Improved data assimilation for algal bloom dynamics simulation in the Three Gorges Reservoir using particle filter.

Algal blooms have been increasingly prevalent in recent years, especially in lakes and reservoirs; their accurate prediction is essential for preserving water quality. In this study, the observed chlorophyll a (chl-a) levels were assimilated into the Environmental Fluid Dynamics Code (EFDC) of algal bloom dynamics by using a particle filter (PF), and the state variables of water quality and model parameters were simultaneously updated to achieve enhanced algal bloom predictive performance. The developed data assimilation system for algal blooms was applied to Xiangxi Bay (XXB) in the Three Gorges Reservoir (TGR). The results show that the ensemble mean accuracy and reliability of the confidence intervals of the predicted state variables, including chl-a and indirectly updated phosphate (PO4), ammonium (NH4), and nitrate (NO3) levels, were considerably improved after PF assimilation. Thus, PF assimilation is an effective tool for the dynamic correction of parameters to represent their inherent variations. Increased assimilation frequency can effectively suppress the accumulation of model errors; therefore, the use of high-frequency water quality data for assimilation is recommended to ensure more accurate and reliable algal bloom prediction.

[Temporal-spatial Distribution and Risk Assessment of Polycyclic Aromatic Hydrocarbons in Soil of Xiangxi Bay in Three Gorges Reservoir Area].

The US Environmental Protection Agency has listed 16 polycyclic aromatic hydrocarbons (PAHs) for priority control. Soil samples from Xiangxi Bay in the Three Gorges Reservoir Area (the water fluctuation zone, WLFZ; upper edge of water fluctuation zone, UEWLFZ; sediments) were analyzed for the concentration of these PAHs, using high performance liquid chromatography (HPLC). The results showed that the soil samples of Xiangxi Bay could be ranked, based on the concentration of PAHs, in the following order:UEWLFZ>WLFZ>sediment. The composition of PAHs varied from the three regions, with 3- and 4-ring PAHs dominating in sediments and 4- and 5-ring PAHs dominating in soil from the WLFZ and UEWLFZ. The composition of PAHs in soil from the WLFZ exhibited a higher coefficient of variation and a weaker correlation with the composition of PAHs in soil from the UEWLFZ and sediment. Soil from the three regions showed varying seasonal distributions of PAHs, which is closely related to the quantity and types of energy consumption in each season. PAHs in sediment from sites at the same altitude showed no evident differences, whereas WLFZ and UEWLFZ soil had higher levels of PAHs at the sites near Xiakou Town and the Yangtze River Estuary. Isomer ratio analysis showed that the sources of PAHs in Xiangxi Bay vary between seasons and regions, with incomplete combustion of fossil fuels and biomass forming the main sources in the soil of Xiangxi Bay. The lifetime carcinogenic risk assessment shows that PAHs in sediment, WLFZ, and UEWLFZ have a potential risk to human through ingestion and dermal contact, with PAHs in the soil of UEWLFZ posing the highest carcinogenic risk. The results provide a theoretical reference for the prevention and control of contamination by PAHs in Xiangxi Bay of the Three Gorges Reservoir area.

[Impact of Rainfall-Runoff Events on Methane Emission from Xiangxi Bay of the Three Gorges Reservoir].

Methane is an important greenhouse gas and whether reservoirs act as a source or sink of methane has attracted great attention worldwide. However, unrepresentative sampling periods and a lack of consideration of unfavorable weather conditions have limited the accurate estimation of CH4 emission from reservoirs. This study focused on the middle reach of Xiangxi Bay in the Three Gorges Reservoir to track an entire rainfall-runoff event via on-site measurements in the summer of 2019, and initiatively investigated the impact of rainfall and inflow processes on methane concentration and emission. Results showed that from before to after the rainfall event, methane flux at the air-water interface ranged between 0.011 and 0.326 mg·(m2·h)-1, indicating a net source of methane to the atmosphere. Both wind velocity and rainfall affected methane evasion from the surface by altering the gas transfer velocity, with the effect of wind being more prominent. Methane concentrations at the bottom layer significantly increased when rainfall-induced density flow from the watershed arrived at the sampling section. This was likely due to methane export from upstream and along the flow path. During this event, discharge was too small to destratify the water column, and methane was strongly oxidized as it diffused upwards, having little impact on surface methane concentrations and air-water methane flux.

Nitrate repletion during spring bloom intensifies phytoplankton iron demand in Yangtze River tributary, China.

Most aquatic systems show characteristic seasonal fluctuations in the total nutrient pool supporting primary productivity. The nutrient dynamics essentially exacerbate critical demand for the counterpart micronutrients towards achieving ecosystem equilibrium. Herein, the phytoplankton demand for iron (Fe) uptake under high concentration of nitrate-nitrogen during spring in Xiangxi Bay, China, was studied. Our result confirmed that significant Fe concentrations (P = 0.01) in both autumn (0.62 ± 0.02 mgL-1) and winter (0.06 ± 0.03 mgL-1) relative to spring (0.004 ± 0.01 mgL-1) are linked to the low NO3-N paradigms during autumn and winter. As NO3-N showed a sharp increase in spring, a dramatic reduction in the Fe pool was observed in the entire tributary, driving the system to a critical Fe limited condition. Bioassay study involving Fe additions both alone and in combinations led to maximum growth stimulation with biomass as chla (16.44 ± 0.82 µgL-1) and phytoplankton cell density (6.75 × 106 cellsL-1) which differed significantly (P = 0.03) with the control. Further, the study demonstrated that Fe additions triggered biomass productions which increased linearly with cell densities. The P alone addition caused biomass production (15.26 ± 2.51 µgL-1) greater than both NO3-N (9.15 ± 0.66 µgL-1) and NH4+N (13.65 ± 1.68 µgL-1) separate additions but reported a low aggregate cell density (3.18 × 106 cellsL-1). This indicates that nutrient and taxonomic characteristics e.g., high cell pigment contents rather than just the cell bio-volume also determine biomass. The Bacilliarophyta, Chlorophyta, and Cryptophyta with the total extinction of Cyanophyta characterized the bloom in spring. The anthropogenic NO3-N input into XXB would have driven to higher NO3-N than NH4+N situation, and incapacitated the Cyanophyta that preferentially utilize NH4+N. Our study provides a useful report for incorporation into the monitoring programs for prudent management of phytoplankton bloom and pollution across the eutrophic systems.

Study on nutrient limitation of phytoplankton growth in Xiangxi Bay of the Three Gorges Reservoir, China.

After the impoundment of the Three Gorges Reservoir (TGR), algal blooms in the sidearm tributaries have resulted from increasing nutrient loads along the major tributaries. Field sampling and in situ nutrient addition bioassay were implemented to examine the nutrient limitation of phytoplankton growth and bloom initiation during autumn in Xiangxi Bay of the TGR. Result shows that P is the primary limiting nutrient for algal growth and bloom in Xiangxi Bay during autumn. The treatment involving the combination of N, P and Si had a significant (p < .05) additional effect on the growth of phytoplankton. The N, P, Si combined treatment increased growth by 10-50% relative to the N and P treatments from day 1 to day 4, respectively. Trace metal additions involving Fe, Zn, Mn, and Cu and/or in combination with N, P, and Si initially resulted in an extremely low growth rate which later increased significantly (p < .05) towards the end of the study. The present study provides an insight into the responses of different phytoplankton taxa in autumn under nutrient conditions in the tributary bay. The nutrient limitation study is recognized as the first step to mitigating the bloom while proposing an effective nutrient control strategy. The outcome of which can provide the basis for formulating sustainable watershed management. Multiple nutrients reductions with P as primary concern are required for a lasting management solution to the risk of bloom in the TGR.

[Distribution Characteristics and Release Fluxes of Phosphorus Forms in Xiangxi Bay Sediments in the Three Gorges Reservoir Before and After Impoundment].

The sediment storage environment in tributaries has been altered by impoundment of water in the Three Gorges Reservoir area, affecting the distribution of phosphorus forms in sediment and processes at the sediment-water interface. Through collection of sediment and overlying water samples in Xiangxi Bay in August 2016 (before impoundment) and October (after impoundment), the distribution characteristics of sedimentary phosphorus and the environmental conditions of storage before and after impoundment were analyzed. Fluxes of PO43--P at the sediment-water interface were also estimated. Results show that pH increased, alkalinity and reducibility were enhanced, and Eh in sediments decreased after impoundment. The relative content of phosphorus in sediments changed as follows:NaOH-P > HCl-P > OP to HCl-P > OP > NaOH-P; this could be attributed to changes in the depositional environment. Compared to pre-impoundment values, TP values after impoundment in sediment, overlying water ρ(PO43--P), and interstitial water ρ (PO43--P) were 1.3 times, 3.7 times, and 8.3 times higher, increasing the risk of nutrient release in sediments of Xiangxi Bay. The manifestation of PO43--P in sendiments of Xiangxi River generally is "source" pre-impoundment and post-impoundment, but the PO43--P diffusive flux increased from -0.0029-0.0059 mg·(m2·d)-1 pre-impoundment to 0.0067-0.1071 mg·(m2·d)-1 post-impoundment. The release of phosphorus from sediments at the bottom of Xiangxi Bay increased after impoundment.

[Effect of the Rainfall on Extinction of Cyanobacteria Bloom and Its Mechanism Analysis].

There were three rainfall events with different intensity in the Xiangxi Bay (XXB) from May 24 to June 2 in 2016. The factors such as hydrodynamics, water temperature, optical properties, and chlorophyll a concentrations during the rainfall events were analyzed. During the May 27 moderate rain period, the upstream flow of the reservoir bay increased by 1.9 times and the average mixing layer depth in the whole reservoir increased 8.2 m, compared to those before the rainfall event. During the June 1 light rain period, the average mixing layer depth in the whole reservoir increased 1.6 m and the average chlorophyll concentration reduced 2.02 µg·L-1, compared with those before the rainfall event. During the June 2 heavy rain period, the upstream flow of the reservoir bay increased by 4 times, the average mixing layer depth in the whole reservoir increased 7.9 m and the average chlorophyll concentration reduced 14.64 µg·L-1, compared with those before the rainfall event. The algae moved from the upstream to the downstream with water that reduced the concentration of algae in the XXB. The water temperature stratification weakened during the rain event and the average mixing layer depth in the whole reservoir increased, destroying the algal growth environment. After the rainfall, under suitable light and temperature conditions for 2-3 d, the water temperature stratification of the reservoir was recovered and rapid growth and reproduction of algae occurred. As a result, the chlorophyll concentrations in the reservoir increased. Rainfall has a periodic inhibitory effect on the outbreak of algal blooms; however, it cannot fundamentally solve the problem of tribal bay blooms.

[Characteristics of Nitrogen Release at the Sediment-Water Interface in the Typical Tributaries of the Three Gorges Reservoir During the Sensitive Period in Spring].

Overlying water and sediment interstitial water samples were acquired to study the nitrogen release between sediments and water interfaces in Xiangxi Bay in April 2016 during the Sensitive Period in spring. The spatial distribution of different forms of nitrogen in the sediment was analyzed, the diffusion fluxes of different forms of nitrogen in the sediments and water systems were also measured, and a correlation analysis with environmental factors was conducted. The results show that overlying water and sediment interstitial water ρ(TN) ranges from 1.10 to 6.90 mg·L-1 and 6.19 to 32.57 mg·L-1 respectively; indicating the nitrogen concentrations in the overlying and interstitial water of sediments have a certain variation along the process and vertically. The interstitial water nitrogen concentrations in the upstream area are higher than those in the downstream area. The interstitial water ρ(NH4+-N) in the sediment is significantly larger than that in the overlying water, but the interstitial water ρ(NO3--N) in the sediment is slightly smaller than that in the overlying water. Xiangxi Bay sediment acts as a source of NH4+-N; however, for NO3--N it is a sink. The diffusive fluxes of NH4+-N range from 2.70 to 4.72 mg·(m2·d)-1; and the diffusive fluxes of NO3--N range from -1.61 to -0.62 mg·(m2·d)-1. Nitrogen is mainly present in the form of ammonium nitrogen in the sediment of Xiangxi Bay. The ρ(NH4+-N) in the sediment ranges from 69.97-1185.97 mg·kg-1, ρ(NO3--N) ranges from 2.78-38.17 mg·kg-1, and the ρ(NH4+-N) in sediments in the surface at 0-8 cm changes with the same trend.