The continuous increased stability of sediment dissolved organic matter implies ecosystem degradation of lakes in the cold and arid regions.

The characteristics of lake dissolved organic matter (DOM) pool and lake ecosystem interact, and studying the responses between sediment DOM characteristics and lake ecosystem changes may shed light on the inherent connection between ecosystem evolution and carbon biogeochemical cycles. Lakes in cold and arid regions are sensitive to changes and accumulate large amounts of carbon as DOM, which may provide a window into more explicit relationships between ecosystem evolution and changes in sediment DOM characteristics in time dimension. However, considerable blind spots exist in the responses between the sediment DOM and ecosystem evolution on time scale and the underlying mechanisms. In this study, multiple approaches were combined to investigate the relationship between the variation trend of sediment DOM characteristics and the evolution of fragile lake ecosystems across three different lake ecosystems in cold and arid regions of China. A strong positive relationship between sediment DOM stabilities, especially humification, and ecosystem degradation was found, consistent for the three lakes. Ultra-high-resolution mass spectrometry and structural equation modeling revealed that the changes of ecosystems affected sediment DOM stability through direct pathways (0.24), such as the contents of terrestrial DOM in lake DOM pool, and indirect pathways, including algae-mediated (0.43) and salinity-mediated pathways (0.22), which all increased the contents of refractory DOM in the lake DOM pool and sediments. Based on the fact that DOM stability changes could act on the ecosystem in turn, a possible positive feedback mechanism between ecosystem degradation and increased DOM stability was further inferred. These results suggested that the continuous increased stability of sediment DOM in may implies ecosystem degradation of lakes in the cold and arid regions. This study provides a new perspective for recognizing ecosystem evolution through sediment DOM and improves the understanding of the interaction of lake ecosystem evolution and the biogeochemical cycle of DOM.

Spatial gradients and molecular transformations of DOM, DON and DOS in human-impacted estuarine sediments.

Dissolved organic matter (DOM) constitutes the most active fraction in global carbon pools, with estuarine sediments serving as significant repositories, where DOM is susceptible to dynamic transformations. Anthropogenic nitrogen (N) and sulfur (S) inputs further complicate DOM by creating N-bearing DOM (DON) and S-bearing DOM (DOS). This study delves into the spatial gradients and transformation mechanisms of DOM, DON, and DOS in Pearl River Estuary (PRE) sediments, China, using combined techniques of UV-visible spectroscopy, Excitation-emission matrix (EEM) fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and microbial high-throughput sequencing. Results uncovered a distinct spatial gradient in DOM concentration, aromaticity (SUVA254), hydrophobicity (SUVA260), the content of substituent groups including carboxyl, carbonyl, hydroxyl and ester groups (A253/A203) of chromophoric DOM (CDOM), and the abundances of tyrosine/tryptophan-like protein and humic-like substances in fluorophoric DOM (FDOM). These all decreased from upper to lower PRE, accompanied by a decrease in O3S and O5S components, indicating seaward reduction in the contribution of terrestrial OM, especially anthropogenic inputs. Additionally, sediments exhibited a reduction in molecular diversity (number of formulas) of DOM, DON, and DOS from upper to lower PRE, with molecules tending towards a lower nominal oxidation state of carbon (NOSC) and higher bio-reactivity (MLBL), molecular weight (m/z) and saturation (H/C). While molecular composition of DOM remained similar in PRE sediments, the relative abundance of lignin-like substances decreased, with a concurrent increase in protein-like and lipid-like substances in DON and DOS from upper to lower PRE. Mechanistic analysis identified the joint influence of terrestrial OM, anthropogenic N/S inputs, and microbial processes in shaping the spatial gradients of DOM, DON, and DOS in PRE estuarine sediments. This study contributes valuable insights into the intricate spatial gradients and transformations of DOM, DON, and DOS within human-impacted estuarine sediments.

Seaward alteration of arsenic mobilization mechanisms based on fine-scale measurements in Pearl River estuarine sediments.

Identification of key As mobilization processes in estuarine sediments is challenging due to the transitional hydrodynamic condition and the technical restriction of obtaining fine-scale results. Herein, high-resolution (µm to mm) and in situ profiling of As with associated elements (Fe, Mn, and S) by the diffusive gradients in thin-film (DGT) technique were applied and coupled with pore water and solid phase analysis as well as microbial high-throughput sequencing, to ascertain the driving mechanisms of As mobilization in the sediments of Pearl River Estuary (PRE). Significant diffusion fluxes of As from sediment to water were observed, particularly in the upper estuary. With the seaward increase of salinity, the driving mechanism of As mobilization gradually shifted from microbial-induced dissimilatory Fe reduction to saltwater-induced ion exchange. Correspondingly, the dominant Fe-reducing bacteria (FeRB) in sediments changed from the genera Clostridium_sensu_stricto_1 and Bacillus to Ferrimonas and Deferribacter. The presence of dissolved sulfide in deeper sediments contributes to As removal through the formation of As-S precipitates as supported by theoretical calculations. Fine-scale findings revealed seaward changes of As mobilization mechanism in the sediments of a human-impacted estuary and may benefit the understanding of As biogeochemical behavior in estuaries worldwide.

Overlooked great role of wind erosion in terrestrial dissolved organic matter input to lake ecosystem in cold and arid regions.

Lakes in cold and arid regions have weak hydrological connectivity to their watersheds and serious soil erosion by wind, and are particularly sensitive to changes in underlying surface process and global climate changes, which may result in unique carbon cycles across the terrestrial-aquatic interface and ecological impacts. However, the roles of input pathways of terrestrial dissolved organic matter (TDOM) to lakes in cold and arid regions, especially the effects of possible TDOM input caused by wind erosion, has not been fully understood. Taking a typical lake in cold and arid regions as an example, this study comprehensively investigated the characteristics and contributions of dissolved organic matter (DOM) input from different TDOM input pathways, ultimately highlighted the impacts of wind erosion from composition characteristics, historical changes and universal proofs. The results showed that DOM introduced by wind erosion contributed 37.34 % to all TDOM input, and exhibited the strongest humification and aromaticity, and greatest molecular weight and stability. The considerable input and refractory characteristics led to the differences in the TDOM distribution and DOM composition between the near-wind and far-wind shores of the lake. Moreover, historical analysis showed that wind erosion became the main way driving the changes in buried terrestrial organic matter in the lake through a combined action of precipitation and land cover changes after 2008. The pervasive importance of wind erosion pathways on TDOM inputs in the cold and arid regions was further demonstrated through the proof from two other representative lakes. The findings also shed light on the possible impacts of wind erosion on material distribution, aquatic productivity, and energy input in lake ecosystems. The study provides new insights to broaden the content of global lake-landscape interactions and regional ecosystem conservation.

[Kinetic Release Characteristics of Organic Phosphorus of Sediment-water and Water Quality Risks].

To reveal the characteristics of organic phosphorus release from lake sediments and its potential impact on water quality, six lake sediments from Yunnan Plateau and the middle and lower reaches of the Yangtze River in China were selected. We studied the differences in the kinetics of dissolved organic phosphorus (DOP) and dissolved inorganic phosphorus (SRP) release from sediments. The effects of organic phosphorus morphology and dissolved organic matter (DOM) characteristics on sediment phosphorus release were investigated, and the water quality risks of sediment DOP release were discussed. The results showed that:① the release kinetics of sediment DOP and SRP were similar; both followed the second-order kinetic model, starting with a rapid release phase, followed by a slow release, and the release curve gradually leveled off and reached the maximum release. ② The release of organic phosphorus was related to organophosphorus morphology and organic matter. Active organic phosphorus (LOP) and medium active organic phosphorus (MLOP) were the DOP forms mainly released into the overlying water during the rapid release phase. The proportion of LOP and MLOP to total organic phosphorus (DTP) decreased in the late release stage, whereas the proportion of non-active organic phosphorus (NLOP) increased; further, the degree of humification and aromaticity of organic matter gradually increased with phosphorus release, and its activity decreased, resulting in a slower release rate at the later stage. ③ Compared with that of SRP, the risk of DOP release was higher, accounting for 47%-77% of the total amount of DTP. It was also found that the higher the nutrient level of the lake, the greater the release of DOP and the higher the water quality risk. Therefore, not only the release of inorganic phosphorus but also that of organic phosphorus should be of concern in the process of phosphorus release from lake sediments to prevent the underestimation of phosphorus release and water quality risk.

Cognizing and characterizing the organic phosphorus in lake sediments: Advances and challenges.

Organic phosphorus (OP) is one of the main forms of phosphorus in lake ecosystems. Mounting evidence has shown that sediment OP has become a major but underestimated issue in addressing lake eutrophication and algal bloom. However, a holistic view of sediment OP remains missing. This review aims to provide an overview of progress on the studies of OP in lake sediments, focusing on the contribution of OP to internal P loading, its potential role in algal bloom, and the migration and transformation. In addition, this work systematically summarized current methods for characterizing OP content, chemical fraction, composition, bioavailability, and assessment of OP release in sediment, with the pros and cons of each method being discussed. In the end, this work pointed out following efforts needed to deepen the understanding of sediment OP, namely: (1) In-depth literature review from a global perspective regarding the contribution of sediment OP to internal P loading with further summary about its pattern of distribution, accumulation and historical changes; (2) better mathematical models for describing drivers and the linkages between the biological pump of algal bloom and the replenishment of sediment OP; (3) fully accounting the composition and molecular size of OP for better understanding its transformation process and mechanism; ; (4) developing direct, high-sensitivity and combined techniques to improve the precision for identifying OP in sediments; (5) establishing the response of OP molecular properties and chemical reactivity to OP biodegradability and designing a comprehensive and accurate composite index to deepen the understanding for the bioavailability of OP; and (6) integrating fundamental processes of OP in current models to better describe the release and exchange of P in sediment-water interface (SWI). This work is expected to provide critical information about OP properties and deliver perspectives of novel characterization methods.

Novel insights into molecular composition of organic phosphorus in lake sediments.

Organic phosphorus (Po) plays a key role in eutrophication and ecological equilibrium in lake systems. However, characterizing the composition of Po in lake sediments has been a bottleneck hindering further understanding of the biogeochemical cycle of Po. Here, multiple methods of 31P NMR spectroscopy and molecular weight (MW) ultrafiltration were combined to detect Po composition characteristics from a novel angle in ten lake sediments of China. The results showed that sediment Po mainly consisted of monoester (mono-P, 14±8.8% of the NaOH-EDTA total P on average), diester (di-P, 1.4±1.4%) and phosphonate (phos-P, 0.1±0.1%), while the abundance of Po was largely underestimated by 31P NMR methods. Some specific species of mono-P were successfully determined, and the contents of these species followed a decreasing order: inositol hexakisphosphate (IHP6) > RNA mononucleotides (RNA-mnP) > ß-glycerophosphate (ß-gly) > D-glucose 6-phosphate (Glu-6) > α-glycerophosphate (α-gly), which was largely dependent upon their bioreactivity. A significant relationship between MW and Po components was observed despite the great differences among sediment samples. For refractory Po components, IHP6 was mainly rich in the MW < 3 kDa while phos-P was almost only detected in the MW > 3 kDa, which largely attributed to their metal binding affinities and characteristics. The abundance of bioreactive Po species (α-gly, ß-gly, Glu-6, di-P) in high MW (HMW, > 3 kDa) were all higher than that of low MW (LMW, < 3 kDa) due to microbial degradation and self-assembly. If the HMW organic molecules were biologically and chemically more reactive than its LMW counterparts, the high percentage of α-gly, ß-gly, glu-6 and di-P in the HMW portion would highlights their high reactivity from the perspective of MW. These insights revealed the dynamics of the MW distribution of Po components and provide valuable information to better understand the Po composition and bioreactivity in sediments.

Water inflow and endogenous factors drove the changes in the buffering capacity of biogenic elements in Erhai Lake, China.

Buffering capacity could provide a comprehensive view to recognize the response between external loads and water quality and help address the significant challenges associated with the reduction of lake pollution. However, quantification of the dynamic change in the holistic buffering capacity of biogenic elements in lakes and its driving mechanisms has not been fully understood. Taking Erhai Lake in China as an example, this study quantified the long-term (2000-2019) dynamic changes in buffering capacity and revealed key driving forces for the changes in buffering capacity. The results showed that nitrogen buffering capacity (NBC) and organic buffering capacity (CODBC) decreased during the past 20 years, while phosphorus buffering capacity (PBC) did not change significantly. Endogenous factors are the main controlling factors of buffering capacity. Specifically, algal biomass drove the change in NBC (interpretation rate of 62.2%); the adsorption and sedimentation effects of sediments maintained the relative stability of PBC (56.30%) while algal biomass indirectly impacted the PBC (1.69% only) by affecting the redox environment of the sediments; and algae-derived organic matter and refractory organic matter accumulation dominated the change in CODBC (61.4% and 32.8%, respectively). Water inflow is another controlling factor for NBC and CODBC due to dilution of lake water. This study indicated that the accumulation of endogenous loads and a decrease in water inflow drove the decrease in the lake's buffering capacity (mainly NBC and CODBC), which could help explain why the decrease in external loads in Erhai Lake has not yet reversed the trend of water quality decline. Our study highlights the importance of comprehensive buffering capacity improvement instead of simple external load control to optimize lake environmental management. In the future, attention should be given to controlling endogenous loads, especially preventing algal blooms, and to optimizing hydrodynamic conditions to cope with the decrease in water inflow.

Trends of the response-relationship between net anthropogenic nitrogen and phosphorus inputs (NANI/NAPI) and TN/TP export fluxes in Raohe basin, China.

The intensification of anthropogenic nitrogen (N) and phosphorus (P) inputs profoundly affects water environmental quality. Hence it is pivotal to clarify the response relationship between riverine TN/TP export and anthropogenic N/P inputs to provide strategies guidance in N/P management. Based on the variation of net anthropogenic N and P inputs (NANI/NAPI) in the Raohe basin from 1990 to 2018, we constructed the response relationship between NANI/NAPI and total nitrogen and phosphorus (TN/TP) export fluxes in the riverine, which successfully predicted N and P export at the basin scale management. We found N export ratio (ratio of TN export to NANI) increased with slight fluctuation and was mainly affected by the combined effects of Nfer (fertilizer N inputs) and Ndep (atmospheric N deposition) etc., while the decrease of P export ratio (ratio of TP export to NAPI) was mainly due to intensive retention effect of the soil and sediment induced by anthropogenic influence to P transportation process. These results indicate that the downstream aquatic systems take a high risk of increasing N load pressure and the basin systems suffer a danger from rising P load pressure. Therefore, it is recommended to concentrate more on downstream aquatic systems during the N management strategy implementation and pay closer attention to the whereabouts of P in the basin system.

Molecular weight driving bioavailability and intrinsic degradation mechanisms of dissolved organic phosphorus in lake sediment.

The sediment dissolved organic phosphorus (DOP) for the "internal phosphorus (P) loading" has raised intensive concern, but its bioavailability and intrinsic degradation mechanism have not been fully elucidated. In this work, multi-techniques were combined to construct the response of sediments DOP's bioavailability to molecular weight (MW) based on ten lakes of China, thereby elucidating the intrinsic degradation mechanism of sediment DOP. A high percentage (74.5% on average) and significantly positive correlations with respect to different MWs were observed, highlighting the importance of DOP to dissolved P in sediments. DOP is mainly composed of a low MW (LMW) portion (63.8%) and the substances are primarily derived from microbial sources. Bioavailable DOP species were closely related to MW, with labile monoester P and diester P decreased with decreasing MW. Analysis of environmental processes showed that microbial utilization capacity and the characteristics of dissolved organic matter (DOM) with different MWs were the dominant drivers in determining the bioavailability of DOP. That is, microorganisms exhibit high DOM utilization capacity in LMW portion, promoting the degradation and transformation of bioavailable DOP species. Furthermore, the increased humic and fulvic-like substances by microbial degradation might in turn inhibit the enzymatic hydrolysis of LMW-DOP. This pattern explains why the contents of LMW-DOP are very high, but it contains less bioavailable DOP. By studying the bioavailability of sediment DOPs with different MWs, it is found that, under natural conditions, labile monoester and diester P in LMW-DOP have a high tendency to degrade than those in HMW-DOP. The results further show that, microbial utilization and DOM characteristics, as well as their linkage with DOP's bioavailability and degradability, have important implications for assessing DOP's degradation potential. The insights from this study might shed light on more effective strategies for mitigating the risks of "internal P loading".

Dynamic and driving evolution of lake basin pressure in cold and arid regions based on a new method: A case study of three lakes in Inner Mongolia, China.

Static single-path assessment methods usually underestimate the lake basin pressure (LBP). Considering the cumulative transmission of multiple paths, how to assess the dynamic changes of LBP and reveal the driving evolution is not clear. Here, taking Hulun, Daihai and Wuliangsuhai lake basins in the cold and arid region as the study case, we established an LBP assessment method that coupled multiple driving paths. For the first time, this study reveals the dynamic and driving evolution of LBP and found that rapid economic development and accelerated urbanization dramatically increased the LBPs in the three lakes. Specifically, the LBPs in the three lake basins has increased by 90%-270 % in the past 32 years, and they experienced a stable stage driven by climate (1987-1992), followed by a slight increasing stage driven by agriculture (1993-2004) and finally a significant increase stage (2005-2018) driven by industry and urbanization. Different degrees of warming, populations and development intensities of agriculture and livestock were the main factors driving the spatial differences in LBPs in the three lake basins. The LBPs in the Hulun, Daihai and Wuliangsuhai Lake Basins exhibited phase driving, continuous driving and ineffective driving characteristics to the water environment changes, respectively, which were related to lake basin governance. Compared with the driving paths of water quality and water quantity, the LBP was most strongly transmitted through the water ecological path. Rapid economic development and accelerated urbanization will bring greater LBPs to the lake basins. Lake management should promote the construction of water environmental protection mechanisms that correspond to urbanization, such as land use, and continue to strengthen watershed governance to alleviate the impact of LBP, especially the impact on the service functions of water ecosystem. Our method quantified the LBPs that were transmitted from different driving paths and provided action priorities for watershed management.

Implications of phosphorus partitioning at the suspended particle-water interface for lake eutrophication in China's largest freshwater lake, Poyang Lake.

Nutrient partition, especially for phosphorus (P), has been prominently changed that was caused by variation of river-lake relationship during the post-Three Gorges Reservoir and catchment alternations. Changes in proportion of total particulate phosphorus (TPP) and total dissolved phosphorus (TDP) might accelerate lake eutrophication, but limited attention has been paid to P partition over suspended particle (SP) levels. Data analysis showed that SP concentration presented a positive effect on TPP in wet season and soluble reactive phosphorus (SRP) in dry season, indicating seasonal physical and chemical variations. Based on this phenomenon, we proposed a hypothesis that the SP levels would affect TDP and TPP proportions by partition in aqueous-solid. It was found that using the parabola models to fit the sorption relationships of SRP and TDP (R2 > 0.6, p < 0.01), the maximum sorption capacity (Qmax) was 64.54 mg/kg and 60.52 mg/kg at 400 mg/L of SP level, respectively. In addition, the partition coefficients (KP) of TDP and SRP were logarithmically increased with SP levels, indicating that higher SP levels (>400 mg/L) would hinder the sorption process. Furthermore, enhancing turbulence lead to less sorption of SRP and TDP at high SP levels (>800 mg/L). The sorption of SRP and TDP related to the presence of Fe/Al oxy-hydroxides were enriched in the Fe/Al-P fraction (47% of TP). The findings of this study indicated that the low SP levels would increase P bioavailability for alga and is not conducive for lake eutrophication management.

Emerging water pollution in the world's least disturbed lakes on Qinghai-Tibetan Plateau.

Qinghai-Tibet Plateau (QTP) Lake Region has largest abundance and size distribution of lakes in China. Being relatively away from major human activities, the water quality of these lakes has not attracted concerns in the past. However, dramatic climate change and intensified anthropogenic activities over the past 30 years have exerted multiple pressures on the water environment of the lakes, resulting in elevated nutrient concentrations in major freshwater lakes of the region. Rapid water quality deterioration and eutrophication of the lakes were first found in Lake Hurleg in the northeast of the plateau. Analyses of driving forces associated with these changes indicate that both the intrinsic characteristics of the QTP lakes and climate change were responsible for the vulnerability to human activities than other lakes in different regions of China, with accelerated urbanization and extensive economic development in the lake basin playing a decisive role in creating water pollution events. Under combination pressures from both natural and anthropogenic effect, the increasing rate of nutrient concentrations in Lake Hurleg has been 53-346 times faster than in Lake Taihu and Lake Dianchi during the deterioration stage. The result suggests the current development mode of Lake Hurleg basin is not suitable for setting protection targets for the QTP lake region more broadly due to its extremely poor environmental carrying capacity. To stop worsening the lake water environment condition, it is necessary to review the achievements made and lessons learned from China's fight against lake pollution and take immediate measures, inform policies into the development mode in the QTP lake region, and avoid irreversible consequences and ensure good water quality in the "Asian Water Tower."

[Spatio-Temporal Variation of Release Flux of Sediment Nitrogen and Phosphorus in High-Risk Period of Algal Bloom in Lake Erhai].

Spatial and temporal characteristics of release fluxes of sediment nitrogen (N) and phosphorus (P) were investigated in the high-risk period of algal blooms in Lake Erhai. Moreover, the influence factors were examined. Results show that the release flux of N and P increased in recent years, exhibiting a clear increase in the period from 2009 to 2013, and a slight increase in the period since 2013. The release flux of dissolved total nitrogen (DTN) ranged between 11.71-14.15 mg·(m2·d)-1, within which the release flux of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) were 6.39-8.42 mg·(m2·d)-1 and 5.31-5.73 mg·(m2·d)-1, accounting for 58% and 42% of the DTN, respectively. The release flux of dissolved total phosphorus (DTP) ranged between 0.11-0.14 mg·(m2·d)-1, within which the release flux of dissolved organic phosphorus (DOP) and dissolved inorganic phosphorus (DIP) were 0.04-0.05 mg·(m2·d)-1 and 0.07-0.09 mg·(m2·d)-1, accounting for 34% and 66% of the DTP, respectively. The distribution of release flux of N showed a decreasing order:south > north > middle, while P was north > middle > south. The release flux of N increased by 17%, 13% and 23%, and the release flux of P increased by 19%, 28%, and 29% in north, middle, and south part of Lake Erhai from 2009 to 2018. Comparing the years 2009, 2013 and 2018, although the contents of N and P were stable, the release flux of N and P in the sediment was enhanced due to increasing pH and decreasing DO. Therefore, the increasing release of nitrogen and phosphorus from sediments, caused by changes in the water environment factors, should be paid attention to for the protection of Lake Erhai.

Response of phosphorus fractionation in lake sediments to anthropogenic activities in China.

In this study, geochemical fractionation is used to establish the relationship between sediment phosphorus (P) pools and anthropogenic activities based on the dated sediment from a suite of lakes in China. Our extraction results showed that the inorganic P fractions, including Ca-P (46% of TP) and Fe/Al-P (24%), constitute most of the P in sediments. Soil erosion dynamics and geographic location are the dominant factors controlling the historical distribution and partitioning of Ca-P in sediments, while over the last few decades, industrial and domestic effluents were the leading factor controlling Fe/Al-P. The organic P (Po) fractions, NaHCO3-Po, HCl-Po, and Ful-Po accounted for only 11%, 16% and 12% of Po on average, respectively, whereas Hum-Po and Res-Po made up the dominant Po fraction (59%) and were the main factors controlling Po dynamics due to fertilizer and livestock breeding. Thus, the historical fraction of P in the sediment core can be used as an indicator of anthropogenic activities. Ca-P decreased in the top layers of the cores because of the implementation of the Soil and Water Conservation Law in China since 1991. However, Fe/Al-P and Po continuously increased in the lakes from the economic backward area over the last few decades, which is largely due to enhanced point sources of pollution and an increase in the intensity of agricultural practices. As a potential P source, the massive accumulation of Fe/Al-P and Po would be released into the overlying water to further facilitate eutrophication via increasing pH and alkaline phosphatase and decreasing in the dissolved oxygen concentration. Therefore, in order to control eutrophication more effectively and efficiently, it is essential that the accumulation of sediment Fe/Al-P and Po be decreased immediately and domestic wastewater, poultry excreta, and fertilizer loss must be more carefully controlled, especially in economically backward areas.

[Correlation Analysis of Water Quality Between Lake Inflow and Outflow: A Case Study of Poyang Lake].

The relationships between inflow and outflow water quality data for Poyang Lake from 1996 to 2016 are discussed and the main influencing factors are identified. TN and TP were the main factors causing a decline in water quality in Poyang Lake during the study period. The water quality of both the inflow and outflow rivers was generally good between 1996 and 2003; however, water quality declined over the study period, which is attributed to an increase in nutrients loads in the watershed. From 2004 to 2011, the water quality of the "Five Rivers" decreased significantly, which caused the water quality of Poyang Lake to decline. Due to the high purification capacity of Poyang Lake, the water quality of the outflow during this period was relatively good. A decline in water quality after this point was affected by pollution loads and hydrological conditions. Specifically, from 2012 to 2016, water quality in Poyang Lake and of the inflow water declined further. This was combined with a decrease in the water-purification capacity of the lake due to changes in the hydrological conditions, resulting in lower water quality at the outflow. Overall, the water quality of the inflow river has been closely related to the water quality in Poyang Lake. The concentrations of TN were significantly higher in the southern and eastern areas of Poyang Lake compared to the western areas. Higher nutrient loading from the Ganjiang River and the Xinjiang River has been an important driver. The concentrations of TP in the southern area of the lake have been significantly higher than in the eastern and western areas. This is attributed to comparatively high TP loads in the Ganjiang River and the Fuhe River. Compared to the changes in hydrological conditions, variations in nutrient loading have had a greater effect on water quality in the lake.

The potential role of sediment organic phosphorus in algal growth in a low nutrient lake.

The role of sediment-bound organic phosphorus (Po) as an additional nutrient source is a component of internal P budgets in lake system that is usually neglected. Here we examined the relative importance of sediment Po to internal P load and the role of bioavailable Po in algal growth in Lake Erhai, China. Lake Erhai sediment extractable Po accounted for 11-43% (27% average) of extractable total P, and bioavailable Po accounted for 21-66% (40%) of Po. The massive storage of bioavailable Po represents an important form of available P, essential to internal loads. The bioavailable Po includes mainnly labile monoester P and diester P was identified in the sequential extractions by H2O, NaHCO3, NaOH, and HCl. 40% of H2O-Po, 39% of NaHCO3-Po, 43% of NaOH-Po, and 56% of HCl-Po can be hydrolyzed to labile monoester and diester P, suggesting that the bioavailability of Po fractions was in decreasing order as follows: HCl-Po > NaOH-Po > H2O-Po > NaHCO3-Po. It is implied that traditional sequential fractionation of Po might overestimate the availability of labile Po in sediments. Furthermore, analysis of the environmental processes of bioavailable Po showed that the stabler structure of dissloved organic matter (DOM) alleviated the degradation and release of diester P, abundant alkaline phosphatase due to higher algal biomass promoted the degradation of diester P. The stability of DOM structure and the degradation of diester P might responsible for the spatial differences of labile monoester P. The biogeochemical cycle of bioavailable Po replenishs available P pools in overlying water and further facilitate algal growth during the algal blooms. Therefore, to control the algal blooms in Lake Erhai, an effective action is urgently required to reduce the accumulation of Po in sediments and interrupt the supply cycle of bioavailable Po to algal growth.

[Effects and Differences of the Release of Dissolved Organic and Inorganic Phosphorus in Different Sediments Covered by Different Materials of Erhai Lake].

In this work, the effects of four covering materials on the release of total dissolved phosphorus (DTP), dissolved organic phosphorus (DOP), and soluble reactive P (SRP) in different sediments of Erhai Lake were simulated. The results showed that the max release of DTP was reduced in covering material, which attributed to the changes of pH, Eh and characteristics of dissolved organic matter (DOM) by the effect of covering material. The application of iron oxide material significant reduced the release of DTP in the northern and southern part of the lake, with decrease rate of 44.3% and 35.7%, respectively. by contrast, the application of aluminum oxide material significant reduced the release of DTP in the middle part sediment, with decrease rate of 29.6%. Furthermore, the release of SRP and DOP in different sediments has significant difference after added different material. In northern part of sediment, the release of SRP and DOP reduced by 35.6% and 36.2% after added iron oxide material. This is because iron oxide can reduce the pH and Eh but increase the availability of DOM in northern, and then benefits for inhibiting the release of SRP and DOP. In the middle, the release of sediment SRP and DOP reduced by 28.9% and 31.6% after added aluminum oxide material. This is because the aluminum oxide can facilitate the availability of DOM in middle, and then inhibits the release of SRP and DOP. In southern part of the lake, the release of sediment SRP and DOP reduced by 47.4% and 16.5% after added iron oxide material. This is largely attributed to the effect of iron oxide on the pH and Eh. Therefore, to control the release of P in the sediment from Lake Erhai, iron oxide material should be selected in the northern and southern parts, whereas aluminum oxide should be selected in the middle part of the lake.

Response of sediment organic phosphorus composition to lake trophic status in China.

Organic phosphorus (Po) constitutes the most important fraction of P in lake sediments, and the compositional properties of Po affect its behavior in lake ecosystems. In this study, 31P NMR, FT-IR spectroscopy, and UV-visible absorbance spectroscopy were combined to identify the dynamic composition of sediment Po across two sets of lakes in China ranging from oligotrophic to eutrophic, and their possible effects on lake eutrophication were evaluated. The results showed that sediment Po content (accounting for 24-75% of TP) was positively correlated with trophic status in both Eastern Plain and Yun-Gui Plateau lakes of China, and the linear relationship was more stable compared to total P (TP), implying that sediment Po may be a superior indicator of trophic status than TP. The Po component, phosphonate accounted for only 0.4% or less of Po, while the monoester P and diester P, accounted for 2-24% and 0.5-5% of Po, respectively, and were the main factors causing Po to increase with the increasing trophic status. The factors were closely related to the enhanced organic sewage load and intensification of contemporary sedimentation of phytoplankton. As trophic status increased, sediment Po might integrate into larger amounts of aromatic substances and functional groups, which could enhance the stability of Po in sediments. Furthermore, sediments from lakes with higher trophic status exhibited a higher degree of humification and molecular weights, which impart resistance to biodegradation, and therefore, reduced the risk of sediment Po release. However, the massive accumulation of bioavailable Po (monoester and diester P) allows possible degradation, supporting algal growth and maintains eutrophic status because there is abundant alkaline phosphatase in eutrophic lakes. Thus, to control lake eutrophication more effectively, targeted actions are urgently required to reduce the accumulation and degradation of Po in lake sediment.

Characteristics and effects of dissolved organic phosphorus from different sources on the water quality of Erhai Lake in Southwest China.

To establish the influence of dissolved organic phosphorus (DOP) from different sources on the water quality of Erhai Lake in August 2014, enzymatic hydrolysis technology was used to characterize enzymatically hydrolyzable phosphorus (EHP). The results indicate that the DOP represents a significant source of the bioavailable P in the lake water and its different sources of Erhai Lake, and the bioavailability of the P pool may be underestimated by up to 55% if DOP is ignored and only the soluble-reactive phosphorus (SRP) is considered. The significant differences in DOP characteristics from the different sources may be related to the origin of the DOP, regional pollution, and environmental factors. The DOP from the sediment porewater can act as an important source of DOP in the lake water based on its high DOP content and high DOP loads from the sediment release. In addition, the highest EHP loads from the sediment release can threaten the water quality of Erhai Lake and sustain the algal blooms when they occur. Therefore, for the protection of Erhai Lake, the contributions of not only the SRP but also the DOP to the bioavailable P pool should be considered, emphasizing the eutrophication risk caused by DOP from sediments, especially in the middle section of Erhai Lake. The effects of DOP from the inflowing rivers on the water quality should also not be ignored, due to its high bioavailability.