogen forms of(RN) HN and at each Total sampling Nitrogen site (TN) was in diSuspendedfferent. The Solids mean values of AN, AAN, ASN, and HUN were 152.4, 50.6, 14.3, and 561.3 mg/kg, respectively. HUN had a lowerFigure decomposition 5 illustrates degreethe spatial than distribution other HN forms of residual such as nitrogen AN, and (RN) belonged and total to the nitrogen form that (TN) was in suspendednot easily mineralizable. solids. However, HUN decreased as the soil mineralization test proceeded. This indicates that HUN can also become bioavailable through biodegradation [40]. HUN had the highest proportion of HN (72.1%), followed by AN and AAN (19.6% and 6.5%, respectively). The suspended solids produce a certain amount of AN during the process of acid hydrolysis. The proportion of AN in the sampling sites of the eastern Zone A, the western Zone B, and the northern Zone D was relatively high, exceeding 28% in all cases. According to Wang et al. [41], AN is positively correlated with fixed ammonium nitrogen and organic matters. Therefore, we can infer that the organic matter and fixed ammonium nitrogen contents in suspended solids are important factors affecting AN. AAN (i.e., one of the most identifiable nitrogen-containing organic compounds), which is a kind of mineralizable nitrogen, and also a kind of stable SON [42]. Amino acids are mainly derived from proteins and polypeptides in organic matters. Thus, the OM content and its compounds will likely affect AAN contents directly. Suspended solid ASN mainly originates from the cell contents of microorganisms and aquatic insects. Its main component is glucosamine, followed by lactosamine; in fact, the sum of these two amines is close to the amount of nitrogen in the amino sugar fraction [43]. The results show that amino sugars are rarely found in higher plants [44,45], and are instead mainly concentrated in the chitin of microorganisms and insects. ASN represented the smallest fraction, with only 1.84%. Sustainability 2020, 12, 5026 9 of 27

The mean values of HN in Zones A, B, C, and D were 681.9, 697.3, 814.9, and 903.5 mg/kg, respectively. Moreover, the spatial distribution was ranked as Zone D > C > B > A.

3.2.3. Spatial Distribution of Residual Nitrogen (RN) and Total Nitrogen (TN) in Suspended Solids Figure5 illustrates the spatial distribution of residual nitrogen (RN) and total nitrogen (TN) in Sustainabilitysuspended 20 solids.20, 12, x FOR PEER REVIEW 9 of 28

FigureFigure 5.5. SpatialSpatial distributiondistribution ofofresidual residual nitrogen nitrogen (RN) (RN) and and total total nitrogen nitrogen (TN) (TN in) in suspended suspended solids solids in Lihu Lake. in Lihu Lake.

RN in suspended solids is mainly comprised of organic heterocyclic compounds or organic nitrogennitrogen-bound-bound compounds w withith heterocyclic or aromatic rings [44]. [44]. This This nitrogen nitrogen fraction fraction mainly mainly comes from humic structur structuralal compounds with a higher degree of condensation. The RN content was was foundfound to be 189.0–1435.0189.0–1435.0 mg/kg,mg/kg, with a mean value of 710.3 710.3 mg mg/kg./kg. The highest RN content wwasas observed in the Changguangxi Bridge sampling site, which is is in in the the south south of of Lihu Lihu Lake, Lake, follow followeded by thethe Jincheng Bay sampling site in the eastern Lihu Lake. Zone Zone A A exhibited exhibited the the lowest lowest RN RN mean mean value value (517.8 mg/kg). mg/kg). The RN spatial distribution increased fr fromom west to east (Zone D > C > BB >> A).A). The TN content in suspended solids ranged from 7 758.958.9 to 30 3098.198.1 mg/kg, mg/kg, with a mean value of 1663.2 mg/kg. mg/kg. Moreover, Moreover, the the spatial spatial distribution distribution of of TN TN content content in in suspended suspended solids solids varied varied greatly. greatly. The TN content in the easter easternn Zone A was the highest, highest, exhibiting exhibiting concentrations concentrations that that were were 1.86 times times higher than the mean va valuelue of the entire lake. The The mean mean values values of of the the four four zones zones (A, (A, B, B, C, C, D) were 1380.2, 1403.6, 1663.0, and 2168.9 mgmg/kg,/kg, respectively. The average average proportions proportions of of various various nitrogen nitrogen forms forms in each in each zone zone are illustrated are illustrated in Figure in Fig6. Basedure 6. Basedon the on proportion the proportion of the content of the ofcontent each nitrogen of each formnitrogen in the form TN, in HN the had TN, the HN highest had the proportion, highest proportion,accounting foraccounting 28.4–60.8%, for with 28.4 an–60.8% average, with of 48.4%,an average followed of 48.4%, by RN, followed accounting by forRN, 22.2–55.0%, accounting with for 22.an2 average–55.0%, of with 40.55%. an average EN had of the 40.55%. smallest EN proportion, had the smallest accounting proportion, for only 11.03%.accounting The for content only of11.03%. EN in TheArea content D was relativelyof EN in Area high, D which was relatively was likely high, due to which the high was TN likely content due into thethe overlyinghigh TN content water of in Zone the overlyingD. This conclusion water of wasZone based D. This on conclusion the Surface was Water based Environment on the Surface Quality Water Standard Environment of China, Quality which Standardclassiﬁed theof China, aforementioned which classified area as Classthe aforementioned IV–V. The suspended area a solidss Class adsorb IV–V. inorganicThe suspended nitrogen solids (e.g., adsorbammonia inorganic nitrogen nitrogen and nitrate (e.g., nitrogen) ammonia onto nitrogen the surfaces and of nitrate the particles nitrogen) via ontoexchangeable the surface adsorption.s of the particles via exchangeable adsorption. 2200

2000 RN HN 1800 EN

） 1600

1400

1200

1000

800

600 nitrogen content (mg / kg / content (mg nitrogen 400

200

0 Zone A Zone B Zone C Zone D Figure 6. Contents of various nitrogen forms in each zone. Sustainability 2020, 12, x FOR PEER REVIEW 9 of 28

Figure 5. Spatial distribution of residual nitrogen (RN) and total nitrogen (TN) in suspended solids in Lihu Lake.

RN in suspended solids is mainly comprised of organic heterocyclic compounds or organic nitrogen-bound compounds with heterocyclic or aromatic rings [44]. This nitrogen fraction mainly comes from humic structural compounds with a higher degree of condensation. The RN content was found to be 189.0–1435.0 mg/kg, with a mean value of 710.3 mg/kg. The highest RN content was observed in the Changguangxi Bridge sampling site, which is in the south of Lihu Lake, followed by the Jincheng Bay sampling site in the eastern Lihu Lake. Zone A exhibited the lowest RN mean value (517.8 mg/kg). The RN spatial distribution increased from west to east (Zone D > C > B > A). The TN content in suspended solids ranged from 758.9 to 3098.1 mg/kg, with a mean value of 1663.2 mg/kg. Moreover, the spatial distribution of TN content in suspended solids varied greatly. The TN content in the eastern Zone A was the highest, exhibiting concentrations that were 1.86 times higher than the mean value of the entire lake. The mean values of the four zones (A, B, C, D) were 1380.2, 1403.6, 1663.0, and 2168.9 mg/kg, respectively. The average proportions of various nitrogen forms in each zone are illustrated in Figure 6. Based on the proportion of the content of each nitrogen form in the TN, HN had the highest proportion, accounting for 28.4–60.8%, with an average of 48.4%, followed by RN, accounting for 22.2–55.0%, with an average of 40.55%. EN had the smallest proportion, accounting for only 11.03%. The content of EN in Area D was relatively high, which was likely due to the high TN content in the overlying water of Zone D. This conclusion was based on the Surface Water Environment Quality Standard of China, which classified the aforementioned area as Class IV–V. The suspended solids Sustainability 2020, 12, 5026 10 of 27 adsorb inorganic nitrogen (e.g., ammonia nitrogen and nitrate nitrogen) onto the surfaces of the particles via exchangeable adsorption. 2200

2000 RN HN 1800 EN

） 1600

1400

1200

1000

800

600 nitrogen content (mg / kg / content (mg nitrogen 400

200

0 Zone A Zone B Zone C Zone D

Sustainability 2020, 12, x FORFigure PEER REVIEW 6.6. ContentsContent s of various nitrogen forms in each zone.zone. 10 of 28 3.3. Spatial Distribution of Phosphorus Forms in Suspended Solids 3.3. Spatial Distribution of Phosphorus Forms in Suspended Solids 3.3.1. Spatial Distribution of Inorganic Phosphorus in Suspended Solids 3.3.1. Spatial Distribution of Inorganic Phosphorus in Suspended Solids The spatial distribution of weakly adsorbed inorganic phosphorus (WA-P ), potentially active The spatial distribution of weakly adsorbed inorganic phosphorus (WA-Pi), poti entially active inorganicinorganic phosphorus phosphorus (PA-P (PA-iP),i), iron iron (Fe)- (Fe)- oror aluminumaluminum (Al) (Al)-bound-bound inorganic inorganic phosphorus phosphorus (Fe/Al (Fe-P/Al-Pi), i), andand calcium calcium (Ca)-bound (Ca)-bound inorganic inorganic phosphorus phosphorus (Ca-P (Ca-Pi)i) isis shownshown in Fig Figureure 7. As As illustrat illustrateded in in Fig Figureure 7, the7 inorganic, the inorganic phosphorus phosphorus contents contents in in the the suspended suspended solidssolids of of Lihu Lihu Lake Lake exhibited exhibited overall overall lower lower concentrationsconcentrations in thein the west west than than in in the the east, east, and and the the highest highest valuesvalues were mostly concentrated concentrated in in Zone Zone D. D.

FigureFigure 7. Spatial7. Spatial distribution distribution of of inorganicinorganic phospho phosphorusrus in in suspended suspended solids solids of Lihu of Lihu Lake Lake..

WA-Pi is mainly derived from interstitial water or inorganic phosphorus types that adhere to other phases, such as carbonate, oxide, hydroxide, or clay mineral particles, in a physically adsorbed state, and is thus a form of phosphorus that can be easily transported [46,47]. The WA-Pi content in suspended solids of Lihu Lake ranges from 0.6 to 24.7 mg/kg, with a mean value of 10.3 mg/kg. Notably, its fraction only accounts for 2.7% of the total inorganic phosphorus and 1.6% of the total phosphorus. The highest values of WA-Pi occurred in the central region of Zone A and Zone D, whereas the lowest values were mainly concentrated in the western part of Zone A and Zone B in western Lihu Lake. PA-Pi in suspended solids refers to the fraction of active Fe-P and Al-P salts leached by a NaHCO3 solution during hydrolysis. PA-Pi can be used as an eﬀective indicator of the bioavailable phosphorus content in suspended solids [48]. In this study, PA-Pi content in the suspended solids ranged from 19.3 to 128.6 mg/kg, with an arithmetic mean value of 75.2 mg/kg. This proportion accounted for 19.68% of the inorganic phosphorus and 11.71% of total phosphorus. Fe/Al-Pi refers to the inorganic phosphorus extracted by a NaOH solution, and includes phosphorus combined with Fe/Al oxides or hydroxides. When the suspended solid overlying water interface is under anaerobic conditions, Fe/Al-Pi can be converted into soluble phosphorus, which is then recycled into the water column [49]. The Fe/Al-Pi content was 21.9–236.1 mg/kg, with a mean value of 111.1 mg/kg. Ca-Pi refers to the phosphorus fraction in the sediments that is associated with phosphorus-bearing minerals, such as authigenic apatite, calcium carbonate deposits, or biological skeletons [50]. Notably, this fraction is known to be released in acidic environments. However, the pH of the lake’s suspended solids is generally weakly alkaline, and thus has little eﬀect on the release of Ca-Pi from the suspended solids to the water column. Therefore, Ca-Pi in the suspended solids is not readily bioavailable to aquatic organisms. Ca-Pi contents were found to range from 93.9 to 307.8 mg/kg, with a mean value of 185.4 mg/kg.

3.3.2. Spatial Distribution of Organic Phosphorus in Suspended Solids Figure8 illustrates the spatial distribution of organic phosphorus in suspended solids. Similarly to WA-Pi, weakly absorbed organic phosphorus (WA-Po) is also derived from interstitial water or an organic phosphorus form that adheres to other phases, such as carbonate, oxide, hydroxide, and clay ore particles, by physical adsorption. Additionally, it is a form that is extremely easy to mineralize and transfer [51]. In this study, the content of WA-Po in suspended solids of Lihu Lake ranged from 1.9 to 11.0 mg/kg, with an average value of 4.7 mg/kg. The potentially active organic phosphorus (PA-Po) fraction is mainly comprised of phosphorus-containing compounds, such as nucleic acids, phospholipids, and phosphosaccharides extracted from NaHCO3 solutions. [52,53]. Moreover, it can be used as an eﬀective phosphorus source for the growth of phytoplankton and aquatic organisms. The PA-Po content in suspended solids of Lihu Lake ranged from 9.2 to 45.2 mg/kg, with an average value of 24.4 mg/kg. Medium active organic phosphorus (MA-Po) is the potentially bioavailable phosphorus form in suspended solids, which is composed of medium active organic phosphorus extracted by a NaOH solution (MANaOH-Po) and by a hydrochloric acid solution (MAHCl-Po), including calcium phytate, magnesium phytate, and some phosphorus-containing compounds combined with fulvic acids. These compounds can be further decomposed into inorganic phosphorus through the action of hydrolases, such as phosphatase and phytase [54]. The average MA-Po value was 65.3 mg/kg, ranging from 36.3 to 103.4 mg/kg. Sustainability 2020, 12, x FOR PEER REVIEW 11 of 28 ranged from 19.3 to 128.6 mg/kg, with an arithmetic mean value of 75.2 mg/kg. This proportion accounted for 19.68% of the inorganic phosphorus and 11.71% of total phosphorus. Fe/Al-Pi refers to the inorganic phosphorus extracted by a NaOH solution, and includes phosphorus combined with Fe/Al oxides or hydroxides. When the suspended solid overlying water interface is under anaerobic conditions, Fe/Al-Pi can be converted into soluble phosphorus, which is then recycled into the water column [49]. The Fe/Al-Pi content was 21.9–236.1 mg/kg, with a mean value of 111.1 mg/kg. Ca-Pi refers to the phosphorus fraction in the sediments that is associated with phosphorus-bearing minerals, such as authigenic apatite, calcium carbonate deposits, or biological skeletons [50]. Notably, this fraction is known to be released in acidic environments. However, the pH of the lake’s suspended solids is generally weakly alkaline, and thus has little effect on the release of Ca-Pi from the suspended solids to the water column. Therefore, Ca-Pi in the suspended solids is not readily bioavailable to aquatic organisms. Ca-Pi contents were found to range from 93.9 to 307.8 mg/kg, with a mean value of 185.4 mg/kg.

Sustainabilityand transfer2020 [51]., 12, 5026In this study, the content of WA-Po in suspended solids of Lihu Lake ranged12 from of 27 1.9 to 11.0 mg/kg, with an average value of 4.7 mg/kg.

Figure 8. Spatial distribution of organic phosphorus in suspended solids of Lihu Lake.

Non-active organic phosphorus (NA-Po) is a relatively stable phosphorus form in suspended solids, which is composed of non-active organic phosphorus extracted by NaOH solutions (NANaOH-Po) and residual phosphorus (R-Po), including humic acid in humus, organic phosphorus in humin, iron-aluminum salts containing 4–6 kinds of phosphoric acids in inositol, and a small amount of phosphorus-containing aluminum or calcium encapsulated by an Fe2O3 adhesive ﬁlm [55]. In this study, the range of NA-Po in suspended solids was 134.0–274.1 mg/kg, and the average value was 172.3 mg/kg, accounting for 67.7% of the total organic phosphorus and 25.1% of the total phosphorus. Additionally, the results demonstrated that the majority of NA-Po was R-Po, and the proportion of R-Po in NA-Po ranged from 64.9% to 91.1%, with an average of 80.4%. The total phosphorus contents of suspended solids in Lihu Lake were similar to those in the Taihu Lake basin [56]; when compared with the sediment in Taihu Lake [57], the contents were signiﬁcantly higher. The proportions of various phosphorus forms in each zone are illustrated in Figure9. In this study, inorganic phosphorus was the primary form of phosphorus in suspended solids, with contents ranging from 144.1 to 684.5 mg/kg, and an average of 381.9 mg/kg, accounting for 58.6% of the total phosphorus. Inorganic phosphorus was dominated by Ca-Pi, accounting for 49.6% of the inorganic phosphorus fraction. Furthermore, organic phosphorus was mainly composed of R-Po in inactive organic phosphorus, also accounting for more than 50% of the total organic phosphorus, which is consistent with previous studies [56]. Organic phosphorus (OP) mainly derives from terrestrial sources Sustainability 2020, 12, x FOR PEER REVIEW 12 of 28

Figure 8. Spatial distribution of organic phosphorus in suspended solids of Lihu Lake.

The potentially active organic phosphorus (PA-Po) fraction is mainly comprised of phosphorus-containing compounds, such as nucleic acids, phospholipids, and phosphosaccharides extracted from NaHCO3 solutions. [52,53]. Moreover, it can be used as an effective phosphorus source for the growth of phytoplankton and aquatic organisms. The PA-Po content in suspended solids of Lihu Lake ranged from 9.2 to 45.2 mg/kg, with an average value of 24.4 mg/kg. Medium active organic phosphorus (MA-Po) is the potentially bioavailable phosphorus form in suspended solids, which is composed of medium active organic phosphorus extracted by a NaOH solution (MANaOH-Po) and by a hydrochloric acid solution (MAHCl-Po), including calcium phytate, magnesium phytate, and some phosphorus-containing compounds combined with fulvic acids. These compounds can be further decomposed into inorganic phosphorus through the action of hydrolases, such as phosphatase and phytase [54]. The average MA-Po value was 65.3 mg/kg, ranging from 36.3 to 103.4 mg/kg. Non-active organic phosphorus (NA-Po) is a relatively stable phosphorus form in suspended solids, which is composed of non-active organic phosphorus extracted by NaOH solutions (NANaOH-Po) and residual phosphorus (R-Po), including humic acid in humus, organic phosphorus in humin, iron-aluminum salts containing 4–6 kinds of phosphoric acids in inositol, and a small amount of phosphorus-containing aluminum or calcium encapsulated by an Fe2O3 adhesive film [55]. In this study, the range of NA-Po in suspended solids was 134.0–274.1 mg/kg, and the average value was 172.3 mg/kg, accounting for 67.7% of the total organic phosphorus and 25.1% of the total phosphorus. Additionally, the results demonstrated that the majority of NA-Po was R-Po, and the proportion of R-Po in NA-Po ranged from 64.9% to 91.1%, with an average of 80.4%. The total phosphorus contents of suspended solids in Lihu Lake were similar to those in the Taihu Lake basin [56]; when compared with the sediment in Taihu Lake [57], the contents were significantly higher. The proportions of various phosphorus forms in each zone are illustrated in Figure 9. In this study, inorganic phosphorus was the primary form of phosphorus in suspended solids, with contents ranging from 144.1 to 684.5 mg/kg, and an average of 381.9 mg/kg, accounting for 58.6% of the total phosphorus. Inorganic phosphorus was dominated by Ca-Pi, accounting for 49.6% of the Sustainabilityinorganic phosphorus2020, 12, 5026 fraction. Furthermore, organic phosphorus was mainly composed of R13-P ofo in 27 inactive organic phosphorus, also accounting for more than 50% of the total organic phosphorus, which is consistent with previous studies [56]. Organic phosphorus (OP) mainly derives from andterrestrial biological sources processes, and biological and OP content processes, is generally and OP regarded content as is a generally better eutrophication regarded as indicator a better thaneutrophication TP. In this indicator study, the than contents TP. In of this inorganic study, andthe organiccontents phosphorus of inorganic with and directorganic bioavailability phosphorus rangedwith direct between bioavailability 22.0–153.3 ranged and 12.6–51.2 between mg 2/2.0kg,–153.3 respectively, and 12.6 accounting–51.2 mg/kg, for r 7.5–18.3%espectively, and accounting 1.5–7.4%. Thefor 7.5 content–18.3% of and organic 1.5–7.4% phosphorus. The content with direct of organic bioavailability phosphorus was with relatively direct low, bioavailability which may was be duerelatively to the low, strong which interaction may be due between to the the strong suspended interaction solids betwe anden water the suspended in the lake. solids This and facilitates water thein the decomposition lake. This facilitates of WA-P theo and decomposition PA-Po into small-molecule of WA-Po and compounds PA-Po into or small phosphate,-molecule which compounds are then releasedor phosphate, into the which overlying are then water. released into the overlying water.

600 350 NA-Po 150 PA-Po Ca-Pi 300 MA-Po WA-Po

500 Fe/Al-Pi (b) ） (c) ） PA-P PA-P (a) ） o i PA-Pi 120 250 WA-Po WA-Pi 400 WA-Pi

200 90 300 150 60 200 100

organic phosphorus (mg / kg / (mg phosphorus organic 30

inorganic phosphorus (mg / kg / (mg phosphorus inorganic 100

50 bioavailabekg / (mg phosphorus

0 0 0 Zone A Zone B Zone C Zone D Zone A Zone B Zone C Zone D Zone A Zone B Zone C Zone D Figure 9. ContentContentss of inorganic ( a), organic ( b), and bioavailable (c) phosphorus forms in each zone.zone.

In nature, organic phosphorus occurs mainly in the form of monoester or diester phosphorus. Previous studies showed that hydroxyl-bound phosphorus in monoester and diester organic phosphorus has similar properties to those of orthophosphate, both of which absorb and desorb onto particles via hydroxyl-phosphorus interactions [58]. Furthermore, organic phosphorus shares similar adsorption and desorption mechanisms with inorganic phosphorus in suspended solids. In fact, phosphorus is always in a state of dynamic equilibrium between the suspended solids and overlying water. They can transform each other under certain conditions, and some forms of phosphorus in suspended solids can be released into the overlying water through physical, chemical, and biological processes, thereby adopting forms that can be readily utilized by organisms. In this study, we first observed that WA-Pi, WA-Po, PA-Pi, and PA-Po were bioavailable phosphorus compounds that could be released into overlying water and participate in water recycling, which affects the nutritional status and primary productivity of the water body. Secondly, Fe/Al-Pi and MA-Po can be converted under certain conditions. Specifically, Fe/Al-Pi can be released to meet the needs of plankton growth when the suspended solid–water interface presents anaerobic conditions, whereas MA-Po can directly generate inorganic phosphorus under the action of hydrolases, such as phosphatase and phytase, or otherwise be mineralized to soluble active phosphorus via microorganism activity. Generally, Fe/Al-Pi and MA-Po are regarded as potentially bioavailable phosphorus forms [59,60]. Lastly, Ca-Pi and NA-Po are considered to be stable forms, and therefore cannot be easily converted into bioavailable phosphorus forms, especially in lakes that are already affected by eutrophication. Therefore, the bioavailability of various forms of phosphorus in suspended solids must be further studied via multiple methods, such as mineralization and enzymatic hydrolysis, in order to establish a better understanding of nutrient dynamics in eutrophic lakes.

3.4. Correlation Analysis Indicative of Nutrient Sources Suspended solids in lakes originate from numerous sources, and the correlation between various forms of nitrogen and phosphorus can reﬂect the link between each fraction. Furthermore, this approach can reﬂect the sources of nitrogen and phosphorus forms in suspended solids to some extent. In order to explore the possible sources of nitrogen and phosphorus forms in suspended solids, Pearson correlation tests were conducted between all fractions. The analysis results are summarized in Table1; Table2. Sustainability 2020, 12, x FOR PEER REVIEW 15 of 28 SustainabilitySustainabilitySustainability 20 20Sustainability2020, ,1 12022, ,20x x FOR ,FOR 1 220, xPEER 20PEER FOR, 12 , REVIEWPEER REVIEWx FOR REVIEW PEER REVIEW 1515 ofof 281528 of 2815 of 28

Table 1. Correlations between nitrogen forms in suspended solids. TableTableTable 1.1. CorrelationCorrelation 1.Table Correlation 1.s s Correlation betweenbetweens between nitrogennitrogens between nitrogen formsforms nitrogen forms inin suspendedsuspended informs suspended in solids solidssuspended .solids. . solids . ETN E-NH4+-N E-NO3--N SON HN AN AAN ASN HUN RN TN + + + - - - ETNETN ETNEE- -NHNHETNE44+--NH -NN E4 E-ENHN--NONO 4E3-3-N-NO-NN E3 --SONNSONNO 3 SON- N HNHN SON HN ANANHN AN AANAANAN AAN ASNASN AAN ASN HUNHUN ASNHUN RNHUNRN RN TNTN RN TN TN ETN 1 ETNETN ETN ETN1 1 1 1 E-NH4+-N 0.794 ** 1 + + + − EE--NHNHE44+--NH-NN E4 -NHN0.794 0.7944 -N 0.794 ** ** **0.794 11 ** 1 1 E-NO3 NO3 -N 0.736 ** 0.549 ** 1 − − − EE--NONOE33 - NONO3E33− -NO-NNON 330.736- 0.736NNO 3 0.736 **-**N **0.5490.7360.549 0.549 **** **0.549 11 ** 1 1 SON 0.539 ** −0.078 0.359 1 SONSON SON SON0.5390.539 0.539 **** **0.539−0−0 .078.078 **−0 .078−0 0.3590.359.078 0.359 0.35911 1 1 HN 0.440 * 0.198 0.759 ** 0.365 1 HNHN HN HN0.4400.440 0.440 ** *0.4400.198 0.198 *0.198 0.7590.1980.759 0.759 **** **0.7590.3650.365 0.365** 0.36511 1 1 AN 0.577 ** 0.403 * 0.745 ** 0.325 0.854 ** 1 ANAN AN AN0.5770.577 0.577 **** **0.5770.4030.403 0.403** ** *0.4030.745 0.745 * 0.745 ** ** **0.7450.3250.325 0.325** 0.8540.854 0.325 0.854 **** **0.854 11 **1 1 AAN 0.178 0.374 −0.004 −0.201 −0.216 −0.050 1 AANAANAAN AAN0.1780.178 0.178 0.3740.1780.374 0.374 0.374−0−0.004.004 −0 .004−0−0−0 .201.201.004−0 .201−0−0−0.216 .216.201−0 .216 −0−0−0 .050.050.216−0 .050−0 11.050 1 1 ASN −0.045 0.030 −0.039 −0.115 0.218 0.234 0.301 1 ASNASN ASN ASN−0−0.045.045 −0 .045−0 0.0300.030.045 0.030 0.030−0−0.039.039 −0 .039−0−0−0 .115.115.039−0 .1150.2180.218−0 .115 0.218 0.2340.234 0.218 0.234 0.3010.3010.234 0.301 10.3011 1 1 HUN 0.265 −0.014 0.630 ** 0.371 0.929 ** 0.659 ** −0.517 ** 0.073 1 HUNHUNHUN HUN0.2650.265 0.265 −0−00.265.014.014−0 .014−0 0.6300.630.014 0.630 ** ** **0.6300.3710.371 0.371** 0.9290.929 0.371 0.929 **** 0.6590.659 **0.929 0.659 **** ** −0−0 **0.659.517.517 −0 ****.517 0.073−00.073 **.517 0.073 ** 110.073 1 1 RN 0.643 ** 0.558 ** 0.814 ** 0.216 0.769 ** 0.816 ** 0.072 0.089 0.593 ** 1 RNRN RN RN0.6430.643 0.643 **** **0.5580.6430.558 0.558 **** **0.5580.814 0.814 ** 0.814 **** **0.8140.2160.216 0.216** 0.7690.769 0.216 0.769 **** 0.8160.816 **0.769 0.816 **** ** **0.8160.0720.072 **0.072 0.0890.089 0.072 0.089 0.5930.593 0.089 0.593 **** **0.593 11 **1 1 TN 0.656 ** 0.481 ** 0.862 ** 0.338 0.911 ** 0.888 ** −0.036 0.140 0.764 ** 0.959 ** 1 TNTN SustainabilityTN TN0.6560.656 2020 0.656 ****, 12, 5026**0.4810.6560.481 0.481 **** **0.4810.862 0.862 ** 0.862 **** **0.8620.3380.338 0.338** 0.9110.911 0.338 0.911 **** 0.8880.888 **0.911 0.888 **** ** **−00.888−0 .036.036 −0** .036 0.1400.140−0 .036 0.140 0.764 0.764 0.140 0.764 **** 0.9590.959 **0.764 0.959 **** ** 1**0.9591 1 ** 1 14 of 27 Table 2. Correlations between phosphorus forms in suspended solids. TableTableTable 2.2. CorrelationCorrelation 2.Table Correlation 2.s s Correlation betweenbetweens between phosphorusphosphoruss between phosphorus phosphorus formsforms forms inin suspendedsuspended informs suspended in solids solidssuspended solids.. . solids . Table 1. Correlations between nitrogen forms in suspended solids. WA-Pi PA-Pi Fe/Al-Pi Ca-Pi WA-Po PA-Po MA-Po NA-Po IP OP TP WA -PWAi -PPAWAi -P-PAPi i -PFe/Ali PAFe/Al--PPi i -Fe/AlPCai -P-Cai Pi -PWAi Ca--PWAPoi -PPAWAo -P-PAoP o -PMAoPA --PMAPoo -PNAMAo -P-NAPo o -PoNA IP -PoIP OPIP OPTP OPTP TP WA-Pi PA-Pi Fe/Al-Pi Ca-Pi + WA-Po PA-Po MA-Po NA-Po IP OP TP WA-Pi 1 ETN E-NH4 -N E-NO3−-N SON HN AN AAN ASN HUN RN TN WAWA--PPWAi i -PWAi 11 -Pi 1 1 PA-Pi 0.910 ** 1 ETN 1 PA-PPAi -P0.910i PA-0.910 P**i **0.910 1 **1 1 i PA-Pi 0.910 ** 1+ Fe/Al-P 0.728 ** 0.817 ** 1 E-NH4 -N 0.794 ** 1 Fe/AlFe/Al-Pii -0.728Fe/AlPi 0.728 **-P i 0.817 **0.728 0.817 ** ** **0.817 1 ** 1 1 Ca-Pi 0.670 ** 0.616 ** 0.626 ** 1 Fe/Al-P 0.728 ** E-NO0.8173 ** 1 Ca-PCai -P0.670i Ca-0.670P **i 0.616 **0.670 0.616 ** ** 0.626**0.6160.736 0.626 **** **0.6260.549 1 ** ** 1 1 1 o Ca-Pi 0.670 ** NO0.6163−-N ** 0.626 ** 1 WA-P 0.344 0.234 0.201 0.142 1 WA-PWAo -PWAo0.344 -P0.344 o SON0.234 0.3440.234 0.539 0.2010.234 **0.201 0.1420.2010.0780.142 0.1421 0.359 1 1 1 PA-Po 0.256 0.293 0.374 0.332 0.209 1 WA-Po 0.344 0.234 0.201 0.142− 1 HN 0.440 * 0.198 0.759 ** 0.365 1 PAPA--PPPAoo -Po0.2560.256PA -P 0.256 o 0.2930.293 0.256 0.293 0.374 0.3740.293 0.374 0.332 0.3320.374 0.332 0.2090.209 0.332 0.209 0.20911 1 1 MA-Po 0.346 0.492 * 0.563 ** 0.419 * 0.294 0.159 1 AN 0.577 ** 0.403 * 0.745 ** 0.325 0.854 ** 1 MA-PMAo -PMAo0.346 -P0.346 o 0.492 0.3460.492 * 0.563 *0.492 0.563 ** * **0.4190.563 0.419 *** 0.294*0.419 0.294 * 0.159 0.2940.159 0.1591 1 1 NA-Po 0.568 ** 0.634 ** 0.822 ** 0.467 * 0.181 0.377 * 0.414 * 1 MA-Po 0.346 AAN0.492 * 0.5630.178 ** 0.4190.374 * 0.2940.004 0.159 0.201 1 0.216 0.050 1 NA-PNAo -P0.568oNA -0.568 P**o 0.634 **0.568 0.634 ** ** 0.822**0.634 0.822 **** **0.4670.822 0.467 *** 0.181*0.467 − 0.181 * 0.377 0.1810.377 −* 0.414 *0.377 0.414 * *− *0.414 1 * 1− 1 NA-Po 0.568 ** 0.634ASN ** 0.8220.045 ** 0.4670.030 * 0.1810.039 0.377 * 0.1150.414 * 0.2181 0.234 0.301 1 IP 0.844 ** 0.868 ** 0.899 ** 0.887 ** 0.212 0.380 * 0.529 ** 0.707 ** 1 − − − IPIP IP 0.8440.844IP 0.844 ** ** HUN0.8680.868 **0.844 0.868 **** ** 0.899 0.899**0.8680.265 0.899 ** **** 0.887 0.887**0.899 0.014 0.887 ** **** 0.212**0.8870.212 0.630 0.212 ** **0.3800.380 0.212 0.380 **0.371 0.5290.529 *0.380 0.529 **** 0.929* 0.7070.707 **0.529 ** 0.707 **** ** 0.659 **0.707 11 ** **1 0.517 1 ** 0.073 1 OP 0.588 ** 0.663 ** 0.856 ** 0.530 ** 0.285 0.499 ** 0.612 ** 0.960 ** 0.757 ** 1 − − OPOP OP0.588 0.588OP 0.588 **** 0.6630.663 RN**0.588 0.663 **** ** 0.856 0.856**0.6630.643 0.856 ** **** 0.530 0.530**0.8560.558 0.530 ** ****** 0.285**0.5300.285 0.814 0.285 ** **0.4990.499 0.285 0.499 ****0.216 0.6120.612 **0.499 0.612 **** **0.769 0.9600.960 **0.612 ** 0.960 **** ** 0.816 0.7570.757 **0.960 ** 0.757 **** ** **0.7570.072 11 **1 0.0891 0.593 ** 1 TP 0.805 ** 0.848 ** 0.936 ** 0.816 ** 0.249 0.443 * 0.588 ** 0.834 ** 0.975 ** 0.884 ** 1 TN 0.656 ** 0.481 ** 0.862 ** 0.338 0.911 ** 0.888 ** 0.036 0.140 0.764 ** 0.959 ** 1 TPTP TP0.805 0.805TP 0.805 ** ** 0.8480.848 **0.805 0.848 **** ** 0.936 0.936**0.848 0.936 ** **** 0.816 0.816**0.936 0.816 ** **** 0.249**0.8160.249 0.249 ** 0.4430.443 0.249 0.443 ** 0.5880.588 *0.443 0.588 **** * 0.8340.834 **0.588 0.834 **** ** 0.9750.975 **0.834 0.975 **** ** 0.8840.884 −**0.975 0.884 **** ** 1**10.884 1 ** 1 In Table 1; Table 2, the numbers represent the mean correlation coefficients; ** means p < 0.01, and indicates an extremely significant correlation; * means p < 0.05, and p p InIn TableTableIn Table 11;; In Table Table 1Table; Table 22,, 1thethe; 2 Tablenumbersnumbers, the numbers2, the In representrepresent Tablenumbers represent1, the thethe numbersrepresent meanmean the mean corre representcorre the lation lationcorremean the lation coefficients coefficientscorre mean coefficientslation correlation;; coefficients**** meansmeans coe; ** ﬃmeans cients;pp ; <<** 0.01,0.01, meansp ** < means andand0.01, p indicates indicates

Table 2. Correlations between phosphorus forms in suspended solids.

WA-Pi PA-Pi Fe/Al-Pi Ca-Pi WA-Po PA-Po MA-Po NA-Po IP OP TP

WA-Pi 1 PA-Pi 0.910 ** 1 Fe/Al-Pi 0.728 ** 0.817 ** 1 Ca-Pi 0.670 ** 0.616 ** 0.626 ** 1 WA-Po 0.344 0.234 0.201 0.142 1 PA-Po 0.256 0.293 0.374 0.332 0.209 1 MA-Po 0.346 0.492 * 0.563 ** 0.419 * 0.294 0.159 1 NA-Po 0.568 ** 0.634 ** 0.822 ** 0.467 * 0.181 0.377 * 0.414 * 1 IP 0.844 ** 0.868 ** 0.899 ** 0.887 ** 0.212 0.380 * 0.529 ** 0.707 ** 1 OP 0.588 ** 0.663 ** 0.856 ** 0.530 ** 0.285 0.499 ** 0.612 ** 0.960 ** 0.757 ** 1 TP 0.805 ** 0.848 ** 0.936 ** 0.816 ** 0.249 0.443 * 0.588 ** 0.834 ** 0.975 ** 0.884 ** 1 In Table2, the numbers represent the mean correlation coe fficients; ** means p < 0.01, and indicates an extremely significant correlation; * means p < 0.05, and indicates a significant correlation. represents an extremely significant positive correlation; represents a significant positive correlation; represents a positive correlation. Sustainability 2020, 12, 5026 15 of 27

The correlation analysis results showed that TN was significantly positively correlated with EN, HN, and RN (p < 0.01, n = 28). Among these, the correlation coefficient between TN and RN was the highest (r = 0.959), indicating that the TN content in suspended solids was mainly modulated by RN. + E-TN is significantly positively correlated with E-NH4 -N, E- NO3−-N, and E-SON (p < 0.01, + n = 28). Among these, the correlation coefficient of E-TN and E-NH4 -N was the highest (r = 0.794). + Additionally, E-NH4 -N was one of the main components of E-TN, and thus, it can be inferred that E-TN + + is mainly controlled by E-NH4 -N. Among the different nitrogen forms, E-NH4 -N was significantly positively correlated with E-NO3−-N (r = 0.549, p < 0.01, n = 28), but not significantly positively + correlated with E-SON. This indicates that E-NH4 -N and E-NO3−-N can transform into each other under certain conditions., mainly because of the DO (Dissolved Oxygen) content in Lihu Lake (DO = + 7.05), which facilitates denitrification. Moreover, there was no correlation between E-SON, E-NH4 -N, and E-NO3−-N. This may be because the transformation between inorganic and organic nitrogen occurs mainly through anaerobic microorganism-mediated mineralization; however, the conditions in the overlying water were not favorable for the growth of anaerobic microorganisms. Furthermore, H-TN was significantly positively correlated with AN and HUN (r = 0.854, r = 0.929, p < 0.01, n = 28), meaning that the amount of H-TN in suspended solids mainly depended on AN and HUN. Among the different nitrogen forms, only AN and HUN had a positive correlation (r = 0.659, p < 0.01, n = 28), indicating that the two forms may not only share a similar source, but frequent transformations likely occur between them. The correlations among the remaining forms were not significant, indicating that they may derive from different sources. Particularly, AAN and HUN showed a significant negative correlation (r = 0.517, p < 0.01, n = 28), suggesting that AAN and HUN may be antagonistic to each − other or derive from different sources. To explore the potential sources of phosphorus in suspended solids, a correlation analysis among the phosphorus forms was performed. There was a significant positive correlation between TP and inorganic phosphorus, as well as MA-Po and NA-Po (p < 0.01), meaning that the TP content was mainly affected by inorganic phosphorus, MA-Po, and NA-Po. Moreover, the correlation between TP and various forms of inorganic phosphorus was more significant, indicating that TP in suspended solids is mainly controlled by inorganic phosphorus. In other words, the various forms of inorganic phosphorus originate from sediment re-suspension. Furthermore, IP, MA-Po, and NA-Po were significantly positively correlated (p < 0.01), indicating that they may share the same source. Particularly, the correlation between WA-Pi and PA-Pi with high biological activity was the most significant (r = 0.910, p < 0.01, n = 28), suggesting that these two forms likely share the same source. We also observed a significant correlation between OP and MA-Po, as well as MA-Po, and NA-Po (p < 0.01). However, WA-Po did not have a significant correlation with the contents of other phosphorus forms. This may be because a fraction of OP is bioavailable phosphorus, which mainly comes from agricultural non-point source pollution [61]. However, NA-Po mainly derives from insoluble phosphorus minerals, and, therefore, cannot be easily utilized by aquatic organisms. Based on these observations, we concluded that the sources of OP in suspended solids were quite variable in the studied lake. Previous studies [62,63] showed that the dry and wet deposition amount of TN was 11.0 t/a, whereas that of TP was 0.2 t/a. Moreover, the release fluxes of nitrogen and phosphorus in sediments by resuspension were approximately 20–30 t/a and 1–2 t/a, respectively. The sum of the two accounted for more than 90% of the nitrogen and phosphorus loads in Lihu Lake. Dry and wet nitrogen deposition is mainly attributable to dissolved total nitrogen (DTN) (91.4%), whereas the contribution of dissolved total phosphorus (DTP) to phosphorus deposition is relatively lower, with an average of 65.1%. This is also consistent with the fact that TN was found to be mainly dissolved nitrogen, whereas TP was largely comprised of the particulates in the water of Lihu Lake. These observations also reflect the contribution of suspended solids to nitrogen and phosphorus pollution in Lihu Lake. The prevention and control of endogenous pollution in lakes is a complicated and difficult undertaking. Internal loading and eutrophication affect each other. Therefore, to improve the water quality of Lihu Lake, it is necessary Sustainability 2020, 12, x FOR PEER REVIEW 17 of 28 an important supplier to these two nitrogen forms, and can indirectly provide a continuous nitrogen source for overlying water. In suspended solids and surface sediments, various forms of nitrogen Sustainability 2020, 12, 5026 16 of 27 were mainly significantly linked to ammonium nitrogen in the overlying water, indicating that both of them contribute to the reduction of nitrogen in water to ammonia nitrogen. Some studies [65,66] haveto consider shown notthat only algae the can interaction selectively between utilize nitrogen water and in sediments,water. This but is because also the ammonium impact of suspended nitrogen doessolids not on need pollution to be loads.reduced when assimilated, whereas nitrate nitrogen needs to be reduced before its use. Therefore, ammonia nitrogen promotes the growth of algae more effectively. The inorganic nitrogen3.5. Analysis in water of Influencing is mainly Factors composed of ammonia nitrogen, which is not correlated with the concentrationWater is the of medium suspended that solids.connects Therefore, the various although environmental the RN components content in of sus a lakepended [64]. Therefore, solids is significantlythe characteristics positively of the correlated overlying with water ammonia must have nitrogen an important (p < 0.01), eff RNect oncannot the release be directly of nitrogen converted and tophosphorus ammonia in nitrogen suspended and, solids. consequently, In order to suspended further characterize solids cannot the relationship directly promote between algae theoverlying growth. Therefore,water, suspended we can solids,infer that and the surface sediment sediments, has a greater Pearson’s effect correlation on the nitrogen analyses in were water conducted compared between with suspendedphysical and solids. chemical In indicescontrast, and high the concentrationsnitrogen and phosphorus of suspended forms solids in suspended lead to solids water and turbidity, surface therebysediments. limiting The resultsalgae photosynthesis are illustrated inand Figure inhibiting 10. their growth to some extent.

1.0 1.0 0.90T 0.09 * 0.12 0.72 0.80 0.59 0.66 0.84 0.98 0.70 0.56 0.62 0.90T 0.46 0.60 0.19 0.49 0.36 0.72 0.79 ** 0.17 0.64 0.33 0.46 0.8 DO* 0.09 0.06 * 1.00 0.44 0.30 0.88 0.53 0.56 0.09 0.29 0.15 0.8DO* * * *** 0.13 0.22 0.43 0.45 0.13 0.60 0.58 0.06 0.24

pH** * *** * 0.49 0.75 0.39 0.32 0.13 0.86 0.07 0.43 0.17 0.6pH** 0.39 0.41 0.59 0.38 0.68 0.74 0.43 0.10 0.86 0.90 0.94 0.91 0.6

ORP*** 0.30 * 0.46 0.10 0.91 0.92 0.85 0.76 0.96 0.27 0.90 0.51 ORP0.4*** 0.36 0.25 0.85 0.53 0.43 0.40 0.13 0.06 0.67 0.50 0.87 0.73 0.4

SD* 0.71 0.33 0.76 * 0.65 0.99 0.91 0.39 0.59 0.25 0.75 0.43 0.2SD* 0.12 0.10 0.64 0.12 0.86 0.92 0.49 0.35 0.23 0.29 0.60 0.36 0.2

Chl.a* 0.70 0.71 0.96 0.22 0.19 0.09 0.96 0.99 0.36 0.07 0.29 0.13 Chl.a0.0 * 0.13 0.17 0.66 * 0.84 0.71 0.80 0.57 0.50 0.19 0.62 0.29 0.0

(a) (b)

COD*** * ** ** 0.43 0.32 0.18 0.90 0.30 0.50 * 0.18 * COD-0.2*** * * 0.10 * 0.85 0.95 0.22 * 0.91 0.47 0.28 0.35 -0.2

TN-water*** 0.07 * 0.07 0.90 0.08 * 0.63 0.66 0.15 ** 0.05 ** TN-water-0.4*** *** *** 0.07 * 0.88 0.74 0.62 0.30 0.06 * 0.22 0.07 -0.4 DTN*** * * * 0.39 0.06 * 0.79 0.45 0.17 *** * ** DTN*** *** ** ** ** 0.20 0.17 0.47 0.19 0.54 * * * -0.6 -0.6 nitrate*** 0.52 0.85 0.61 0.19 0.24 0.21 0.39 0.41 0.24 0.12 0.24 0.15 nitrate*** ** * * ** 0.30 0.27 0.98 0.50 0.41 0.15 0.06 0.08 -0.8 -0.8 ammonia*** 0.09 0.11 * 0.69 * 0.05 0.45 0.55 0.07 *** * ** ammonia*** *** *** ** ** 0.53 0.43 0.45 0.26 0.72 0.10 0.06 0.06 -1.0 -1.0 0.09 *** *** *** ** * ** 0.37 0.82 0.17 *** *** *** *** *** *** *** *** ** * 0.62 0.08 0.37 *** *** *** HN AN RN HN AN RN ETN SON AAN ASN HUN TN-SS ETN SON AAN ASN HUN E-NH4+-NE-NO3--N E-NH4+-NE-NO3--N bioavailable N bioavailableTN-sediments N

1.0 1.0 0.90T 0.48 0.69 0.87 ** 0.96 0.50 0.37 0.59 0.38 0.16 0.76 0.82 0.90T 0.74 0.31 0.72 ** 0.36 0.54 0.44 0.63 0.16 0.07 0.94 0.30 0.8 0.8 DO0.92 0.88 0.50 0.98 0.19 0.35 * 0.17 0.25 0.80 0.43 0.37 0.96 DO0.92 0.72 0.82 0.38 0.32 ** 0.59 0.83 0.72 0.89 0.74 0.66 0.79 0.6 0.6 pH0.74 0.24 0.23 0.12 0.19 0.72 ** 0.94 0.10 0.09 0.12 0.09 0.07 pH0.74 0.65 0.62 0.98 0.36 0.83 0.97 0.13 0.67 0.39 0.50 0.29 0.74 0.4 0.4 ORP* 0.13 0.07 ** 0.30 0.57 0.07 0.48 0.19 0.05 * 0.13 * ORP* 0.28 * 0.21 0.45 0.72 0.65 0.84 0.42 0.15 0.14 0.44 0.06 0.2 0.2 SD0.13 * * ** 0.20 0.95 0.84 0.20 0.13 * * 0.12 * SD0.13 * 0.09 0.20 0.18 0.85 0.49 0.51 0.56 0.13 0.07 0.88 0.15

0.0 0.0 (c) Chl.a0.82 0.07 * * *** 0.31 0.37 0.48 0.11 ** ** 0.10 * (d) Chl.a0.82 0.10 ** 0.76 *** 0.97 0.37 0.60 0.29 ** ** 0.29 ** -0.2 -0.2 COD0.80 0.17 0.21 ** 0.21 0.63 * 0.87 ** * * * 0.09 COD0.80 0.59 0.08 0.24 0.17 0.43 0.90 0.79 0.10 0.05 0.08 0.08 0.13 -0.4 -0.4 TP-water* 0.05 * ** ** 0.28 * 0.54 0.23 * ** 0.14 * TP-water* 0.10 ** 0.28 * 0.84 0.80 0.61 0.56 * ** 0.67 ** -0.6 -0.6 0.08PP 0.05 * ** ** 0.32 0.06 0.66 0.14 ** ** 0.11 * 0.08PP 0.14 ** 0.34 ** 0.88 0.92 0.83 0.43 * ** 0.47 * -0.8 -0.8 IP-water*** 0.46 0.27 0.74 0.93 0.14 0.27 0.29 0.32 0.88 0.72 0.76 0.19 IP-water*** 0.80 * 0.67 0.96 0.29 0.34 0.07 0.30 0.96 0.53 0.10 * -1.0 -1.0 0.46 *** *** *** *** 0.07 0.19 0.07 ** *** *** ** *** 0.80 *** 0.08 *** ** 0.47 0.46 0.52 0.42 *** *** 0.46 ** PA-Pi Ca-Pi IP-SS PA-Pi Ca-Pi WA-Pi Fe/Al-Pi WA-PoPA-PoMA-PoNA-PoTP-SS OP-SS WA-Pi Fe/Al-Pi WA-PoPA-PoMA-PoNA-Po bioavailable P TP-sedimentsIP-sedimentsOP-sedimentsbioavailable P

Figure 10. Relationships between nitrogen and phosphorus forms in suspended solids and surface Figure 10. Relationships between nitrogen and phosphorus forms in suspended solids and surface sediments and physicochemical indices in water. The numbers in the figure indicate p values. (a) sediments and physicochemical indices in water. The numbers in the figure indicate p values. (a) correlation between nitrogen forms in suspended solids and water, (b) correlation between nitrogen correlation between nitrogen forms in suspended solids and water, (b) correlation between nitrogen forms in surface sediments and water, (c) correlation between phosphorus forms in suspended solids forms in surface sediments and water, (c) correlation between phosphorus forms in suspended solids and water, (d) correlation between phosphorus forms in surface sediments and water. * means p < 0.05, and water, (d) correlation between phosphorus forms in surface sediments and water. * means p < ** means p < 0.01, *** means p < 0.001. Red cells indicate positive correlations and blue cells indicate 0.05, ** means p < 0.01, *** means p<0.001. Red cells indicate positive correlations and blue cells negative correlations. indicate negative correlations. Although suspended solids have a strong capacity to release EN and HN, they exhibited no significant correlation with TN in overlying water. In contrast, RN in suspended solids showed a positive correlation with TN, DTN, and ammonium nitrogen in the overlying water. This was mainly Sustainability 2020, 12, 5026 17 of 27 because the EN and HN contents were less than that of RN, which also demonstrates that RN was the main nitrogen source in the overlying water. The content of bioavailable N forms in suspended + solids is positively correlated with NH4 -N in the water, which means that under certain conditions, + the suspended solids can affect the content of ammonia in water. TN and NH4 -N in the water + had a significantly positive correlation with EN and E-NH4 -N in the surface sediments (p < 0.01), and this was consistent with Sun’s [47] observations in Erhai Lake. EN is a nitrogen form that is easily released into the overlying water if the conditions are favorable. Therefore, this relationship indicated that the nitrogen in the overlying water mainly originated from the surface sediments, and + + the release of NH4 -N from surface sediments played an important role in the supply of NH4 -N to + the overlying water. Both NH4 -N and NO3−-N in the surface sediment had significant correlations with HN. This indicates that the mineralization and decomposition of HN in the surface sediments is an important supplier to these two nitrogen forms, and can indirectly provide a continuous nitrogen source for overlying water. In suspended solids and surface sediments, various forms of nitrogen were mainly significantly linked to ammonium nitrogen in the overlying water, indicating that both of them contribute to the reduction of nitrogen in water to ammonia nitrogen. Some studies [65,66] have shown that algae can selectively utilize nitrogen in water. This is because ammonium nitrogen does not need to be reduced when assimilated, whereas nitrate nitrogen needs to be reduced before its use. Therefore, ammonia nitrogen promotes the growth of algae more effectively. The inorganic nitrogen in water is mainly composed of ammonia nitrogen, which is not correlated with the concentration of suspended solids. Therefore, although the RN content in suspended solids is significantly positively correlated with ammonia nitrogen (p < 0.01), RN cannot be directly converted to ammonia nitrogen and, consequently, suspended solids cannot directly promote algae growth. Therefore, we can infer that the sediment has a greater effect on the nitrogen in water compared with suspended solids. In contrast, high concentrations of suspended solids lead to water turbidity, thereby limiting algae photosynthesis and inhibiting their growth to some extent. Chlorophyll a (Chl.a) is an important indicator of primary productivity. In this study, the correlation coefficients between Chl.a and various forms of phosphorus in suspended solids were all found to be positive. Among them, the correlation coefficient between Chl.a and inorganic phosphorus was the highest (r = 0.567). In other words, when the external environment changes, part of the inorganic phosphorus fraction is released, thereby increasing the content of phosphorus in water. Algae are parts of the suspended solids; in the meanwhile, phosphorus in suspended solids plays an important role in algae growth. Particularly, its correlation coefficient with Ca-Pi was the highest (r = 0.616). This is because areas with high Chl.a contents are indicative of high photosynthetic activity and active phytoplankton growth, which translates to an accelerated rate of biological residue decomposition and a subsequent increase in Ca-Pi that result from the combination of biological residues with calcium in the environment. The bioavailable phosphorus forms in the sediment can be released into the water, and then affect the algae growth. All of the nitrogen forms in suspended solids and sediment are not correlated with Chl.a, which means that nitrogen in suspended solids and sediment does not directly affect the primary productivity of the lake. The correlation between the phosphorus components in the water and phosphorus forms in the suspended solids/surface sediments demonstrated that the content of phosphorus in water was affected by both suspended solids and sediment. ρ(TP) in the overlying water was affected by various phosphorus forms in the suspended solids, especially inorganic phosphorus. Notably, particulate phosphorus (PP) was found to be the main component of TP in the overlying water, and ρ(PP) was mainly affected by inorganic phosphorus in the suspended solids. Moreover, the correlation coefficient of Fe/Al-Pi was the highest (r = 0.503), indicating that Fe/Al-Pi was the main form affecting TP content in the water. The increase of ω(PA-Pi) in the surface sediments leads to increases in ρ(IP) in the overlying water. This observation is highly relevant, as IP is known to be easily utilized by algae, which leads to cyanobacterial blooms. The bioavailable phosphorus forms in sediment affect the nutrient level by affecting the content of IP in water. Sustainability 2020, 12, x FOR PEER REVIEW 18 of 28

Chlorophyll a (Chl.a) is an important indicator of primary productivity. In this study, the correlation coefficients between Chl.a and various forms of phosphorus in suspended solids were all found to be positive. Among them, the correlation coefficient between Chl.a and inorganic phosphorus was the highest (r = 0.567). In other words, when the external environment changes, part of the inorganic phosphorus fraction is released, thereby increasing the content of phosphorus in water. Algae are parts of the suspended solids; in the meanwhile, phosphorus in suspended solids plays an important role in algae growth. Particularly, its correlation coefficient with Ca-Pi was the highest (r = 0.616). This is because areas with high Chl.a contents are indicative of high photosynthetic activity and active phytoplankton growth, which translates to an accelerated rate of biological residue decomposition and a subsequent increase in Ca-Pi that result from the combination of biological residues with calcium in the environment. The bioavailable phosphorus forms in the sediment can be released into the water, and then affect the algae growth. All of the nitrogen forms in suspended solids and sediment are not correlated with Chl.a, which means that nitrogen in suspended solids and sediment does not directly affect the primary productivity of the lake. The correlation between the phosphorus components in the water and phosphorus forms in the suspended solids/surface sediments demonstrated that the content of phosphorus in water was affected by both suspended solids and sediment. ρ(TP) in the overlying water was affected by various phosphorus forms in the suspended solids, especially inorganic phosphorus. Notably, particulate phosphorus (PP) was found to be the main component of TP in the overlying water, and ρ(PP) was mainly affected by inorganic phosphorus in the suspended solids. Moreover, the correlation coefficient of Fe/Al-Pi was the highest (r = 0.503), indicating that Fe/Al-Pi was the main form affecting TP content in the water. The increase of ω(PA-Pi) in the surface sediments leads to increases in ρ(IP) in the overlying water. This observation is highly relevant, as IP is known to be Sustainability 2020, 12, 5026 18 of 27 easily utilized by algae, which leads to cyanobacterial blooms. The bioavailable phosphorus forms in sediment affect the nutrient level by affecting the content of IP in water. 3.6. Comparison Analysis between Suspended Solids and Surface Sediments 3.6. Comparison Analysis Between Suspended Solids and Surface Sediments Nutrients in the sediments can be released into the overlying water through diﬀusion, convection, and otherNutrients processes, in the causing sediments secondary can pollution be released [67 ].in Moreover,to the overlying suspended water solids through are an important diffusion, partconvection, of aquatic and ecosystems, other proc andesses, their causing nutrient second concentrationsary pollution can [ reﬂect67]. Moreover, the water suspended quality of asolids lake [ 68are]. Inan order important to more part precisely of aquatic control ecosystems, the endogenous and their pollution nutrient of concentrations a lake, it is necessary can reflect to the compare water thequality contents of a oflake various [68]. In nutrient order to forms more in precisely suspended control solids the with endogenous those in surface pollution sediments. of a lake, These it is resultsnecessary are illustrated to compare in the Figure contents 11. of various nutrient forms in suspended solids with those in surface sediments. These results are illustrated in Figure 11.

180 580 1800 570 Suspended solids Suspended solids Suspended solids 160 560 1600 (a) (b) Surface Sediments (c) Surface Sediments Surface Sediments 550 140 1400

120 300 1200

100 250 1000

80 200 800

60 150 600

40 100 Nitrogen Content/mg/kg 400 Exchangeable nitrogen/mg/kg Exchangeable Hydrolysable nitrogen/mg/kg Hydrolysable 150 20 50 100 50 0 0 0 + - E-NH4 E-NO3 SON STN AN AAN ASN HUN EN HN RN TN

250 Suspended solids 350 (d) (e) Suspened solids Surface sediments Surface sediments 300 200

250 150 200

100 150

100

Organic phosphorus/(mg/kg) Organic Inorganic phosphorus Inorganic /(mg/kg) 15 10 10 5 0 0 WA-Pi PA-Pi Fe/Al-Pi Ca-Pi IP WA-Po PA-Po MA-Po NA-Po OP

Figure 11. Comparison between the contents of nitrogen and phosphorus forms in the suspended solids with those in surface sediments.

Liu [69] examined the correlation between the content of various nitrogen forms and particle size in sediments of the Dan River. The results showed that finer sediment particles translated to higher contents of nitrogen forms. Therefore, the nitrogen adsorption capacity of suspended solids is stronger than that of surface sediments. Moreover, EN is the main and most active form involved in the transportation and migration of nitrogen in suspended solids/surface sediments. It is also the main source of bioavailable nitrogen in suspended solids and surface sediments, and + the NH4 -N and NO3−-N content in suspended solids is typically lower than in surface sediments. This is because the nitrogen concentration in the interstitial water of surface sediments is higher than that in the overlying water due to the influence of OM degradation by benthic organisms. + Therefore, suspended solids adsorb less NH4 -N and NO3 NO3−-N than surface sediments. Moreover, low O2 concentrations in sediments favor the growth of denitrifying bacteria, thereby promoting + the denitrification process and the subsequent accumulation of large amounts of NH4 -N in the surface + sediments. Thus, the NH4 -N content was higher than that of NO3 NO3−-N. The OM content affects the number of adsorptive sites in sediments. Studies have shown that sediments with high organic matter contents have more adsorptive sites, which can adsorb more AN and AAN. In consequence, organic matter affects AN and AAN contents. The COD (Chemical Oxygen Demand) content in the overlying water of Lihu Lake (COD = 4.41 mg/L) is lower than that of the interstitial water. Therefore, the contents of AN and AAN in the suspended solids are lower than those of surface sediments. Generally speaking, there are three main sources of bioavailable nitrogen in suspended + solids/surface sediments: Inorganic nitrogen (NH4 -N, NO3−-N), dissolved organic nitrogen, and mineralizable organic nitrogen (mainly AAN). In this study, EN, AN, and AAN occurred as bioavailable Sustainability 2020, 12, 5026 19 of 27 nitrogen, and their contents in suspended solids were lower than those in surface sediments. In other words, the nitrogen in surface sediments was more bioavailable than that in suspended solids. Upon comparing the RN content in the suspended solids with that in the surface sediments, we observed that the RN content in suspended solids of each sampling site was generally more than twice that of the surface sediments. This was because the oxidation environment of the suspended solids was likely stronger than that of surface sediments, and a strong oxidation environment enhances the stability of RN in sediments [70]. In general, the contents of various forms of phosphorus except for MA-Po in suspended solids were higher than in the surface sediments. That is, the phosphorus in suspended solids was more bioavailable than that in surface sediments. This relationship between suspended solids and surface sediments was consistent with the observations of Jin [37] and Zan [61]. This was mainly attributed to the continuous wind–wave disturbances in the studied sites, which cause the surface sediments to be constantly disturbed. For this reason, only low-density substances (including phytoplankton, zooplankton, organic detritus, algae fragments, and relatively light minerals) remain in the water column for long periods. Higher WA-P contents in suspended solids indicate that the adsorption of inorganic phosphate and dissolved organic phosphorus onto suspended solids was higher than that of the surface sediments. Compared with surface sediments, suspended solid particles are smaller and their specific surface area increases when they enter the water body after being disturbed. Therefore, this increase in surface potential energy facilitates adsorption. This is consistent with the results of a simulation test, which demonstrated that the amount of adsorption and desorption of phosphate increased with decreased particle sizes. Previous studies [71] have shown that the content of inorganic phosphate and dissolved organic phosphorus in interstitial water was much higher than that in overlying water, which results in the diffusion and release of dissolved phosphorus from the interstitial water to the overlying water. According to a field investigation in Lake Arresø, a shallow lake in Denmark [72], the increase of nutrients released from dynamic suspension can reach up to 20–30 times the original value. This kind of release is likely aggravated by wave and wind disturbances, even though the content of dissolved phosphorus in the lake remains stable. Therefore, given that a large amount of released dissolved phosphorus can be re-adsorbed onto the suspended solids, the phosphorus content dissolved in the water likely remains relatively stable. This conclusion also supports the phosphate buffering mechanism proposed by Froelich [73]. In a few words, this rapid adsorption is beneficial to maintaining the balance of phosphorus in the water. Nevertheless, the suspended solids can also adsorb the dissolved phosphorus from the surface sediments, which is then released under certain conditions. The NA-Po content in suspended solids was significantly higher than that of the surface sediments, which may be due to the large amount of loose and amorphous inorganic macromolecular flocculate materials or organic detritus from surface sediment entering the water, increasing the organic phosphorus content in water. These substances not only have poor precipitation themselves, but also lead to poor precipitation of suspended solids after being adsorbed, which further leads to significant increases in the NA-Po content of suspended solids. According to the correlation analysis, the dissolved phosphorus content in the water of Lihu Lake is affected by both suspended solids and sediment. However, since high phosphorus contents are found in suspended solids, we should pay more attention to the suspended solids.

4. Conclusions This study investigated the contents, spatial distribution, and sources of nitrogen and phosphorus forms in the suspended solids of Lihu Lake, and comparatively analyzed the diﬀerent eﬀects of suspended solids and surface sediments on water quality. After a comprehensive analysis and discussion of the results, our main conclusions were the following:

(1) The content of TN in the suspended solids of Lihu Lake was 758.9–3098.1 mg/kg and showed a decreasing trend from east to west. The proportions of various N forms in the suspended solids Sustainability 2020, 12, 5026 20 of 27

of the study areas were ranked as follows: Hydrolyzable nitrogen (HN) > residual nitrogen (RN) > exchangeable nitrogen (EN). The total phosphorus (TP) ranged from 294.8 to 1066.4 mg/kg, the contents of IP and OP in suspended solids were 381.9 and 259.8 mg/kg, respectively, and Ca-Pi and NA-Po were the major components of IP and OP. Almost all P forms exhibited higher contents in the west than in the east. This is because several environmental dredging and ecological restoration projects have been carried out in western Lihu Lake. (2) The correlation analysis of various forms of nitrogen and phosphorus in suspended solids indicated that there were different sources of suspended nitrogen for Lihu Lake, and TN in suspended solids was mainly affected by RN. Inorganic phosphorus in suspended solids likely originated from a common source, whereas organic phosphorus came from different sources. Phosphorus in suspended solids was controlled by both IP and OP, but was mainly affected by IP. (3) The nitrogen in sediments was more bioavailable than that in suspended solids. In particular, once the physical and biochemical conditions of the sediment change, the EN in the sediment + can be released and can diffuse into the overlying water, affecting the contents of NH4 -N and TN in water. The nitrogen forms in SS change continuously through rapid transformations when water quality conditions change. The contents of phosphorus in water were affected both by suspended solids and sediment. The bioavailable phosphorus content in suspended solids was higher than that in sediments, and the suspended solids influenced the ρ(TP) by affecting particulate phosphorus. Suspended solids act as a medium that continually carries phosphorus from sediments to water and provides a substrate that enables phosphorus to remain in the water column for long periods.

Although many studies have been conducted on the nutrient concentrations in water and sediment, little is known about the nutrients in suspended solids. The spatial distribution characteristics of nitrogen and phosphorus forms in suspended solids were analyzed in this study. The impact on eutrophication was studied based on statistical analysis of these data. Future studies will hopefully be on the biogeochemical mechanisms, so as to provide theoretical support for controlling the eutrophication.

Author Contributions: Q.Z. provided the concept and designed the article content. J.L. performed the experiments and wrote the manuscript; Q.Z. verified the results and contributed to the proofreading of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (No. U1803241, No. 51709238, and No. 51779230) Acknowledgments: We thank Jiang’s contribution to this article. Conflicts of Interest: The authors declare no conflict of interest.

Appendix A

Table A1. The abbreviations and their meanings in this study.

Abbreviation Meaning SS suspended solids TSS total suspended solids OM organic matter TN total nitrogen DTN dissolved total nitrogen PN particulate nitrogen TP total phosphorus DTP dissolved total phosphorus PP particulate phosphorus IP inorganic phosphorus OP organic phosphorus EN exchangeable nitrogen Sustainability 2020, 12, 5026 21 of 27

Table A1. Cont.

Abbreviation Meaning + E-NH4 -N exchangeable ammonium nitrogen E-NO3−-N exchangeable nitrate nitrogen SON exchangeable soluble organic nitrogen HN acid-hydrolyzable nitrogen AAN amino acid nitrogen ASN amino sugar nitrogen AN acid-hydrolyzable ammonium nitrogen HUN acid-hydrolyzable unknown nitrogen RN residual nitrogen WA-Pi weakly adsorbed inorganic phosphorus PA-Pi potentially active inorganic phosphorus Fe/Al-Pi iron (Fe)- or aluminum (Al)-bound inorganic phosphorus Ca-Pi calcium (Ca)-bound inorganic phosphorus WA-Po weakly absorbed organic phosphorus PA-Po potentially active organic phosphorus MA-Po medium active organic phosphorus NA-Po non-active organic phosphorus R-P residual phosphorus Sustainability 2020, 12, x FORo PEER REVIEW 22 of 28

5g suspended solids sample

2mol/L KCl (25mL), 2h shaking

2 rinse, H2O exchangeable nitrogen (EN) 2g residue

6mol/L HCl (pH=8.0±0.2), exchangeable ammonia exchangeable nitrate exchangeable total nitrogen exchangeable soluble organic ℃ + - 120 24h (E-NH4 -N) (E-NO3 -N) (E-TN) nitrogen (SON)

2 rinse, H2O acid hydrolysable nitrogen (HN) 1g residue

hydrolysable ammonia amino acid nitrogen amino sugar nitrogen total hydrolysable nitrogen hydrolysable unknown nitrogen (AN) (AAN) (ASN) (THAN) (HUN)

residual nitrogen (RN)

Figure A1. Measurement procedure for nitrogen fractionation in the suspended solids. Figure A1. Measurement procedure for nitrogen fractionation in the suspended solids. Table A2. Determination methods for nitrogen forms in suspended solids. Table A2. Determination methods for nitrogen forms in suspended solids. Index Determination Method Reference Index DeterminationAlkaline potassium Methodpersulfate digestionReferenceHJ 636-2012 from Ministry of Ecology EN, HN AlkalineUV potassium spectrophotometric persulfate method and Environment, PRC HJ 636-2012 from Ministry of Ecology and EN, HNE-NH +-N,digestion UV HJ 535-2009 from Ministry of Ecology 4 Nessler’s reagent spectrophotometryEnvironment, PRC AN spectrophotometric method and Environment, PRC E-NHE-NO4+-N, Nessler’s reagent HJ 535-2009HJ/T from 346-2007 Ministry from Ministryof Ecology of Ecology and 3 Ultraviolet spectrophotometry AN NO3−-N spectrophotometry Environment,and Environment, PRC PRC E-NOSON3 SON = EN- E-NH +-N- E-NO -NHJ/T 346-2007 from Ministry of Ecology and Ultraviolet spectrophotometry4 3− − NO3 -N Environment,1. Shao PRC J.L.; Li Q.W.; Dong B.S., Liu H.S. SON SON = EN- E-NH4+-N- E-NO3−-N Determination of total free amino acid AAN Ninhydrin colorimetric 1. Shaoin J.L tea.; by Li NinhydrinQ.W.; Dong colorimetry. B.S., Liu ChinaH.S. Food Additives 2008, 02, 162–165. Determination of total free amino acid in tea by AAN Ninhydrin colorimetric Ninhydrin colorimetry. China Food Additives 2008, 02, 162–165. Elson–Morgen colorimetric Bremner J.M. Organic forms of nitrogen. Madison: ASN method American Society of Agronomy, 1965: 1148–1178. HUN HUN = TH-AN-AAN-ASN HJ 717 - 2014 from Ministry of Ecology and RN, TN Modified Kjeldahl method Environment, PRC Sustainability 2020, 12, 5026 22 of 27

Table A2. Cont.

Index Determination Method Reference Bremner J.M. Organic forms of nitrogen. ASN Elson–Morgen colorimetric method Madison: American Society of Agronomy, 1965: 1148–1178. HUN HUN = TH-AN-AAN-ASN HJ 717 - 2014 from Ministry of Ecology RN, TN Modiﬁed Kjeldahl method Sustainability 2020, 12, x FOR PEER REVIEW and Environment, PRC 23 of 28

Step 1:

1mol/L NH4Cl solution (50mL), weakly adsorbed inorganic 1g Suspended solids sample 2h shaking phosphorus (WA-Pi) 5 000r/min,10min 0.45 μm filter film weakly adsorbed organic phosphorus (WA-Po=WA-Pt-WA-Pi) 2 rinse, 20mL NaCl saturated each Step 2:

0.5mol/L NaHCO3 solution (50mL), potentially active inorganic Residue from step 1 1h shaking phosphorus (PA-Pi) 5 000r/min,10min 0.45 μm filter film potentially active organic phosphorus (PA-Po=PA-Pt-PA-Pi)

2 rinse, 20mL NaCl saturated each Step 3:

0.1mol/L NaOH solution (50mL), iron (Fe) or aluminum (Al)-bound Residue from step 2 16h shaking inorganic phosphorus (Fe/Al-Pi) 5 000r/min,10min 0.45 μm filter film organic phosphorus extracted by NaOH solution (NaOH-Po=NaOH-Pt-Fe/Al-Pi) 0.1mol/L HCl (pH=3.0) medium active organic phosphorus extracted by NaOH solution 2 rinse, 20mL NaCl saturated each (MANaOH-Po=MANaOH-Pt-MANaOH-Pi)

non-active organic phosphorus 40℃ to dry extracted by NaOH solution (NANaOH-Po=NaOH-Po-MANaOH-Po) Step 4:

0.5g residue from step 3 1mol/L HCl solution (50mL), calcium (Ca)-bound inorganic 16h shaking phosphorus (Ca-Pi) 5 000r/min,10min 0.45 μm filter film medium active organic phosphorus extracted by HCl solution (MAHCl-Po=MAHCl-Pt-MAHCl-Pi) 2 rinse, 20mL NaCl saturated each Step 5:

0.2g residue from step 4 residual phosphorus (R-Po)

Figure A2. MeasurementMeasurement procedure for phosphorus fractionation in the suspended solids.solids. Table A3. Determination methods for phosphorus forms in suspended solids. Table A3. Determination methods for phosphorus forms in suspended solids. Index Measurement Reference Index Measurement Reference phosphomolybdenum blue HJ 593-2010 from Ministry of inorganicinorganic phosphorusphosphomolybdenum blue HJ 593-2010 from Ministry of Ecology spectrophotometric method Ecology and Environment, PRC phosphorus spectrophotometric method and Environment, PRC ammonium molybdate GB 11893-89 from Ministry of TP ammonium molybdate GB 11893-89 from Ministry of Ecology TP spectrophotometric method Ecology and Environment, PRC spectrophotometric method and Environment, PRC organic phosphorus OP = TP IP organic − OP = TP − IP phosphorus Sustainability 2020, 12, 5026 23 of 27 Sustainability 2020, 12, x FOR PEER REVIEW 24 of 28

1.0

0.20 0.8 y =0.003x+0.452 2 y2=0.0009x+0.099 r =0.208 0.15 0.6 2 r2=0.331

0.4 0.10

y1=0.0007x+0.087 0.2 0.05 y1=0.0021x+0.291 nitrogen (mg/L) nitrogen ρ(PP) r1=0.260 ρ(PN) r =0.182 phosphorus (mg/L) ρ(TN) 1 ρ(TP) 0.0 fitting curve with PN 0.00 fitting curve with PP fitting curve with TN fitting curve with TP 95% confidence interval of PN 95% confidence interval of PP -0.2 -0.05 95% confidence interval of TN 95% confidence interval of TP

0 10 20 30 40 50 0 10 20 30 40 50 ρ(TSS) (mg/L) ρ(TSS) (mg/L)

Figure A3. Relationship between annual mean total suspended solids (TSS), nitrogen, and phosphorus Figure A3. Relationship between annual mean total suspended solids (TSS), nitrogen, and in the water of Lihu Lake. phosphorus in the water of Lihu Lake. Table A4. Basic information of water quality of Lihu Lake. Table A4. Basic information of water quality of Lihu Lake. Index Unit Min Max Average SD 1 Index Unit Min Max Average SD 1 T ◦C 18.50 19.30 18.88 0.22 pH T / °C 7.8418.50 19.30 8.37 18.88 8.150.22 0.09 DOpH mg/L/ 3.607.84 8.37 8.50 8.15 7.050.09 0.96 ORPDO mVmg/L 166.903.60 8.50 247.20 7.05 197.720.96 22.59 CODORP mg /LmV 3.42166.90 247.20 8.06 197.72 4.4122.59 0.82 3 Chl.a CODmg /m mg/L 6.303.42 8.06 50.54 4.41 16.200.82 11.11 TSS mg/L 4 47 27 13.69 3 TNChl.a mg /Lmg/m 0.266.30 50.54 0.90 16.20 0.5311.11 0.20 DTNTSS mg /Lmg/L 0.044 47 0.4127 0.1813.69 0.11 + NH4 -NTN mg/Lmg/L 0.030.26 0.90 0.25 0.53 0.110.20 0.05 NO3−-NDTN mg /Lmg/L 0.0080.04 0.41 0.069 0.18 0.0480.11 0.007 TP mg/L 0.05 0.23 0.12 0.04 NH4+-N mg/L 0.03 0.25 0.11 0.05 DTP mg/L 0.004 0.114 0.032 0.031 - IPNO3 - mgN /Lmg/L 0.00060.008 0.069 0.0205 0.048 0.00870.007 0.006 TP mg/L 0.05 0.23 0.12 0.04 1 standard deviation. DTP mg/L 0.004 0.114 0.032 0.031 IP mg/L 0.0006 0.0205 0.0087 0.006 Table A5. General characteristics of nitrogen1 standard forms deviation in surface. sediments of Lihu Lake (content unit: mg/kg). Table 5. General characteristics of nitrogen forms in surface sediments of Lihu Lake (content unit: Index Min Max Average SD mg/kg). ETN 33.42 431.19 144.50 95.27 + E-NH4 -NIndex 3.12Min Max 222.66 Average 75.31SD 56.92 E-NO3−-NETN 4.9933.42 431.19 85.45 144.50 21.5995.27 15.89 SON 17.76 126.22 47.60 30.42 E-NH4+-N 3.12 222.66 75.31 56.92 HN 477.91 1156.33 701.96 158.99 E-NO3--N 4.99 85.45 21.59 15.89 AN 208.28 698.38 335.19 108.17 AANSON 10.1017.76 126.22 383.07 47.60 162.9130.42 94.27 ASNHN 3.49477.91 1156.33 29.17 701.96 11.79158.99 5.81 HUNAN 62.29208.28 698.38 281.15 335.19 192.08108.17 61.17 RNAAN 010.10 383.07 1163.30 162.91 451.1794.27 315.24 TN 516.34 2551.14 1297.64 497.75 ASN 3.49 29.17 11.79 5.81 HUN 62.29 281.15 192.08 61.17 RN 0 1163.30 451.17 315.24 TN 516.34 2551.14 1297.64 497.75 Sustainability 2020, 12, 5026 24 of 27

Table A6. General characteristics of phosphorus forms in surface sediments of Lihu Lake (content unit: mg/kg).

Index Min Max Average SD

WA-Pi 1.40 20.50 6.37 4.50 PA-Pi 10.34 107.15 53.85 24.84 Fe/Al-Pi 40.93 243.13 106.93 51.85 Ca-Pi 62.10 348.53 163.43 80.71 WA-Po 0.67 10.69 3.01 1.92 PA-Po 0.03 54.00 9.52 10.11 MA-Po 50.97 142.15 91.29 18.85 NA-Po 76.93 244.99 137.07 45.83

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