Mixing regime shapes the community assembly process, microbial interaction and proliferation of cyanobacterial species Planktothrix in a stratified lake

Lake mixing influences aquatic chemical properties and microbial community composition, and thus, we hypothesized that it would alter microbial community assembly and interaction. To clarify this issue, we explored the community assembly processes and cooccurrence networks in four seasons at two depths (epilimnion and hypolimnion) in a mesotrophic and stratified lake (Chenghai Lake), which formed stratification in the summer and turnover in the winter. During the stratification period, the epilimnion and hypolimnion went through contrary assembly processes but converged to similar assembly patterns in the mixing period. In a highly homogeneous selection environment, species with low niche breadth were filtered, resulting in decreased species richness. Water mixing in the winter homogenized the environment, resulting in a simpler microbial cooccurrence network. Interestingly, we observed a high abundance of the cyanobacterial genus Planktothrix in the winter, probably due to nutrient redistribution and Planktothrix adaptivity to the winter environment in which mixing played important roles. Our study provides deeper fundamental insights into how environmental factors influence microbial community structure through community assembly processes.

Radon traced seasonal variations of water mixing and accompanying nutrient and carbon transport in the Yellow-Bohai Sea

Radon (²²²Rn) is a natural radioactive tracer widely utilized to evaluate water exchange and mixing processes; however, minimal studies have investigated the ²²²Rn distribution in the Yellow-Bohai Sea (YBS) and its behavior is poorly understood. In this study, the seasonal distribution of ²²²Rn in the YBS was investigated. The results found that the ²²²Rn distribution in surface waters is significantly affected by rivers, while ²²²Rn activity in bottom waters is highly affected by submarine groundwater discharge. The eddy diffusivity and advection velocities of the YBS were obtained utilizing an improved 1D steady-state ²²²Rn diffusion-advection model. The average horizontal eddy diffusivities in the wet (August 2015) and dry (November 2014) seasons were 4.54 × 10⁸ and 2.28 × 10⁸ cm² s^-1 in dry season, respectively and the average vertical eddy diffusivity was 4.99 cm² s^-1. The dissolved inorganic nutrient (N, P, and Si) and dissolved inorganic carbon flux outputs from vertical eddy diffusion were determined to be 4.85, 0.29, 3.59, and 61.6 mmol m^-2 d^-1, respectively. These results demonstrate that eddy diffusion tracing in coastal ocean is conducive to interpreting water mixing processes and can be utilized to understand offshore nutrient and carbon transport better.

Behaviour of heavy metals and natural radionuclides in the mixing of phosphogypsum leachates with seawater

Phosphogypsum (PG) is disposed worldwide in large stacks usually placed in coastal zones, as in the case of Huelva (SW of Spain), where around 100 Mt of PG are stored on the salt marshes of the Tinto River estuary covering a surface of about 1000 ha. This management generates the weathering of PG, and due to its high acidity (pH ≈ 2) and pollutant load can provoke significant emissions into their surroundings. In this work were evaluated by laboratory experiments the effects of pH increase in the behaviour of heavy metals and natural radionuclides during the mixing of phosphogypsum leachates with seawater. The acidic phosphogypsum leachates showed concentrations of heavy metals from two to three orders of magnitude higher than natural continental waters, and natural radionuclides (U-isotopes and ²¹⁰Po) from four to five orders of magnitude higher than unperturbed aquatic systems. Major elements and some heavy metals as Mn, Ni, Cd, As, Sb and Co showed a conservative behaviour during the neutralisation of the leachates with seawater, remaining in the liquid phase, while other ones as Al, Fe, Cr, Zn, Cu, Pb precipitated and/or were adsorbed onto the solid phase. The U-isotopes and ²¹⁰Po showed a clear non-conservative behaviour probably due to coprecipitation/adsorption processes onto the formed precipitates, but while ²¹⁰Po reached a total removal at pH ≈ 7, U- isotopes after a total removal at pH ≈ 5 returned into the liquid phase due to redissolution/desorption processes at near neutral pH. The formed precipitates, mainly composed by iron phosphates particles, showed heavy metal and natural radionuclide concentrations from one to three orders of magnitude higher than unperturbed soils. All these facts demonstrate the serious environmental impact produced by the PG stacks into their surroundings and the urgency of effective restoration measures.

Study of water mixing in the coastal waters of the western Taiwan Strait based on radium isotopes

Radium is considered to be a useful tracer for studying the physical processes of seawater. In this work, three naturally occurring radium isotopes, (224)Raex, (226)Ra and (228)Ra, were measured in the coastal zone of the western Taiwan Strait during the summer seasons. Based on the distributions of the three radium isotopes and the salinity, we conclude that the water mixing pattern in the study area in summer consists of diluted water flowing from the Jiulong River to the open sea towards the east and southeast, and open sea seawater flowing inward from south to north. The submarine ground water discharges in the estuarine region, as suggested by the radium and salinity data. The residence times of the Jiulong River estuary, ranging from 7 to 49 d, were estimated using the radium isotope pairs (224)Raex and (226)Ra.

A variable reaction rate model for chlorine decay in drinking water due to the reaction with dissolved organic matter

A second order kinetic model for simulating chlorine decay in bulk water due to the reaction with dissolved organic matter (DOM) was developed. It takes into account the decreasing reactivity of dissolved organic matter using a variable reaction rate coefficient (VRRC) which decreases with an increasing conversion. The concentration of reducing species is surrogated by the maximum chlorine demand. Temperature dependency, respectively, is described by the Arrhenius-relationship. The accuracy and adequacy of the proposed model to describe chlorine decay in bulk water were evaluated and shown for very different waters and different conditions such as water mixing or rechlorination by applying statistical tests. It is thus very well suited for application in water quality modeling for distribution systems.

Uranium behaviour in an estuary polluted by mining and industrial effluents: the Ría of Huelva (SW of Spain)

This paper describes a comprehensive study of the behaviour of U in the Ría of Huelva estuary, formed by the Tinto and Odiel rivers. This ecosystem is conditioned by two hydrochemical facts: one connected with the acid mining drainage (AMD) generated in the first section of the river basins, and another one related to the fertilizer industry located at the estuary. AMD gives a singular character to these rivers; low pH and high redox potential that keep high amounts of toxic elements and radionuclides in dissolution. Most of the data for dissolved U in estuaries indicate conservative mixing, but there are examples of non-conservative behaviour attributed to oxidation/reduction processes or solubility variations. In the Ría of Huelva estuary the U shows a non-conservative behaviour due to solubility changes produced by variations in the pH. A complete removal of riverine dissolved U is observed in a pH range of 4-6. At higher pH values, U release from suspended matter, and probably also from sediments into the dissolved phase is found.