Characterization of phosphorus forms in a Eutrophic Lake, China

Identifying the drivers of chlorophyll-a dynamics in a landscape lake recharged by reclaimed water using interpretable machine learning

The water quality of lakes recharged by reclaimed water is affected by both the fluctuation of reclaimed water quality and the biochemical processes in the lakes, and therefore the main controlling factors of algal blooms are difficult to identify. Taking a typical landscape lake recharged by reclaimed water as an example and using the spatiotemporal distribution characteristics and correlation analysis of water quality indexes, we propose an interpretable machine learning framework based on random forest to predict chlorophyll-a (Chl-a). The model considered nutrient difference indexes between reclaimed water and lake water, and further used feature importance ranking and partial dependence plot to identify nutrient drivers. Results show that the NO3--N input from reclaimed water is the dominant nutrient driver for algal bloom especially at high temperatures, and the negative correlation between NO3--N and Chl-a in the lake water is the consequence of algal bloom rather than the cause. Our study provides new insights into the identification of eutrophication factors for lakes recharged by reclaimed water.

Suspended phosphorus sustains algal blooms in a dissolved phosphorus-depleted lake

The expansion of algal bloom in surface waters is a global problem in the freshwater ecosystem. Differential reactivity of organic phosphorus (P_o) compounds from organic debris, suspended particulate matter (SPM), and sediment towards hydrolysis can dictate the extent of supply often limited inorganic P (P_i) for algal growth, thereby controlling the extent of bloom. Here, we combined solution P-31 nuclear magnetic resonance (³¹P NMR), sequential extraction, enzymatic hydrolysis, and 16S rRNA measurements to characterize speciation and biogeochemical cycling of P in Lake Erhai, China. Lower ratios of diester-P/monoester-P in SPM in January (mean 0.09) and July (0.14) than that in April (0.29) reflected the higher degree of diester-P remineralization in cold and warm months. Both H₂O-P_i and P_o were significantly higher in SPM (mean 1580 mg ·kg^-1 and 1618 mg ·kg^-1) than those in sediment (mean 8 mg ·kg^-1 and 387 mg ·kg^-1). In addition, results from enzymatic hydrolysis experiments demonstrated that 61% P_o in SPM and 58% in sediment in the H₂O, NaHCO₃, and NaOH extracts could be hydrolyzed. These results suggested that H₂O-P_i and P_o from SPM were the primarily bioavailable P sources for algae. Changes of P_i contents (particularly H₂O-P_i) in algae and alkaline phosphatase activity (APA) during the observation periods were likely to be controlled by the strategies of P uptake and utilization of algae. P remobilization/remineralization from SPM likely resulted from algae and bacteria (e.g., Pseudomonas). Collectively, these results provide important insights that SPM P could sustain the algal blooms even if the dissolved P was depleted in the water column.

Characteristics and distribution of phosphorus in surface sediments of a shallow lake

Phosphorus (P) in sediments plays an important role in shallow lake ecosystems and has a major effect on the lake environment. The mobility and bioavailability of P primarily depend on the contents of different P forms, which in turn depend on the sedimentary environment. Here, sediment samples from Baiyangdian (BYD) lake were collected and measured by the Standards, Measurements, and Testing procedure and Phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMR) to characterize different P forms and their relationships with sediment physicochemical properties. The P content in the sediments varied in different areas and had characteristics indicative of exogenous river input. Inorganic P (334-916 mg/kg) was the dominant form of P. The 31P NMR results demonstrated that orthophosphate monoesters (16-110 mg/kg), which may be a source of P when redox conditions change, was the dominant form of organic P (20-305 mg/kg). The distribution of P forms in each region varied greatly because of the effects of anthropogenic activities, and the regions affected by exogenous river input had a higher content of P and a higher risk of P release. Principal component analysis indicated that P bound to Fe, Al, and Mn oxides and hydroxides (NaOH-P) and organic P were mainly derived from industrial and agricultural pollution, respectively. Redundancy analysis indicated that increases in pH lead to the release of NaOH-P. Organic matter plays an important role in the organic P biogeochemical cycle, as it acts as a sink and source of organic P.

Removal effects of aquatic plants on high-concentration phosphorus in wastewater during summer

Aquatic plants are widely used in depth treatment of wastewater; however, the phosphorus (P) removal mechanisms of aquatic plants at high temperatures in summer are not well understood. Eight aquatic plants, including two floating species (Ludwigia peploides and Hydrocharis dubia) and six emergent species (Lythrum salicaria, Sagittaria sagittifolia, Canna indica, Sparganium stoloniferum, Rotala rotundifolia, and Ludwigia ovalis), were treated with five P solutions (3.0, 3.5, 4.0, 4.5, and 5.5 mg L^-1) for 5 weeks in a greenhouse during summer at air temperatures ranging from 25 to 35 °C. H. dubia, L. peploides, L. salicaria, and S. sagittifolia showed high water P removal efficiencies (exceeded 95%). Furthermore, their corresponding residual P concentrations in water were almost lower than the limit value of 0.2 mg L^-1 of Grade III in the Chinese Environmental Quality Atandards for Surface Water (GB3838-2002). Plants have different water P removal paths. For example, H. dubia enriched more P with water P concentration increasing significantly. As the culture time increased, the water pH fluctuated significantly in the fall, and then H. dubia used the produced H⁺ enrich P. L. peploides did not enrich P, but proliferated rapidly, to remove P from water by increasing its fresh weight (FW). L. salicaria and S. sagittifolia showed two paths of enrich-P and FW increase. During the growth process of L. salicaria, the stem diameter and leaf length increased with an increase in P concentration in water or plant or both; however, the height and root length of L. peploides were reduced. Moreover, SOD and CAT activities responded to high P concentrations in water or high temperatures or both, which protected against oxidative damage. These findings could offer theoretical foundation and practical guidance for selection of aquatic plant species in depth treatment of wastewater during summer.

Biogeochemical dynamics of particulate organic phosphorus and its potential environmental implication in a typical "algae-type" eutrophic lake

Organic phosphorus (P_o) plays a very important role in the process of lake eutrophication, but there is still a lack of knowledge about the internal cycle of P_o in suspended particulate matter (SPM) dominated by algal debris. In this study, the characterization of bioavailable P_o by sequential extraction and enzymatic hydrolysis showed that 45% of extracted TP was P_o in SPM of Lake Dianchi, and 43-98% of total P_o in H₂O, NaHCO₃ and NaOH fractions was enzymatically hydrolyzable P_o (EHP, H₂O-EHP: 31-53%). Importantly, labile monoester P was the main organic form (68%) of EHP, and its potential bioavailability was higher than that of diester P and phytate-like P. According to the estimation of P pools in SPM of the whole lake, the total load of P_i plus EHP in the H₂O extract of SPM was 74.9 t and had great potential risk to enhance eutrophication in the lake water environment. Accordingly, reducing the amount of SPM in the water during the algal blooming period is likely to be a necessary measure that can successfully interfere with or block the continuous stress of unhealthy levels of P on the aquatic ecosystem.

Fuel from within: Can suspended phosphorus maintain algal blooms in Lake Dianchi

Extensive algal bloom in the surface water is a pressing issue in Lake Dianchi that causes lake restoration to be difficult owing to complex and variable phosphorus (P) sources in the water column. P released from algae, suspended particles (SS), and sediment can provide sustainable P sources for algal blooms. However, little is known regarding the dynamic of P speciation in these substances from different sources. In this study, solution ³¹P nuclear magnetic resonance (³¹P NMR) and chemical sequential extraction were employed to identify P speciation in algae, SS, and sediment during different periods. Results showed that dissolved inorganic P (P_i) directly accumulated in algae in the form of orthophosphate (ortho-P) and pyrophosphate (pyro-P). Algae preferentially utilized P_i, followed by organic P (P_o) in the water column when the P_i was insufficient during growth and reproduction. The ³¹P NMR spectra demonstrated that ortho-P, orthophosphate monoesters (mono-P), orthophosphate diesters (diester-P), and pyro-P dominated the P compounds across the samples tested. Increasing remineralization of SS mono-P driven by intense alkaline phosphatase activities was caused by increasing P needs of algae and pressure of P supply in the water column. The higher ratios of diester-P to mono-P in sediment (mean 0.55) than those in algae (mean 0.07) and SS (mean 0.11 in surface water, 0.14 in bottom water) suggested that the degradation and regeneration occurred within these P compounds during or after sedimentation. P_i content in algae during growth and reproduction was controlled by its P absorption and utilization strategies. Results of this study provide insights into the dynamic cycling of P in algae, SS, and sediment, explaining the reason for algal blooms in the surface water with low concentrations of dissolved P.

Organic phosphorus regeneration enhanced since eutrophication occurred in the sub-deep reservoir

Lake eutrophication remains a serious environmental problem of global significance, and phosphorus (P) plays a key role in lake eutrophication. Internal P loading, as a result of P release from sediments, is gathering more and more recognition as an important source governing the P availability in these ecosystems. Anoxic condition can promote the release of P associated with Fe oxides, which has already been a consensus. However, it is still unknown whether the anoxic conditions induced by eutrophication act to intensify or weaken the regeneration of organic P (P_org) in sediments. We selected the Hongfeng Reservoir, a typical sub-deep lake, to study the regeneration behaviours of C and P in the sediments buried before and after eutrophication. The results showed that P_org did not significantly increase with the rapid increase in organic C (C_org) since eutrophication occurred. Furthermore, the organic C/P ratio was much higher in sediments buried after eutrophication than in those buried before, which indicated that P_org regeneration had been significantly enhanced since eutrophication occurred. Based on C/P ratios, our estimation suggested that the P_org regeneration and P release from sediment to water approximately enhanced 45.2% ± 8.7% and 34.5% ± 9.8%, respectively. Elevated primary productivity (algae) and the corresponding hypoxic/anoxic condition, both caused by eutrophication, promoted P biogeochemical cycle in the sub-deep reservoir. This study further verifies the significant contribution of regenerated P_org to the internal P load, and highlights the importance of controlling P release from sediments in order to restore clear water ecosystems in sub-deep lakes or reservoirs.

Effects of suspended solids on cyanobacterial bloom formation under different wind fields

Wind waves and suspended solids (SS) generated by the resuspension of sediments are ubiquitous characteristics of lake ecosystems. However, their effects on phytoplankton remain poorly elucidated in shallow eutrophic lakes. Laboratory experiments were carried out to investigate the responses of Microcystis aeruginosa to SS under static (wind speed of 0 m/s) and breeze (wind speed of 3 m/s) conditions. Results showed that 50 mg/L SS can promote the growth of M. aeruginosa, accelerate the formation of colonies, and increase the floating rate under no-wind conditions. Comparing with static environment, breeze can significantly increase the growth rate of M. aeruginosa and benefit the formation of larger colonies of algae cells. Driven by wind and SS, the buoyancy of the cyanobacteria community in different experimental groups was obviously different. The specific performance was that low SS concentration and breeze were in favor of the floating of cyanobacteria, while high SS concentration went against the floating of algal cells. As a conclusion, wind speed of 3 m/s and 20-50 mg/L SS have a synergistic effect on the formation of cyanobacterial blooms. This study can provide an improved current understanding of bloom formation and turbidity management strategies in shallow eutrophic lakes.

Remote monitoring of total dissolved phosphorus in eutrophic Lake Taihu based on a novel algorithm: Implications for contributing factors and lake management

Understanding the spatiotemporal dynamics of total dissolved phosphorus concentration (C_TDP) and its regulatory factors is essential to improving our understanding of its impact on inland water eutrophication, but few studies have assessed this in eutrophic inland lakes due to a lack of suitable bio-optical algorithms allowing the use of remote sensing data. We developed a novel semi-analytical algorithm for this purpose and tested it in the eutrophic Lake Taihu, China. Our algorithm produced robust results with a mean absolute square percentage error of 29.65% and root mean square error of 9.54 μg/L. Meanwhile, the new algorithm demonstrates good portability to other waters with different optical properties and could be applied to various image data, including Moderate Resolution Imaging Spectroradiometer (MODIS), Medium Resolution Imaging Spectrometer (MERIS), and Ocean and Land Color Instrument (OLCI). Further analysis based on Geostationary Ocean Color Imager observations from 2011 to 2020 revealed a significant spatiotemporal heterogeneity of C_TDP in Lake Taihu. Correlation analysis of the long-term trend between C_TDP and driving factors demonstrated that air temperature is the dominant regulating factor in variations of C_TDP. This study provides a novel algorithm allowing remote-sensing monitoring of C_TDP in eutrophic lakes and can lead to new insights into the role of dissolved phosphorus in water eutrophication.

Distribution characteristics of phosphorus-containing substances in a long running aerobic granular sludge-membrane bioreactor with no sludge discharge

This work aimed at revealing the distribution characteristics of phosphorus (P) containing substances in an aerobic granular sludge-membrane bioreactor (AGS-MBR). During the long running period (180 days) with no sludge discharge, AGS was successfully cultivated on day 20, and the system performed well in removing organic pollutants and total nitrogen (TN). However, the removal of total P (TP) showed a fluctuant tendency, and P was found to distribute in all the phases of the system. In the intracellular phase, it occupied the largest ratio all through the period. In AGS, inorganic P (IP) was measured to be about 74.4-77.8% of TP, with non-apatite IP (NAIP) composing 57.5-69.6%, while in organic P (OP), the ratio of monoester and diester phosphate was in the range of 19-26.9% and 12-13.5%, respectively. The presence of highly releasable and bioavailable P (NAIP + OP) in AGS implied that it might be a potential P resource for utilization.

Phosphorus supply level is more important than wheat variety in safe utilization of cadmium-contaminated calcareous soil

Screening and cultivating crop varieties with low Cd accumulation is an effective way to safely utilize the Cd slightly contaminated soil. The characteristics and mechanism of Cd uptake by 13 wheat varieties in two calcareous soils with similar Cd contamination level but different P supply level were studied. The grain Cd concentration of almost all varieties in low-P soil was significantly higher than that in high-P soil and exceeded the maximum level of 0.2 mg kg^-1 recommended by the Codex Alimentarius Commission. The pH value of low-P soil was significantly lower than that of high-P soil by 0.27 units, while leaf [Mn] (proxy for rhizosphere carboxylates) and the activities of soil alkaline phosphatase and phytase were significantly higher than those of high-P soil by 35%, 55%, and 286%, respectively. The exchangeable Cd concentration in low-P soil was 2.93 times higher than that in high-P soil, while the Cd concentration of oxides and organic species was significantly lower than that in high-P soil by 21% and 64%, respectively, collectively increasing soluble Cd concentration in low-P soil by 38%. In low-P calcareous soil, P mobilization induced the change of root-zone microenvironment, resulting in the mobilization of Cd.

Analysis of the solubility of phosphate rock from Aipe (Colombia) via formation of 2Na-EDTA complex

Phosphate rock (PR) is the main source of phosphorous used in fertilizers for Colombian soils. In many regions of Colombia, PR is applied directly to the soil, which affects eutrophication problems and phosphorus losses due to runoff, because to the low solubility of phosphorus in this georesource. In this article, phosphate rock samples from Colombia were treated with ethylenediamine tetra-acetic acid disodium salt dihydrate (2Na-EDTA) at different concentrations. PR obtained from the Media Luna Mine, located in Aipe, Huila (Colombia), was characterized using Infrared Spectroscopy, X-ray Diffraction and Scanning Electron Microscopy. Carbonate fluorapatite Ca_9.74(PO₄)_5.45F_2.05(CO₃)_0.53 (0.5%-61.5 %), hydroxyapatite - Ca₅(PO₄)₃OH (26.6 %-84.0 %) and quartz- SiO₂ (13.2 %) were the minerals found in the largest composition in the two samples of PR. The PR was crushed to powder (~125 μm) and it was treated with disodium EDTA dihydrate solutions at different concentrations [0.0025-0.1000 M]. Water-soluble phosphate was determined by UV-Vis spectrophotometry based on the ASTM -4500P method. The modified absolute solubility index (ASI*) was calculated for each EDTA treatment used in the phosphate rock, considering the total phosphorus solubilized in H₂SO₄ (40 % v/v) at 30 °C. The statistical analysis showed that there were significant differences between the treatments used, where (16.87 %) was presented highest ASI* for the treatment with 2Na-EDTA (0.353 M).

Seasonal variations, potential bioavailability, and ecological risk of phosphorus species in the coastal sediments of the Makran

Eutrophication and algal blooms occur frequently in coastal waters of the Makran, for which phosphorus is an important driving factor. Therefore, in this study, sequential extraction procedure was used to evaluate abundance of five phosphorus chemical species. Total phosphorus (TP) concentration ranged from 672.3 to 2031.9 μg/g in pre-monsoon and 771.7 to 2095.7 μg/g in post-monsoon. More than 87% of TP was inorganic P and contribution of organic phosphorus was low. Potential bioavailable P (PBAP) comprised of 52.1 and 35.5% of TP in the pre and post-monsoon, respectively. Average phosphorus pollution index was higher than one in all stations and seasons, implying that sediments in the study area are polluted by phosphorus and are under eutrophication risk. Thus, in the future, foregoing information will be important for assessment of phosphorus loading and pollution in the Makran.

Phosphorus compounds in the dissolved and particulate phases in urban rivers and a downstream eutrophic lake as analyzed using 31P NMR

Phosphorus (P) discharges from human activities result in eutrophication of lakes. We investigated whether the forms of phosphorus (P) in rivers with high effluent loads flowing through urban areas of Sapporo, Japan, were transformed when transported downstream into a eutrophic lake, namely Lake Barato. We hypothesized that the inorganic P supplied from the rivers might be transformed to organic forms in the lake. The results showed that soluble reactive phosphorus (SRP) and particulate inorganic phosphorus (PIP) dominated in the river discharge to the lake. Suspended solids in the rivers were rich in iron (Fe) so PIP was associated with Fe. A comparison of the concentrations at the river mouth and 4.5 km downstream showed that the concentrations of SRP and PIP were lower at 4.5 km downstream than at the river mouth, whereas the concentrations of organic P (i.e., dissolved organic phosphorus and particulate organic phosphorus) were similar. The results from solution ³¹P nuclear magnetic resonance spectroscopy of lake water showed that pyrophosphate was only present in the particulate fraction, while orthophosphate diesters (DNA-P) were only present in the dissolved fraction. Riverine samples contained orthophosphate (ortho-P) only, while lake samples contained ortho-P, orthophosphate monoesters, and DNA-P. The results suggest that the P forms, particularly those of dissolved P, shifted from inorganic to organic forms as the water was discharged from the river to the lake.

Phosphorus-based metabolic pathway tracers in surface waters

Trophic status in surface waters has been mostly monitored by measuring soluble reactive phosphorus (SRP) and total phosphorus (TP). Additional to these common parameters, a two-dimensional ion chromatography mass spectrometry (2D-IC-MS) method was used to simultaneously measure soluble phosphate (Pi), pyrophosphate (PPi), and eleven phosphate-containing metabolites (P-metabolites) in Lake Ontario and its tributaries. From the additional P species, PPi, adenosine 5'-monophosphate (AMP), glucose 6-phosphate (G-P), D-fructose 6-phosphate (F-P), D-fructose 1,6-biphosphate (F-2P), D-ribulose 5-phosphate (R-P), D-ribulose 1,5-bisphosphate (R-2P), and D-(-)-3-phosphoglyceric acid (PGA) were detected and quantified in the lake and river samples. The additional multivariate statistical analysis identified similarities between samples collected at different locations. The presence of R-P, R-2P, and F-2P in Lake Ontario tributaries seems to be mainly related to the Calvin cycle, while the lack of all these three P-metabolites and higher PGA levels than G-P in Toronto Harbour samples seems to be the result of depleted Calvin cycle, pentose phosphate, and glycolysis metabolic pathways.

Assessment and management of lake eutrophication: A case study in Lake Erhai, China

Some wastewater sources, such as agricultural waste and runoff, and industrial sewage, can degrade water quality. This study summarises the sources and corresponding mechanisms that trigger eutrophication in lakes. Additionally, the trophic status index and water quality index (WQI) which are effective tools for evaluating the degree of eutrophication of lakes, have been discussed. This study also explores the main nutrients (nitrogen and phosphorus) driving transformations in the water body and sediment. Lake Erhai was used as a case study, and it was found to be in a mesotrophic state, with N and P co-limitation before 2006, and only P limitation since 2006. Finally, effective measures to maintain sustainable development in the watershed are proposed, along with a framework for an early warning system adopting the latest technologies (geographic information systems (GIS), remote sensing (RS)) for preventing eutrophication.

Species of iron in the sediments of the Yellow River and its effects on release of phosphorus

Species of Fe in the sediments from 21 sections of the Yellow River have been analyzed utilizing the so-called sequential extraction approach. The correlation between iron and phosphorus was analyzed, and the risk of release phosphorus was predicted. The results show that the content of residual iron (Fe_residual) in the sediments is the highest, accounting for more than 92.55% of total iron (∑Fe) (the sum of the iron contents in each form is the total iron). Ca-bound phosphorous (P_Ca) is equal to the sum of authigenic calcium and phosphorus (P_aut) and detritus calcium phosphorus (P_De), accounting for more than 73.01% of ∑P, in which P_De accounts for more than 90.91% of P_Ca, which is the main component of P_Ca and the main component of ∑P; Fe_water in sediments is closely related to P_org, and Fe_ex is closely related to P_ex and P_Al, which is an important factor to control the release of endogenous phosphorus. The extremely significant positive correlation between Fe_residual and P_aut and P_De in sediments is that they are affected by complex geological conditions in the Yellow River Basin. Judging by the ratio of ∑Fe/∑P combined with the influence of human factors, the release of phosphorus in sediments of most reaches of the Yellow River is controlled by iron, while the release of phosphorus is inhibited. Sediments in Xigu section H4, Zhongwei section H5, Haibowan section H7, three Sanshenggong section H8, Liulin section H13, and Tongguan section H16 of the Yellow River have certain phosphorus release risks, because phosphorus release is not only controlled by iron but also influenced by human factors, such as industrial and agricultural production level and artificial dam construction.

CoWO4-x -Based Photothermal Membranes for Solar-Driven Water Evaporation and Eutrophic Lake Water Purification

Solar-driven water evaporation has been proven to be a promising and efficient method for the energy crisis and clean water shortage issues. Herein, we strategically design and fabricate a novel nonstoichiometric CoWO_4-x -deposited foam nickel (NF) membrane (CoWO_4-x @NF) that possesses all the desirable optical, thermal, and wetting properties for efficient water evaporation and purification. The broadband absorption of CoWO_4-x nanoparticles (NPs) obtained by hydrogen reduction contributes to light-to-heat conversion, while NF with a three-dimensional porous structure can support CoWO_4-x NPs and ensure the rapid flow of water molecules during the water evaporation process. We systematically explore and compare the outdoor water evaporation performance of the pure water group, NF group, and CoWO_4-x @NF group, and the results show that CoWO_4-x @NF performs well under natural sunlight irradiation (water evaporation: 2.91 kg m^-2). Significantly, under solar irradiation, the remarkable reduction of Cyanophyta and Euglenophyta in lake water is achieved in the CoWO_4-x @NF membrane-administered group, and these two algae are the main factors for eutrophication of the lake water. Our work highlights the great potentials of the CoWO_4-x @NF membrane as a device for realizing outdoor solar energy-driven water evaporation and proposes a new strategy for purifying the eutrophication of the lake water.

Dynamics of phosphorus composition in suspended particulate matter from a turbid eutrophic shallow lake (Lake Chaohu, China): Implications for phosphorus cycling and management

Particulate phosphorus (P) dominates the total P (TP) content in lacustrine water columns and is a primary source of dissolved P in turbid eutrophic shallow lakes. However, the spatiotemporal variability of P compositions in suspended particulate matter (SPM) remains poorly understood. In this study, we applied chemical extraction and solution ³¹P nuclear magnetic resonance (³¹P NMR) to assess the seasonal variations of SPM P compositions from a shallow turbid lake (Lake Chaohu, China) and its main river tributaries. P fractionation analysis indicated that mobile P (the sum of labile-P, iron-bound P, and organic P) accounted for >60% of the TP in SPM and showed high spatiotemporal variability throughout the year-long field investigation. In most seasons, riverine SPM (in urban rivers or rivers with high flow) contained a higher mobile P content than that of the lake and was therefore a dominant source of lacustrine mobile particulate P. Solution ³¹P NMR identified five types of P compounds in SPM, with highest contributions from orthophosphate. Organic P components and concentrations showed high seasonal variability, and elevated p values occurred during the summer algal bloom. The correlation analysis between organic and inorganic P fractions inferred the possible degradation of organic P into reactive inorganic components of SPM. Consequently, biological or chemical processes would further transform the labile inorganic P into soluble reactive phosphorus, which is readily utilized by lacustrine algae. Our results suggest that the labile forms of P in SPM were highly dynamic and significantly contributed to the eutrophication of the turbid shallow lake.

Understanding the long-term trend of particulate phosphorus in a cyanobacteria-dominated lake using MODIS-Aqua observations

Information on the long-term trends in phosphorus (P) in lake waters is critical for clarifying transformation and biogeochemical cycling processes of P. We developed and validated an empirical model for deriving particulate phosphorus (PP, a dominant form of P) from MODIS-Aqua (Moderate Resolution Imaging Spectroradiometer) images. Subsequently, the long-term trend in PP in Lake Taihu from 2003 to 2017 and the driving factors were clarified. Based on the spectral index of the combination of remote sensing reflectance at 645 nm and 859 nm, a simple linear model was developed to derive PP for turbid cyanobacteria-dominated inland waters from MODIS-Aqua data (R² = 0.65; RMSE = 0.048 mg/L). Long-term MODIS observations show that PP demonstrated distinct spatial variations in Lake Taihu, with higher PP levels in cyanobacterial bloom-sensitive regions. There was a clear increasing trend in the PP of Lake Taihu, and the yearly average PP value increased from 0.089 mg/L in 2003 to 0.10 mg/L in 2017. A relatively strong positive correlation between 15-year spatially averaged PP data and algal bloom frequency revealed that cyanobacterial blooms mainly controlled the PP spatial variations. The daily average water temperature was significantly correlated with the daily PP derived from MODIS-Aqua data, indicating that the increasing water temperature was responsible for the PP increasing trend. An increase in water temperature can facilitate algal growth, thus resulting in a significant change in aquatic biogeochemical conditions, inducing more P release from the sediment and thereby increasing water PP levels. Our study has significant implications for understanding and controlling the P biogeochemical cycle in a cyanobacterial-dominated lake under the background of global warming.

Effects of Organic Phosphorus on Methylotrophic Methanogenesis in Coastal Lagoon Sediments With Seagrass (Zostera marina) Colonization

Methanogens are the major contributors of greenhouse gas methane and play significant roles in the degradation and transformation of organic matter. These organisms are particularly abundant in Swan Lake, which is a shallow lagoon located in Rongcheng Bay, Yellow Sea, northern China, where eutrophication from overfertilization commonly results in anoxic environments. High organic phosphorus content is a key component of the total phosphorus in Swan Lake and is possibly a key factor affecting the eutrophication and carbon and nitrogen cycling in Swan Lake. The effects of organic phosphorus on eutrophication have been well-studied with respect to bacteria, such as cyanobacteria, unlike the effects of organic phosphorus on methanogenesis. In this study, different sediment layer samples of seagrass-vegetated and unvegetated areas in Swan Lake were investigated to understand the effects of organic phosphorus on methylotrophic methanogenesis. The results showed that phytate phosphorus significantly promoted methane production in the deepest sediment layer of vegetated regions but suppressed it in unvegetated regions. Amplicon sequencing revealed that methylotrophic Methanococcoides actively dominated in all enrichment samples from both regions with additions of trimethylamine or phytate phosphorus, whereas methylotrophic Methanolobus and Methanosarcina predominated in the enrichments obtained from vegetated and unvegetated sediments, respectively. These results prompted further study of the effects of phytate phosphorus on two methanogen isolates, Methanolobus psychrophilus, a type strain, Methanosarcina mazei, an isolate from Swan Lake sediments. Cultivation experiments showed that phytate phosphorus could inhibit methane production by M. psychrophilus but promote methane production by M. mazei. These culture-based studies revealed the effects of organic phosphorus on methylotrophic methanogenesis in coastal lagoon sediments and improves our understanding of the mechanisms of organic carbon cycling leading to methanogenesis mediated by organic phosphorus dynamics in coastal wetlands.

A novel Mg(OH)2 binding layer-based DGT technique for measuring phosphorus in water and sediment

Diffusive gradients in thin films (DGT) have gained wide attention for in situ measurement of reactive phosphorus species (PO4) in natural water, sediments and potentially soils. In this study, a novel Mg(OH)2 binding gel was formed using magnesium hydroxide obtained by in situ hydration of calcined magnesium oxide. Laboratory scale experiments showed that the novel Mg(OH)2 gel had a homogeneous dispersion of fine particles of Mg(OH)2 with a particle size of 2-5 μm. With 10 mL of 2.0 mol L-1 NaOH as the eluting agent, the optimal elution efficiency of PO4 on the Mg(OH)2 gel was 72 ± 5%. There were linear relationships between the accumulated PO4 mass and the applied PO4 concentration (0.1 to 20 mg P per L), time (0 to 24 h) and temperature (22 to 40 °C). The capacity of the Mg(OH)2 binding layer was determined to be 99.5 μg P per disc. Tests in synthetic seawater, Chaohu Lake and Yihai Pond confirmed that Mg(OH)2-DGT was able to accurately measure phosphorus up to 10 days. This was indicated by the good agreements between the concentrations measured by DGT (CDGT) technology and by an ex situ chemical method in solution (Csoln), with a CDGT/Csoln ratio between 0.91 and 1.09.