Internal Phosphorus Research Articles

Effective elimination of internal phosphorus pollution with MgFe-LDHs hybrid material in eutrophic water: The chemical forms, transformation mechanism, and dynamic relationships

The internal phosphorus (P) pollution is a major cause of water eutrophication. Although the traditional powders materials, with good performance, were difficult to recycle due to its shape, and the reaction mechanism between materials and internal P was rarely elucidated. In this work, a hydrotalcite composite material (MgFe-LDHs hybrid gel microspheres) was prepared via chemical crosslinking to eliminate internal P pollution both in overlying water and sediments effectively. Benefiting from the strong coordination between iron hydroxide on the MgFe-LDHs surface and internal P, the P adsorption capacity was up to 1256.38 mg/kg under natural conditions, the IP (NaOH-P and HCl-P) and OP were 78 % and 22 %, respectively. During 40-days remediation experiment, with the addition of MgFe-LDHs hybrid gel microspheres, whatever in the mixed or the capped, promoted a conversion from stable P to soluble active P of NaOH-P in sediments, and thus contributing to an easy adsorption of NaOH-P by MgFe-LDHs hybrid gel microspheres (The NaOH-P in sediments decreased by 16 % and 22 %, respectively). Meanwhile, the excessive stable HCl-P (only a minor portion) that could not be adsorbed by MgFe-LDHs remained in the form of Ca-P in sediments. Additionally, the MgFe-LDHs hybrid gel microspheres improved the microbial community structure, and the microorganism was involved in the elimination of internal P simultaneously. In summary, this study proposed a promising remediation approach to resolve the problem of internal P pollution in eutrophic water with high efficiency, cost-effective, and environmentally friendly.

A landscape limnology approach to assessing controls on soluble reactive phosphorus in sediment porewater and internal loading risk

Sediment nutrients can be mobilized to overlying water via internal loading, which can be important to aquatic productivity. Using data from 143 Canadian lakes, we show high (~2400-fold) variation of soluble reactive phosphorus (SRP) concentrations in surficial sediment porewater, with results suggesting internal phosphorus loading (IPL) is also likely to vary widely. Consistent with past work at smaller scales, we show that lake depth, pH, trophic status, and bulk sediment Al:P and Fe:P influence porewater SRP, and IPL. Median porewater SRP concentration in lakes with high Al:P (molar ratios >10) were 4.8-fold smaller than in lakes with lower Al:P. In lakes where bulk sedimentary Fe:P molar ratios were >10 porewater SRP was 3.9-fold lower than in lakes with lower Fe:P. High pH (>7.8), along with hyper-eutrophic lakes were associated with higher porewater SRP. Finally, shallow lakes (<4 m depth) had median porewater SRP concentration 6-fold higher than deep lakes (>9 m depth). Important regional differences emerged, linked to regional variation in pH, soils, lake depth and trophic status, and paralleling areas of poor water quality. For example, median porewater SRP in lakes from the Boreal Plains and Prairies ecozones (dominated by Chernozems/Mollisols) was 64-fold and 44-fold higher than in the Boreal Shield (dominated by Podzols/Spodosols) (respectively), although we note that IPL risk is likely important across many ecozones. Using national data, we found in-lake measurements (particularly pH, and salinity) showed strong capacity in predicting porewater SRP (explaining 60–72 % of the variance in the data). Importantly, watershed predictors showed good predictive power, explaining ~50 % of variance in porewater SRP using variables including soil types, and % agriculture. Porewater SRP can be predicted with reasonable accuracy using easily measured variables, as can estimates of internal phosphorus loading, suggesting that landscape limnology holds strong potential in helping to inform lake management by informing understanding of in-lake nutrient sources.

Phosphorus resource partitioning underpins diversity patterns and assembly processes of microbial communities in plateau karst lakes

Eutrophication triggered by internal phosphorus (P) poses a substantial threat to the biodiversity of organisms in freshwater ecosystems. However, little is known about the linkages between P resource partitioning and microbial succession, especially in karst sediments. Here, we studied the diversity patterns and assembly processes of bacterial and archaeal communities in sediment cores from two historically hyper-eutrophicated karst lakes, Hongfeng Lake and Aha Lake, and investigated the relative contribution of P fractions to them. Our null and neutral models consistently indicated that bacterial and archaeal community assembly was judged to be deterministic rather than stochastic. We found a monotonically decreasing pattern for bacterial Shannon diversity toward deep sediments in Aha Lake, but U- or hump-shaped patterns for archaea in Hongfeng and Aha Lakes. Intriguingly, the community dissimilarity Bray–Curtis of bacteria and archaea consistently increased with increasing depth distance, with slopes of 0.0080 and 0.0069 in Hongfeng Lake and 0.0078 and 0.0087 in Aha Lake, respectively. Such cross-taxon congruence was well-supported by equivalent ecological processes (i.e., environmental selection). For bacteria and archaea, Shannon diversity was primarily affected by the total P (TP) fractions such as the loosely adsorbed TP or calcium-bound TP and sediment TP. Their community composition was significantly (P < 0.05) affected by calcium-bound inorganic P (Pi), loosely adsorbed Pi and reductant-soluble Pi. Although sediment properties were important, bacterial and archaeal diversity or community composition were well-explained by the Pi fractions, with high direct or indirect effects. In particular, Pi fractions exhibited stronger effects on bacterial and archaeal characteristics than organic P fractions. Taken together, our study provides novel insights into the ecological importance of P resource partitioning to microbial succession, which has crucial implications for disentangling the biogeochemical processes of P cycling in aquatic ecosystems.

Interception of internal phosphorus release from sediments by lanthanum-modified shrimp shell biochar in two application modes

Lanthanum (La)-modified biochar has been widely investigated as a phosphate adsorbent material. However, its effectiveness and mechanism as an amendment material to manage sediment internal phosphorus (P) release under anoxic conditions is still unclear. Thus, a lanthanum carbonate-modified shrimp shell biochar (LBC-1) was synthesized for controlling sediment P release, and the efficacy, mechanism, and sediment microbial community impact of LBC-1 under two application modes (addition and capping) were investigated. The maximum adsorption capacity of LBC-1 for phosphate was 79.5 mg/g, superior to most La-modified biochar materials. The generation of LaPO4 and Ca5(PO4)3(OH) precipitates and the formation of inner-sphere complexes were the main adsorption mechanisms of LBC-1 for phosphate. Both the LBC-1 addition and capping treatments successfully blocked sediment P release under anoxic conditions, with the removal efficiencies of DGT-labile P in the overlying water being 97.7–99.7% and 97.5–99.5%, respectively. The conversion of the mobile P forms to the stable HCl-P form in sediments played an important role in the interception of internal P release by the LBC-1 addition treatment. Furthermore, the LBC-1 addition treatment also prevented internal P release by effectively adsorbing DGT-labile P in the interstitial water due to the excellent P adsorption capacity of LBC-1. However, the prevention of P release from sediments by the LBC-1 capping treatment was mainly attributed to the efficient adsorption of DGT-labile P by LBC-1 at the sediment/overlying water interface. The release of P driven by sulfate-reducing bacteria (SRB) was a key mechanism for the migration of P from sediment to interstitial water. The LBC-1 addition treatment reduced the relative abundance of SRB in the surface sediment and might inhibit the release of sediment P into the interstitial water, whereas the LBC-1 capping treatment had little effect on the relative abundance of SRB. In practice, the LBC-1 capping was easier to manipulate than the LBC-1 addition. These findings confirmed that sediment remediation using the LBC-1 capping treatment has great potential to manage internal P loading in eutrophic water bodies.

Suppression of phosphorus release from sediment by Vallisneria natans combined with iron- and zirconium-modified bentonites: Effectiveness, mechanism, and response of sedimentary microbial communities

The efficiency and mechanism in the reduction of internal phosphorus loading from sediment by Vallisneria natans combined with iron- and zirconium-modified bentonites (abbreviated as FeBT and ZrBT, respectively) were studied, and the responses of the microbial communities in sediments to Vallisneria natans combined with FeBT and ZrBT were also explored. ZrBT had stronger phosphate adsorption ability than FeBT. The greatest phosphate adsorption capacities for FeBT and ZrBT in the simultaneous presence of common ions can attain 1.60 and 4.29 mg P/g, respectively, and those for the loaded iron in FeBT and zirconium in ZrBT can attain 55.2 mg P/g Fe2O3 and 164 mg P/g ZrO2, respectively. The combined utilization of FeBT and Vallisneria natans and that of ZrBT and Vallisneria natans can effectively lower the releasing risk of phosphorus from sediment into the overlying water (OW), and the combined use of ZrBT and Vallisneria natans had a higher controlling efficiency than that of FeBT and Vallisneria natans. The passivation of diffusion gradient in thin film (DGT)-labile phosphorus in the sediment layer was vital to the control of phosphorus release from sediment by Vallisneria natans combined with FeBT and ZrBT. The bacterial communities in the sediments under the conditions of FeBT + Vallisneria natans and ZrBT + Vallisneria natans can exert well ecological functions. This work indicates that the combined utilization of ZrBT and Vallisneria natans is a better method to lower the releasing risk of phosphorus from sediment into OW than that of FeBT and Vallisneria natans.

Linking sediment geochemistry with catchment processes, internal phosphorus loading and lake water quality

Research in the field of sediment geochemistry suggests potential linkages between catchment processes (land use), internal phosphorus (P) loading and lake water quality, but evidence is still poorly quantified due to a limited amount of data. Here we address the issues based on a comprehensive data set from 27 lakes in southern Finland. Specifically, we aimed at: 1) elucidating factors behind spatial variations in sediment geochemistry; 2) assessing the impact of diagenetic transformation on sediment P regeneration across lakes based on the changes in the vertical distribution of sediment components; 3) exploring the role of the sediment P forms in internal P loading (IL), and 4) determining the impact of IL on lake water quality. The relationship between sediment P concentration and field area percentage (FA%) was statistically significant in (mainly eutrophic) lakes with catchments that included more than 10 % of fields. We found that sediment iron-bound P (Fe-P) increased with increasing FA%, which agrees with the high expected losses from the cultivated areas. Additionally, populated areas increased the pool of sediment Fe-P. Internal P loading was significantly positively related to both sediment Fe-P and sediment organic P (Org-P). However, Org-P was not significant (as the third predictor) in models that had a trophic state variable as the first predictor and Fe-P as the second predictor. Further, the vertical profiles of sediment components indicated a role of diagenetic transformations in the long-term sediment P release, especially in lakes with deeper maximum depth and longer water residence time. Finally, IL was significantly positively correlated to water quality variables including phytoplankton biomass, its proportion of cyanobacteria, chlorophyll a concentration and trophic state index. Our findings suggest that reduction of P losses from the field and populated areas will decrease internal P loads and increase water quality through a reduced pool of Fe-P.

Bottom sediments as an indicator of the restoration potential of lakes—a case study of a small, shallow lake under significant tourism pressure

The study covered a small, shallow lake, intensively used for recreation (sailing, tourist services and port infrastructure). This study aimed to determine the spatial differentiation of bottom sediments and the potential for phosphorus release in five zones, differing mainly in the type of recreation, depth, direct catchment management, shoreline management and macrophyte presence. The results were used to propose protective and restoration measures to improve the water quality of the studied lake. The innovation in the study was the detailed analysis of bottom sediments, which can be a significant source of pollution besides the external load from the catchment and tourist pressure, in the planned management of this ecosystem. Examination of the physicochemical properties of the bottom sediments showed a clear variation in both composition and potential for internal phosphorus loading. The sediments from the profundal zone, where the most boating activity was observed, together with the sediments from the shallow zone where the boats dock (mooring zone), had the highest potential to supply phosphorus to the bottom waters. This fact was demonstrated by the highest total phosphorus (TP) concentrations in sediments (up to 1.32 mgPg−1 DW) and the content of the most mobile fractions (up to 33%). The other zones associated with the marina, fuel zone, tributary and canal were not significant sources of phosphorus to the ecosystem. Based on the above results, a restoration method involving the removal of bottom sediments from the bottom zone was proposed, supported, of course, by protective measures in the catchment (maintaining a buffer zone around the lake and limiting the inflow of pollutants with tributary waters). The proposed measures with sustainable tourist pressure should improve water quality and thus contribute to protecting this valuable natural landscape.

Estimating internal phosphorus loading for a water quality model using chemical characterisation of sediment phosphorus and contrasting oxygen conditions

The Finnish Archipelago Sea (AS) has long been subject to intensive anthropogenic phosphorus (P) loading. The area suffers from seasonal hypoxia and cyanobacterial blooms despite reductions in nutrient discharge from the catchment and point sources. Internal loading may even dominate the P budget. Previous estimates of internal P loading have limitations (e.g., in spatial coverage and infrequent measurements). We present the first area-wide estimates of the magnitude of internal P loading based on the long-term release of P stored in the sediments.Modelling the internal P loading in the AS is challenging due to the complexity of biogeochemical processes in the sediment-water interface, as well as the heterogenic topography of the seafloor. Instead, we calculated estimates of internal P loading based on data from previous studies on sequential chemical extraction of sediment P, sediment physical characteristics (e.g., organic content, location of muddy seabed substrates), and near-bottom oxygen (O2) conditions. The estimates in three scenarios of contrasting O2 conditions were based on potentially mobile P pools in the sediments, recycled from sediment to water (i.e., loosely-bound or exchangeable P, P bound to reducible iron oxy(hydr)oxides, and labile organic P). The potentially mobile P pools were determined by chemical extraction methods (modified from Psenner et al., 1984 and Ruttenberg, 1992). The internal P loading under presumable O2 conditions was estimated to be fivefold that of waterborne P input to the AS; comparable to previous estimates for hypoxic areas in the Baltic Sea.Our estimates revealed wide spatial variability in the internal P loading, depending on O2 conditions and seabed sediment substrate. The site-specific P release estimates are included in a water quality model used by regional authorities, which increases the model's reliability for estimating the impact of human activities on the water quality across the AS.

Lake ecosystem tipping points and climate feedbacks

Abstract. Lakes and ponds experience anthropogenically forced changes that may be non-linear and sometimes initiate ecosystem feedbacks leading to tipping points beyond which impacts become hard to reverse. In many cases climate change is a key driver, sometimes in concert with other stressors. Lakes are also important players in the global climate by ventilating a large share of terrestrial carbon (C) back to the atmosphere as greenhouse gases and will likely provide substantial feedbacks to climate change. In this paper we address various major changes in lake ecosystems and discuss if tipping points can be identified, predicted, or prevented, as well as the drivers and feedbacks associated with climate change. We focus on potential large-scale effects with regional or widespread impacts, such as eutrophication-driven anoxia and internal phosphorus (P) loading, increased loading of organic matter from terrestrial to lake ecosystems (lake “browning”), lake formation or disappearance in response to cryosphere shifts or changes in precipitation to evaporation ratios, switching from nitrogen to phosphorus limitation, salinization, and the spread of invasive species where threshold-type shifts occur. We identify systems and drivers that could lead to self-sustaining feedbacks, abrupt changes, and some degree of resilience, as opposed to binary states not subject to self-propelling changes or resilience. Changes driven by warming, browning, and eutrophication can cause increased lake stratification, heterotrophy (browning), and phytoplankton or macrophyte mass (eutrophication), which separately or collectively drive benthic oxygen depletion and internal phosphorus loading and in turn increase greenhouse gas (GHG) emissions. Several of these processes can feature potential tipping point thresholds, which further warming will likely make easier to surpass. We argue that the full importance of the vulnerability of lakes to climate and other anthropogenic impacts, as well as their feedback to climate, is not yet fully acknowledged, so there is a need both for science and communication in this regard.

Control of phosphorus release from sediment by iron/aluminum co-modified zeolite: efficiency, mechanism, and response of microbial communities in sediment.

The efficiency of iron/aluminum co-modified zeolite (FeAl-Z) covering and amendment for controlling the internal loading of phosphorus (P) from sediment to the overlying water (OW) and its controlling mechanism were explored. The response of the composition of sedimentary microbial communities in sediment and their function to the FeAl-Z capping and amendment was also examined. FeAl-Z showed good removal performance for phosphate in aqueous solution. The maximum phosphate adsorption quantity for FeAl-Z at pH 7 attained 11.2mg P/g. The release of sediment endogenous phosphorus to OW can be successfully restrained by the FeAl-Z covering and amendment, and the suppression ability of FeAl-Z covering was stronger than that of FeAl-Z amendment. Under the capping or amendment condition, FeAl-Z can effectively inactivate the labile phosphorus measured by diffusion gradient in thin film (DGT-LP) in the overlying water and surface sediment. The added FeAl-Z transformed redox-sensitive phosphorus (BD-P) to metal oxide-bound phosphorus (NaOH-IP) and residual phosphorus (Res-P) in sediment, which increased the stability of inorganic phosphorus in the sediment. The passivation of soluble reactive phosphorus (SRP) and DGT-LP in the surface sediment by FeAl-Z significantly contributed to the inhibition of sediment endogenous phosphorus release to OW by the FeAl-Z capping, and the passivation of SRP, DGT-LP and mobile phosphorus in the surface sediment played a pivotal role in the control of sediment internal phosphorus release by the FeAl-Z amendment. The FeAl-Z amendment and capping did not increase the liberation risk of Fe from sediment, and the microorganisms in the sediments under the conditions of FeAl-Z amendment and covering still can perform good ecological functions. Results of this research demonstrate that FeAl-Z capping has high application potential in the control of phosphorus transfer from sediment to OW.

Phosphorus release from newly inundated soils and variation in benthic algal nutrient limitation induced by rising water levels of Qinghai Lake, China.

The mobilization of internal phosphorus (P) plays a crucial role in transitioning nutrient limitations within lake ecosystems. While previous research has extensively examined P release in littoral zones influenced by fluctuating water levels, there is a paucity of studies addressing the implications of sustained water level rise in this context, particularly as it pertains to nutrient limitations in benthic algae. To address this gap, we conducted an integrated study in Qinghai Lake. In the field sampling and microcosm experiment, we found that P concentrations are elevated in areas subjected to short-term inundation compared to those enduring prolonged inundation, primarily due to the dissolution of sedimentary P fractions. The results of nutrient diffusing substrata (NDS) bioassays indicated that benthic algae in Qinghai Lake displayed either P limitation or NP co-limitation. The transition from P limitation to NP co-limitation suggested that internal P release may serve to ameliorate nutrient limitations in benthic algae. This phenomenon could potentially contribute to the proliferation of Cladophora in the littoral zones of Qinghai Lake, thereby posing long-term implications for the lake's aquatic ecosystem, particularly under conditions of sustained water level rise.

Historical Phosphorus Mass and Concentrations in Utah Lake: A Case Study with Implications for Nutrient Load Management in a Sorption-Dominated Shallow Lake

Utah Lake is unusual due to its large surface area, shallow depth, phosphorus-rich sediments, and well-mixed, unstratified waters. This creates conditions where water column phosphorous concentrations tend toward equilibrium, with lake sediments containing high concentrations of geologic phosphorus. To help understand the potential impact of phosphorous load reductions, we computed a time history of phosphorus mass in the lake using state and federal records of lake volume, dissolved phosphorus concentrations, and outflow. We show that historically, Utah Lake phosphorus concentrations have remained stable over time, in the range of 0.02 to 0.04 mg/L, despite large changes in lake volume and internal phosphorus mass. We performed sorption calculations using data from the literature, demonstrating that it would take unrealistically large load changes to alter water column phosphorus concentrations under sorption processes. The sorption model produces results consistent with historical data that show relatively constant phosphorous concentrations despite large lake volume changes. We show, through several lines of evidence, that water column phosphorus concentrations are insensitive to external loads. Phosphorous load reduction is unlikely to have a significant effect on phosphorus concentrations in Utah Lake and, by extension, in other sorption-dominated shallow lakes with phosphorus-rich sediment.

Internal phosphorus loading alters nutrient limitation and contributes to cyanobacterial blooms in a polymictic lake

Clear Lake, a medium-sized hypereutrophic, polymictic lake in northern California, has had recurring harmful cyanobacteria blooms (HCBs) for over a century despite reductions in external phosphorus (P) loadings. Internal P loading can alter nutrient availability and limitation supporting HCBs but is rarely quantified or compared with external loads. We have quantified external P loads from 2019 to 2021 for the three main tributaries (accounting for 46% of the flow) and internal loadings using two methods: a P mass balance and modeled release rates of soluble reactive phosphorus from oxic and anoxic sediments. In addition, we combined high-frequency in situ measurements of water temperature and dissolved oxygen, discrete grab sampling for nutrient chemistry, and remote sensing to explore the potential drivers of the observed variability and provide a comprehensive view of the spatiotemporal dynamics of HCBs. By understanding the relative contribution of external and internal nutrient loadings and the relationship between environmental parameters and HCBs, interannual bloom variability can be better predicted. Comparative estimates of external and internal phosphorus loading indicate that internal sources accounted for 70–95% of the total P input into the system during the study period. Contrary to other lakes, the intensity of the summer bloom season was correlated to the timing and duration of anoxia rather than the magnitude of spring runoff. Internally released P shifted the system from phosphorus to nitrogen limitation during the summer, potentially favoring the proliferation of nitrogen-fixing cyanobacteria.

Recovery of lakes from eutrophication: changes in nitrogen retention capacity and the role of nitrogen legacy in 10 Danish lakes studied over 30 years

AbstractUsing data on 23 Danish lakes, we conducted mass balances to develop total nitrogen (TN) models for predicting annual mean TN in lakes based on external TN loading and found high predictability when including lake hydraulic retention time and mean depth in the model. We further used a unique 30-year mass balance data series from 10 Danish lakes with contrasting mean depths and hydraulic retention times to elucidate the effect of external TN loading reduction and N legacy on lake TN. We found that the TN retention percentage during the 30 years was generally not sensitive to an often major reduction in the external TN loading; it overall followed the pattern of the above model predictions, suggesting a low TN legacy effect. Moreover, the TN retention percentage was not affected by changes in TP. Our results, therefore, show a fast response to TN loading reduction, indicating that we can expect an immediate effect on lake water quality in shallow lakes suffering from internal phosphorus loading during re-oligotrophication provided that inorganic N is low enough to become a growth-limiting nutrient.

Internal phosphorus loading in a chain of eutrophic hardwater lakes in Saskatchewan, Canada

Sediments can act as a source or sink of phosphorus (P) for the water column of lakes. In iron (Fe)-rich softwater lakes, redox processes are important contributors to sediment P flux. However, the contribution of redox processes to P flux in hardwater lakes, with high pH and high concentrations of redox-insensitive calcium (Ca) is unknown. Intact sediment cores, collected in different seasons (summer or fall) from a chain of eutrophic hardwater lakes in southeastern Saskatchewan, Canada, were used to quantify sediment P fluxes in laboratory incubations under hypoxic or oxic conditions at temperatures consistent with the season of sample collection. Geochemical analyses determined concentrations of sediment total (TP) and organic P (TPo), organic matter (OM), total Ca and magnesium, and total and extractable manganese, Fe and aluminum. Sediment P pools were determined using sequential fractionation and solution 31P nuclear magnetic resonance spectroscopy. Sediment P fluxes were significantly higher in sediment cores incubated under hypoxic conditions (−24.4 to 28.5 mg P m−2 d−1) than oxic conditions (−60.3 to 14.2 mg P m−2 d−1). There were significant seasonal and lake differences for TP, TPo and cation concentrations, with Ca the dominant cation in all but one lake. Phosphate bound in the redox-sensitive pool was the only sediment P fraction that significantly differed among the lakes (0.10 to 0.18 mg P g−1 d.w.; 9 to 16 % of TP), with an inverse relationship to sediment P flux. Principal component analysis suggests that high concentrations of internally-generated TPo forms and OM in surface sediments play a key role in internal P loading in these lakes. However, sediment Ca appears to have an overriding effect on sediment P, partially masking the impact of redox control on internal P loading in these hardwater prairie lakes.

Effect of Combined Application of Lanthanum-Based Capping Material and Biochemical Oxidant on Control of Internal Phosphorus

In situ capping and biochemical oxidation are two of the most commonly used methods for internal phosphorus (P) control, but there are few studies on the combined use of these two methods. In this study, two lanthanum (La)-based materials, La-modified bentonite (LMB) and the La-modified attapulgite and chitosan composite (LMA&amp;C), were combined with calcium nitrate (CN) to investigate the effect on sediment P control. Results showed that SRP removal rates by LMB+CN and LMA&amp;C+CN were much higher than that by LMB, LMA&amp;C and CN alone. LMB+CN and LMA&amp;C+CN decreased DGT-labile P flux both in overlying water and surface 65 mm sediment, while LMB and LMA&amp;C had an effect only on overlying water and surface 10~40 mm sediment, and CN was effective only on sediments. LMB and LMA&amp;C transformed NH4Cl-P, BD-P, NaOH-SRP and NaOH-NRP in surface 20 mm sediment to HCl-P and Residual-P on day 60. CN transformed NaOH-SRP and NaOH-NRP to BD-P, resulting in the increase in NH4Cl-P. LMB+CN and LMA&amp;C+CN sequestered P in the surface 20 mm mainly as HCl-P and Residual-P, and mainly as BD-P in −20~−60 mm. Results indicate that the combination of capping by La-based material and oxidation by CN is a promising method for sediment P control.

The combined effects of lanthanum-modified bentonite and Vallisneria spiralis on phosphorus, dissolved organic matter, and heavy metal(loid)s

The use of lanthanum-modified bentonite (LMB) combined with Vallisneria spiralis (V∙s) (LMB + V∙s) is a common method for controlling internal phosphorus (P) release from sediments. However, the behaviors of iron (Fe) and manganese (Mn) under LMB + V∙s treatments, as well as the associated coupling effect on P, dissolved organic matter (DOM), and heavy metal(loid)s (HMs), require further investigations. Therefore, we used in this study a microelectrode system and high-resolution dialysis technology (HR-Peeper) to study the combined effects of LMB and V∙s on P, DOM, and HMs through a 66-day incubation experiment. The LMB + V∙s treatment increased the sediment DO concentration, promoting in-situ formations of Fe (III)/Mn (IV) oxyhydroxides, which, in turn, adsorbed P, soluble tungsten (W), DOM, and HMs. The increase in the concentrations of HCl-P, amorphous and poorly crystalline (oxyhydr) oxides-bound W, and oxidizable HMs forms demonstrated the capacity of the LMB + V∙s treatment to transform mobile P, W, and other HMs forms into more stable forms. The significant positive correlations between SRP, soluble W, UV254, and soluble Fe (II)/Mn, and the increased concentrations of the oxidizable HMs forms suggested the crucial role of the Fe/Mn redox in controlling the release of SRP, DOM, and HMs from sediments. The LMB + V∙s treatment resulted in SRP, W, and DOM removal rates of 74.49, 78.58, and 54.78 %, which were higher than those observed in the control group (without LMB and V∙s applications). On the other hand, the single and combined uses of LMB and V·s influenced the relative abundances of the sediment microbial communities without exhibiting effects on microbial diversity. This study demonstrated the key role of combined LMB and V∙s applications in controlling the release of P, W, DOM, and HMs in eutrophic lakes.

Inhibitory effects and mechanisms of insoluble humic acids on internal phosphorus release from the sediments

Release of phosphorus (P) from the sediments plays a critical role in the eutrophication of aquatic environments. Humic acids (HA), as the main form of carbon storage in the sediments, has essential impacts on the biogeochemical cycle of phosphorus in aquatic systems. Nevertheless, previous studies mainly concentrated on the competitive adsorption of HA solution and P on metal oxides and soils, with little attention paid to the effects of insoluble humic acids (IHA) on P sorption by and release from the sediments. Herein, an investigation on the rivers and lakes in Sichuan Province, China, found that there was a significantly positive correlation between the maximum P adsorption capacity (Qmax) of sediments and IHA contents (p < 0.01), but a significantly negative correlation between the zero equilibrium P concentration (EPC0) and IHA concentrations (p < 0.01). This indicated that IHA might have an inhibitory effect on the release of P from the sediments, which was verified by batch adsorption experiments and static incubation experiments. Adsorption experiments indicated that IHA can promote P adsorption by sediments. With the increase of IHA addition (from 0 to 20 mg/g) in the sediments, Qmax of sediments increased (from 0.516 to 0.911 mg/g), while EPC0 decreased greatly (from 0.264 to 0.005 mg/L). Increases in Fe (Ⅲ) bound-P, Al bound-P and humic bound-P caused by IHA were responsible for this promoting effect. Incubation experiments illustrated that IHA addition can efficiently inhibit P release from the sediments. After 32 days incubation, P concentration in the overlying water of control group (without IHA addition) was 0.651 mg/L, which was 13.29–40.69 times higher than those (0.016–0.049 mg/L) in the test groups (with 5 %-20 % IHA addition). The analysis of P species in sediments showed that transformation from loosely adsorbed-P and Fe (Ⅲ) bound-P to Al bound-P and humic bound-P was responsible for this inhibition of P release by IHA. This study demonstrated that IHA, differing from readily degradable or dissolved organic matter, have great inhibitory effects on internal P release, which provided a novel insight into the association between carbon burial and internal P release and even the management of water eutrophication.

How a reservoir modulates downstream water quality under declining upstream loading and progressing climate change

Reservoirs regulate water flow and pollutant transport in catchments. However, climate change can significantly impact their ability to perform this function. This study analysed a multi-decadal time series of data to examine the complex relationship between climate and nutrient pollution trends in the Möhne reservoir catchment. The study aimed at understanding the effect of the reservoir on downstream nutrient pollution in the face of a changing climate. The analysis revealed that upstream nutrient concentrations were higher than downstream, indicating a general nutrient-trapping effect of the reservoir. Upstream stations exhibited a declining trend in total nitrogen (TN) and total phosphorus (TP) concentrations. This was due to improved wastewater management and reduced nutrient mobilisation resulting from decreasing surface runoff and streamflow. At the downstream station, whereas TN concentrations decreased, TP concentrations mildly increased. These opposite downstream trends were likely due to rising temperatures and declining dissolved oxygen concentration within the reservoir, which might have favoured nitrogen denitrification and internal phosphorus loading, causing the decline and increase in downstream TN and TP concentrations, respectively. The contrasting downstream TN and TP trends alter the nutrient stoichiometry, which can profoundly affect the ecosystem's biogeochemical functioning. Therefore, in a warming climate, reservoirs may modulate nitrogen and phosphorus nutrients differently, leading to ecological discontinuities along river networks due to changes in TN-to-TP ratios. The study highlights the need to develop adaptable and precise nutrient pollution management strategies in reservoir catchments to address the challenges of climate change effectively.

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.