Hydrodynamic controls on nitrogen distribution and removal in aquatic ecosystems

Distinct response of nitrogen metabolism to exogenous cadmium (Cd) in river sediments with and without Cd contamination history

The role of metal resistance on nitrogen metabolism function and community resilience against Cd is important for elucidating the evolutionary dynamics of key ecological functions in river ecosystems. In this study, the response of nitrogen transforming function to Cd exposure in river sediments from the Yangtze River Basin with varying levels of heavy metal contamination history (Cd-contaminated and Cd-free sediments) was compared to understand how Cd influenced nitrogen metabolism under varying metal resistance conditions. The results showed that chronic and persistent Cd pollution of sediments caused an elevation of transport efflux metal resistance genes (MRGs) and a reduction in the uptake MRGs, leading to a stronger tolerance to Cd for Cd-contaminated sediment than Cd-free ones. Specifically, denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) respectively responded to Cd through different mechanisms. Exogenous Cd (5-100 mg kg-1) influenced denitrification rates (-70 %-100 % deviation to control group) by regulating key genera (Thiobacillus, Magnetospirillum, Sideroxydans etc.) and gene clusters for denitrification. Both adaptive nature of anammox bacteria and co-regulation of key genera (Candidatus_Scalindua, Candidatus_Jettenia, Planctomyces etc.) and gene hzsA were drivers of differential responses in sediments from various contamination history. Environmental factors rather than contamination history, key genera or genes were probably critical ones determining Cd-resistance in DNRA, being more tolerant to Cd in sediments with higher TOC and NH4+. Stimulation of N2O reduction process (genera Gemmatimonas and Gemmatirosa and genes nosZ) in Cd-contaminated sediments by exogenous Cd lowered N2O emission risk, whereas the reverse was true for Cd-free sediments. These results enrich our understanding about the linkages among MRGs and nitrogen reduction functions in river.

Fate variations of Per- and polyfluoroalkyl substances in diverse aquatic environments: An overlooked influence of hydrodynamics

Per- and polyfluoroalkyl substances (PFASs) have become a significant global issue; nevertheless, information regarding the hydrodynamic effect on their catchment-scale fate remains lacking. Thus, this study investigated PFASs in water and paired sediment samples from diverse aquatic habitats within the Qinhuai River Basin (QRB), where high concentrations of PFASs are ubiquitous. Rarity score analysis reveals that PFASs were diffusely distributed across the QRB, yet specific sites were identified as emission hotspots. The sediment-water and suspended particulate matter-water partitioning coefficients of PFASs both exhibited significant correlations with chemical structures, ambient variables, land use, and flow velocity (p < 0.05). Flow velocity can promote the liberation of PFASs from particles into water, reducing their accumulation capacity; hence, the higher partitioning coefficients of PFASs were observed in relatively low-velocity aquatic systems, such as lakes, reservoirs, and ponds. A partial least-squares structural equation model was employed to further elucidate their effect pathways and magnitudes on partitioning coefficients. In addition, the primary sources of PFASs were identified, emphasizing their complexity. The ecological risks of PFASs were assessed, indicating priority PFAS species (long-chain PFCAs and HFPO-TA) for management and suggesting water as the preferable environmental medium for regulation. This is the first field investigation to quantify the significance of hydrodynamic influences on the catchment-scale fate of PFASs, improving our understanding of their distribution and behaviors from the perspective of environmental hydraulics.

The influence of turbulence caused by hydraulic structures on the community assembly of epilithic biofilms in rivers

The assembly mechanisms of riverine biofilm communities in river systems represent a central question in aquatic microbial ecology. However, the influence of turbulence on the assembly of generalists and specialists within biofilms remains poorly understood. This study aimed to address this gap by examining a river with multiple spur dikes, using high-throughput sequencing, ecological network analysis, and partial least squares path modeling to explore the assembly process and community structure of biofilms. The results revealed that turbulence intensity (0.029 m/s) and kinetic energy (0.0018 m2/s2) were significantly higher at the heads of spur dikes compared to the tails. Notably, hydrodynamic parameters explained 6.50% of biofilm community variance, highlighting their underappreciated role as deterministic drivers of microbial assembly. Habitat specialists exhibited heightened sensitivity to hydrodynamic fluctuations, occupying central positions in co-occurrence networks. Additionally, turbulence intensity and kinetic energy emerged as the primary drivers of community assembly, influencing critical ecological processes such as homogeneous selection, drift and dispersal limitation. At the head of spur dikes, a high turbulence region, the weakened impact of homogeneous selection, combined with an increase in dispersal limitation, created conditions that particularly favored habitat generalists. Conversely, low turbulence dike tails supported specialist proliferation via strengthened deterministic selection and nutrient-driven niche partitioning. Furthermore, the partial least squares path modeling confirmed that turbulence dominates the assembly process of microbial specialists and generalists. This study revealed the pivotal role of turbulence in shaping biofilm assembly and driving the spatial differentiation of generalists and specialists, offering fresh insights into the complex interplay between hydrodynamics and microbial ecology in rivers impacted by hydraulic structures. These findings significantly enhance the understanding of biofilm assembly mechanisms and their broader implications for effective river ecosystem management.

Seasonal dynamics of groundwater discharge: Unveiling the complex control over reservoir greenhouse gas emissions

The pronounced topographical differences, giving rise to numerous water bodies, also endow these formations with substantial hydraulic gradients, leading to pronounced groundwater discharge within their low-lying, natural reservoir settings. However, the dynamics of groundwater discharge in reservoirs and their impact on greenhouse gas (GHG) production and emission under different conditions remain unclear. This study focuses on a reservoir in southeastern China, where we conducted seasonal field observations alongside microcosm incubation experiments to elucidate the relationship between greenhouse gas emissions and groundwater discharge. Based on the radon (222Rn) mass balance model, groundwater discharge rates were estimated to be 2.14 ± 0.49 cm d-1 in autumn, 4.04 ± 2.09 cm d-1 in winter, 2.55 ± 1.32 cm d-1 in spring, and 2.61 ± 1.93 cm d-1 in summer. Groundwater discharge contributes on average to 31.23 % of CH4, 35.65 % of CO2, and 11.26 % of N2O emissions across all seasons in the reservoir. Groundwater primarily influences GHG emissions by directly inputting carbon and nitrogen, as well as by altering aquatic chemical conditions and the environment of dissolved organic matter (DOM), exerting significant effects particularly during spring and autumn seasons. Especially, in winter, higher groundwater discharge rates influence microbial activity and environmental conditions in the water body, including the C/N ratio, which somewhat reduces its enhancement of greenhouse gas emissions. This study provides an in-depth exploration of greenhouse gas emissions from reservoirs and examines the impact of groundwater on these emissions, aiming to reduce uncertainties in understanding greenhouse gas emission mechanisms and carbon and nitrogen cycling.

Shifts of the pond area ratio for lowland polders: Implication for nutrient control

Shifts for natural ecosystems were increasingly concerned due to its profound impacts on ecosystem services. Ponds within lowland artificial watersheds (polders) play a critical role in nitrogen (N) and phosphorus (P) cycling. From the perspective of N & P control in management practices, it is needed to determine an optimal pond area ratio for polders. For this purpose, our study proposed a process-based modelling framework to investigate the response of polder N & P loss to pond area, and thus to determine the threshold value of pond area ratio to achieve maximum N & P reduction for polders. The proposed framework included two process-based models (NDP and PDP) specially developed to describe N & P dynamics in lowland polders. To evaluate the proposed performance of the framework, it was applied to 171 polders in Zhong River Watershed in Lake Taihu Basin, eastern China. Our investigation results revealed that the correlation between polder N & P reduction rate and pond area ratio had an abrupt shift of 13.6 %, 14.7 % for N & P, respectively. Therefore, polders with a pond area ratio of 13.6-14.7 % had the largest N & P reduction (5.27 and 0.19 kg/ha). Polder size affected P reduction rate, with smaller polders (<200 ha) showing a higher P reduction rate, while it did not affect N reduction rate. Compared with annual precipitation, rainy-season precipitation more significantly (P<0.01) determined polder N & P reduction. This study demonstrated the use of our process-based framework in characterizing the shifts for the pond area ratio for polders, and thus provided technical support for N & P control of lowland areas in water management practices.

Improving denitrification estimation by joint inclusion of suspended particles and chlorophyll a in aquaculture ponds

The denitrification process in aquaculture systems plays a crucial role in nitrogen (N) cycle and N budget estimation. Reliable models are needed to rapidly quantify denitrification rates and assess nitrogen losses. This study conducted a comparative analysis of denitrification rates in fish, crabs, and natural ponds in the Taihu region from March to November 2021, covering a complete aquaculture cycle. The results revealed that aquaculture ponds exhibited higher denitrification rates compared to natural ponds. Key variables influencing denitrification rates were Nitrate nitrogen (NO3--N), Suspended particles (SPS), and chlorophyll a (Chla). There was a significant positive correlation between SPS concentration and denitrification rates. However, we observed that the denitrification rate initially rose with increasing Chla concentration, followed by a subsequent decline. To develop parsimonious models for denitrification rates in aquaculture ponds, we constructed five different statistical models to predict denitrification rates, among which the improved quadratic polynomial regression model (SQPR) that incorporated the three key parameters accounted for 80.7% of the variability in denitrification rates. Additionally, a remote sensing model (RSM) utilizing SPS and Chla explained 43.8% of the variability. The RSM model is particularly valuable for rapid estimation in large regions where remote sensing data are the only available source. This study enhances the understanding of denitrification processes in aquaculture systems, introduces a new model for estimating denitrification in aquaculture ponds, and offers valuable insights for environmental management.

Coupled processes involving ammonium inputs, microbial nitrification, and calcite dissolution control riverine nitrate pollution in the piedmont zone (Qingshui River, China)

Rivers in agricultural countries widely suffer from diffuse nitrate (NO3-) pollution. Although pollution sources and fates of riverine NO3- have been reported worldwide, the driving mechanisms of riverine NO3- pollution associated with mineral dissolution in piedmont zones remain unclear. This study combined hydrogeochemical compositions, stable isotopes (δ18O-NO3-, δ15N-NO3-, δ18O-H2O, and δ2H-H2O), and molecular bioinformation to determine the pollution sources, biogeochemical evolution, and natural attenuation of riverine NO3- in a typical piedmont zone (Qingshui River). High NO3- concentration (37.5 ± 9.44 mg/L) was mainly observed in the agricultural reaches of the river, with ~15.38 % of the samples exceeding the acceptable limit for drinking purpose (44 mg/L as NO3-) set by the World Health Organization. Ammonium inputs, microbial nitrification, and HNO3-induced calcite dissolution were the dominant driving factors that control riverine NO3- contamination in the piedmont zone. Approximately 99.4 % of riverine NO3- contents were derived from NH4+-containing pollutants, consisted of manure & domestic sewage (74.0 % ± 13.0 %), NH4+-synthetic fertilizer (16.1 % ± 8.99 %), and soil organic nitrogen (9.35 % ± 4.49 %). These NH4+-containing pollutants were converted to HNO3 (37.2 ± 9.38 mg/L) by nitrifying bacteria, and then the produced HNO3 preferentially participated in the carbonate (mainly calcite) dissolution, which accounted for 40.0 % ± 12.1 % of the total riverine Ca2+ + Mg2+, also resulting in the rapid release of NO3- into the river water. Thus, microbial nitrification could be a new and non-negligible contributor of riverine NO3- pollution, whereas the involvement of HNO3 in calcite dissolution acted as an accelerator of riverine NO3- pollution. However, denitrification had lesser contribution to natural attenuation for high NO3- pollution. The obtained results indicated that the mitigation of riverine NO3- pollution should focus on the management of ammonium discharges, and the HNO3-induced carbonate dissolution needs to be considered in comprehensively understanding riverine NO3- pollution in piedmont zones.

Decreasing denitrification rates poses a challenge to further decline of nitrogen concentration in Lake Taihu, China

Nitrogen (N) concentrations in many lakes have decreased substantially in recent years due to external load reduction to mitigate harmful algal blooms. However, little attention has been paid to the linkage between the lakes' nitrogen removal efficiency and improved water quality in lakes, especially the variation of denitrification rate (DNR) under decreasing N concentrations. To understand the efficiency of N removal under improving water quality and its influence on the N control targets in Lake Taihu, a denitrification model based on in situ experimental results was developed and long-term (from 2007 to 2022) water quality and meteorological observations were used to estimate DNR and relate it to the amount of N removal (ANR) from the lake. The concentration of total nitrogen (TN) in Lake Taihu decreased from 3.28 mg L-1 to 1.41 mg L-1 from 2007 to 2022 but the reduction showed spatial heterogeneity. The annual mean DNR decreased from 45.6 μmol m-2 h-1 to 4.2 μmol m-2 h-1, and ANR decreased from 11.85×103 t yr-1 to 1.17×103 t yr-1 during the study years. N budget analysis suggested that the amount of N removed by denitrification accounted for 23.3 % of the external load in 2007, but decreased to only 4.0 % in 2022. Thus, the contribution of N removal by internal N cycling decreased significantly as water quality improved. Notably, the proportion of ANR in winter to total ANR increased from 14 % in 2007 to 23 % in 2022 due to warming. This could potentially lead to N deficiencies in spring and summer, thus limiting the availability of N to phytoplankton. A TN concentration of less than 1.0 mg L-1 in the lake and 1.5 mg L-1 in the inflowing lake zones in spring contribute to local N-limitation in Lake Taihu for cyanobacteria control. Our study revealed a general pattern that N removal efficiency decreases with improved water quality, which is instructive for eutrophic lakes in nitrogen management.

Mass variations and transfer process of shrimp farming pollutants in aquaculture drainage systems: Effects of DOM features and physicochemical properties

The expansion of aquaculture produces increasing pollutant loads, necessitating the use of drainage systems to discharge wastewater into surface water. To assess the mass variations and transfer process of aquaculture wastewater, an entire aquaculture drainage investigation lasting for 48 h was conducted, focusing on the nutrients, heavy metals, dissolved organic matter (DOM), and physicochemical properties of drainage in a commercial shrimp farm. The findings revealed that early drainage produced more heavy metals, total nitrogen (TN), dissolved organic nitrogen (DON), and feed-like proteins from aquaculture floating feed and additives, whereas late drainage produced more PO43--P and total dissolved phosphorus (TP). A few pollutants, including DON, Cu, and feed-like proteins, were effectively removed, whereas the contents of TN, dissolved inorganic nitrogen, and Zn increased in the multi-level aquaculture drainage system. Limited dilution indicated that in-stream transfer was the main process shaping pollutant concentrations within the drainage system. In the lower ditches, NO3--N, heavy metals, and feed-like proteins exhibited evident in-stream attenuation, while TN and NH4+-N underwent significant in-stream enrichment processes, especially in ditch C, with the transfer coefficient values (vf) of -1.74E-5 and -2.04E-5. This indicates that traditional aquaculture drainage systems serve as nitrogen sinks, rather than efficient nutrient purge facilitators. Notably, DOM was identified as a more influential factor in shaping the in-stream transfer process in aquaculture drainage systems, with an interpretation rate 40.79% higher than that of the physiochemical properties. Consequently, it is necessary to eliminate the obstacles posed by DOM to pollutant absorption and net zero emissions in aquaculture drainage systems in the future. ENVIRONMENTAL IMPLICATIONS: Nutrients, heavy metals, and dissolved organic matter are hazardous pollutants originating from high-density aquaculture. As the sole conduit to natural waters, aquaculture drainage systems have pivotal functions in receiving and purifying wastewater, in which the in-stream transfer process is affected by ambient conditions. This field study investigated the spatial variations, stage distinctions, effects of physicochemical properties, and dissolved organic matter (DOM) features. This finding suggests that the aquaculture drainage system as a nitrogen sink and DOM source. While the DOM is the key factor in shaping the in-stream transfer process, and obstacles for pollutant elimination. This study helps in understanding the fate of aquaculture pollutants and reveals the drawbacks of traditional aquaculture drainage systems.

Ditch level-dependent N removal capacity of denitrification and anammox in the drainage system of the Ningxia Yellow River irrigation district

Drainage networks, consisting of different levels of ditches, play a positive role in removing reactive nitrogen (N) via self-purification before drainage water returns to natural water bodies. However, relatively little is known about the N removal capacity of irrigation agricultural systems with different drainage ditch levels. In this study, we employed soil core incubation and soil slurry 15N paired tracer techniques to investigate the N removal rate (i.e., N2 flux), denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) rates in the Ningxia Yellow River irrigation district at various ditch levels, including field ditches (FD), paddy field ditches (PFD), lateral ditches (LD1 and LD2), branch ditches (BD1, BD2, BD3), and trunk ditches (TD). The results indicated that the N removal rate ranged from 44.7 to 165.22 nmol N g-1 h-1 in the ditches, in the following decreasing order: trunk ditches > branch ditches > paddy field ditches > lateral ditches > field ditches. This result suggested that the N removal rate in drainage ditches is determined by the ditch level. In addition, denitrification and anammox were the primary pathways for N removal in the ditches, contributing 68.40-76.64 % and 21.55-30.29 %, respectively, to the total N removal. In contrast, DNRA contributed only 0.82-2.15 % to the total nitrate reduction. The N removal rates were negatively correlated with soil EC and pH and were also constrained by the abundances of denitrification functional genes. Overall, our findings suggest that the ditch level should be considered when evaluating the N removal capacity of agricultural ditch systems.