Effects of dissolved oxygen and nutrient loading on phosphorus fluxes at the sediment-water interface in the Hai River Estuary, China

Seasonal variation regulate the endogenous phosphorus release in sediments of Shijiuhu Lake via water-level fluctuation

Freshwater lakes undergo substantial alterations of the phosphorus (P) cycle in the water-sediment ecosystem due to thermal change. The impact process of seasonal fluctuation on P cycling in sediments has been scarcely investigated. P forms in sediments from a freshwater lake in China were analyzed using sequential extraction technique. The vertical distribution of soluble reactive P (SRP), Fe2+, and S2- in the interstitial water was measured using diffusion gradient technique (DGT). Fick's Law and DIFS model were used to obtain the diffusion fluxes of SRP and the kinetic parameters in the water-sediment system. The results showed that total P (TP) concentrations in the solid sediments varied from 207.5, 266.6 and 130.3 mg/kg to 614.7, 1053.1, and 687.6 mg/kg in winter, spring, and summer, respectively. The concentrations of individual P forms in spring were higher than those in other seasons, with Fe-bound P (Fe-P) concentration being the highest across all seasons. Notably, significant variations of SRP concentrations were found in the interstitial water between sedimentary depths of approximately 2 cm and 6 cm, particularly in the summer. Furthermore, higher diffusion fluxes of SRP through the interface were found in summer. A stable anaerobic environment failed to develop in spring with high water level, preventing the desorption of solid Fe-P and diffusion of Fe2+ into the water due to the afflux and deposition of P-containing particulate into deeper sediment layers along with organic material. Under extreme high-temperature in summer, decreased rainfall and rising temperatures boosted the activity of aquatic organisms in the water, thereby reducing P fixation by sediments and leading to P release. This process increased the risk of P excess and potential eutrophication in the water. Generally, clarifying the resupplying processes of endogenous P in sediment systems experiencing seasonal variations is critical for eutrophication management of lakes.

Water quality assessment and pollution source apportionment using multivariate statistical and PMF receptor modeling techniques in a sub-watershed of the upper Yangtze River, Southwest China

Rapid industrial and agricultural development as well as urbanization affect the water environment significantly, especially in sub-watersheds where the contaminants/constituents present in the pollution sources are complex, and the flow is unstable. Water quality assessment and quantitative identification of pollution sources are the primary prerequisites for improving water management and quality. In this work, 168 water samples were collected from seven stations throughout 2018-2019 along the Laixi River, a vital pollution control unit in the upper reaches of the Yangtze River. Multivariate statistics and positive matrix factorization (PMF) receptor modeling techniques were used to evaluate the characteristics of the river-water quality and reveal the pollution sources. Principal component analysis was employed to screen the crucial parameters and establish an optimized water quality assessment procedure to reduce the analysis cost and improve the assessment efficiency. Cluster analysis further illustrates the spatiotemporal distribution characteristics of river-water quality. Results indicated that high-pollution areas are concentrated in the tributaries, and the high-pollution periods are the spring and winter, which verifies the reliability of the evaluation system. The PMF model identified five and six potential pollution sources in the cold and warm seasons, respectively. Among them, pollution from agricultural activities and domestic wastewater shows the highest contributions (33.2% and 30.3%, respectively) during the cold and warm seasons, respectively. The study can provide theoretical support for pollutant control and water quality improvement in the sub-watershed, avoiding the ecological and health risks caused by the deterioration of water quality.

New insights into pollution source analysis using receptor models in the upper Yangtze river basin: Effects of land use on source identification and apportionment

To effectively control pollution and improve water quality, it is essential to accurately analyze the potential pollution sources in rivers. The study proposes a hypothesis that land use can influence the identification and apportionment of pollution sources and tested it in two areas with different types of water pollution and land use. The redundancy analysis (RDA) results showed that the response mechanisms of water quality to land use differed among regions. In both regions, the results indicated that the water quality response relationship to land use provided important objective evidence for pollution source identification, and the RDA tool optimized the procedure of source analysis for receptor models. Positive matrix decomposition (PMF) and absolute principal component score-multiple linear regression (APCS-MLR) receptor models identified five and four pollution sources along with their corresponding characteristic parameters. PMF attributed agricultural nonpoint sources (23.8%) and domestic wastewater (32.7%) as the major sources in regions 1 and 2, respectively, while APCS-MLR identified mixed sources in both regions. In terms of model performance parameters, PMF demonstrated better-fit coefficients (R²) than APCS-MLR and had a lower error rate and proportion of unidentified sources. The results show that considering the effect of land use in the source analysis can overcome the subjectivity of the receptor model and improve the accuracy of pollution source identification and apportionment. The results of the study can help managers clarify the priorities of pollution prevention and control, and provide a new methodology for water environment management in similar watersheds.

Long-term remote observations of particulate organic phosphorus concentration in eutrophic Lake Taihu based on a novel algorithm

Monitoring the long-term spatiotemporal variations in particulate organic phosphorus concentration (C_POP) is imperative for clarifying the phosphorus cycle and its biogeochemical behavior in waters. However, little attention has been devoted to this owing to a lack of suitable bio-optical algorithms that allow the application of remote sensing data. In this study, based on Moderate Resolution Imaging Spectroradiometer (MODIS) data, a novel absorption-based algorithm of C_POP was developed for eutrophic Lake Taihu, China. The algorithm yielded a promising performance with a mean absolute percentage error of 27.75% and root mean square error of 21.09 μg/L. The long-term MODIS-derived C_POP demonstrated an overall increasing pattern over the past 19 years (2003-2021) and a significant temporal heterogeneity in Lake Taihu, with higher value in summer (82.06 ± 3.81 μg/L) and autumn (78.74 ± 3.8 μg/L), and lower C_POP in spring (79.52 ± 3.81 μg/L) and winter (81.97 ± 3.8 μg/L). Spatially, relatively higher C_POP was observed in the Zhushan Bay (85.87 ± 7.5 μg/L), whereas the lower value was observed in the Xukou Bay (78.95 ± 3.48 μg/L). In addition, significant correlations (r > 0.6, P < 0.05) were observed between C_POP and air temperature, chlorophyll-a concentration and cyanobacterial blooms areas, demonstrating that C_POP was greatly influenced by air temperature and algal metabolism. This study provides the first record of the spatial-temporal characteristics of C_POP in Lake Taihu over the past 19 years, and the C_POP results and regulatory factors analyses could provide valuable insights for aquatic ecosystem conservation.

Insights into the role of the microbial community lifestyle strategies in variations of the dissolved organic matter molecular composition along an effluent-dominated river

Urbanization has dramatically changed the quality and quantity of dissolved organic matter (DOM) fluxes in rivers, thereby affecting the diversity and lifestyle strategies of microbial communities. However, relationships between DOM molecular composition and microbial lifestyle strategies in effluent-dominated rivers are poorly understood. Herein, we investigated the variations in DOM structure and composition of an effluent-dominated river and further revealed how these changes alter the abundance and lifestyle strategies of microbial communities. Results demonstrated that macromolecular (MW > 35 kDa) humic-like substances constituted the major components of effluent-dominated riverine DOM. Also, due to the degradation of humic-like substances, the accumulation of protein-like substances was observed from upstream to downstream areas, corresponding to an apparent decrease in overall aromaticity. The abundance of bacterial, Actinobacteria, and eukaryotic was higher in the upstream and midstream areas but relatively lower in the downstream area. The response of bacterial and Actinobacteria communities to the changes in DOM composition was more prominent as compared to that of eukaryotic. Based on multivariate statistical analysis, the decrease in aromatic components (MW > 35 kDa) was mainly attributed to the degradation of Proteobacteria and Actinobacteria (K-strategists), resulting in a decrease in their relative abundance along the river course. Proteins and polysaccharides (15 kDa < MW < 35 kDa, MW < 6 kDa) were more easily utilized by Firmicutes and Bacteroidetes (r-strategists), leading to an increase in their relative abundance. With the decrease of macromolecular humic-like substances and the increase of protein-like substances, river microbial communities shifted from K-strategists to r-strategists. This work unveils the evolution of DOM in an effluent-dominated river and the influence of the degradation of macromolecular humic-like substances on r/K-strategists.

Physicochemical, structural analysis of coal discards (and sewage sludge) (co)-HTC derived biochar for a sustainable carbon economy and evaluation of the liquid by-product

This study focused on the hydrothermal treatment (HTC) of coal tailings (CT) and coal slurry (CS) and the co-hydrothermal treatment (Co-HTC) of CT, CS and sewage sludge to assess the potential for increasing the carbon content of the hydrochar produced as an enabler for a sustainable carbon economy. The optimal combination methodology and response surface methodology were used to study the relationship between the important process parameters, namely temperature, pressure, residence time, the coal-to-sewage-sludge ratio, and the carbon yield of the produced hydrochar. The optimized conditions for hydrochar from coal tailing (HCT) and hydrochar from coal slurry (HCS) (150 °C, 27 bar, 95 min) increased fixed carbon from 37.31% and 53.02% to 40.31% and 57.69%, respectively, the total carbon content improved from 42.82 to 49.80% and from 61.85 to 66.90% respectively whereas the ash content of coal discards decreased from 40.32% and 24.17% to 38.3% and 20.0% when compared CT and CS respectively. Optimized Co-HTC conditions (208 °C, 22.5bars, and 360 min) for Hydrochar from the blend of coal discards and sewage sludge (HCB) increased the fixed carbon on a dry basis and the total carbon content from 38.67% and 45.64% to 58.82% and 67.0%, when compared CT and CS respectively. Carbonization yields for HCT, HCS, and HCB were, respectively, 113.58%, 102.42%, and 129.88%. HTC and Co-HTC increase the calorific value of CT and CS, to 19.33 MJ/kg, 25.79 MJ/kg, respectively. The results further show that under Co-HTC conditions, the raw biomass undergoes dehydration and decarboxylation, resulting in a decrease in hydrogen from 3.01%, 3.56%, and 3.05% to 2.87%, 2.98%, and 2.75%, and oxygen from 8.79%, 4.78, and 8.2% to 5.83%, 2.75%, and 6.00% in the resulting HCT, HCS, and HCB, respectively. HTC and Co-HTC optimal conditions increased the specific surface area of the feedstock from 6.066 m²/g and 6.37 m²/g to 11.88 m²/g and 14.35 m²/g, for CT and CS, respectively. Total pore volume rose to 0.071 cm³/g from 0.034 cm³/g, 0.048 cm³/g, and 0.09 cm³/g proving the ability of HTC to produce high-quality hydrochar from coal discards alone or in conjunction with sewage sludge as precursors for decontamination of polluted waters, soil decontamination applications, solid combustibles, energy storage, and environmental protection.

The role of organic nutrients in structuring freshwater phytoplankton communities in a rapidly changing world

Carbon, nitrogen, and phosphorus are critical macroelements in freshwater systems. Historically, researchers and managers have focused on inorganic forms, based on the premise that the organic pool was not available for direct uptake by phytoplankton. We now know that phytoplankton can tap the organic nutrient pool through a number of mechanisms including direct uptake, enzymatic hydrolysis, mixotrophy, and through symbiotic relationships with microbial communities. In this review, we explore these mechanisms considering current and projected future anthropogenically-driven changes to freshwater systems. In particular, we focus on how naturally- and anthropogenically- derived organic nutrients can influence phytoplankton community structure. We also synthesize knowledge gaps regarding phytoplankton physiology and the potential challenges of nutrient management in an organically dynamic and anthropogenically modified world. Our review provides a basis for exploring these topics and suggests several avenues for future work on the relation between organic nutrients and eutrophication and their ecological implications in freshwater systems.

Understanding the Variation of Bacteria in Response to Summertime Oxygen Depletion in Water Column of Bohai Sea

Aiming to reveal the variation in bacteria community under oxygen depletion formed every summer in water column of central Bohai Sea, a time-scenario sampling from June to August in 2018 at a 20-day interval along one inshore-offshore transect was settled. Water samples were collected at the surface, middle, and bottom layer and then analyzed by high-throughput sequencing targeting both 16S rRNA and nosZ genes. Compared to the surface and middle water, oxygen depletion occurred at bottom layer in August. In top two layers, Cyanobacteria dominated the bacterial community, whereas heterotrophic bacteria became dominant in bottom water of Bohai Sea. Based on the time scenario, distinct community separation was observed before (June and July) and after (August) oxygen depletion (p = 0.003). Vertically, strict stratification of nosZ gene was stably formed along 3 sampling layers. As a response to oxygen depletion, the diversity indices of both total bacteria (16S rRNA) and nosZ gene-encoded denitrification bacteria all increased, which indicated the intense potential of nitrogen lose when oxygen depleted. Dissolved oxygen (DO) was the key impacting factor on the community composition of total bacteria in June, whereas nutrients together with DO play the important roles in August for both total and denitrifying bacteria. The biotic impact was revealed further by strong correlations which showed between Cyanobacteria and heterotrophic bacteria in June from co-occurrence network analysis, which became weak in August when DO was depleted. This study discovered the variation in bacteria community in oxygen-depleted water with further effort to understand the potential role of denitrifying bacteria under oxygen depletion in Bohai Sea for the first time, which provided insights into the microbial response to the world-wide expanding oxygen depletion and their contributions in the ocean nitrogen cycling.

Seasonal and spatial variations in nutrients under the influence of natural and anthropogenic factors in coastal waters of the northern Yellow Sea, China

Analysis of the common and most influential natural and anthropogenic activities on the spatiotemporal variation in nutrients at a multiannual scale is important. Eleven cruises from 2015 to 2017 were carried out to better elucidate the seasonal and spatial variations in nutrients, as well as the impact factors on dissolved inorganic nitrogen (DIN), phosphorus (DIP) and silicate (DSi). Both nutrient concentrations and forms showed similar and significant seasonal variations over the 3 years, and were closely related to the biomass and species of phytoplankton. Terrestrial inputs had significant effects on the spatial distribution of nutrients throughout the year, especially in the surface water, which showed DIN > DIP>DSi. In summer, shellfish aquaculture and hypoxia jointly affected the spatial distribution of nutrients. The bottom water nutrient concentrations in the aquaculture area were 1.1-2.3 times higher than those outside of the aquaculture area. Seasonal hypoxia can increase the release of DSi and NH₄⁺ from the sediment to the water. In summary, anthropogenic activities and physical conditions jointly influenced the nutrient distributions.

Riverine flux of dissolved phosphorus to the coastal sea may be overestimated, especially in estuaries of gated rivers: Implications of phosphorus adsorption/desorption on suspended sediments

The flux of terrestrial dissolved inorganic phosphorous (DIP, i.e. PO₄^3-) via rivers into coastal seas is usually calculated by simply multiplying its concentration with the corresponding water flow at the river mouth. Subsequent adsorption/desorption of DIP onto suspended sediment and the influence of salinity in the estuary are often overlooked. A series of DIP adsorption/desorption experiments under different salinities (0, 5, 15, 30) and suspended sediment concentrations (1-40 g L^-1) were conducted in order to assess the potential influence of these factors on the overall DIP loading to the coastal zone. The effect of different sea-salt ions on DIP adsorption/desorption was also assessed by comparing different experimental solutions (NaCl solution, artificial seawater and real seawater). In estuaries, the adsorption of DIP to suspended sediments was greater than desorption, and the net adsorption increased with increasing concentration of suspended sediments and salinity. This enhanced DIP adsorption onto suspended sediment reduces the riverine discharge of DIP to coastal ecosystems. Disregarding this process, especially for the gated estuaries with high sediment resuspension, potentially leads to an overestimation of the terrestrial DIP input to the coastal region.

Eukaryotic communities in coastal water from Shenzhen in South China

Eukaryotic microorganisms are ubiquitous in the marine environment, and have a wide variety of ecosystem functions. Shenzhen is one of the most developed cities in South China, but the eukaryotic communities in the water along its coastlines remain poorly understood. The study applied 18S rRNA gene ITS (internal transcribed spacer) sequencing to identify the eukaryotic community from twenty sites of Shenzhen coast water. The alpha-diversity of the samples between these sites were significantly different, and the seawater of eastern coast had higher alpha-diversity compared to that of the western coast. The abundance of Chlorophyta was notably higher in the seawater of western coast, but Picozoa was relatively depleted. Specifically, Cryptocaryon, Pseudovorticella, and Cyclotella were significantly higher in the water of western coast, while Guinardia, Minutocellus, and Amoebophrya were increased in eastern samples. The spatially variations of eukaryotic microorganism community in the seawater of Shenzhen coast were associated with the water quality. The results have important significance for the understanding of coastal eukaryotic community, their interaction network, and build a foundation for future management and protection of coastal water quality.

In-situ responses of phytoplankton to graphene photocatalysis in the eutrophic lake Xingyun, southwestern China

Graphene photocatalysis is receiving increased attention for its potential to be used as a novel green technology for mitigating harmful algae in highly eutrophic waters. However, graphene is seldom applied to in situ aquatic ecosystems for environmental applications. Here, the impacts of graphene photocatalysis on phytoplankton and environmental conditions were evaluated through an in situ macrocosm experiment in the eutrophic Lake Xingyun, southwestern China. The graphene photocatalysis treated area had significantly reduced conductivity, total nitrogen (TN), total phosphorus (TP) and dissolved phosphorus concentrations, as well as increased dissolved oxygen (DO) concentrations. The abundances of all species of the genus Microcystis were significantly reduced in the graphene photocatalysis-treated area; in contrast, the abundances of all species of the diazotrophic genera, including Anabaena and Aphanizomenon, greatly increased after treatment with graphene photocatalysis. Eukaryotic algae, especially Chlorophyta, Euglenophyta and Pyrrophyta, as well as Cryptophyta, had significantly higher abundances in the graphene photocatalysis-treated area, whereas most of the eutrophic diatom species had lower abundances in the treated area. These observed differences in eukaryotic algae between the two groups might be related to their sensitivity to graphene photocatalysis and their tolerance of nutrients. Generally, graphene photocatalysis can make a great contribution to the improvement of eutrophic water, as evidenced by the reduction in cyanobacteria abundance and phosphorus concentration, as well as the increase in species richness and the dissolved oxygen concentration in the treated area. However, the mechanisms underlying these differences in phytoplankton community structure and environmental conditions require further study.

Removal process of phosphorus during the settlement of particulates with runoff and its implication for reservoir management

Substantial particulates and phosphorus (P) loads are carried into the reservoir by flood runoff and the P exchange between water and settling particulates under variable water conditions in the reservoir after flooding is critical to the removal of active P from water. To investigate the impact of particulate sedimentation on P changes in reservoir water, runoff samples were collected at four locations in the upstream channels of two reservoirs after a rainstorm. Two batches of particulate sedimentation simulations were conducted separately in four plexiglass columns to analyze the changes of water P and environmental factors during particulate sedimentation. The results showed that the contents of total P (TP), total particulates P (TPP), and phosphate (PO₄^3-) decreased with the settlement of particulates. The correlation between the environmental factors and the amount of PO₄^3- in water changed from uncorrelated to correlated with particulate settlement, implying that the rapid settling of particulates might weaken the effect of environmental factors on P exchange between water and particulates. Particulates firstly release PO₄^3- rapidly in river or reservoir and then adsorb PO₄^3- slowly during settlement in reservoirs. Coarse particulates release more and adsorb less PO₄^3- during settlement, and fine particulates play an important role in the removal of water PO₄^3- due to the slow sedimentation rates. Therefore, to mitigate the reactive P content of reservoir water, the loss of coarse particulates from the watershed should be controlled, and the discharge of water with particulates downstream should also be avoided during the flood season.

High-throughput sequencing reveals the spatial distribution variability of microbial community in coastal waters in Shenzhen

Seashore habitats are located between terrestrial and marine ecosystems, which are a hotspot for anthropogenic impacts. Shenzhen is one of the most developed cities in south China, but the microbial functions of its coastal ecosystems remain poorly understood. The study applied 16S rRNA gene sequencing methods to identify the bacterial community from twenty sites of Shenzhen inshore waters. The microbial structure of the samples between eastern Shenzhen and western Shenzhen seashores is notably different, suggesting the spatial variability. Proteobacteria, Cyanobacteria, Actinobacteria, and Bacteroidetes were dominant phyla in the community, and the relative abundance of Bacteroidetes was significantly higher in eastern seashores. Specifically, samples from western Shenzhen contained much more Prochlorococcus, while Synechococcus was more abundant in eastern samples. Moreover, the metabolism of terpenoids and polyketides, and transport and catabolism were significantly more abundant in eastern samples, while antibiotic-resistant pathways were enriched in western samples. The results have important significance to understand bacterial ecosystem of coastal water and promote water quality management and protection activity in Shenzhen. This study can also help developing an optimal strategy for the green economy development and the policy planning of Guangdong-Hong Kong-Macao Greater Bay Area.

Electrolysis-enhanced ecological floating bed and its factors influencing nitrogen and phosphorus removal in simulated hyper-eutrophic water

To enhance ammonia nitrogen (NH3-N) and phosphate (PO43--P) removal in hyper-eutrophic water, electrolysis-enhanced ecological floating bed (EEEFB) was designed with a Mg-Al alloy anode, a Ir-Ta-Ti metal oxide-coated titanium anode, and an Fe anode with the same graphite cathode. The results showed that the Mg-Al alloy anode with graphite cathode had a better ability to enhance NH3-N and PO43--P removal. When the current density was 0.37 mA·cm-2, the electrolysis time was 24 h/d, and the net removal rates of NH3-N and PO43--P were 62% and 99.4%, respectively. In winter, the purification efficiencies of NH3-N and PO43--P were as high as 7388.4 mg·m-2 and 4297.5 mg·m-2, respectively, by EEEFBs which were significantly higher than the traditional ecological floating bed (p < 0.05). Scanning electron microscopy (SEM) and X-ray spectrometry confirmed that the PO43--P was deposited in the sediment of EEEFBs with Mg-Al alloy anode and Fe anode.

Flood impact on the transport, transition, and accumulation of phosphorus in a reservoir: A case study of the Biliuhe Reservoir of Northeast China

Stormflow runoff is the most important agent for phosphorus (P) input to reservoirs, as the particulates contained in runoff carry a substantial amount of P. The settling process of particulates affects the P content of water, and the distribution of particulates determines the P distribution in reservoir sediment. An understanding of flood impacts on the transport, transition, and accumulation of P in a reservoir is critical to reservoir management. In this study, water samples before and after flooding and sediment samples after flooding were collected from Biliuhe Reservoir in Northeast China. P content and load in the water and P-fractions and particle sizes of the sediments were analyzed. Results showed that total particulate P (TPP) increased sharply from 1.56 to 26.72 t after flooding, whereas dissolved organic P (DOP) decreased markedly from 3.24 to 1.17 t, which was largely caused by biological uptake directly or indirectly before flooding. Orthophosphate (PO₄^3-) shared a similar trend with TPP, indicating that PO₄^3- could be adsorbed onto settling particulates, helping to reduce the reactive P introduced by flooding. Reservoir sediment showed a fining trend downstream and the clay fraction exhibited an obvious correlation with P-fractions, demonstrating that the distribution of particulate matter determined P distribution in the sediment. This study also found that particulates from the largest tributary (Biliu River) were only minimally transported from its reservoir entrance to the dam because of a longer travel distance, while contrastingly, particulates from a smaller tributary (Bajia River) were maximally carried to the dam because of a shorter distance. Our fundings suggests that surface water in the reservoir should be released prior to flooding in order to mitigate control of P in the water, moreover, it is necessary to strengthen the effectiveness of pollutant control projects at the reservoir entrance of the Bajia River.

Evaluation of heavy metals stability and phosphate mobility in the remediation of sediment by calcium nitrate

The injection of oxidants is one of the useful remediation technologies for eliminating hydrogen sulfide (H₂ S) and ammonia (NH₃ / NH 4 + ) in aquatic sediments. In the current work, the impact of calcium nitrate injection on the release of heavy metals associated with phosphate was evaluated in a column test of sediment with overlying water at a volume ratio of 1:1 for 131 days. Sulfide was significantly oxidized by calcium nitrate, as its amount was reduced substantially by 85% from the 20th to the 30th day, with a decrease in the oxidation-reduction potential to -68 mV and a simultaneous increase in pH to 9.83. Over 50% of the mobile Zn, Pb, and Cu were reprecipitated in the sediment when the phosphate was partially released. It is proposed that the heavy metal immobilization was related to the phosphate content in the pore water due to the precipitation of heavy metals and phosphorus on the surface of Fe hydroxide particles after oxidation. This is supported by chemical fraction analysis of the heavy metals in the sediment, which indicated increased residual fractions of heavy metals. Our results provide an insight into the remediation of sediment by oxidation with a self-stabilization of heavy metals and phosphate. PRACTITIONER POINTS: Effective removal of sulfide after calcium nitrate injection was achieved. Metal immobilization was related to the phosphate content in pore water. Over 50% of mobile Zn, Pb, and Cu might be reprecipitated in sediment. Oxidizable fraction of heavy metal predominantly transformed to its residual fraction.

The formation of a metalimnetic oxygen minimum exemplifies how ecosystem dynamics shape biogeochemical processes: A modelling study

Metalimnetic oxygen minima are observed in many lakes and reservoirs, but the mechanisms behind this phenomena are not well understood. Thus, we simulated the metalimnetic oxygen minimum (MOM) in the Rappbode Reservoir with a well-established two-dimensional water quality model (CE-QUAL-W2) to systematically quantify the chain of events leading to its formation. We used high-resolution measured data to calibrate the model, which accurately reproduced the physical (e.g. water level and water temperature), biogeochemical (e.g. nutrient and oxygen dynamics) and ecological (e.g. algal community dynamics) features of the reservoir, particularly the spatial and temporal extent of the MOM. The results indicated that around 60% of the total oxygen consumption rate in the MOM layer originated from benthic processes whereas the remainder originated from pelagic processes. The occurrence of the cyanobacterium Planktothrix rubescens in the metalimnion delayed and slightly weakened the MOM through photosynthesis, although its decaying biomass ultimately induced the MOM. Our research also confirmed the decisive role of water temperature in the formation of the MOM since the water temperatures, and thus benthic and pelagic oxygen consumption rates, were higher in the metalimnion than in the hypolimnion. Our model is not only providing novel conclusions about the drivers of MOM development and their quantitative contributions, it is also a new tool for understanding and predicting ecological and biogeochemical water quality dynamics.

Assessment of iron-modified calcite/zeolite mixture as a capping material to control sedimentary phosphorus and nitrogen liberation

Calcite/zeolite mixture (CZ) can be used to construct a capping layer for the simultaneous management of phosphorus (P) and nitrogen (N) liberation from sediments into the overlying water (OVER-water). However, its control efficiency of sedimentary P release still needs to be improved. To address this issue, an iron-modified CZ (Fe-CZ) was synthesized, characterized, and employed as a capping material to simultaneously prevent P and N release from sediments into OVER-water. Batch and microcosm incubation experiments were performed to study the efficiency and mechanism for the control of P and N release from sediments by capping Fe-CZ. Results showed that sediment capping with Fe-CZ resulted in the significant reduction of soluble reactive P (SRP) and ammonium-N (NH₃-N) in OVER-water, with reduction rates of 77.8-99.7% and 54.0-96.7%, respectively. Furthermore, the Fe-CZ capping layer decreased the SRP concentration in the pore water (PORE-water) at depth of 0-30 mm and reduced the concentration of PORE-water NH₃-N at depth of 0-50 mm. Moreover, the Fe-CZ capping layer gave rise to the great decrement of the concentration of the labile P measured by DGT (diffusive gradient in thin films) technology (P-DGT) in the profile of OVER-water and sediment. Additionally, the Fe-CZ capping resulted in the reduction of redox-sensitive P (P-BD) in the 0-50 mm sediment and caused the transformation of P-BD to calcium-bound P (P-HCl) and residual P (P-RES) in the 0-10 mm sediment as well as to P-RES in the 10-20 mm sediment. Results of this work indicate that the Fe-CZ capping has a high potential for the simultaneous management of P and N release from sediments, and the decrease of the contents of sediment P-DGT, sediment P-BD, PORE-water SRP and PORE-water NH₃-N as well as the conversion of mobile P to more stable P in the top sediment should have a significant role in the simultaneous interception of sedimentary P and N liberation into OVER-water by the Fe-CZ capping.

Formation mechanism of the Microcystis aeruginosa bloom in the water with low dissolved phosphorus

The utilization of phosphorus by algae in the low-phosphorus state has drawn wide concerns due to the high risk of forming algal blooms. The cyanobacteria Microcystis aeruginosa (M. aeruginosa) grew well under low-phosphorus condition by hydrolyzing dissolved organic phosphorus (DOP) to dissolved inorganic phosphorus (DIP) through alkaline phosphatase (AP). There was a negative correlation between DIP concentration and AP activity of algae. AP activity significantly increased at 0-3 d (p < 0.05), and reached the peak values of 43.06 and 49.11 King unit/gprot on day 5 for DIP (0.1 mg/L) and DOP (4.0 mg/L), respectively. The relative expression of phosphate transporter gene increased with decreasing phosphorus concentrations. The catalase activity under low-phosphorus condition increased significantly (p < 0.05) after one week, and was generally higher than 0.15 U/mgprot on day 14. Understanding the utilization efficiency and mechanism of DIP and DOP in the low-phosphorus state would help to inhibit the formation of algal blooms.

Combined use of calcium nitrate addition and anion exchange resin capping to control sedimentary phosphorus release and its nitrate‑nitrogen releasing risk

Calcium nitrate (Ca(NO₃)₂) addition can be used to control the release of phosphorus from sediments, however it can also cause an increase in the concentration of nitrate‑nitrogen (NO₃^--N) in the water column. The risk of NO₃^--N release from the Ca(NO₃)₂-injected sediments may be reduced by the placement of the anion exchange resin (AER) capping layer. In this study, the effectiveness of the combined use of Ca(NO₃)₂ addition and AER capping to prevent the liberation of phosphorus from sediments was investigated, and the reduction of the risk of NO₃^--N released from the Ca(NO₃)₂-injected sediment by the AER capping was also evaluated. The combined application of Ca(NO₃)₂ addition and AER capping could tremendously reduce the amount of soluble reactive phosphorus (SR-P) in the overlying water, with SR-P reduction rates of 75.9-98.7%. Furthermore, it could cut down the contents of high-resolution diffusive gradients in thin films (DGT)-labile phosphorus in the sediments, resulting in the formation of phosphorus static layer in the upper sediments. The combined treatment using Ca(NO₃)₂ and AER had a relatively small effect on the contents of mobile phosphorus in the sediments, but it could greatly increase the amount of residual phosphorus in the top 30mm sediments (increased by 27.7-42.9%). The amount of NO₃^--N in the overlying water under the action of the combined treatment method using Ca(NO₃)₂ and AER was much lower than that under the action of the single Ca(NO₃)₂ treatment during the early stage of sediment remediation. In conclusion, the combined use of Ca(NO₃)₂ addition and AER capping is a more promising strategy for the control of sedimentary phosphorus release than the single use of Ca(NO₃)₂ addition from the point of view of both the control efficiency of P release from sediments and the releasing risk of the added nitrate.

Seasonal Release Potential of Sediments in Reservoirs and its Impact on Water Quality Assessment

Reservoir sediments act as important receivers and sources for nutrients in the overlaying water. To determine the sediments adsorption and release characteristics and their impacts on water quality, surface sediments samples collected from Biliuhe reservoir in August 2015 were studied through simulation experiments in a laboratory. The results demonstrated that the equilibrium concentrations (EC₀) of ammonia nitrogen (NH₄⁺–N) and total phosphorus (TP) were 8.29 mg/L and 0.025 mg/L, respectively, which were both higher than the average concentrations in the overlying water. Therefore, the sediments of Biliuhe reservoir mainly acted as the pollution sources for NH₄⁺–N and TP, and the release to water showed a seasonal variation. When potential release amounts were considered, the average concentrations of NH₄⁺–N and TP in the overlaying water could reach 6.4 mg/L and 0.21 mg/L, respectively, which significantly exceeded the current contents. Further, water quality exhibited a decreased tendency after taking the release potential of nutrients into account of water quality assessment. Among the samples, 42% and 33% of them in summer and autumn exceeded the third level of the National Surface Water Quality Standards of China (GB3838-2002). The results indicated that sediments release potential had an unignored influence on water quality during various seasons.

Spatio-temporal Variation in Nutrient Profiles and Exchange Fluxes at the Sediment-Water Interface in Yuqiao Reservoir, China

Nutrients released from sediments have a significant influence on the water quality in eutrophic lakes and reservoirs. To clarify the internal nutrient load and provide reference for eutrophication control in Yuqiao Reservoir, a drinking water source reservoir in China, pore water profiles and sediment core incubation experiments were conducted. The nutrients in the water (soluble reactive P (SRP), nitrate-N (NO₃⁻-N), nitrite-N (NO₂⁻-N), and ammonium-N (NH₄⁺-N)) and in the sediments (total N (TN), total P (TP) and total organic carbon (TOC)) were quantified. The results show that NH₄⁺-N was the main component of inorganic N in the pore water. NH₄⁺-N and SRP were higher in the pore water than in the overlying water, and the concentration gradient indicated a diffusion potential from the sediment to the overlying water. The NH₄⁺-N, NO₃⁻-N, and SRP fluxes showed significant differences amongst the seasons. The NH₄⁺-N and SRP fluxes were significantly higher in the summer than in other seasons, while NO₃⁻-N was higher in the autumn. The sediment generally acted as a source of NH₄⁺-N and SRP and as a sink for NO₃⁻-N and NO₂⁻-N. The sediments release 1133.15 and 92.46 tons of N and P, respectively, to the overlying water each year.

Temporal and spatial variations and impact factors of nutrients in Bohai Bay, China

The temporal and spatial distributions of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSi), and their long-term changes were investigated in Bohai Bay (BHB) in spring, summer, and autumn (2013-2014). The high DIN values were consistently distributed in the western inshore waters, mainly determined by terrestrial factors, e.g., riverine input, while DIP and DSi were mostly distributed in the southern coastal waters, the central BHB, or near the sea port Caofeidian in northern BHB, largely related to non-terrestrial factors, e.g., sediment release. Based on the nutrient distribution, BHB could be partitioned into western and eastern parts, with -15 m depth as the separation. The long-term variations of nutrients since 2000 showed an increase in DIN and decreases in DIP and DSi. Relatively slow changes in DIN and DIP and a rapid decrease in DSi were exhibited in summer, which was associated with precipitation and sediment release.

Recent developments in bioreactor scale production of bacterial polyhydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are biological plastics that are sustainable alternative to synthetic ones. Numerous microorganisms have been identified as PHAs producers. They store PHAs as cellular inclusions to use as an energy source backup. They can be produced in shake flasks and in bioreactors under defined fermentation and physiological culture conditions using suitable nutrients. Their production at bioreactor scale depends on various factors such as carbon source, nutrients supply, temperature, dissolved oxygen level, pH, and production modes. Once produced, PHAs find diverse applications in multiple fields of science and technology particularly in the medical sector. The present review covers some recent developments in sustainable bioreactor scale production of PHAs and identifies some areas in which future research in this field might be focused.