Bubbles dominated the significant spatiotemporal variability and accumulation of methane concentrations in an ice-covered reservoir.

Climate-sensitive ice-covered reservoirs are critical components of methane (CH4) release. However, the mechanisms that influence CH4 dynamics during ice-covered periods remain poorly studied. To investigate the effects of bubbles on CH4 dynamics, we conducted intensive field and incubation experiments in an ice-covered reservoir (ice growth, stability, and melt period) in Northeast China. We found that the mean dissolved CH4 concentrations in the ice (625.9 ± 2419.7 nmol L-1) and underlying water (1218.9 ± 2678.9 nmol L-1) were high, making them atmosphere CH4 sources. The visible bubble bands (bubble area) in the riverine zone and the vertical profile of the CH4 concentration in the ice reflect the distribution of trapped bubbles. The mean CH4 concentration in the ice of the bubble area (1674.8 ± 3926.8 nmol L-1) was 2 orders of magnitude higher than that of no-bubble area (53.7 ± 9.2 nmol L-1). Moreover, a large amount of CH4 accumulated under the ice in the bubble area. These findings suggest that bubbles determine the CH4 storage in ice and CH4 accumulation in the underlying water. Ice growth increases CH4 storage in ice and the underlying water because of the entrapment and re-dissolution of CH4 bubbles. However, ice melting releases the CH4 accumulated in the ice and underlying water. A comparison of the field and incubation experiments indicated that the deep-water environment of the reservoir had a CH4 burial effect. Stepwise regression analysis revealed that higher sediment organic matter content, median particle size, and porosity increased the production and release of CH4 bubbles, trapping more CH4 bubbles in ice. Overall, this study improves the mechanistic understanding of CH4 dynamics and predictability of CH4 emissions during ice-covered periods.

Spatiotemporal variability and environmental effects of greenhouse gases, nutrients, and dissolved carbons in an ice-covered reservoir.

Ice cover restructures the distribution of substances in ice and underlying water and poses non-negligible environmental effects. This study aimed to clarify the spatiotemporal variability and environmental effects of methane (CH4), nitrous oxide (N2O), total nitrogen (TN), total phosphorus (TP), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) in ice and water columns during different ice-covered periods. We surveyed the ice-growth, ice-stability, and ice-melt periods in an ice-covered reservoir located in Northeast China. The results showed that underlying water (CH4: 1218.9 ± 2678.9 nmol L-1 and N2O: 19.3 ± 7.3 nmol L-1) and ice (CH4: 535.2 ± 2373.1 nmol L-1 and N2O: 9.9 ± 1.5 nmol L-1) were sources of atmospheric greenhouse gases. N2O concentrations were the highest in the bottom water of the reservoir while CH4 accumulated the most below the ice in the riverine zone. These can be attributed to differences in the solubilities and relative molecular masses of the two gases. Higher concentrations of N2O, TN, TP, DOC, and DIC were recorded in the underlying water than those in the ice due to the preferential redistribution of these substances in the aqueous phase during ice formation. Additionally, we distinguished between bubble and no-bubble areas in the riverine zone and found that the higher CH4 concentrations in the underlying water than those in the ice were due to CH4 bubbles. In addition, we reviewed various substances in ice-water systems and found that the substances in ice-water systems can be divided into solute exclusion and particle entrapment, which are attributed to differences between dissolved and particulate states. These findings are important for a comprehensive understanding of substances dynamics during ice-covered periods.

Contrasting effects of temperature and biological processes on aragonite saturation state at the A4HDYD station in the North Yellow Sea.

To explore the effects of seawater temperature and biological processes on aragonite saturation state (Ωarag), eight field surveys were conducted from December 2014 to November 2015 at A4HDYD station in the North Yellow Sea (NYS). Low regional yearly initial values of seawater Ωarag (1.26-1.76) were obtained in winter due to low seawater temperature. During spring to early autumn, surface-water Ωarag increased from 2.02 to 3.47 for the increasing seawater temperature and biological production, whereas bottom-water Ωarag remained as low as 1.44-1.79 due to low seawater temperature, net community respiration and the influence of the North Yellow Sea Cold Water Mass (NYSCWM). Following the collapse of stratification in late autumn, the seawater was well-mixed with an average Ωarag of 2.17. The annual cycle of seawater temperature dominated the seasonal variations of Ωarag, counteracted by biological production increasing Ωarag of 0.49 in surface water in March and strengthened by net community respiration decreasing Ωarag of 0.11-0.41 below the pycnocline during summer and autumn. It is, therefore, mid-to-high latitude coastal areas impacted by human activities are likely subject to ocean acidification due to the combined effects of low seawater temperature and net community respiration.

Potential of Mie-Fluorescence-Raman Lidar to Profile Chlorophyll a Concentration in Inland Waters.

Vertical distribution of phytoplankton is crucial for assessing the trophic status and primary production in inland waters. However, there is sparse information about phytoplankton vertical distribution due to the lack of sufficient measurements. Here, we report, to the best of our knowledge, the first Mie-fluorescence-Raman lidar (MFRL) measurements of continuous chlorophyll a (Chl-a) profiles as well as their parametrization in inland water. The lidar-measured Chl-a during several experiments showed good agreement with the in situ data. A case study verified that MFRL had the potential to profile the Chl-a concentration. The results revealed that the maintenance of subsurface chlorophyll maxima (SCM) was influenced by light and nutrient inputs. Furthermore, inspired by the observations from MFRL, an SCM model built upon surface Chl-a concentration and euphotic layer depth was proposed with root mean square relative difference of 16.5% compared to MFRL observations, providing the possibility to map 3D Chl-a distribution in aquatic ecosystems by integrated active-passive remote sensing technology. Profiling and modeling Chl-a concentration with MFRL are expected to be of paramount importance for monitoring inland water ecosystems and environments.

Satellite-estimated air-sea CO2 fluxes in the Bohai Sea, Yellow Sea, and East China Sea: Patterns and variations during 2003-2019.

The Bohai Sea (BS), Yellow Sea (YS), and East China Sea (ECS) together form one of the largest marginal sea systems in the world, including enclosed and semi-enclosed ocean margins and a wide continental shelf influenced by the Changjiang River and the strong western boundary current (Kuroshio). Based on in situ seawater pCO2 data collected on 51 cruises/legs over the past two decades, a satellite retrieval algorithm for seawater pCO2 was developed by combining the semi-mechanistic algorithm and machine learning method (MeSAA-ML-ECS). MeSAA-ML-ECS introduced semi-analytical parameters, including the temperature-dependent seawater pCO2 (pCO2,therm) and upwelling index (UISST), to characterise the combined effect of atmospheric CO2 forcing, thermodynamic effects, and multiple mixing processes on seawater pCO2. The best-selected machine learning algorithm is XGBoost. The satellite-derived pCO2 achieved excellent performance in this complicated marginal sea, with low root mean square error (RMSE = 20 µatm) and mean absolute percentage deviation (APD = 4.12 %) for independent in situ validation dataset. During 2003-2019, the annual average CO2 sinks in the BS, YS, ECS, and entire study area were 0.16 ± 0.26, 3.85 ± 0.68, 14.80 ± 3.09, and 18.81 ± 3.81 Tg C/yr, respectively. Under continuously increasing atmospheric CO2 concentration, the BS changed from a weak source to a weak sink, the YS experienced interannual fluctuations but did not show significant trend, while the ECS acted as a strong sink with CO2 absorption increased from ∼10 Tg C in 2003 to ∼19 Tg C in 2019. In total, CO2 uptake in the entire study area increased by 85 % in 17 years. For the first time, we present the most refined variation in the satellite-derived pCO2 and air-sea CO2 flux dataset. These complete ocean carbon sink statistics and new insights will benefit further research on carbon fixation and its potential capacity.

Shipborne oceanic high-spectral-resolution lidar for accurate estimation of seawater depth-resolved optical properties.

Lidar techniques present a distinctive ability to resolve vertical structure of optical properties within the upper water column at both day- and night-time. However, accuracy challenges remain for existing lidar instruments due to the ill-posed nature of elastic backscatter lidar retrievals and multiple scattering. Here we demonstrate the high performance of, to the best of our knowledge, the first shipborne oceanic high-spectral-resolution lidar (HSRL) and illustrate a multiple scattering correction algorithm to rigorously address the above challenges in estimating the depth-resolved diffuse attenuation coefficient Kd and the particulate backscattering coefficient bbp at 532 nm. HSRL data were collected during day- and night-time within the coastal areas of East China Sea and South China Sea, which are connected by the Taiwan Strait. Results include vertical profiles from open ocean waters to moderate turbid waters and first lidar continuous observation of diel vertical distribution of thin layers at a fixed station. The root-mean-square relative differences between the HSRL and coincident in situ measurements are 5.6% and 9.1% for Kd and bbp, respectively, corresponding to an improvement of 2.7-13.5 and 4.9-44.1 times, respectively, with respect to elastic backscatter lidar methods. Shipborne oceanic HSRLs with high performance are expected to be of paramount importance for the construction of 3D map of ocean ecosystem.

Seasonal evolution and controlling factors of bottom oxygen depletion in the Bohai Sea.

A coupled physical-biogeochemical model is used to investigate the seasonal evolution and controlling factors of oxygen depletion in the Bohai Sea (BS). Comparisons show that the model reproduces observed spatiotemporal variations of important physical and biogeochemical variables well. Bottom oxygen in the BS shows an annual cycle with significant drawdown in summer and enhanced replenishment in fall. Two oxygen-depleted regions off Qinhuangdao (QHD) and the Yellow River estuary (YRE) develop separately and experience higher oxygen depletion rates and longer durations of low-oxygen conditions. The evolution of oxygen depletion is primarily controlled by stratification and biological oxygen consumption but is also modulated by lateral transport. Strong stratification is established earlier than oxygen depletion and maintains its development. The biological oxygen consumption determines the two oxygen-depleted regions under stratified conditions. Lateral transport influenced by anticyclonic circulations favors an expansion of oxygen depletion off QHD but alleviates oxygen depletion off the YRE.

Multiple factors dominate the distribution of methane and its sea-to-air flux in the Bohai Sea in summer and autumn of 2014.

The Bohai Sea is well-known as a source of atmospheric methane (CH4). However, the main regulate factors of the spatiotemporal distribution of CH4 and its sea-to-air flux remain largely unknown. In this study, the observed CH4 concentration ranged from 4.8 to 32.7 nmol/L and 3.1 to 15.2 nmol/L in August and November of 2014, respectively. The main factors that influence the distribution of CH4 and its sea-to-air flux were stratification, solubility, and current structure for the mid-west depression basins, the permanent well-mixed seawater column and CH4 source strength for the centre shallow ridge zone, and the upwelling for the east depression basin, respectively. Meanwhile, wind also plays an important role in sea-to-air CH4 flux in the study area except the centre shallow ridge zone. Upwelling made the east depression basin the most intensive source of CH4, with a flux of 2 to 4 times higher than the other sub-regions.

Low dissolved oxygen in the Pearl River estuary in summer: Long-term spatio-temporal patterns, trends, and regulating factors.

Several large semi-enclosed seas and coastal bodies have formed seasonal hypoxic water masses over large areas. The dominant cause for such formations is believed to be the increasing inflow of nutrients from watersheds and urban areas into estuaries and coastal waters. Several studies have reported hypoxic events in the Pearl River estuary (PRE). However, hypoxia events appear to be episodic, which are neither seasonal nor estuary-wide. The reasons for such occurrences are not understood fully. The objectives of this study are to use 24-year time series data obtained during 1988-2011 to analyze the long-term variability in dissolved oxygen (DO) in summer over the entire estuary from urban areas to estuarine coastal waters and examine regulating factors for the occurrences of hypoxia. The results showed that various regulating factors were responsible for the variability of DO in different regions of the PRE. DO decreased in the urban reach, upstream and main estuarine zones of the PRE from about 2000. It was hypoxic (DO <2.0 mg L-1) in the urban reach zone and periodic low DO (DO <3.5 mg L-1) and episodic hypoxic in the upstream zone after 2000, mainly influenced by the increasing sewage discharge. The main estuarine zone experienced episodic low DO and sporadic hypoxia after 2000. The ecosystem buffering capacity was characterized by turbidity, well vertical mixing and short residence time which diluted the pressure for the formation of hypoxia. The eastern shelf appeared to be more vulnerable to hypoxia because of its deeper topography, plume front position, and stratification of the water column compared with the western shelf. Its bottom waters experienced seasonal low DO but seldom hypoxia. In comparison, low DO rarely occurred in the western shelf because of its shallow topography and short residence time. In brief, 24-year series data captured from 1988 to 2011 showed a lack of seasonal and estuary-wide hypoxia in the PRE.

Diagnosis of CO2 dynamics and fluxes in global coastal oceans.

Global coastal oceans as a whole represent an important carbon sink but, due to high spatial-temporal variability, a mechanistic conceptualization of the coastal carbon cycle is still under development, hindering the modelling and inclusion of coastal carbon in Earth System Models. Although temperature is considered an important control of sea surface pCO2, we show that the latitudinal distribution of global coastal surface pCO2 does not match that of temperature, and its inter-seasonal changes are substantially regulated by non-thermal factors such as water mass mixing and net primary production. These processes operate in both ocean-dominated and river-dominated margins, with carbon and nutrients sourced from the open ocean and land, respectively. These can be conceptualized by a semi-analytical framework that assesses the consumption of dissolved inorganic carbon relative to nutrients, to determine how a coastal system is a CO2 source or sink. The framework also finds utility in accounting for additional nutrients in organic forms and testing hypotheses such as using Redfield stoichiometry, and is therefore an essential step toward comprehensively understanding and modelling the role of the coastal ocean in the global carbon cycle.

Aragonite saturation state variation and control in the river-dominated marginal BoHai and Yellow seas of China during summer.

Based on a survey conducted from June to July 2013, aragonite saturation state variation and control in the river-dominated marginal BoHai and Yellow seas were investigated. Surface water Ωarag ranged from 2.0-3.8, whereas subsurface water Ωarag was generally lower than 2.0. Temperature changes had a strong influence on Ωarag through induced CO2 solubility changes in seawater. Riverine freshwater input decreased Ωarag in the Changjiang and Yalu river estuaries, but induced higher Ωarag in the Yellow River estuary. Biological processes had opposite effects on Ωarag, whereby elevated biological production led to the highest Ωarag in the South Yellow Sea surface water, whereas net community respiration/remineralization induced low Ωarag in subsurface water. Stratification affected the level and scale of low Ωarag in subsurface water. By the year 2100, surface water with Ωarag > 2.0 will disappear except for the Yellow River estuary, and most of the subsurface water will develop substantial aragonite undersaturation.

Aragonite saturation state and dynamic mechanism in the southern Yellow Sea, China.

Based upon surveys conducted in November 2012 and June 2013, the distribution and dynamics of aragonite saturation state (Ωarag) were investigated in the southern Yellow Sea (SYS) of China. In summer, surface water Ωarag ranged from 2.1-3.8 and enhanced biological production fueled by Changjiang River freshwater input increased Ωarag to 3.8 in the southern SYS. However, subsurface water Ωarag was <2.0 in the central SYS. During autumn, surface water Ωarag was 2.0-2.9, lower than that in summer due to ventilation between surface and low Ωarag (1.0-1.4) subsurface waters in the central SYS. Community respiration and/or aerobic remineralization dominated low Ωarag in subsurface waters, while water stratification influenced the level and scale of acidity accumulation. By the end of this century, waters with Ωarag>2.0 could disappear from the SYS with increasing atmospheric CO2, while bottom waters Ωarag may become undersaturated due to the impact of eutrophication.

Enhanced methane emissions from oil and gas exploration areas to the atmosphere--the central Bohai Sea.

The distributions of dissolved methane in the central Bohai Sea were investigated in November 2011, May 2012, July 2012, and August 2012. Methane concentration in surface seawater, determined using an underway measurement system combined with wavelength-scanned cavity ring-down spectroscopy, showed marked spatiotemporal variations with saturation ratio from 107% to 1193%. The central Bohai Sea was thus a source of atmospheric methane during the survey periods. Several episodic oil and gas spill events increased surface methane concentration by up to 4.7 times and raised the local methane outgassing rate by up to 14.6 times. This study demonstrated a method to detect seafloor CH4 leakages at the sea surface, which may have applicability in many shallow sea areas with oil and gas exploration activities around the world.

Dioxin-like compounds in sediments from the Daliao River Estuary of Bohai Sea: distribution and their influencing factors.

The concentrations, compositional profiles, and potential ecological risk of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs) and polychlorinated naphthalenes (PCNs) in sediments of the Daliao River Estuary were investigated. Total concentrations of PCDD/Fs, PCBs, and PCNs were in the range of 11.3-133.2 ng/kg dry weight (dw), 1 971-37 632 ng/kg dw and 33.1-284.4ng/kg dw, respectively. The total TEQ values varied from 0.37 to 4.08ng/kg dw, with the dominant contributions by PCDD/Fs, then by PCBs and PCNs. The spatial distributions of PCDD/Fs, PCBs and PCNs in the river estuary were much related to hydrodynamic conditions. The risk of contamination to the deeper sea was increased in the flood seasons. Moreover, our data confirmed that both organic matter in sediments and molecular properties of dioxin-like compounds were the factors which strongly influenced the partition behavior of these dioxin-like compounds between sediments and water phase in the estuarine zone.