The phosphorus release pathways and their mechanisms driven by organic carbon and nitrogen in sediments of eutrophic shallow lakes

Forms and Migration Mechanisms of Phosphorus in the Ice, Water, and Sediments of Cold and Arid Lakes

Phosphorus (P) is a crucial nutrient in lake ecosystems and organic phosphorus (Po) is a significant component. However, the distribution characteristics and migration behaviour of Po in ice-water-sediment systems under freezing and thawing conditions in cold and arid regions remain unclear. This study aims to investigate the forms of Po and its contribution to endogenous P pollution. We selected three lakes (Dai, Hu, and Wu Lake) and employed phosphorus nuclear magnetic resonance (31P-NMR) techniques to analyse the following: (1) The total phosphorus (TP) content, which was the highest in the water from Dai Lake (0.16 mg/L), with substantial seasonal variation observed in Wu Lake, where P content was four times higher in summer than in winter because of farmland drainage. (2) Eutrophication analysis, which indicated that Dai Lake had significantly higher eutrophication levels than Wu Lake, with P being the controlling factor in Dai Lake and both N and P in Wu Lake. The proportion of Po in the TP content was 90%, 70%, and 55% for Wu, Dai, and Hu Lake, respectively, indicating that Po was the main component of eutrophic lakes. (3) 31P-NMR, which revealed that orthophosphate (Ortho-P) and monoester phosphate (Mon-P) were the main P components in the winter, with a higher P content in Dai Lake. Ortho-P has a higher content in ice, indicating that inorganic phosphorus (Pi) migration is the main factor in ice-water media. Mon-P showed multiple peaks in Dai Lake, indicating a complex composition of adenosine monophosphate and glucose-1-phosphate. (4) The ice-water phase change simulation experiments, which showed that phosphate was the least repelled in ice, while pyrophosphate (Pyro-P) and macromolecular P were more repelled. Adding sediment enhanced the migration of P but did not change the repulsion of macromolecular P, suggesting the molecular structure as the main influencing factor. These results provide important scientific evidence for the quantitative analysis of Po pollution in lake water environments, aiding in P load reduction and risk prevention and control.

Impact of varying dissolved organic carbon load on sediment phosphorus release and its periodic mechanisms

Phosphorus (P) release from sediment poses a severe challenge for eutrophication management in the aquatic environment. The dissolved organic carbon (DOC) concentrations in riverine ecosystems have shown an increasing trend due to intensified climate change and anthropogenic activities, while their impact on sediment P cycling remains unclear. To investigate the effects of different DOC loads on sediment P release and the underlying mechanisms, we conducted a two-month experiment in 15 plexiglass tanks, with five gradient-increasing target DOC concentrations set according to reality: control (S0), 5 mg/L (S5), 10 mg/L (S10), 15 mg/L (S15), and 20 mg/L (S20). The results demonstrated that: i) DOC enrichment promoted the sediment P mobilization and release, with the underlying mechanisms exhibited periodic characteristics. ii) reduced dissolved oxygen (DO) concentration and stimulated alkaline phosphatase activity (APA) were likely the primary and sustained facilitating mechanisms. While after the termination of DOC load, elevated pH level was also considered a contributing factor when chlorophyll a (Chl a) ranged between 5.9 μg/L and 7.7 μg/L iii) ultimate concentration of total P (TP) in the overlying water depended on DOC load. After DOC addition was terminated, decreased TP concentrations were observed when DOC concentration was in the range of 5-15 mg/L, which may be attributed to the direct uptake of P by phytoplankton counteracting the minor promotion of P release induced by anoxic conditions. However, when DOC concentrations exceeded 15-20 mg/L, there were notable increments in TP concentrations. Our findings provide further insight into the response mechanisms of sediment P release to the increasing organic C load in natural ecosystems. The impact of broader C forms or C loads on sediment P cycling needs to be fully elucidated and even quantified in future studies, especially through large-scale field investigations to further clarify the coupled roles between C and P.

Evaluate climate change and anthropogenic activities influencing geochemical variations in sediment between and within the avulsion period in the Lower Yellow River avulsion channels

The geochemical data from sediments in avulsion channels provide historical evidence of climate change and human-induced alterations in river basin environments. The present study focused on the particle size of sediments in cores and the level of geochemical variation in avulsion channels of the Lower Yellow River Delta (YRD), China. The sediment samples were collected in a depth range of 20-400 cm in avulsion channels. The collected samples were analyzed for sediment particle size and geochemical composition using standard methods. The results demonstrated rapid increases in agriculture practices, rainfall pattern changes, and terrestrial sediment runoff reduction in river basins after the 1960s. The reduced sediment loads in the Sanmenxia Reservoir significantly changed the sediment grain size and geochemical levels in the avulsion channel from August 1960.8 to January -1961.1. In particular, TC, TN, and C/N levels decreased with increasing sediment depth. The C/N values of <12 denoted completely reduced terrestrial sources of organic matter in the channel during the August 1960-January 1961 period compared to the July 1953-August 1960 period. The two-way ANOVA p-values were (p <0.016-p<0.001) strong between the avulsion periods but had no significant variation within the avulsion periods. We emphasize that this study provides a close interplay of different historical periods of geochemical variation in avulsion channel sediments in the alluvial fan YRD, and we argue that the evolution of the middle upstream river basin was subjective by climate change and human developmental actions, which impacted the YRD. In particular, reservoir-interrupted water flow and sediment reduction impacts associated with geochemical fluctuations are documented in the YRD.

Variations of sediment organic phosphorus and organic carbon during the outbreak and decline of algal blooms in Lake Taihu, China

In this study, sediment organic phosphorus (OP) and organic carbon (OC) in Lake Taihu, China, as well as their relationships, were analyzed during the outbreak and decline of algal blooms (ABs) over a five-month field study. The results showed synchronous temporal changes in the sediment OP and OC contents with the development of ABs. In addition, there was a significant positive correlation between the sediment OP and OC (p < 0.01), suggesting simultaneous deposition and consumption during the ABs outbreak. The sediment OP and OC contents decreased significantly at the early and last stages of the ABs outbreak and increased at the peak of the ABs outbreak and during the ABs decline. These temporal variation patterns suggest that the sediment OC and OP contents did not consistently increase during the ABs outbreak, even though algae are an important source of organic matter in sediments. The depletion or enrichment of OC and OP in sediments may also depend on the scale of the ABs outbreak. The obtained results revealed significant differences in the sediment OC and OP contents between the months (p < 0.05). In addition, OP in the sediments was dominated by orthophosphate diester (phospholipids and DNA-P) and orthophosphate monoester during the ABs outbreak and decline, respectively. The active OC contents and proportions in the sediments in the ABs outbreak were significantly lower than those observed in the ABs decline period, demonstrating the significant impacts of the ABs outbreak and decline on the sediment OC and OP in Lake Taihu.

Phosphorus mining from marine sediments adopting different carbon/nitrogen strategies driven by anaerobic reactors: The exploration of potential mechanism and microbial activities

To investigate the possibility of phosphorus (P) recovery from marine sediment and explore the role of the carbon: nitrogen ratio in affecting the internal P release under anaerobic conditions, we experimented with the external addition of carbon (acetic acid and glucose) and ammonia nitrogen (NH4-N) to expose P release mechanisms. The 24-day anaerobic incubations were conducted with four different carbon: nitrogen dosing groups including no NH4-N addition and COD/N ratios of 100, 50, and 10. The P release showed that extra NH4-N loading significantly suppressed the decomposition of P (p < 0.05) from the marine sediment, the maximum P release was 4.07 mg/L and 7.14 mg/L in acetic acid- and glucose-fed systems, respectively, without extra NH4-N addition. Additionally, the results exhibited that the imbalance of carbon: nitrogen not only failed to induce the production of organic P mineralization enzyme (alkaline phosphatase) in the sediment but also suppressed its activity under anaerobic conditions. The highest enzyme activity was observed in the group without additional NH4-N dosage, with rates of 1046.4 mg/(kg∙h) in the acetic acid- and 967.8 mg/(kg∙h) in the glucose-fed system, respectively. Microbial data analysis indicated that a decrease in the abundance of P release-regulating bacteria, including polyphosphate-accumulating organisms (Rhodobacteraceae) and sulfate-reducing bacteria (Desulfosarcinaceae), was observed in the high NH4-N addition groups. The observed reduction in enzyme activity and suppression of microbial activity mentioned above could potentially account for the inhibited P decomposition in the presence of high NH4-N addition under anaerobic conditions. The produced P-enriched solution from the bioreactors may offer a promising source for future recovery endeavors.

Benthic primary production decreases internal phosphorus loading from lake sediments under light supplement

In aquatic ecosystems, light penetrating the sediment surface in shallow lakes may regulate the internal phosphorus (P) release through benthic primary production, which subsequently affects oxidation, pH levels, and alkaline phosphatase activity in the upper sediment. To study the effects of light exposure on the P dynamics at the sediment-water interface under eutrophic conditions, a two-month mesocosm experiment was conducted in twelve cement tanks (1000 L each). The tanks were equipped with Light-Emitting Diode (LED) lights, and surface sediments collected from eutrophic Lake Nanhu (China) were exposed to four different light intensities (0, 50, 100, 200 μmol m-2 s-1). The results revealed that: 1) Both the total phosphorus concentration and the phosphorus release flux from the sediment were lower in the light treatments (mean value, 0.59-0.71 mg L-1 and 0.00-0.01 mg m-2 d-1, respectively) than in the control treatment (0.77 mg L-1 and 0.01 mg m-2 d-1, respectively), indicating that light supplement could decrease the internal P release. 2) Benthic primary production promoted by light directly absorbed soluble reactive phosphorus and decreased the internal P release. The resulting improved production could also increase dissolved oxygen concentrations at the sediment-water interface, thus indirectly inhibiting internal P release. 3) The relative contributions of direct absorption and indirect inhibition on the internal P release ranged between 23% to 69% and 31% to 77% depending on the light intensity.

The effect of novel aquaculture mode on phosphorus sorption-release in pond sediment

The emergence of aquaculture modes has brought considerable changes to the aquaculture landscape and profoundly influenced environmental processes. However, there is limited research on nutrient cycling in emerging aquaculture modes. This study investigated the characteristics and mechanisms of sediment phosphorus (P) sorption-release in traditional earthen pond culture (TEP) and pond-tank culture mode (PTC), which represents novel aquaculture modes. The results showed that under higher nutrient load, the PTC did not show significant differences in nutrient concentration in water and sediments compared to TEP. Although there are no significant differences in overlying water P concentration between the modes throughout the entire aquaculture period, the trends of its variation over time are different, which significantly affected the P sorption-release characteristics of sediment. Additionally, correlation analysis suggested that calcium-bound P and hot NaOH-extractable organic P may affect the sorption-release characteristics of sediment as active P fractions. The change in redox condition caused by enzyme-mediated organic matter decomposition (such as protein and lipids) is also an important reason for sediment P release. However, the P fractions and organic matter content showed no significant differences between the two modes. Sediment microbial analysis showed that TEP exhibited a significant dominance of inorganic P-solubilizing bacteria, especially Actinobacteria and Bacilli classes. PTC had a higher proportion of organic P-solubilizing bacteria, primarily in the Bacteroidia class. The quantitative results of the key functional gene phoD in organic P decomposition also showed that the abundance in PTC was significantly higher than that in TEP. This suggested that microbial differences may be another reason for differences in P sorption-release behavior. This study revealed the differences in P sorption-release characteristics and mechanisms between the TEP and PTC, which holds positive implications for water quality and pollution management in novel aquaculture modes.

Mechanisms of high ammonium loading promoted phosphorus release from shallow lake sediments: A five-year large-scale experiment

The unprecedented global increase in the anthropogenic-derived nitrogen (N) input may have profound effects on phosphorus (P) dynamics and may potentially lead to enhanced eutrophication as demonstrated in short-term mesocosm experiments. However, the role of N-influenced P release is less well studied in large-scale ecosystems. To gain more insight into ecosystem effects, we conducted a five-year large-scale experiment in ten ponds (700-1000 m2 each) with two types of sediments and five targeted total N concentrations (TN) by adding NH4Cl fertilizer (0.5, 1, 5, 10, and 25 mg N L-1). The results showed that: (ⅰ) The sediment P release increased significantly when TN exceeded 10-25 mg N L-1. (ⅱ) The most pronounced sediment P release increase occurred in summer and from sediments rich in organic matter (OMSed). (ⅲ) TN, algal biomass, fish biomass, non-algal turbidity, sediment pH, and OMSed were the dominant factors explaining the sediment P release, as suggested by piecewise structural equation modeling. We propose several mechanisms that may have stimulated P release, i.e. high ammonium input causes a stoichiometric N:P imbalance and induce alkaline phosphatase production and dissolved P uptake by phytoplankton, leading to enhanced inorganic P diffusion gradient between sediment and water; higher pelagic fish production induced by the higher phytoplankton production may have led increased sediment P resuspension through disturbance; low oxygen level in the upper sediment caused by nitrification and organic decomposition of the settled phytoplankton and, finally, long-term N application-induced sediment acidification as a net effect of ammonium hydrolysis, nitrification, denitrification; The mechanisms revealed by this study shed new light on the complex processes underlying the N-stimulated sediment P release, with implications also for the strategies used for restoring eutrophicated lakes.

Eutrophication control of large shallow lakes in China

Large shallow lake refers to a polymictic system that is often well mixed without stratification during summer. Similar to a small and deep lake, a large and shallow lake has a high nutrient retention rate. Differing from a small and deep lake, it has an extensive sediment-water interface and internal loading from sediment, which has led to high susceptibility to eutrophication. There are many large and shallow freshwater lakes in the middle and lower Yangtze River (MLYR), China, experienced eutrophication and cyanobacteria blooms. To address this issue, a variety of methods focused on in-lake physical and biogeochemical processes was explored. The main gains of these studies included: (1) shallow lakes in the floodplain of the Yangtze River are prone to eutrophication because of their high trophic conditions; (2) wind-induced waves determine sediment resuspension, downward dissolved oxygen penetration, and upward soluble reactive nutrient mobilization, while wind-driven currents regulate the spatial distribution of water quality metrics and algal blooms; (3) the low P loss of shallow lakes via sedimentation and high N loss via denitrification lead to a low N:P ratio and N and P colimitation, which demonstrated the significance of dual N and P reduction for eutrophication control in shallow lakes; (4) extensive submerged macrophyte could suppress internal loading in large, shallow waters, but nutrient loading must be reduced and water clarity must be increased; and (5) climate warming promotes cyanobacterial blooms through positive feedback to exacerbate eutrophication in shallow lakes. The lack of action to address the challenges of non-point source pollution and internal loading from the sediment has led to limited effectiveness of eutrophication control in large shallow lakes under climate warming. In the future, the management of large shallow eutrophic lakes in China must combine social sciences (economic development) with natural technology (pollution reduction) to achieve sustainability.

Effects of calcination on the environmental behavior of sediments by phosphorus speciation and interface characterization

Dredged sediments derived from eutrophicated lakes poses hardness of sludge disposal and ecological risks. The proper pretreatment and utilization of dredged sediments presented a challenge. In this study, Dianchi Lake sediments were dredged, thermally treated and utilized as particle capping material in batch experiments. The effects of calcination on phosphorus speciation and sediment-water interface environment as well as P immobility mechanism were predominantly explored. The microstructures and chemical compositions of calcined sediments were investigated, indicating the porosity and mineralization components were greatly enhanced. The fractional analysis of phosphorus revealed that the calcination process reduced the percentage of unsteady phosphorus, transforming into stable inert phosphorus fractions (Al-P, Ca-P and Res-P), respectively, thereby minimized its mobility and eutrophication risk. Interestingly, calcination temperatures of 700 °C and 800 °C resulted in smaller releasing potentials and equilibrium phosphorus concentrations, despite having lower adsorption capacities than 550 °C. Furthermore, the results of redox potential monitoring showed that the thermally treated Dianchi Lake sediments could enhance the redox potential and dissolved oxygen in the surface sediment, indicating the amelioration of interfacial environment. The practical monitoring experiments confirmed the capping depressed the DTP to 0.031 mg L^-1. The investigation of this study provided explicit evidence of Ca coupled P and aerobic Fe bound P strengthened the immobilization effects, and the development of sediment calcination demonstrates a promising strategy for alleviating the burden of endogenous pollution and improving aerobic environment, which are of great significance for lake ecological remediation.

Metagenomics reveals biogeochemical processes carried out by sediment microbial communities in a shallow eutrophic freshwater lake

Introduction

As the largest shallow freshwater lake in the North China Plain, Baiyangdian lake is essential for maintaining ecosystem functioning in this highly populated region. Sediments are considered to record the impacts of human activities.

Methods

The abundance, diversity and metabolic pathways of microbial communities in sediments were studied by metagenomic approach to reveal patterns and mechanism of C, N, P and S cycling under the threat of lake eutrophication.

Results

Many genera, with plural genes encoding key enzymes involved in genes, belonging to Proteobacteria and Actinobacteria which were the most main phylum in bacterial community of Baiyangdian sediment were involved in C, N, S, P cycling processes, such as Nocardioides (Actinobacteria), Thiobacillus, Nitrosomonas, Rhodoplanes and Sulfuricaulis (Proteobacteria).For instance, the abundance of Nocardioides were positively correlated to TN, EC, SOC and N/P ratio in pathways of phytase, regulation of phosphate starvation, dissimilatory sulfate reduction and oxidation, assimilatory sulfate reduction, assimilatory nitrate reduction and reductive tricarboxylic acid (rTCA) cycle. Many key genes in C, N, P, S cycling were closely related to the reductive citrate cycle. A complete while weaker sulfur cycle between SO₄ ^2- and HS^- might occur in Baiyangdian lake sediments compared to C fixation and N cycling. In addition, dissimilatory nitrate reduction to ammonia was determined to co-occur with denitrification. Methanogenesis was the main pathway of methane metabolism and the reductive citrate cycle was accounted for the highest proportion of C fixation processes. The abundance of pathways of assimilatory nitrate reduction, denitrification and dissimilatory nitrate reduction of nitrogen cycling in sediments with higher TN content was higher than those with lower TN content. Besides, Nocardioides with plural genes encoding key enzymes involved in nasAB and nirBD gene were involved in these pathways.

Discussion

Nocardioides involved in the processes of assimilatory nitrate reduction, denitrification and dissimilatory nitrate reduction of nitrogen cycling may have important effects on nitrogen transformation.

Nutrients and Environmental Factors Cross Wavelet Analysis of River Yi in East China: A Multi-Scale Approach

The accumulation of nutrients in rivers is a major cause of eutrophication, and the change in nutrient content is affected by a variety of factors. Taking the River Yi as an example, this study used wavelet analysis tools to examine the periodic changes in nutrients and environmental factors, as well as the relationship between nutrients and environmental factors. The results revealed that total phosphorus (TP), total nitrogen (TN), and ammonia nitrogen (NH4+-N) exhibit multiscale oscillation features, with the dominating periods of 16-17, 26, and 57-60 months. The continuous wavelet transform revealed periodic fluctuation laws on multiple scales between nutrients and several environmental factors. Wavelet transform coherence (WTC) was performed on nutrients and environmental factors, and the results showed that temperature and dissolved oxygen (DO) have a strong influence on nutrient concentration fluctuation. The WTC revealed a weak correlation between pH and TP. On a longer period, however, pH was positively correlated with TN. The flow was found to be positively correct with N and P, while N and P were found to be negatively correct with DO and electrical conductance (EC) at different scales. In most cases, TP was negatively correlated with 5-day biochemical oxygen demand (BOD₅) and permanganate index (COD_Mn). The correlation between TN and COD_Mn and BOD₅ was limited, and no clear dominant phase emerged. In a nutshell, wavelet analysis revealed that water temperature, pH, DO, flow, EC, COD_Mn, and BOD₅ had a pronounced influence on nutrient concentration in the River Yi at different time scales. In the case of the combination of environmental factors, pH and DO play the largest role in determining nutrient concentration.

Response mechanism of microorganisms to the inhibition of endogenous pollution release by calcium peroxide

To further explore the response mechanism of microorganisms to the synchronous control of nitrogen and phosphorus release from sediments by CaO₂, the spatiotemporal changes in the physical, chemical and biological indicators of the overlying water, interstitial water and sediments in each reactor were measured in the experiment. The experiment results showed that CaO₂ could increase the ammonia monooxygenase activity, nitrite oxidase activity and Nitrospira abundance in the sediment near its dosing position, and enhanced the activities of nitrate reductase and nitrite reductase at a certain distance from the dosing position, thereby promoting nitrogen removal in sediments through the alternating process of nitrification and denitrification. At the same time, the increase of alkaline phosphatase activity and Saccharimonadales abundance in the test groups accelerated the hydrolysis of organic phosphorus, and the P immobilization in sediments was realized through the subsequent precipitation reaction of Ca²⁺ and PO₄^3- under alkaline conditions. In addition, the enhanced activities of dehydrogenase and catalase ensured that CaO₂ would not cause great killing effect on microorganisms when improving the hypoxic conditions and inhibiting endogenous release. As a result, the dissolved product of CaO₂ such as Ca(OH)₂ and H₂O₂ reduced the nutrients concentration and killed the algae, which kept the algae density and chlorophyll a concentration at a low level throughout the test groups. Therefore, this study systematically clarified the microbial mechanism of CaO₂ synchronously controlling the release of nitrogen and phosphorus from sediments, which provided a new idea for the remediation of endogenous pollution in the water system.

Internal nitrogen and phosphorus loading in a seasonally stratified reservoir: Implications for eutrophication management of deep-water ecosystems

Water eutrophication is a serious global issue because of excess external and internal nutrient inputs. Understanding the intensity and contribution of internal nitrogen (N) and phosphorus (P) loading in deep-water ecosystems is of great significance for water body eutrophication management. In this study, we combined intact sediment core incubation, high-resolution peeper (HR-Peeper) sampling, and analysis of N and P forms and other environmental factors in the water column and sediments to evaluate the contributions of internal N and P loading to water eutrophication by N and P fluxes across the sediment-water interface (SWI) of the Panjiakou Reservoir (PJKR), a deep-water ecosystem where eutrophication threatens the security of the local drinking water supply in North China. The results indicated that the PJKR showed obvious thermal and dissolved oxygen (DO) stratification in the warm seasons and full mixing in the cold seasons. The mean DO concentration was 9.9 and 3.55 mg/L in the epilimnion and hypolimnion, respectively, in warm seasons and 10.7 mg/L in cold seasons. The sediment acted as a source of soluble reactive phosphorus (SRP), NH₄⁺-N, and NO₂^--N and a sink of NO₃^--N. The SRP fluxes were 5.28 ± 4.34 and 2.30 ± 1.93 mg m^-2·d^-1 in warm and cold seasons, respectively, and the dissolved inorganic nitrogen (DIN) fluxes were -0.66 ± 47.84 and 44.04 ± 84.05 mg m^-2·d^-1. Seasonal hypoxia accelerated the release of P rather than N from the sediments in warm seasons, which came mainly from Fe-P and Org-P under anoxic conditions. The strong negative NO₃^--N flux (diffusion from the water column to the sediment) implied an intensive denitrification process at the SWI, which can counteract the release flux of NH₄⁺-N and NO₂^--N, resulting in the sediment acting as a weak dissolved inorganic nitrogen (DIN) source for the overlying water. We also found that internal N loading accounted for only ∼9% of the total N loading, while internal P loading accounted for 43% of the total P loading of the reservoir. Our results highlight that efforts to manage the internal loading of deep-water ecosystems should focus on P and seasonal hypoxia.

Extracellular enzyme stoichiometry reveals carbon and nitrogen limitations closely linked to bacterial communities in China’s largest saline lake

Saline lakes possess substantial carbon storage and play essential roles in global carbon cycling. Benthic microorganisms mine and decompose sediment organic matter via extracellular enzymes to acquire limiting nutrients and thus meet their element budgets, which ultimately causes variations in sediment carbon storage. However, current knowledge about microbial nutrient limitation and the associated organic carbon changes especially in saline lake remains elusive. Therefore, we took Qinghai Lake, the largest saline lake of China, as an example to identify the patterns and drivers of microbial metabolic limitations quantified by the vector analyses of extracellular enzyme stoichiometry. Benthic microorganisms were dominantly colimited by carbon (C) and nitrogen (N). Such microbial C limitation was aggravated upon the increases in water salinity and sediment total phosphorus, which suggests that sediment C loss would be elevated when the lake water is concentrated (increasing salinity) and phosphorus becomes enriched under climate change and nutrient pollution, respectively. Microbial N limitation was predominantly intensified by water total nitrogen and inhibited by C limitation. Among the microbial drivers of extracellular enzyme investments, bacterial community structure consistently exerted significant effects on the C, N, and P cycles and microbial C and N limitations, while fungi only altered the P cycle through species richness. These findings advance our knowledge of microbial metabolic limitation in saline lakes, which will provide insights towards a better understanding of global sediment C storage dynamics under climate warming and intensified human activity.

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.

Drivers for primary producers' dynamics: New insights on annual benthos pelagos monitoring in anthropised freshwater marshes (Charente-Maritime, France)

Wetlands, especially marshes, support many services such as carbon catchment control or water purification led by primary producers such as phytoplankton and microphytobenthos (PB). The impact of the sedimentary compartment, as source and sink of essential nutrients for the water column, is often neglected in the study of their dynamics and water purification capacity of the systems. This work compared monthly (between February 2020 and April 2021) the benthic and pelagic primary producers' dynamics in two anthropised freshwater marshes (Marans and Genouillé), with the simultaneous follow-up of physico-chemical parameters of the water column and nutrient fluxes at the sediment-water (SWI) interface. It was suggested a strong contribution of phytoplankton (pumping) and the benthic compartment (denitrification) to the water purification of these two nitrates (NO₃^-)-rich marshes. Total phytoplankton production fluctuated between ∼5 (winter) and 1500 mg C m^-3 d^-1 (fall) at Marans and between 40 (winter) and ∼750 mg C m^-3 d^-1 (spring) at Genouillé. At Marans, soluble reactive phosphorus (SRP) benthic effluxes (-2.10¹ to -6.10² µmol m^-2 d^-1 in fall and summer, respectively) coincided with phytoplankton bloom periods. These effluxes were inhibited by NO₃^- penetration in the sediment (0 to 5.10⁴ µmol m^-2 d^-1), by inhibiting iron respiration. At Genouillé, inhibition of SRP effluxes depended on denitrification rate and on P stocks in the sediment, where slight SRP effluxes (-10¹ µmol m^-2 d^-1) could have co-occurred with slight NO₃^- influxes (5.10² µmol m^-2 d^-1) in spring. The presence of PB (between 10-60 and 40-120 mg g_sed^-1 at Marans and Genouillé, respectively), suggested a strong contribution of the benthic compartment to the total primary production (benthic and pelagic through resuspension processes) in these environments. This work encourages to consider the benthos and the pelagos as a unicum to provide better sustainable management of such systems and limit eutrophication risks in coastal areas.

Nitrogen and Phosphorous Retention in Tropical Eutrophic Reservoirs with Water Level Fluctuations: A Case Study Using Mass Balances on a Long-Term Series

Nitrogen and phosphorous loading drives eutrophication of aquatic systems. Lakes and reservoirs are often effective N and P sinks, but the variability of their biogeochemical dynamics is still poorly documented, particularly in tropical systems. To contribute to the extending of information on tropical reservoirs and to increase the insight on the factors affecting N and P cycling in aquatic ecosystems, we here report on a long-term N and P mass balance (2003–2018) in Valle de Bravo, Mexico, which showed that this tropical eutrophic reservoir lake acts as a net sink of N (−41.7 g N m−2 y−1) and P (−2.7 g P m−2 y−1), mainly occurring through net sedimentation, equivalent to 181% and 68% of their respective loading (23.0 g N m−2 y−1 and 4.2 g P m−2 y−1). The N mass balance also showed that the Valle de Bravo reservoir has a high net N atmospheric influx (31.6 g N m−2 y−1), which was 1.3 times the external load and likely dominated by N2 fixation. P flux was driven mainly by external load, while in the case of N, net fixation also contributed. During a period of high water level fluctuations, the net N atmospheric flux decreased by 50% compared to high level years. Our results outlining water regulation can be used as a useful management tool of water bodies, by decreasing anoxic conditions and net atmospheric fluxes, either through decreasing nitrogen fixation and/or promoting denitrification and other microbial processes that alleviate the N load. These findings also sustain the usefulness of long-term mass balances to assess biogeochemical dynamics and its variability.

Interactions of Vallisneria natans and Iron-Oxidizing Bacteria Enhance Iron-Bound Phosphorus Formation in Eutrophic Lake Sediments

Submerged macrophyte restoration and in situ phosphorus (P) passivation are effective methods for the control of internal P loading from sediments. This study explored the synergistic effects of Vallisneria natans and iron (Fe)-oxidizing bacteria (IOB) on internal P loading from eutrophic freshwater lake sediments by taking into account Fe-bound P (FeP) formation and associated bacterial community structures. Sediment samples were prepared in glass tanks under four treatments, namely no V. natans planting or IOB inoculation (control), planting V. natans without IOB inoculation (Va), planting V. natans with IOB inoculation (Va-IOB), and planting V. natans with autoclaved IOB inoculation (Va-IOB[A]). Compared with the control, all three treatments with V. natans (Va, Va-IOB, and Va-IOB[A]) had significantly decreased organic matter contents and increased redox potential in sediments (p < 0.05), at the rapid growth and mature stages of V. natans. Planting V. natans with and without IOB inoculation also decreased the total P (TP) and Fe–P concentrations in sediments. Conversely, Fe³⁺ concentrations, Fe³⁺/Fe²⁺ ratios, and the proportions of Fe–P in TP all increased in sediments planted with V. natans, especially under the Va-IOB treatment (p < 0.05). Furthermore, bacterial community diversity increased in sediments due to the presence of V. natans. The relative abundances of IOB (including Acidovorax and Chlorobium) increased from the transplanting to the rapid growth stage of V. natans and then decreased afterwards. In the later stages, the relative abundances of IOB and their ratios to Fe-reducing bacteria were the highest under the Va-IOB treatment. Accordingly, synergistic interactions between V. natans and IOB could enhance Fe–P formation and reduce TP concentrations in eutrophic lake sediments by altering sediment physicochemical properties and Fe oxidation-related bacterial community structures.

Responses of coastal sediment phosphorus release to elevated urea loading

Increased urea is one of the common nitrogen forms polluting coastal waters and affecting nutrient dynamics. To investigate the effects of urea on sediment phosphorus (P) release, we carried out a 2-month mesocosm experiment with six targeted loadings of urea (0-0.6 mg N L^-1 d^-1). Results showed that: i) urea was rapidly transformed into ammonium and then nitrate (NO₃^-). ii) When nitrogen occurred as urea or ammonium, minor P release was observed. iii) After urea were mostly converted to NO₃^-, P release became clearer. iv) NO₃^- had a dual effect by promoting P release through decreasing sediment pH and increasing alkaline phosphatase activity or by inhibiting P release through improving sediment oxidation. v) The overall effects of urea on P release depended on the ultimate NO₃^- concentrations, being prominent when NO₃^- ≥ 11 mg N L^-1. Our findings are of relevance when determining nitrogen reduction targets needed for combating eutrophication.

Spatial-temporal dynamics, ecological risk assessment, source identification and interactions with internal nutrients release of heavy metals in surface sediments from a large Chinese shallow lake

The surface sediment concentrations of heavy metals (Cu, Zn, Pb, Cd, Cr, Hg, and As), major metals (Fe and Mn), and the nutrient concentrations in the interstitial water of Lake Houguan, a large eutrophic shallow lake, were surveyed for three years. The results showed that Cu, Zn, and Fe were significantly higher in the east lake parts, and Cd in November was significantly higher than April. 19% of Hg and all of As were larger than the probable effect concentrations (PECs) according to the consensus-based sediment quality guidelines (SQGs), and the geo-accumulation index (I_geo) indicated As, Hg, and Cd were slightly polluted to severely polluted. The RI value (average 704.2) of the potential ecological risk index (PERI) suggested that heavy metals posed very high ecological risks with most of the contributions induced by Cd and Hg. The consequence of hierarchical clustering analysis (HCA) and principle component analysis (PCA) identified Cd, As, and Pb might originate from urbanization, industrial pollution, and agricultural activity; Hg might be from atmospheric deposition and anthropogenic sources above; Cu, Zn, Cr, Fe, and Mn might be from both natural and anthropogenic sources. The Spearman correlation analysis indicated Pb and As were significantly positively correlated with total nitrogen, while Cd significantly negatively correlated with sulfate; As was significantly correlated with ammonia, sulfate, and nitrate in the interstitial water. These results suggested eutrophication might affect sedimental heavy metals by increasing organic matter or influencing the redox potentials in the sediment.

Effects of consecutive culture of Penaeus vannamei on phosphorus transformation and microbial community in sediment

Phosphorus (P) is highly related to water quality during shrimp culture. Recognizing P transformation in pond-based cultures is crucial for sustainable and healthy aquaculture. However, P transformation remains unclear in the sediment of Penaeus vannamei cultures, although commercial species have been pervasive worldwide. To determine P transformation, samples with different culture years were collected from Zhejiang province, China. Sequential chemical extraction was applied to reveal the composition of inorganic P, while phosphatase activity was used to evaluate the biomineralization of organic P. The results indicated that the consecutive culture of Penaeus vannamei promoted the dissolution potential of sedimentary P. This was attributed to anoxic iron reduction that increased the formation of loosely bound P and Fe (II)-P. However, this phenomenon was dominated by biomineralization, which transformed the organic P to inorganic P. The results suggested that consecutive culture changed the microbial community structure in the sediment as well as the gene functions. The Shannon Wiener index showed that increasing the culture duration significantly decreased the stability of the microbial community. Overall, this study suggests that long-term consecutive culture of Penaeus vannamei may increase the P release potential of the sediment, which increases the risk of pond eutrophication.

Phosphorus supply pathways and mechanisms in shallow lakes with different regime

In order to better understand the pathways and mechanisms of phosphorus (P) supply under different regimes, 12 sampling sites from 4 basins of 2 lakes were studied seasonally from October 2017 to July 2018 in Wuhan City, China. Concentrations of different forms of P and nitrogen (N) in surface and interstitial water, contents of carbon (C), N, P and iron (Fe) compounds as well as related extracellular enzymatic activities, phosphorus sorption, abundance of phosphorus-solubilizing bacteria (PSB), total and specific (containing phosphatase gene) microbial community composition in sediments were analyzed. In lakes with macrophyte dominance, P supply pathway from sediment to water column was blocked. In lakes being early period of regime shifting from macrophyte to algae, exogenous P input was the main P supply mode. In lakes being later period of regime shifting from macrophyte to algae, organic P hydrolysis and calcium-bound P dissociation driven by PSB contributed greatly to P regeneration, which was continuous and trickling. In this process, rapid C and N cycles fueled P regeneration. In lakes with algal dominance, given the significantly higher iron-bound P (Fe(OOH)~P), equilibriums phosphorus concentration and dehydrogenase activity, the main P regeneration pathway might be the desorption of Fe(OOH)~P driven by anoxia, showing the seasonal and pulsed characteristics. In addition, during the process of regime shift from macrophyte to algae, the dominant algal species switched from cyanobacteria to Chlorophyta. P-solubilizing microorganisms correlated with environmental factors, suggesting the coupling of multiple nutrient cycles, especially C, N, P, oxygen (O) and Fe, could effectively increase the pathways diversification and the strength of P regeneration.

The feasibility of recycling drinking water treatment residue as suspended substrate for the removal of excess P and N from natural water

Eutrophication of natural water commonly involves the pollution of both P and N. Here, we developed a new application of drinking water treatment residuals (DWTRs) for suspensions that permits the simultaneous removal of excess P and N from natural water and demonstrates that DWTRs recycling can provide a means for eutrophication control. Based on 364-day continuous flow tests, the suspension application of DWTRs effectively adsorbed P from overlying water under various conditions, decreasing total P concentrations from 0.0739 ± 0.0462 to 0.0111 ± 0.0079-0.0149 ± 0.0106 mg L^-1, which achieved a class Ⅱ level of the China surface water quality standards during the tests. The total N concentrations were also reduced from 1.46 ± 0.63-1.52 ± 0.63 to 0.435 ± 0.185-0.495 ± 0.198 mg L^-1, which achieved a class Ⅲ level during the stable stage of the tests. N removal was closely related to doses of DWTRs and aeration intensities. Effective N removal was mediated by the enriched microbial communities in the suspended DWTRs with simple, stable, and resilient networks, including many taxa associated with the N cycle (e.g., Rhodoplanes, Brevibacillus, and Pseudomonas). Further analysis indicated that both effective P adsorption and functional microbial community construction were closely related to Fe and Al in DWTRs. Suspension application prevented the burial effect of solids sinking from overlying water, which aided the ability of DWTRs to control pollution, and is potentially applicable to other materials for natural water remediation.

The adsorption efficiency of nitrogen and phosphorus by in-situ remediation of modified sediment composite material

Dredged sediment can occupy a large amount of land area, resulting in waste of land resources, and high disposal costs. In response to the problem, this work calcinates and modified the sediment and compounds it with the modified water purification plant sludge, zeolite powder, and bentonite. This is used as a covering material to inhibit the release of nitrogen (N) and phosphorus (P) in the sediment. The results showed that sediment modified composite material covering effectively reduces the release of nitrogen (N) and phosphorus (P) in the sediment, especially the release of P. When the thickness of the covering layer is 3 cm, the reduction rate of total N, NH₄⁺-N, and total P in the overlying water by the modified composite material of sediment is 61.58, 79.59, and 70.34%, respectively. It can be seen that the covering material has a significant effect on the control of the release of N and P in the sediment. Additionally, the reduction of nutrients in the overlying water can overcome the negative effects of temperature rise in controlling the release of N and P in the sediment.

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.

Characteristics of Dissolved Organic Matter and Its Role in Lake Eutrophication at the Early Stage of Algal Blooms—A Case Study of Lake Taihu, China

Decaying algal blooms in eutrophic lakes can introduce organic matter into the water and change nutrient concentrations in the water column. The spatial distribution and composition characteristics, sources, and contribution to eutrophication of dissolved organic matter (DOM) in the overlying water of Lake Taihu, a typical eutrophic lake in China, were analyzed by ultraviolet–visible spectra and three-dimensional fluorescence excitation–emission matrix spectra combined with the statistical decomposition technique, parallel factor analysis. The concentration of DOM was represented by dissolved organic carbon (DOC), and DOC in overlying water of Lake Taihu was 2.86–11.83 mg/L. The colored DOM (CDOM) was characterized by an absorption coefficient at 280 nm (a280) and 350 nm (a350), which were 6.63–29.87 and 1.84–10.41 m−1, respectively. These values showed an increasing trend from southeast to northwest, and the high values were concentrated in the northwest and northern lake areas. The parallel factor analysis (PARAFAC) identified two protein-like (C1: tyrosine-like and C2: tryptophan-like) and one humic-like (C3: humic acid and fulvic acid) fluorescence components for fluorescent DOM (FDOM). The most dominant components were protein-like components (C1 + C2), whose fluorescence intensity contributed 87.55% ± 3.39% to the total fluorescence intensity (Ft) of FDOM (3.38 R.U.). The mean value of the fluorescence index (FI) and index of recent autochthonous contribution (BIX) of DOM was 1.77 and 0.92, and DOC, a280 and fluorescence intensities of FDOM components were all significantly and positively correlated with chl. a, indicating that DOM, CDOM, and FDOM were all mainly derived from algal activities and metabolites. The average humification index of the DOM was 0.66, which indicated a low humification degree. The protein-like DOM was correlated with DON and DOP, and might make great contributions to the continuous occurrence of algal blooms.

Effects of decabromodiphenyl ether on activity, abundance, and community composition of phosphorus mineralizing bacteria in eutrophic lake sediments

Polybrominated diphenyl ethers (PBDEs) are typical persistent organic pollutants (POPs) in the environment. However, little is known about their effects on phosphorus mineralizing bacteria (PMB) in eutrophic lake sediments, despite the critical role of PMB in phosphorus (P) biogeochemical cycling. In this study, we carried out a 60-day microcosm experiment to understand the effects of 2 and 20 mg kg^-1 dry weight decabromodiphenyl ether (BDE-209) on the activity, abundance, diversity, and community composition of PMB in the sediment of Taihu Lake, a typical eutrophic lake in China. The results showed that BDE-209 contamination, regardless of the contamination levels, significantly increased the orthophosphate concentration in overlying water and available phosphorus concentration in sediments on day 60. Such increases may be explained by the stimulatory effects of BDE-209 on alkaline phosphatase (ALP) activity and PMB abundance. Moreover, based on Miseq sequencing of the phoD gene encoding ALP, Actinobacteria was the dominant PMB phylum in all treatments, and BDE-209 significantly increased the diversity of PMB and altered their community composition. In particular, the relative abundances of some PMB genera such as Bradyrhizobium were increased significantly after 60 days of the High treatment. A co-occurrence network analysis further revealed that the high level of BDE-209 contamination strengthened the connectivity and interspecific co-operative relationships in the PMB community. These results will help us to understand the effects of POPs on P biogeochemical cycling in eutrophic lakes and the associated microbial mechanisms.

Impact of calcium peroxide dosage on the control of nutrients release from sediment in the anoxic landscape water

The anoxic and reductive aquatic environment is formed easily in summer due to the global warming, which may accelerate endogenous release. In this experiment, four different dosages of calcium peroxide (CaO₂) were adopted to study the control effects of nutrients release from the sediments in the simulated landscape waters. The results demonstrated that CaO₂ addition could effectively improve the physicochemical properties and microbial composition in sediments, and an obvious improvement was achieved with a larger dosage. It was observed that the surface sediments of experiment groups were oxidized to form a capping barrier between the sediment and overlying water, which might cut off the pollutant diffusion in sediment. Meanwhile, CaO₂ could decrease the nutrients concentration in water obviously, and the reduced effect was positively correlated with the CaO₂ dosage. Compared with the nutrients release fluxes in CK (105.89 mg-TN m^-2 day^-1, 106.48 mg-NH₄⁺-N m^-2 day^-1, 4.14 mg-TP m^-2 day^-1, and 4.30 mg-SRP m^-2 day^-1), the CaO₂ dosages of 0.12 and 0.18 kg m^-2 could entirely inhibit the nutrients release from sediment, and partially reduce the original pollutants in the overlying water. However, 0.18 kg m^-2 CaO₂ would cause a higher increase of pH value and NO₂^--N concentration, and bring potential risk to the aquatic ecosystem. Therefore, 0.12 kg-CaO₂ m^-2-sediment was selected as the optimal dosage by considering the control effect, economic cost, and potential risk comprehensively. In general, this study provided a quantitative usage method of CaO₂, which is convenient and effective to prevent or control the nutrients release from sediment caused by anoxic and reductive condition in summer.

Interactive effects of temperature and nutrients on the phytoplankton community in an urban river in China

Understanding the relative impact sizes of environmental factors and nutrients on the high annual variation of phytoplankton abundance in eutrophic rivers is important for aquatic ecosystem management efforts. In this study, we used phytoplankton dynamic datasets in the eutrophic Fenhe River to show the variations and drivers of phytoplankton abundance under complex, fluctuating environmental conditions during 2012-2017. The temporal and spatial variations of nutrients in the river depicted that the total phosphorus (TP) concentration was higher in the wet season and in downstream. There were increases in total nitrogen (TN) concentration in the normal season and in upstream. The structural equation model (SEM) showed that the phytoplankton abundance increased during the wet season despite the decrease in the TN:TP ratio and was reduced upstream due to the highest TN:TP ratio. Among the environmental variables, water temperature (WT) was an important predictor and positively correlated temporally and spatially to phytoplankton. The interaction of nutrients with the phytoplankton community at different temperature levels indicated that different phytoplankton groups have different nutrient requirements. We can conclude that enhances in temperature and TP concentration will significantly increase phytoplankton abundance and dominance of cyanobacteria and green algae in the future, whereas there was insignificant effect on diatoms. These data indicated that temperature and TP content were the important abiotic factors influencing the phytoplankton growth of the water body, which could provide a reference for the evaluation of environmental alterations in the future.

Coupling between Nitrification and Denitrification as well as Its Effect on Phosphorus Release in Sediments of Chinese Shallow Lakes

The coupling of nitrification and denitrification has attracted wide attention since it plays an important role in mitigating eutrophication in aquatic ecosystems. However, the underlying mechanism is largely unknown. In order to study the coupling relationship between nitrification and denitrification, as well as its effect on phosphorus release, nutrient levels, functional gene abundance and potential rates involved in nitrification and denitrification were analyzed in three shallow urban lakes with different nutrient status. Trophic level was found positively related to not only copy numbers of functional genes of nitrosomonas and denitrifiers, but also the potential nitrification and denitrification rates. In addition, the concentrations of different forms of phosphorus showed a positive correlation with the number of nitrosomonas and denitrifiers, as well as potential nitrification and denitrification rates. Furthermore, the number of functional genes of nitrosomonas exhibited positive linear correlations with functional genes and rate of denitrification. These facts suggested that an increase in phosphorus concentration might have promoted the coupling of nitrification and denitrification by increasing their functional genes. Strong nitrification–denitrification fueled the nitrogen removal from the system, and accelerated the phosphorus release due to the anaerobic state caused by organic matter decomposition and nitrification. Moreover, dissolved organic nitrogen was also released into the water column during this process, which was favorable for balancing the nitrogen and phosphorus ratio. In conclusion, the close coupling between nitrification and denitrification mediated by nitrifier denitrification had an important effect on the cycling mode of nitrogen and phosphorus.

Nutrient regeneration mediated by extracellular enzymes in water column and interstitial water through a microcosm experiment

In coastal lakes the role of microorganisms in driving nutrients regeneration at different water depths and in sediments is not yet fully understood. The dynamics of microbial (algal and bacterial) abundance and bacterial activities involved in organic matter transformation were measured, together with nutrient concentrations, through a microcosm experiment set up using the oligotrophic Faro lake as a study model over a total period of 15 days and with a four-day frequency. Water column at different depths (surface, middle and bottom) and interstitial water obtained by sediment centrifugation were used in appropriate ratios (mixed 1:1 with surface waters) to fill 21-Litre plastic aquaria in order to simulate processes occurring in natural conditions. At early experimental period, the sharp decrease of dissolved organic nutrients and the abundant production of leucine aminopeptidase (LAP) and alkaline phosphatase (AP) in correspondence with high phytoplankton abundance in bottom and interstitial water reflected the relevance of organic nutrients for inorganic nutrients regeneration and phytoplankton growth. Size fractionation of LAP and AP as well as the positive relationship between microbial compartments suggested that bacteria and phytoplankton worked in close reciprocal synergy, and coupling of nitrogen and phosphorus regeneration, especially in bottom and interstitial waters, was observed. At later experimental period, the change in bacterial community, especially the increase of filamentous shaped cells, together with a simultaneous increase of protozoan abundance indicated that nutrient replenishment made the microbial loop structure more competitive. In oligotrophic conditions, such as those in Faro lake, organic nutrient enrichment of bottom and interstitial waters was associated with changes in the bacterial community, with consequent stimulation of extracellular enzymes to support phytoplankton growth. Nutrient availability from microbial regeneration resulted in an increased complexity of the microbial loop structure, with bacteria and phytoplankton adopting specific strategies to respond to the changing environment.

Effect of calcium peroxide on the water quality and bacterium community of sediment in black-odor water

This study investigated how efficiently CaO₂ could treat black-odor landscape water caused by low dissolved oxygen (DO) in a field experiment of 600 m². The study demonstrated that CaO₂ could significantly elevate the DO concentration in waters and the oxidation-reduction potential (ORP) level in sediments (p = 0.003 and p = 0), which is conducive to improving the anoxic environment of landscape water. The concentrations of total chemical oxygen demand (TCOD) and S^2- in overlying and interstitial waters were considerably decreased. The average concentrations of TCOD in the overlying and interstitial waters of the test zone (TZ) were 52.98% and 66.05% of those of the control zone (CZ), and the average concentrations of S^2- in the overlying and interstitial waters of TZ were 29.63% and 39.79% of those of CZ. Meanwhile, CaO₂ could obviously reduce turbidity but increase the transparency in the overlying water. The mean value of turbidity in the overlying water of TZ was 39.46% of that of CZ, whereas the transparency in the overlying water of TZ was 2.07 times that of CZ. Furthermore, CaO₂ changed the microbial community structure in the sediments, where the relative abundance of anaerobic bacteria was decreased but that of the aerobic bacteria was increased with some functional bacteria. In summary, CaO₂ could significantly increase the DO and ORP in black-odor landscape water, obviously inhibit the release of pollutants from sediment, and increase the diversity of microbial strains. Consequently, the black-odor phenomenon of landscape water could be alleviated effectively by adding CaO₂.

Decomposition of cyanobacterial bloom contributes to the formation and distribution of iron-bound phosphorus (Fe-P): Insight for cycling mechanism of internal phosphorus loading

Lake eutrophication and the resulting cyanobacterial blooms have become a global water environment problem. These eutrophic lakes usually have relatively high internal phosphorus loading such as Fe-P to support the formation of cyanobacterial blooms. In order to reveal the mechanisms and processes of phosphorus cycling in lake sediments, in this study, Lake Chaohu was selected as the research area, and the effects of cyanobacterial bloom decomposition on the horizontal distribution pattern of Fe-P was studied by field investigation and laboratory simulations. According to the phosphorus fractions in the sediments, Lake Chaohu can be divided into three lake areas, and the Fe-P content in western Chaohu is the highest (908.6 ± 54.9 mg kg^-1). The contents and proportions of Fe-P were significantly positively correlated with cyanobacterial pigments in sediments, but they negatively correlated with undegraded chl-a, especially when the Fe-P content was <400 mg kg^-1. Based on these statistical analyses, we proposed a hypothesis that the settled cyanobacterial organic matters (COM) could promote the formation of Fe-P. This hypothesis was proved by the simulation experiments of adding COM to the oligotrophic lakeshore clay. The results suggested that the content and proportion of Fe-P in sediments were significantly increased by the COM addition, and also, they were significantly positively correlated with the decomposition of the COM. The formation processes of Fe-P were further confirmed by the analysis of Fourier transform infrared (FT-IR) spectra. Microbial community analysis suggested that the bacterial species including FeOB and genus Pseudomonas might play an important role in the formation of Fe-P. This study suggested that the settled COM could enhance the eutrophication of sediments through a positive feedback cycle. Therefore, it is necessary to carry out bloom removal and sediment dredging simultaneously, and only then the cyanobacterial bloom can be effectively controlled.

Effects and mechanisms of calcium peroxide on purification of severely eutrophic water

In consideration of severe eutrophication of scenic water caused by pollutants released from sediments in summer, calcium peroxide (CaO₂) was adopted as the slow release peroxide to improve the water quality in a field experiment of 600 m². The effect of CaO₂ on the overlying water, interstitial water, sediment, and sediment microorganisms of scenic water was studied. Results for two months indicated that the dissolved oxygen (DO) concentration of the overlying water in the test zone was 3.78 times that in the control zone; the oxidation-reduction potential (ORP) in the overlying water and sediment increased significantly (p = 0.002 and p = 0). Meanwhile, CaO₂ could effectively inhibit the release of nitrogen (N) and phosphorus (P) from the sediment and could obviously reduce the concentrations of N and P in the overlying water by enhancing the microbiological action. Moreover, the average concentrations of total nitrogen and total phosphorus in the overlying water of the test zone were 46.27% and 50.51% of those in control zone, respectively, and the concentrations of N and P in the interstitial water decreased during the entire experiment. In addition, CaO₂ decreased the relative abundance of anaerobic bacteria in the sediment, whereas it increased that of aerobic bacteria and promoted the appearance of the functional bacteria, such as Nitrospirae and Thermodesulfoba. In conclusion, CaO₂ can improve the DO and ORP in the eutrophic water effectively and change the microbial community in the sediment to a certain extent, thereby controlling the pollutants released from the sediment and reducing the N and P concentrations in the overlying water. Thus, CaO₂ can effectively realize the purification and restoration of the severely eutrophic scenic water.

High ammonium loading can increase alkaline phosphatase activity and promote sediment phosphorus release: A two-month mesocosm experiment

In aquatic ecosystems, ammonium is one of the dominant substances in the effluent discharge from wastewater treatment plants and its impact has been widely explored as it is thought, in its toxic form (NH₃), to cause stress on organisms. Little is, however, known about its potential effect on the release of phosphorus (P) from the sediment. In a two-month mesocosm (150 L) experiment, we tested if high loading of ammonium promotes sediment P release and investigated the dominant underlying mechanisms. A gradient of five target ammonium loading levels was used by adding NH₄Cl fertilizer: no addition/control (N0), 3 (N1), 5 (N2), 10 (N3), and 21 (N4) mg NH₄Cl L^-1 (NH₄Cl expressed as nitrogen). We found that: 1) significant sediment P release for N3 and N4 but minor release or retention for N0, N1, and N2 were detected both by the total phosphorus concentration (TP) in the overlying water and in situ measurements of diffusive gradients in thin-films (DGT) at the sediment-water interface; 2) overall, TP correlated significantly and positively with total nitrogen (TN) concentrations in the water. Further correlation and path analyses suggested that stimulated alkaline phosphatase activity (APA) was likely the dominant mechanisms behind the ammonium-induced sediment P release and decreased dissolved oxygen (DO) levels (an approximate reduction from 9.2 to 6.6 mg O2 L^-1) was likely a contributing factor, particularly in the beginning of the experiment.

Different pathways of nitrogen and phosphorus regeneration mediated by extracellular enzymes in temperate lakes under various trophic state

Several Italian and Chinese temperate lakes with soluble reactive phosphorus concentrations < 0.015 mg L^-1 were studied to estimate nitrogen and phosphorus regeneration mediated by microbial decomposition and possible different mechanisms driven by prevailing oligo- or eutrophic conditions. Leucine aminopeptidase (LAP), beta-glucosidase (GLU) and alkaline phosphatase (AP), algal, and bacterial biomass were related to trophic and environmental variables. In the eutrophic lakes, high algal and particulate organic carbon concentrations stimulated bacterial respiration (> 20 μg C L^-1 h^-1) and could favor the release of inorganic phosphorus. High extracellular enzyme activities and phosphorus solubilizing bacteria abundance in sediments accelerated nutrient regeneration. In these conditions, the positive GLU-AP relationship suggested the coupling of carbon and phosphorus regeneration; an efficient phosphorus regeneration and high nitrogen levels (up to 0.067 and 0.059 mg L^-1 NH₄ and NO₃ in Italy; 0.631 and 1.496 mg L^-1 NH₄ and NO₃ in China) led to chlorophyll a peaks of 14.9 and 258.4 μg L^-1 in Italy and China, respectively, and a typical algal composition. Conversely, in the oligo-mesotrophic lakes, very low nitrogen levels (in Italy, 0.001 and 0.005 mg L^-1 NH₄ and NO₃, respectively, versus 0.053 and 0.371 mg L^-1 in China) induced high LAP, while low phosphorus (33.6 and 46.3 μg L^-1 total P in Italy and China, respectively) led to high AP. In these lakes, nitrogen and phosphorus regeneration were coupled, as shown by positive LAP-AP relationship; however, the nutrient demand could not be completely met without the supply from sediments, due to low enzymatic activity and phosphorus solubilizing bacteria found in this compartment.

Investigation on the adsorption of phosphorus in all fractions from sediment by modified maifanite

Sediment phosphorus (P) removal is crucial for the control of eutrophication, and the in-situ adsorption is an essential technique. In this study, modified maifanite (MMF) prepared by acidification, alkalization, salinization, calcination and combined modifications, respectively, were first applied to treat sediment P. The morphology and microstructure of MMF samples were characterized by X-ray fluorescence (XRF), Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM) and Brunauer-Emmett-Teller (BET). Various adsorption parameters were tested, such as dosage of maifanite, time, operation pH and temperature. The adsorption mechanisms were also investigated and discussed. Results showed that CMMF-H2.5-400 (2.5 mol/L H₂SO₄ and calcined at 400 °C) exhibited the highest P adsorption capacity. Thus, it was selected as the in-situ adsorbent material to control the internal P loading. Under the optimal conditions of dynamic experiments, the adsorption rates of TP, IP, OP, Fe/Al-P and Ca-P by CMMF-H2.5-400 were 37.22%, 44.41%, 25.54%, 26.09% and 60.34%, respectively. The adsorption mechanisms analysis revealed that the adsorption of P onto CMMF-H2.5-400 mainly by ligand exchange. Results of this work indicated that the modification treatment could improve the adsorption capacity of maifanite, and CMMF-H2.5-400 could be further applied to eutrophication treatment.

Cause and effect of N/P ratio decline with eutrophication aggravation in shallow lakes

To explore the relationship and cause and effect between eutrophication and the nitrogen (N)/phosphorus (P) ratio, samples from 38 lakes in Wuhan City, China, with differing degrees of eutrophication, were collected for nutrient levels and extracellular enzyme activities (EEA) in the water column from July 2011 to November 2011. The phosphorus fraction, abundance and potential denitrification rate (PDR) as well as community composition of nirS-type denitrifier in sediments of five typical lakes were further analyzed. A higher trophic level index (TSI) corresponded to a lower N/P ratio, which can be attributed to a loss of N and an increase in P. Specifically, in more eutrophic lakes, the enrichment of total organic carbon and all forms of P in sediments could fuel PDR by shaping community composition and increasing the abundance of nirS-type denitrifier as evidenced by correlation and redundancy analysis, ultimately resulting in a loss of N. Meanwhile, iron-bound phosphorus release induced by anoxia and the hydrolysis of organic P accounted for the observed increase of P in the water column. The lower N/P ratio facilitated the production of leucine aminopeptidase, which was unexpectedly induced by high P but not by low N. Similarly, alkaline phosphatase was induced by high N but not by low P. These findings indicate a mutual coupling and interplay between N and P cycling and confirm our hypothesis that P accumulation accelerates N loss in the process of eutrophication.

Interactive effects of environmental factors on phytoplankton communities and benthic nutrient interactions in a shallow lake and adjoining rivers in China

Shallow lakes are vulnerable to eutrophication because of abundant phytoplankton and significant nutrient input from sediments. Previous studies have researched the effect of environmental factors on phytoplankton and phosphorus release from sediment. However, few studies have simultaneously evaluated the interactive effects of environmental factors on phytoplankton communities and the interactions among different sediment nutrients. This paper reports on a 2016 investigation that examined the phytoplankton community and physical and chemical factors in both the water column and sediments in a Chinese shallow lake and its adjoining rivers. Our results indicated that rivers with water gates and lake areas had greater Chlorophyll a concentrations (Chl a) than natural rivers with similar total phosphorus (TP) concentrations; this indicates the importance of residence time on phytoplankton biomass. Although temperature impacted Chl a less than nutrients, its effects were highly species-specific, modulating relationships between nutrients and the abundance of different phytoplankton taxa. The effects of nutrients changed based on phytoplankton biomass and community composition, suggesting that different phytoplankton taxa have different nutrient demands. We predict that increasing residence time, temperature, and nutrients will increase phytoplankton biomass and increase the future dominance of Chlorophyta and Cyanophyta. In the interstitial water, there were no significant seasonal differences in TP, total nitrogen, and soluble reactive silica concentrations. However, ammonia concentrations were higher in the spring and lower in other seasons; nitrate and sulfate were abundant when the ammonia concentration was low. The total iron level in sediments was significantly negatively related with TP at low ammonia and silica concentrations and at high nitrate and sulfate concentrations in the interstitial water. These results indicated that nutrients are closely coupled in the sediments, highlighting the importance of oxidation-reduction potentials on internal nutrient balance.

Effects of Planted Versus Naturally Growing Vallisneria natans on the Sediment Microbial Community in West Lake, China

Submerged macrophytes play an important role in aquatic ecosystems, which has led to an increase in studies on vegetation recovery in polluted lakes from which submerged macrophytes have disappeared. The comparison of microbial communities in sediment cloned with planted and naturally growing submerged macrophytes is an interesting but rarely studied topic. In this investigation, Maojiabu and Xilihu, two adjacent sublakes of West Lake (Hangzhou, China), were selected as aquatic areas with planted and naturally growing macrophytes, respectively. Sediment samples from sites with/without Vallisneria natans were collected from both sublakes. The results showed that sediment total nitrogen and organic matter were significantly lower in the plant-covered sites than that in the non-plant sites in Maojiabu. Additionally, the sediment microbial community characterized by 16S ribosomal RNA (rRNA) sequencing differed more significantly for Maojiabu than for Xilihu. The relative abundances of microbes involved in C, N, and S elemental cycling were significantly higher in the sediments with plants than in those without. Results from both fatty acid methyl ester analysis and 16S rRNA sequencing indicated that vegetation significantly influenced the sulfate-reducing bacteria (SRB). Thus, the gene copies and composition of SRB were explored further. The relative gene abundance of SRB was 66% higher with natural vegetation colonization but was not influenced by artificial colonization. An increase in dominant SRB members from the families Syntrophobacteraceae and Thermodesulfovibrionaceae contributed to the increase of total SRB. Thus, macrophyte planting influences sediment nutrient levels and microbial community more than natural growth does, whereas the latter is more beneficial to sediment SRB.

Nitrogen and phosphorus in sediments in China: A national-scale assessment and review

A national-scale investigation of total nitrogen (TN), total phosphorus (TP), total organic carbon (TOC), and pH in sediments was performed. The sediment samples investigated in this study were collected from 10 major basins in China (Songhua River Basin (SRB), Liao River Basin (LRB), Hai River Basin (HRB), Yellow River Basin (YRB), Huai River Basin (HuRB), Yangtze River Basin (YtRB), Southeastern River Basin (SeRB), Pearl River Basin (PRB), Southwestern River Basin (SwRB), and Northwestern River Basin (NwRB)). And then, a pollution assessment was performed by comparing the data with established sediment quality guidelines (SQGs) and organic nitrogen index values. Results demonstrated that the mean TN content in the sediments of the 10 basins was 1.070g/kg, while the mean TP content was 0.733g/kg. The TN contents displayed significantly positive correlations with the TP contents in the sediments of SRB, LRB, YtRB, SeRB, PRB, and NwRB. Moreover, the concentrations of TN in the sediments of nine basins (SRB, LRB, HRB, YRB, HuRB, YtRB, SeRB, PRB, and NwRB) and TP concentrations of four basins (LRB, YtRB, SeRB, and PRB) were possibly related to the TOC contents, and the distributions of TN concentrations in eight basins (SRB, LRB, HRB, YRB, YtRB, SeRB, PRB, and NwRB) as well as the TP concentrations in LRB might be affected by the pH of sediments. By comparing the data in our study with those obtained in other periods (1990-2013), we found that the TN contamination situation in HuRB and the TP contamination situation in PRB have potentially worsened over time, which deserves more attention. According to the results of SQGs and organic nitrogen index assessment, among the 10 basins, SeRB was the worst watershed polluted by N and HRB was the worst watershed polluted by P.