Simulated bioavailability of phosphorus from aquatic macrophytes and phytoplankton by aqueous suspension and incubation with alkaline phosphatase

Biotic regulation of phoD-encoding gene bacteria on organic phosphorus mineralization in lacustrine sediments with distinct trophic levels

Organic phosphorus (Po) mineralization hydrolyzed by alkaline phosphatase (APase) can replenish bioavailable P load in the sediment water ecosystem of lakes. However, the understanding about the interaction between P load and bacteria community encoding APase generation in the sediment are still limited. Different P pools in the sediments from Taihu Lake, China were measured using sequential extraction procedure. The APAase activity (APA) were obtained accompanying with enzymatic dynamical parameters Vmax and Km. The abundances and diversity of gene phoD-harboring bacterial communities were assessed using high throughput sequencing. The analysis results showed the decrease of potentially bioavailable P fractions including MgCl2-P and Fe-P along sampling gradient southwards together with active P concentrations in the water. Conversely, increasing APA and absolute abundance of phoD gene were found with the decreasing of P loads southwards. Positive correlation (p < 0.05) between absolute abundance and APA indicated that phoD-encoding bacteria manipulated the APA and Po mineralization. Negative correlation (p < 0.01) suggested that the APA was restrained by high P load and was promoted under low P condition. However, higher Vmax and Km values suggested that high mineralization potential of Po maintained the high concentrations of potentially bioavailable P even the APA was restricted. The abundance increase of predominant genus Cobetia (from 15.51 to 24.34 %) mirrored by the reduced Calothrix abundance (from 24.65 to 1036 %) was speculated to be responsible for the APA promotion under low P condition. Higher diversity indices in the high P scenario suggested that high P load stimulated the ecological diversity of gene phoD-encoding bacteria community. Generally, rare taxa such as Burkholderia having high connected degrees in bacterial communities together with abundant genera synergistically manipulated the phoD gene abundance and APase generation. Interaction between P fractions and bacteria encoding phoD gene determined the eutrophication status in the lacustrine ecosystem.

Mechanism of organic phosphorus transformation and its impact on the primary production in a deep oligotrophic plateau lake during stratification

Global warming is leading to extended stratification in deep lakes, which may exacerbate phosphorus (P) limitation in the upper waters. Conversion of labile dissolved organic P (DOP) is a possible adaptive strategy to maintain primary production. To test this, the spatiotemporal distributions of various soluble P fractions and phosphomonesterase (PME)/phosphodiesterase (PDE) activities were investigated in Lake Fuxian during the stratification period and the transition capacity of organic P and its impact on primary productivity were evaluated. The results indicated that the DOP concentration (mean 0.20 ± 0.05 μmol L-1) was significantly higher than that of dissolved inorganic P (DIP) (mean 0.08 ± 0.03 μmol L-1) in the epilimnion and metalimnion, which were predominantly composed of orthophosphate monoester (monoester-P) and orthophosphate diesters (diester-P). The low ratio of diester-P / monoester-P and high activities of PME and PDE indicate DOP mineralization in the epilimnion and metalimnion. We detected a DIP threshold of approximately 0.19 μmol L-1, corresponding to the highest total PME activity in the lake. Meta-analysis further demonstrated that DIP thresholds of PME activities were prevalent in oligotrophic (0.19 μmol L-1) and mesotrophic (0.74 μmol L-1) inland waters. In contrast to the phosphate-sensitive phosphatase PME, dissolved PDE was expressed independent of phosphate availability and its activity invariably correlated with chlorophyll a, suggesting the involvement of phytoplankton in DOP utilization. This study provides important field evidence for the DOP transformation processes and the strategy for maintaining primary productivity in P-deficient scenarios, which contributes to the understanding of P cycles and the mechanisms of system adaptation to future long-term P limitations in stratified waters.

Enhancing phosphorus source apportionment in watersheds through species-specific analysis

Phosphorus (P) is a pivotal element responsible for triggering watershed eutrophication, and accurate source apportionment is a prerequisite for achieving the targeted prevention and control of P pollution. Current research predominantly emphasizes the allocation of total phosphorus (TP) loads from watershed pollution sources, with limited integration of source apportionment considering P species and their specific implications for eutrophication. This article conducts a retrospective analysis of the current state of research on watershed P source apportionment models, providing a comprehensive evaluation of three source apportionment methods, inventory analysis, diffusion models, and receptor models. Furthermore, a quantitative analysis of the impact of P species on watersheds is carried out, followed by the relationship between P species and the P source apportionment being critically clarified within watersheds. The study reveals that the impact of P on watershed eutrophication is highly dependent on P species, rather than absolute concentration of TP. Current research overlooking P species composition of pollution sources may render the acquired results of source apportionment incapable of assessing the impact of P sources on eutrophication accurately. In order to enhance the accuracy of watershed P pollution source apportionment, the following prospectives are recommended: (1) quantifying the P species composition of typical pollution sources; (2) revealing the mechanisms governing the migration and transformation of P species in watersheds; (3) expanding the application of traditional models and introducing novel methods to achieve quantitative source apportionment specifically for P species. Conducting source apportionment of specific species within a watershed contributes to a deeper understanding of P migration and transformation, enhancing the precise of management of P pollution sources and facilitating the targeted recovery of P resources.

Phosphorus nutrition strategies in a Symbiodiniacean species: Implications in coral-alga symbiosis facing increasing phosphorus deficiency in future warmer oceans

Coral reefs thrive in the oligotrophic ocean and rely on symbiotic algae to acquire nutrients. Global warming is projected to intensify surface ocean nutrient deficiency and anthropogenic discharge of wastes with high nitrogen (N): phosphorus (P) ratios can exacerbate P nutrient limitation. However, our understanding on how symbiotic algae cope with P deficiency is limited. Here, we investigated the responses of a coral symbiotic species of Symbiodiniaceae, Cladocopium goreaui, to P-limitation by examining its physiological performance and transcriptomic profile. Under P stress, C. goreaui exhibited decreases in algal growth, photosynthetic efficiency, and cellular P content but enhancement in carbon fixation, N assimilation, N:P ratio, and energy metabolism, with downregulated expression of carbohydrate exporter genes. Besides, C. goreaui showed flexible mechanisms of utilizing different dissolved organic phosphorus to relieve P deficiency. When provided glycerol phosphate, C. goreaui hydrolyzed it extracellularly to produce phosphate for uptake. When grown on phytate, in contrast, C. goreaui upregulated the endocytosis pathway while no dissolved inorganic phosphorus was released into the medium, suggesting that phytate was transported into the cell, potentially via the endocytosis pathway. This study sheds light on the survival strategies of C. goreaui and potential weakening of its role as an organic carbon supplier in P-limited environments, underscoring the importance of more systematic investigation on future projections of such effects.

Two-sided effects of the organic phosphorus phytate on a globally important marine coccolithophorid phytoplankton

Dissolved organic phosphorus (DOP) is a potential source of aquatic eutrophication and pollution because it can potentially stimulate growth in some species and inhibit growth in other species of algae, the foundation of the marine ecosystem. Inositol hexaphosphate (also named phytic acid or PA), an abundant organophosphate, is presumably ubiquitous in the marine environment, but how it affects marine primary producers is poorly understood. Here, we investigated the bioavailability of this DOP to the cosmopolitan coccolithophore Emiliania huxleyi. Our results showed that E. huxleyi cells can take up PA and dissolved inorganic phosphorus (DIP) simultaneously. Absorbed PA can efficiently support algal growth, producing cell yield between DIP and phosphorus (P)-depleted conditions. Accordingly, PA supply as the sole P source highly influences cellular metabolism and nutrient stoichiometry. Particularly, PA-grown cultures exhibited enhanced carbon fixation, increased lipid content, activated energy metabolism, and induced nitrogen assimilation. However, our data suggest that PA may also exert some levels of toxic effects on E. huxleyi. This study provides novel insights into the variable effects of a DOP on marine phytoplankton, which will inform new inquiries about how the complex DOP constituencies in the ocean will shape phytoplankton community structure and function. IMPORTANCE The dissolved organic phosphorus (DOP) utilization in phytoplankton plays vital roles in cellular P homeostasis, P-nutrient niche, and the dynamics of community structure in marine ecosystems, but its mechanisms, potentially varying with species, are far from clear. In this study, we investigated the utilization of a widespread DOP species, which is commonly produced by plants (land plants and marine macrophytes) and released into coastal areas, in a globally distributed bloom-forming coccolithophore species in various phosphorus environments. Using a combination of physiological and transcriptomic measurements and analyses, our experimental results revealed the complex mechanism and two-sided effects of DOP (major algal growth-supporting and minor toxic effects) in this species, providing a novel perspective on phytoplankton nutrient regulation.

Microcystis spp. and phosphorus in aquatic environments: A comprehensive review on their physiological and ecological interactions

Cyanobacterial blooms caused by eutrophication have become a major environmental problem in aquatic ecosystems worldwide over the last few decades. Phosphorus is a limiting nutrient that affects the growth of cyanobacteria and plays a role in dynamic changes in algal density and the formation of cyanobacterial blooms. Therefore, identifying the association between phosphorus sources and Microcystis, which is the most representative and harmful cyanobacteria, is essential for building an understanding of the ecological risks of cyanobacterial blooms. However, systematic reviews summarizing the relationships between Microcystis and phosphorus in aquatic environments are rare. Thus, this study provides a comprehensive overview of the physiological and ecological interactions between phosphorus sources and Microcystis in aquatic environments from the following perspectives: (i) the effects of phosphorus source and concentration on Microcystis growth, (ii) the impacts of phosphorus on the environmental behaviors of Microcystis, (iii) mechanisms of phosphorus-related metabolism in Microcystis, and (iv) role of Microcystis in the distribution of phosphorus sources within aquatic environments. In addition, relevant unsolved issues and essential future investigations (e.g., secondary ecological risks) have been highlighted and discussed. This review provides deeper insights into the relationship between phosphorus sources and Microcystis and can serve as a reference for the evaluation, monitoring, and effective control of cyanobacterial blooms.

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.

Dissolved Organic Phosphorus Removal in Secondary Effluent by Ferrate (VI): Performance and Mechanism

Dissolved organic phosphorus (DOP), which is recalcitrant in municipal wastewater treatment, accounts for 26-81% of dissolved total phosphorus in the effluent. More importantly, the majority of DOP could be bioavailable, potentially threatening the aquatic environment through eutrophication. This study aimed to develop a ferrate (VI)-based advanced treatment to effectively destruct and remove DOP from secondary effluent and use deoxyribonucleic acid (DNA) and adenosine-5'-triphosphate (ATP) as DOP model compounds to explore the relevant mechanisms. The results showed that ferrate (VI) treatment could efficiently destruct and remove 75% of the DOP in secondary effluent from an activated sludge-adopted municipal wastewater treatment plant, under normal operating conditions. Moreover, the coexistence of nitrate, ammonia, and alkalinity barely affected the effectiveness, while the presence of phosphate significantly inhibited DOP removal. The mechanistic study revealed that ferrate (VI)-induced particle adsorption was the dominant way to achieve DOP reduction, rather than oxidating DOP to phosphate and forming precipitation afterward. Meanwhile, DOP molecules could be effectively decomposed into smaller ones by ferrate (VI) oxidation. This study clearly demonstrated that ferrate (VI) treatment could achieve a promising DOP removal from secondary effluent for mitigating the risk of eutrophication in receiving water bodies.

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.

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.

Buffering effect of suspended particulate matter on phosphorus cycling during transport from rivers to lakes

How to maintain harmful algal blooms under phosphate-limitation is still an open question in mesotrophic/eutrophic lakes. Little evidence for the importance of suspended particulate matter (SPM) in mediating phosphorus cycling and contributing to eutrophication has been generated for aquatic ecosystems, especially in coupled river-lake systems. In this study, we examined phosphorus transport and redistribution in a river-lake system in the Lake Erhai basin by establishing the relations between phosphorus distribution and phosphorus sorption behavior on SPM, and predicted how changes in the quality and quantity of SPM might influence phosphorus cycling by laboratory experiments and modeling. During the wet seasons, TP pool shifted from being dominated by total dissolved phosphorus (TDP) in the Miju River and estuary regions (73±5%) to being dominated by total particulate phosphorus (TPP) (74±11%) in Lake Erhai. The detritus-SPM in the Miju River as a P-sink and phytoplankton-SPM in Lake Erhai as a P-source buffered TDP levels during the wet seasons, which attributed to P activity and phytoplankton-POC of SPM. Increasing SPM concentrations could enhance the P-buffering. When C₀ ≤ 5 μmol/L and phytoplankton-SPM ≥ 16 mg/L, P release increased by 50%-300%; when C₀ ≥ 5 μmol/L and detritus-SPM ≥ 16 mg/L, P removal could exceed 30%. This study highlights two distinctive roles of SPM in regulating P cycling during transport from rivers to lakes. Especially the phytoplankton-SPM to buffer phosphate-limitation during algal blooms should not be ignored, which could provide theoretical references for the mechanism of continued algal blooms in mesotrophic lakes.

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) > β-glycerophosphate (β-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, β-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, β-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.

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 simulation of nutrient distribution in lakes during ice cover growth and ablation

Nutrient transport in seasonally ice-covered lakes is an important factor affecting spring algal blooms in eutrophic waters; because phase changes during the ice growth process redistribute the nutrients. In this study, nutrient transport under static conditions was simulated by using two ice thickness models in combination with an indoor freezing experiment under different segregation coefficient conditions for nutrients. A real-time prediction model for nutrient and pollutant concentrations in ice-covered lakes was established to explore the impact of the ice-on period in eutrophic shallow lakes. The results demonstrated that the empirical degree-day model and the high-resolution thermodynamic snow and sea-ice model (HIGHTSI) could both be used to simulate lake ice thickness. The empirical degree-day model performed better at predicting the maximum ice thickness (measured thickness 0.22-0.55 m; simulated thickness 0.48 m), whereas the HIGHTSI model was more accurate when estimating the mean thickness (5-6% error). When simulating ice growth, the HIGHTSI model considered more meteorological factors impacting ice cover ablation; hence, it performed better during the ablation stage relative to the empirical degree-day model. Two non-dynamic nutrient transport models were developed by combining the segregation coefficient model and the ice thickness prediction model. The HIGHTSI nutrient transport model can be used to predict real-time changes in nutrient concentrations under ice cover, and the degree-day model can be used to predict changes in the lake water ecosystem.

Phosphorus Fractions and Release Factors in Surface Sediments of a Tailwater River in Xinmi City, a Case Study

The Shuangji River in Xinmi City is a tailwater-type river. Its main water sources are the effluent from the domestic sewage plant, the effluent from the paper industry sewage plant and the coal well. The construction of wastewater treatment facilities in Xinmi city has significantly reduced the amount of total phosphorus (TP) discharged into Shuangji River. However, phosphorus control in rivers where the overlying waters are predominantly tailwaters is still a challenge, especially as the sediment–water interface’s phosphorus exchange mechanism needs to be investigated in detail. In this study, the content and proportion of each phosphorus fraction in the sediment of a tailwater-type river, the Shuangji River, were determined. It was found that the organic phosphorus (OP) and iron-bound phosphorus (Fe-P) content and proportion were high, and the risk of release was relatively high in the section of the river where the overlying water was the tailwater of a sewage plant. Temperature, pH, dissolved oxygen, and hydraulic disturbance were also found to control phosphorus forms’ transformation and release in the sediment. Elevated temperatures mainly stimulated the release of OP and Fe-P from the sediments. The dissolution of calcium-bound phosphorus (Ca-P) is the main pathway for phosphorus release under acidic conditions, whereas, under alkaline conditions, phosphorus release is mainly controlled by ion exchange between OH− and Fe-P and metal oxide-bound phosphorus (Al-P). Aerobic versus anaerobic conditions cause changes in Fe-P content in the sediment mainly by changing Fe ions’ chemical valence. Hydrodynamic disturbance accelerates labile-P release, but once the hydrodynamic disturbance stops, the overlying water dissolved total phosphorus (DTP) concentration rapidly decreases to a similar concentration as before.

Effects of rising atmospheric CO2 levels on physiological response of cyanobacteria and cyanobacterial bloom development: A review

Increasing atmospheric CO₂ concentration negatively impacts aquatic ecosystems and may exacerbate the problem of undesirable cyanobacterial bloom development in freshwater ecosystems. Elevated levels of atmospheric CO₂ may increase the levels of dissolved CO₂ in freshwater systems, via air-water exchanges, enhancing primary production in the water and catchments. Although high CO₂ levels improve cyanobacterial growth and increase cyanobacterial biomass, the impacts on their internal physiological processes can be more complex. Here, we have reviewed previous studies to evaluate the physiological responses of cyanobacteria to high concentrations of CO₂. In response to high CO₂ concentrations, the pressures of inorganic carbon absorption are reduced, and carbon concentration mechanisms are downregulated, affecting the intracellular metabolic processes and competitiveness of the cyanobacteria. Nitrogen and phosphorus metabolism and light utilization are closely related to CO₂ assimilation, and these processes are likely to be affected by resource and energy reallocation when CO₂ levels are high. Additionally, the responses of diazotrophic and toxic cyanobacteria to elevated CO₂ levels were specifically reviewed. The responses of diazotrophic cyanobacteria to elevated CO₂ concentrations were found to be inconsistent, probably because of differences in other factors in experimental designs. Toxic cyanobacteria tended to be superior to non-toxic strains at low levels of CO₂; however, the specific effects of microcystin on the regulation require further investigation. Furthermore, the effects of increasing CO₂ levels on cyanobacterial competitiveness in phytoplankton communities and nutrient cycling in aquatic ecosystems were reviewed. High CO₂ concentrations may make cyanobacteria less competitive relative to other algal taxa; however, due to the complexity of natural systems and the specificity of algal species, the dominant positions of the cyanobacteria do not seems to be changed. To better understand cyanobacterial responses to elevated CO₂ levels and help control cyanobacterial bloom developments, this review has identified key areas for future research.

Responses of alkaline phosphatase activity to wind-driven waves in a large, shallow lake: Implications for phosphorus availability and algal blooms

Phosphorus is a vital nutrient for algal growth, thus, a better understanding of phosphorus availability is essential to mitigate harmful algal blooms in lakes. Wind waves are a ubiquitous characteristic of lake ecosystems. However, its effects on the cycling of organic phosphorus and its usage by phytoplankton remain poorly elucidated in shallow eutrophic lakes. A mesocosm experiment was carried out to investigate the responses of alkaline phosphatase activity fractions to wind waves in large, shallow, eutrophic Lake Taihu. Results showed that wind-driven waves induced the release of alkaline phosphatase and phosphorus from the sediment, and dramatically enhanced phytoplanktonic alkaline phosphatase activity. However, compared to the calm conditions, bacterial and dissolved alkaline phosphatase activity decreased in wind-wave conditions. Consistently, the gene copies of Microcystis phoX increased but bacterial phoX decreased under wind-wave conditions. The ecological effects of these waves on phosphorus and phytoplankton likely accelerated the biogeochemical cycling of phosphorus and promoted phytoplankton production in Lake Taihu. This study provides an improved current understanding of phosphorus availability and the phosphorus strategies of plankton in shallow, eutrophic lakes.

Effectiveness of dredging on internal phosphorus loading in a typical aquacultural lake

Intensive aquaculture significantly affects the global phosphorus (P) cycle and enhances eutrophication in inland waters. Sediment dredging efficiently removes P-rich sediments from shallow-water eutrophic lakes. However, studies on the effects of sediment dredging on the internal P loading of aquacultural lakes are still lacking. Moreover, the migration and transformation processes of labile P and the mechanisms of sediment P release are unclear. To evaluate dredging effectiveness, we employed two in situ high-resolution sampling techniques to simultaneously measure sediment labile P and porewater soluble reactive P (SRP) and Fe (II) at the millimeter scale. Dredging effectively reduced surface sediment CaP contents and organic matter (OM) below the sediment-water interface (SWI). Moreover, dredging decreased the SRP diffusion flux across the SWI in summer. After dredging, FeP (P bound to Fe, Al, and Mn oxides and hydroxides) and OP (organic P) contents increased by 136% and 48% in the newly formed deposited layer (140 mm thick), respectively. The increased bioavailable P content significantly enhanced the capability of sediment solids to resupply labile P to porewater SRP. The stronger positive correlation between porewater soluble Fe (II) and SRP suggests that Fe redox cycling regulated internal P release. Our results suggest that dredging effectiveness will weaken over time due to the re-deposition of active P, which in turn increases the risk of sediment P release. To curb the release of sediment P, we recommend the implementation of additional in situ restoration techniques that improve the oxide layer of surface sediments and reduce sediment suspension.

Composition characterization and biotransformation of dissolved, particulate and algae organic phosphorus in eutrophic lakes

Characteristics and transformation of organic phosphorus in water are vital to biogeochemical cycling of phosphorus and support of blooms of phytoplankton and cyanobacteria. Using solution ³¹P nuclear magnetic resonance (NMR), combined with field surveys and lab analyses, composition and structural characteristics of dissolved phosphorus (DP), particulate phosphorus (PP) and organic P in algae were studied in two eutrophic lakes in China, Tai Lake and Chao Lake. Factors influencing migration and transformation of these constituents in lake ecosystems were also investigated. A method was developed to extract, flocculate and concentrate DP and PP from lake water samples. Results showed that orthophosphate (Ortho-P) constituted 32.4%-81.3% of DP and 43.7%-54.9% of PP, respectively; while monoester phosphorus (Mono-P) was 13.2%-54.0% of DP and 32.9%-43.7% of PP, respectively. Phosphorus in algae was mostly organic P, especially Mono-P, which was ≥50% of TP. Environmental factors and water quality parameters such as temperature (T), electrical conductivity (EC), pH, secchi depth (SD), dissolved oxygen (DO), chemical oxygen demand (COD_cr), chlorophyll-a (Chl-a), affected the absolute and relative concentrations of various P components in the two lakes. Increased temperature promoted bioavailable P (Ortho-P and Mono-P) release to the lake waters. The results can provide an important theoretical basis for the mutual conversion process of organic P components between various media in the lake water environment.

Simulation and Seasonal Characteristics of the Intra-Annual Heat Exchange Process in a Shallow Ice-Covered Lake

The intra-annual heat exchange process has a considerable influence on the energy circulation, material metabolism, and ecological succession of lakes. The input and output of heat in an ice-covered lake provide the basic dynamic force driving changes in the biochemical state of the lake. Based on the heat balance between the lake surface and the atmosphere, we established a thermodynamic model for calculating the thermodynamic factors of shallow inland lakes during the ice and open seasons. The data of the Ulansuhai Lake, Inner Mongolia, from two years (2012 and 2013) are used to analyze the seasonal characteristics and associated influences of the heat budget on the ecosystem. The results indicated that the monthly mean lake temperature over the past 10 years was 1.7–2.2 °C lower than in the previous 50 years. The absorbed solar radiation reached up to 210 W/m2 in 2012 and 179 W/m2 in 2013, and there were clear differences in the heat budget between the ice-covered and open seasons. The mean net heat fluxes in the ice season were −33.8 and −38.5 W/m2 in 2012 and 2013, respectively; while in the open season water, these fluxes were 62.5 and 19.1 W/m2. In the simulations, the wind was an important factor for intensive evaporation in summer and the main driver of the ice cover formation patterns in winter, involving the transmission and diffusion of material and energy in the lake. The results provide a theoretical foundation for simulating ice cover growth and ablation processes in shallow lakes. They also present data on the ecological evolution in these lacustrine environments.

Effects of suspended particular matters, excess PO43-, and salinity on phosphorus speciation in coastal river sediments

Phosphorus (P) is an essential biogenic element in aquatic ecosystem, and its speciation in sediment may influence the water quality. The composition of P in suspended particular matters (SPM) and sediments were analyzed. Metal ions bonding PO₄^3- and chelating organic P (OP) were explored by Visual MINTEQ simulation and infrared spectroscopy. Inorganic P (IP) mainly comprises orthophosphate and pyrophosphate in SPM. OP mainly includes α-glycerol phosphate, β-Gly, monophosphate, and mononucleotides from aquatic plants in SPM. Cyclotella, Nitzschia, Amphiprore, and terrestrial C₃ plants are the main source of aquatic plants in JH, while they are from Oscillatoria and Merismopedia in JL. These aquatic plants directly determine whether OP or IP is taken to surface sediments during the setting of SPM. The bonding between PO₄^3- and Ca is more preferential than Al and Fe, so the excess PO₄^3- makes Ca compounds bonding IP (Ca-IP) and Al/Fe/Mn (hydr) oxides associated IP (Al/Fe/Mn-IP) dominant, but limited PO₄^3- preferentially contributes more Ca-IP. Metal ions in saline water can firmly cheat with OP via P-OH and/or P=O groups to promote the burial of OP.

Vertical profiles of phosphorus fractions in the sediment in a chain of reservoirs in North China: Implications for pollution source, bioavailability, and eutrophication

The level of eutrophication in reservoirs is dependent on their internal and external P loads. Identifying the P pollution characteristics and its fractional composition in sediments is therefore necessary to determine the potential bioavailability and dominant sources of P for effective water pollution control. In this study, we investigated the P pollution characteristics in the overlying water and sediment in a chain of reservoirs (the Panjiakou (PJK), Daheiting (DHT) and Yuqiao (YQ) Reservoirs) in North China. Our results showed that the P concentrations in the overlying water of the YQ Reservoir was higher than that of the PJK and DHT Reservoirs, but the sediment P loading and P bio-availability were lower than the PJK and DHT Reservoirs. However, the sediment P release risk in the YQ Reservoir was higher than the DHT and PJK Reservoirs. The YQ Reservoir was mainly polluted by internal sediment P release and external sources predominantly derived from the inflowing polluted Sha River Basin. Various forms of P in the DHT Reservoir decreased with depth, and the P in the overlying water column was mainly sourced from internal P release due to sediment accumulation of excess P from human activities. In recent years, the proportion of bio-available P (BAP) in the PJK and YQ Reservoirs had increased, and the proportion of the more inert Al-P and Ca-P in the PJK Reservoir decreased. Ca-P in the YQ Reservoir had also decreased, indicating that inert P has been gradually transformed into active P in the PJK and YQ Reservoirs in recent years. The observed differences in P loading and sedimentary P fractions indicate different pollution characteristics and sources between the three reservoirs. We therefore recommend site-specific remediation strategies for effective control on P pollution in the three eutrophic reservoirs.

Seasonal Variations of Phosphorus Species in the Overlying and Pore Waters of the Tuohe River, China

Overlying sediment and pore waters were collected in summer and winter at upstream (Jintang) and downstream (Neijiang) sites of the Tuohe River, which is one of the five largest tributaries of the Yangtze River in China. Phosphorus species, including soluble reactive phosphorus (SRP), soluble unreactive phosphorus (SUP), and total dissolved phosphorus (TDP), and some diagenetic constituents including dissolved Fe(II), Mn(II), and sulfide in overlying and pore waters, were measured systematically. The seasonal variations and vertical distributions of phosphorus species in overlying and pore waters at both sampling sites were obtained to elucidate some aspects of the transport and transformations of phosphorus. Based on the profiles of pore and overlying waters as well as the TDN/TDP data during an algal bloom in 2007, it was clearly demonstrated that phosphorus was the main factor limiting the phytoplankton growth in the Tuohe River.

Distribution of organic phosphorus species in sediment profiles of shallow lakes and its effect on photo-release of phosphate during sediment resuspension

In this work, the distribution of organic phosphorus (P_o) species in sediment profiles of five shallow lakes was analyzed and its effect on the photo-release of dissolved phosphate (P_i) was investigated during sediment resuspension under simulated sunlight irradiation. The results show that P_o was highly enriched in the surface sediment and gradually decreased as sediment depths increased: 33.10 ± 2.55-96.71 ± 7.60 mg/kg, 33.55 ± 2.34-142.86 ± 5.73 mg/kg, 57.50 ± 3.46-149.68 ± 7.67 mg/kg, 55.18 ± 4.67-168.73 ± 8.31 mg/kg, 98.75 ± 7.56-275.74 ± 10.70 mg/kg for Lake Hou, Lake Tuan, Lake Tangling, Lake Guozheng and Lake Miao, respectively. The photo-release amount of dissolved P_i in the resuspension composed of surface sediments was also higher than that of deep sediment during sediment resuspension under the simulated sunlight irradiation for 9 h. The potential reasons for these results are: (1) difference in morphology and composition of sediments at different depths: the mean particle size of sediment decreased first and then increased as sediment depths increased; (2) difference in composition of P_o species with depths in the sediment profiles: more photolytic P_o species existed in surface sediments confirmed by sequential extraction and ³¹P NMR analysis; and (3) more OH production in the resuspension composed of surface sediment under simulated sunlight irradiation, which directly influence the photo-release of dissolved P_i from photodegradation of organic phosphorus. All of these results indicate that the distribution of organic phosphorus species in the sediment profiles plays an important role in P cycle and its photodegradation during sediment resuspension may be one of the potential pathways for phosphate supplement in shallow lakes.

Three decades of changes in water environment of a large freshwater Lake and its relationship with socio-economic indicators

Tai Lake (Ch: Taihu) has attracted international attention forcyanobacteria blooms. However, the drivers of cultural eutrophication, especially long-term socio-economic indicators have been little researched. The results of research demonstrate how socio-economic development affected quality of water and how it has been improved by anthropogenic activities. This study described variability in indicators of water quality in Tai Lakeand investigated thedrivers. Significant relationships existed between concentrations of annual mean total nitrogen (TN), total phosphorous (TP), chemical oxygen demand (COD) and biological oxygen demand (BOD), and population, per capital gross domestic production (GDP) and sewage discharge (p < 0.05). However, mechanisms causing change varied among TN, TP, COD and BOD. Before 2000, the main contributors to increases in concentrations of TN were human population, GDP and volumes of domestic sewage discharges. After 2000, discharges of industrial sewage become the primary contributor. After 1998, the regressions of annual mean TN, TP and COD on per capital GDP, population and domestic sewage discharge were reversed compared to the former period. Since 1999, an apparent inverted U-shaped relationship between environmental pollution and economic development has developed, which indicated that actions taken by governments have markedly improved quality of water in Tai Lake. The statistical relationship between BOD and per capital GDP didn't conform to the Kuznet curve. The U-shaped Kuznet curve may offer hope for the future that with significant environmental investments a high GDP can be reached and maintained without degradation of the environment, especially through appropriate management of industrial sewage discharge.

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

Organic phosphorus (P_o) constitutes the most important fraction of P in lake sediments, and the compositional properties of P_o 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 P_o 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 P_o 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 P_o may be a superior indicator of trophic status than TP. The P_o component, phosphonate accounted for only 0.4% or less of P_o, while the monoester P and diester P, accounted for 2-24% and 0.5-5% of P_o, respectively, and were the main factors causing P_o 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 P_o might integrate into larger amounts of aromatic substances and functional groups, which could enhance the stability of P_o 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 P_o release. However, the massive accumulation of bioavailable P_o (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 P_o in lake sediment.

Algal uptake of hydrophilic and hydrophobic dissolved organic nitrogen in the eutrophic lakes

Dissolved organic nitrogen (DON) derived from sediments plays an active role in biogeochemical cycling of nutrients in aquatic ecosystems. Sediments from four eutrophic lakes were studied using three-dimensional fluorescence excitation-emission matrix (3DEEM) spectra and supelite XAD-8 macroporous resin separation to investigate the bioavailability of hydrophilic and hydrophobic DON to algae (Microcystis flos-aquae (Wittr.) Kirchner). The results showed that the average loss of DON was <6.0% after dividing DON into hydrophilic and hydrophobic components, demonstrating the utility of XAD-8 resin separation in the study of DON components from lake sediments. The 3DEEM analysis showed that hydrophobic and hydrophilic DON comprised humic- and protein-like materials, respectively. During the incubation period, the bioavailability of hydrophilic DON, which accounted for 59.3%-80.4% of total DON, stimulated algal growth, suggesting that hydrophilic DON was the primary source of organic nitrogen for algae. In contrast, hydrophobic DON increased algal density by only 31.8% of that observed for hydrophilic DON, and had a small (accounted for 20.0%-26.6% of total DON) effect on algal growth over the short-term. The significant differences in algal growth between the two types of DON suggested that they should be considered separately in the eutrophic lake restorations.

The biogeochemical characteristics of phosphorus in coastal sediments under high salinity and dredging conditions

The geochemistry of phosphorus (P) can usually be related to prevailing environmental conditions. To investigate sedimentary P cycling mechanism and biogeochemical characteristics under high salinity and dredging conditions in polluted coastal sediments, thirty-three surface sediment samples were collected from the Jiehe River (JH), Jiaolai River (JL) and their estuarine and offshore areas in the Bohai Sea. Analyses included the Standards, Measurements and Testing method (SMT), Ivanoff organic P (OP) fractionation, and nuclear magnetic resonance (³¹P-NMR) of soluble P and indicated that HCl-P was the dominant fraction in sediments under high salinity stress. However, under dredging conditions in freshwater river sediments, NaOH-P was the dominant fraction. The potential activity of the OP fraction was reactive in freshwater river sediments, while it was unreactive under high salinity conditions. NaOH-P and HCl-P were found to be mainly derived from anthropogenic inputs, whereas both in-situ biological and anthropogenic inputs were important sources of the OP fractions. High salinity had the potential to increase NaOH-P content in acidic river sediments, resulting in the OP being relatively stable with a low risk level. Sediment dredging potentially increased the regeneration of P from HCl-P and OP and increased the potential activity of OP and IP. Seawater was found to induce removal of the NaOH-P and OP from offshore sediments, resulting in the regeneration of the remaining P at a low level.