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

Numerical environmental impact assessment of accidental release of ammonia during bunkering in the Singapore Strait

Ammonia is a promising maritime fuel for decarbonization, but its impact on marine environments is less understood. This study uses a high-resolution numerical model to examine the short-term (from hours to days) effects of ammonia releases during bunkering in Singapore - one of the world's busiest ports. Results show that nearshore shallow-water jetties experience more localized but longer-lasting impacts than open seas due to limited mixing. High discharge volumes exacerbate toxicity, highlighting the need for rapid containment; multi-hose systems with low flow rates are recommended. A dimensionless number and a regression model are developed to estimate the extent and duration of lethal ammonia plumes across varying discharge volumes. The study also highlights the role of tidal dynamics: spring tides enhance rapid but widespread dispersion, while ebb-flood cycles dictate the directional transport of ammonia. Beyond toxicity, ammonia contributes to the local nitrogen cycle, stimulating algal growth in semi-enclosed waters but posing lower risks in open seas due to stronger mixing. The presence of sensitive marine species, such as coral reefs, should be considered in bunkering strategies. These findings provide scientific ground for safer ammonia bunkering practices and support the development of effective emergency response frameworks in coastal port environments.

Overlooked potential of overlying water disturbances on nitrification and denitrification in urban secondary wetlands

Urban secondary wetlands, open urban landscape systems, provide both aesthetic and water purification functions. However, the effects of frequent human disturbances on the nitrification-denitrification potential of these wetlands system remains unclear. This study simulated horizontal, vertical and hybrid flow disturbances, along with scenarios of external NH4+-N input leading to the gradual deterioration of water quality, to investigate the effects of human interference on the nitrification-denitrification potential of urban secondary wetlands. The findings revealed that all modes of flow disturbances significantly enhanced the nitrification and denitrification potential of wetland sediments. Hybrid flow disturbance most significantly improved the nitrification potential, with improvements of 6.01 %-8.84 % compared to the undisturbed reactor (CK), especially as water quality deteriorated from Class Ⅰ to poor Class V. Horizontal flow disturbance most significantly boosted denitrification potential, ranging from 5.21 % to 19.9 % over the CK. As water quality gradually deteriorated, hybrid and horizontal flow disturbances improved the microbial alpha-diversity within the wetland sediments. Hybrid flow disturbance also elevated the abundance of nitrification functional genes such as hao and nxrAB, as well as Hao activity in the sediments, favoring the growth of nitrifiers such as unclassified__c__Deltaproteobacteria. Horizontal flow disturbance, on the other hand, significantly increased the abundance of denitrifiers such as unclassified__d__Bacteria and unclassified__c__Gammaproteobacteri, along with the abundance of denitrification functional genes like nirK, nirS, and nosZ. This study important insights for optimizing the management and improving the ecological functions of urban secondary wetlands.

Dissolved organic carbon can alter coastal sediment phosphorus dynamic: Effects of different carbon forms and concentrations

Coastal waters are receiving increasing loads of dissolved organic carbon (DOC), differing in structural complexity and molecular weights with potential different effects on the phosphorus (P) dynamics in these waters. This study conducted an in-situ investigation in Xiangshan Harbor, China, to explore the patterns of P release in response to DOC inputs. To further elucidate the underlying mechanisms behind the DOC-affected sediment P release, a two-month mesocosm experiment was undertaken with coastal sediment (Xiangshan Harbor) to which acetate, glucose, and humic acid (representing the fermentation product, the simple available carbon, and the refractory humic-like carbon sources, respectively) were separately added to the overlying water at dosages of 0, 5, 10, and 20 mg C L-1. We found that: i) sediment P release showed a non-linear increase with DOC input, a pattern likely due to the diverse forms of DOC in coastal zones, which had varying impacts on P release; ⅱ) significant P release for labile DOC (acetate- and glucose-amended) treatments but retention for humic acid treatments, and the magnitude of P changes mainly depended on the amount of DOC addition; ⅲ) acetate and glucose shared similar P-release-promotion mechanisms, i.e., decreased dissolved oxygen, increased ppk genes in water, and increased P bacteria and alkaline phosphatase activity were the dominant factors behind the P release for both carbon sources, as indicated by piecewise structural equation modelling; ⅳ) humic acid-inhibitory effects on sediment P release, which likely reflect increasing "P-humic acid" complexes that favor P adsorption and sedimentation and form stable "humic acid-enzyme" complexes that reduce the catalytic activity of alkaline phosphatase. Our findings provide new understanding of relationships between loading of DOC with different form/concentration and sediment P dynamics in coastal areas.

Endogenous phosphorus release from plateau lakes responds significantly to temperature variability over the last 50 years

The ecological environment of plateau lakes is very sensitive to temperature changes. Higher temperatures accelerate the cycling processes between lake sediments and water nutrients. Quantitatively investigating the influence mechanism of regional climate change and sediment phosphorus release over a long time series is difficult in revealing the causes of eutrophication in plateau lakes. This paper quantitatively reveals the long-term response mechanism of endogenous phosphorus release to temperature change in Dianchi, the largest plateau eutrophic lake in China, based on nearly 50 years of temperature and sediment phosphorus data from 1964 to 2013, and taking advantage of the Random Forest machine learning algorithm for deep processing of long time series and nonlinear relation. The results showed that: (1) Over the past 50 years, endogenous phosphorus release and temperature showed no trend for 22 years, followed by a consistent, significant increase in both after 1986. (2) Random Forest analysis showed that before the increase of temperature, the contribution to the phosphorus release was weak, while after the mutation, the contribution reached 52.6%, and typically was concentrated from March to August each year. (3) The response relationship between temperature and endogenous phosphorus release had non-linear variation with a threshold interval of 18.3 °C-19.2 °C. This research aims to explore the theoretical scientific knowledge of endogenous phosphorus release processes and complex mechanisms in plateau lakes under changing environments, and further explores the effects of long-term temperature variability on endogenous phosphorus loading in plateau lakes. That is, long-term temperature mutations can alter the internal cycling processes of sedimentary phosphorus by stimulating algal growth, which have a more drastic effect than short-term temperature variations.

The role of microorganisms in phosphorus cycling at river-lake confluences: Insights from a study on microbial community dynamics

River-lake confluences are key zones in the river-lake network, essential for managing contaminant transport and transformation. However, the role of biogeochemical transformations, particularly in phosphorus (P) dynamics, has been underexplored. As a result, this study looks into the dynamics of microbial communities and how important microbes are to the cycling of P. It was revealed that microorganisms contribute differently to phosphorus cycling in different hydraulic regions. Regions with higher-velocity and finer sediment showed increased microbial diversity and enhanced capabilities for organic phosphorus (OP) mineralization and inorganic phosphorus (IP) solubilization due to lower bio-available P (bio-P) concentrations. In areas characterized by flow deflection (FD), flow stagnation (FST), and flow separation (FSE), distinct P fraction distributions were observed: Total phosphorus (TP) and bio-P were found to be more abundant in the FST and FD regions, but residual phosphorus (Res-P) and calcium phosphorus (Ca-P) were more prevalent in the FSE region. Sediment characteristics, including P species like aluminum-phosphorus (Al-P), OP, iron-associate phosphorus (BD-P), and sediment mid-diameter (D50), significantly influence microbial community composition. These results improve our comprehension of the distribution of microbial community distribution and its role in the phosphorus cycle at river-lake confluence, providing useful provide valuable information for managing river-lake confluences and protecting aquatic ecosystems.

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.

Chronic increasing nitrogen and endogenous phosphorus release from sediment threaten to the water quality in a semi-humid region reservoir

The water quality in the drinking water reservoir directly affects people's quality of life and health. When external pollution input is effectively controlled, endogenous release is considered the main cause of water quality deterioration. As the major nitrogen (N) and phosphorus (P) sources in reservoirs, sediment plays a vital role in affecting the water quality. To understand the spatial and temporal variation of N and P in the sediment, this study analyzed the current characteristics and cumulative effects of a semi-humid reservoir, Yuqiao Reservoir, in North China. The N and P concentrations in the reservoir sediment were decreased along the flow direction, while the minimum values were recorded at the central sediment profile. External input and algal deposition were the main factors leading to higher sediment concentrations in the east (Re-E) and west (Re-W) areas of reservoir sediment profiles. According to the long-term datasets, the peaks of both sediment total nitrogen content and deposition rate were observed in the 2010s, which has increased about three times and six times than in the1990s, respectively. Therefore, the increase in phosphorus concentration may be the main reason for eutrophication in water in recent years. The mineralization of organic matter has a significant promoting effect on releasing N and P from sediments, which will intensify eutrophication in water dominated by P and bring huge challenges to water environment management. This study highlights that the current imbalance in N and P inputs into reservoirs and the endogenous P release from sediment will have a significant impact on water quality.

Sediment microbial fuel cell coupled floating treatment wetland for enhancing non-reactive phosphorus removal

The presence of non-reactive phosphorus (NRP) in environmental waters presents a potential risk of eutrophication and poses challenges for the removal of all phosphorus (P) fractions. This study presents the first investigation on the removal performance and mechanism of three model NRP compounds, sodium tripolyphosphate (STPP), adenosine 5'-monophosphate (AMP) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), in the sediment microbial fuel cell-floating treatment wetland (SMFC-FTW). Coupling SMFC with plants proved to be effective at removing NRP via electrochemical oxidation and plant uptake, particularly the challenging-to-degrade phosphonates that contain C-P bonds. Compared with the control group, the removal efficiencies of the model NRP in SMFC were observed to increase by 11.9%-20.8%. SMFC promoted the conversion of NRP to soluble reactive phosphorus (sRP) and the transfer of P to sediment. Furthermore, the electrochemical process enhanced both plant growth and P uptake, and increased P assimilation by 72.6%. The presence of plants in the bioelectrochemical system influenced the occurrence and fate of P by efficiently assimilating sRP and supporting microbial transformation of NRP. Consequently, plants enhanced the removal efficiencies of all P fractions in the overlying water. This study demonstrated that SMFC-FTW is a promising technology to remove various NRP species in environmental waters.

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.