Effects of seasonal deposition-erosion cycle on sedimentary organic carbon remineralization and oxygen consumption in a large-river delta-front estuary.

Seasonal sediment deposition-erosion events are dominant drivers of particle-solute dynamics in large-river delta-front estuaries (LDEs), but their influence on elemental cycles is not yet fully understood. To better constrain the role of deposition-erosion events on elemental cycling in LDEs, benthic fluxes of dissolved inorganic carbon (DIC), oxygen, and pore-water solute profiles were measured over different seasons in the Changjiang LDE. Benthic DIC efflux (23.4 ± 6.0 mmol C m-2 d-1) was greater than oxygen influx (7.5 ± 2.0 mmol O2 m-2 d-1) in summer but less in winter (7.7 ± 1.2 mmol C m-2 d-1 and 10.1 ± 1.5 mmol O2 m-2 d-1, respectively). The additional oxygen consumption in sediments in winter was likely due to the oxidation of inorganic diagenetic reductive products (IDRP) (e.g., NH4+, Fe2+, and Mn2+) in deeper sediments exposed by erosion, which resulted in the development of an "oxygen debt". Sedimentary oxygen respiration accounted for at least 48 % of total oxygen consumption (oxygen consumption in both water column and sediment) in winter and was significantly greater than in summer (∼15 %); this highlighted the importance of winter sediment erosion in oxygen depletion. In addition to IDRP oxidation, the remineralization of resuspended sedimentary organic carbon in water column also contributed to the oxygen consumption. The global dataset on benthic DIC and oxygen fluxes provides evidence that the "oxygen debt" is likely to be widespread in LDEs, exerting a significant impact on global carbon and oxygen cycling.

Preliminary assessment of carbonic acid dissociation constants: Insights from observations in China's east coastal oceans.

Based on observations in China's east coastal oceans, we conducted a preliminary assessment of 16 sets of carbonic acid dissociation constants (K1* and K2*) by comparing spectrophotometrically measured pH values at 25 °C with those calculated from total alkalinity and dissolved inorganic carbon. We obtained that K1* and K2* often performed differently within different salinity ranges, and that the constants of Millero et al. (2002) (M02) demonstrated the best performance for the salinity range of 24-35. In contrast, the often recommended constants of Mehrbach et al. (1973) refit by Dickson and Millero (1987) (DM87-M) and Lucker et al. (2000) (L00) would underestimate pH at salinities of 24-30. This was mainly associated with the higher product of K1* and K2* by DM87-M and L00 than by M02 at this salinity range. Also, we found almost no differences between pH values calculated with DM87-M and L00.

Prokaryote Distribution Patterns along a Dissolved Oxygen Gradient Section in the Tropical Pacific Ocean.

Oceanic oxygen levels are decreasing significantly in response to global climate change; however, the microbial diversity and ecological functional responses to dissolved oxygen (DO) in the open ocean are largely unknown. Here, we present prokaryotic distribution coupled with physical and biogeochemical variables and DO gradients from the surface to near the bottom of a water column along an approximately 12,000-km transect from 13° N to 18° S in the Tropical Pacific Ocean. Nitrate (11.42%), temperature (10.90%), pH (10.91%), silicate (9.34%), phosphate (4.25%), chlorophyll a (3.66%), DO (3.50%), and salinity (3.48%) significantly explained the microbial community variations in the studied area. A distinct microbial community composition broadly corresponding to the water masses formed vertically. Additionally, distinct ecotypes of Thaumarchaeota and Nitrospinae belonging to diverse phylogenetic clades that coincided with specific vertical niches were observed. Moreover, the correlation analysis revealed large-scale natural feedback in which chlorophyll a (organic matter) promoted Thaumarchaeotal biomass at depths that subsequently coupled with Nitrospina, produced and replenished nitrate for phytoplankton productivity at the surface. Low DO also favored Thaumarchaeota growth and fueled nitrate production.

Wastewater inputs reduce the CO2 uptake by coastal oceans.

Every year a large quantity of wastewater is generated worldwide, but its influence on the carbon dioxide (CO2) uptake by coastal oceans is not well understood. Here, sea surface CO2 partial pressure (pCO2) and air-sea CO2 flux were examined in the Jiaozhou Bay (JZB), a temperate coastal bay strongly disturbed by wastewater inputs. Monthly surveys from April 2014 through March 2015 showed that surface pCO2 in the JZB substantially varied both temporally and spatially between 163 µatm and 1222 µatm, with an annual average of 573 µatm. During April-December, surface pCO2 was oversaturated with respect to the atmosphere, with high values exceeding 1000 µatm in the northeastern part of the bay, where seawater salinity was low mainly due to the inputs of wastewater with salinity close to zero. During January-March, surface pCO2 was undersaturated, with the lowest value of <200 µatm also mainly in the northeastern part because of low water temperature and strong biological production. Over an annual cycle, apparently sea surface temperature dominated the monthly variation of surface pCO2 in this shallow bay, while wastewater inputs and related biological production/respiration dominated its spatial variability. Overall, the JZB was a net CO2 source to the atmosphere, emitting 9.6 ± 10.8 mmol C m-2 d-1, unlike its adjacent western part of the Yellow Sea and most of the temperate coastal oceans which are a net CO2 sink. This was possibly associated with wastewater inputs that cause high sea surface pCO2 via direct inputs of CO2 and degradation of organic matter. Thus, from this viewpoint reducing wastewater discharge or lowering CO2 levels in discharged wastewater may be important paths to enhancing the CO2 uptake by coastal oceans in the future.

Long-term variation in nutrients in the South Yellow Sea in response to anthropogenic inputs.

Based on historical data from 1976 to 2019, the effects of anthropogenic activities on long-term changes in nutrients and their ecological effects in the South Yellow Sea were investigated. The dissolved inorganic nitrogen (DIN) concentrations increased continuously from 1990 until the mid-2000s, followed by a shift from an upward trend to a downward trend. The phosphate (PO4-P) and silicate (SiO3-Si) concentrations also showed obvious interannual variations throughout the study period. The concentrations of DIN, PO4-P and SiO3-Si have decreased significantly in recent decade and more. These changes mainly resulted from the reduction in terrestrial input, while the main reason for the decrease in DIN and PO4-P concentrations is the reduction in anthropogenic input. The long-term nutrient changes in the South Yellow Sea have potential ecological impacts on green tide features.

Variations in the nutrient concentration and composition in Liaodong Bay under long-term human activities.

This paper analyzed the long-term variations in nutrients in Liaodong Bay and their potential influencing factors based on historical data from 1978 to 2019. Under the influence of both human activities and natural changes, the concentration of DIN increased approximately 4-fold from the end of the 1990s to the mid-2010s, while DIP and DSi concentrations decreased from the beginning to the end of the 1980s and have since increased again. Asynchronous changes in nutrient levels have led to changes in the nutrient composition, which has caused a series of ecological effects. The total phytoplankton abundance decreased from the 1980s to the end of the 1990s and then increased again. Additionally, the phytoplankton composition shifted from a diatom-dominated to a dinoflagellate-dominated system, and the dominant species of zooplankton changed. Harmful algal blooms (HABs) rarely occurred before the 1980s but have frequently occurred since the end of the 1990s.

Occurrence and distribution of cyclic-alkane-consuming psychrophilic bacteria in the Yellow Sea and East China Sea.

Cyclic alkanes (c-alkanes) are toxic compounds that are abundant in subsurface oil reservoirs and spilled condensate; hence, their environmental risk is significant. Although numerous studies have focused on the decomposition of other compound classes, e.g., acyclic alkanes and aromatic hydrocarbons, very little is known about the biodegradation of c-alkanes in the marine environment. Here, we enriched methylcyclohexane (MCH)-degrading bacteria derived from the cold bottom water (10-20 °C) of China's marginal seas in summer and characterized the changes to the bacterial community using high-throughput amplicon sequencing. MCH-consuming bacteria failed to grow from the warmer surface water (25-29 °C) in the same geographic sites and seasons. Notably, MCH-consuming communities derived from the cold bottom water in the Yellow Sea exhibit distinct structures compared to the other treatments. Furthermore, almost all dominant species in this setting appear to be specifically adapted to deeper cold water as indicated by significantly negative correlations to temperature (P < 0.01). From these results, we proposed that the biodegradation of MCH is effectively limited to the colder waters (10-20 °C) of China's marginal seas, with uncultured psychrophiles acting as the key taxa for MCH decomposition. Overall, this study indicates key functions for uncultivated microbes in the marine environment.

Regime shifts in trophic status and regional nutrient criteria for the Bohai Bay, China.

Identifying trophic status shift and developing nutrient criteria were considered important for controlling coastal eutrophication. Based on data from 1980 to 2018, we used sequential t-test analysis of regime shifts and assessment of coastal trophic status models to detect regime shifts in trophic status in Bohai Bay (China). We identified four distinct periods: unimpaired (1980-1984), minimally impacted (1985-1994), tipping point (1995-1999) and severely degraded (2000-2018). Using the reference conditions at different trophic status, a frequency distribution analysis was performed to develop candidate nutrient criteria. By considering other factors, such as the quantity and quality of data and long-term nutrient variations, we determined recommended criteria for dissolved inorganic nitrogen (15.3 µmol L-1) and phosphorus (0.42 µmol L-1) in Bohai Bay. This study provides a novel and feasible approach to determine reference conditions for the determination of nutrient criteria for coastal waters.

Contribution of the offshore detached Changjiang (Yangtze River) Diluted Water to the formation of hypoxia in summer.

The Changjiang (Yangtze River) Diluted Water (CDW) plume substantially impacts the biogeochemical processes off the estuary and its adjacent area, resulting in considerable environmental and ecological effects. Based on survey data in the northeastern area off the Changjiang Estuary (CE) obtained in the summers of 2008 and 2013, the hypoxia induced by the offshore detached CDW plume and the associated controlling mechanisms were investigated. The results show that the offshore transport of the CDW plume caused a dispersed low-salinity area in the northeastern area off the CE during summer, in sharp contrast with the surrounding high-salinity and high-density waters. There was a hypoxic area with low-pH (i.e., acidification) near the 40-m isobath in bottom waters in the northeastern area off the CE, and its position generally corresponded to the surface offshore CDW plume. In the area affected by the offshore low-salinity water, the surface patch-like phytoplankton bloom and the organic debris produced in situ were the material drivers of the bottom oxygen consumption and led to the corresponding relationship between the bottom hypoxic zone and the high chlorophyll-a (Chl-a) area at the surface. We consider that the local stratification caused by the offshore low-salinity water and the stable environment within the detached CDW plume constituted the external dynamic conditions for maintaining the bottom hypoxia. Our results demonstrate that the offshore detached CDW plume in the northeastern area off the CE may contribute to the formation of a local hypoxic center with low pH. This study would provide basis for understanding of the physical-biogeochemical processes and environmental responses in the offshore areas of the CDW plume.

Oxygen decline in a temperate marginal sea: Contribution of warming and eutrophication.

Dissolved oxygen (DO) decline (i.e., deoxygenation) is an ongoing process in parts of the coastal and open oceans as a result of increased greenhouse gas emissions and nutrient discharges. Yet its controlling mechanisms remain unclear and patchy. Based on continuous observational data collected in a temperate margin, the southern Yellow Sea (SYS), we quantitatively evaluate how deoxygenation responds to warming and eutrophication in different seasons by using an evaluation method that allows us to distinguish the effects of temperature, salinity and biological activities. Results show that during winter, when the water column is vertically well-mixed, and in summer surface waters, deoxygenation is dominated by warming-induced decreases of O2 solubility due to a quick exchange of O2 between the ocean and atmosphere. Moreover, we find a regionally accelerated deoxygenation with enhanced warming along the pathway of the YSCC (Yellow Sea Coastal Current) in winter. In contrast, for bottom waters in summer when O2 exchange is inhibited due to high stratification, deoxygenation appears to be dominated by biological respiration associated with eutrophication. Also, we find the summer bottom deoxygenation can be accelerated by warming, indicating that the bottom waters or the hypolimnion may be vulnerable to deoxygenation in the future. Our study further demonstrates that the deoxygenation mechanisms in shallow coastal oceans are associated with water column structures, i.e., well-mixed vs. stratified water column. Information is assembled into a conceptual model to provide an overview of deoxygenation in temperate marginal systems.

Intra- and inter-seasonal variations in the hydrological characteristics and nutrient conditions in the southwestern Yellow Sea during spring to summer.

Based on field survey in the southwestern Yellow Sea (SWYS) during April-September 2017, the spatiotemporal variations in the hydrological characteristics and nutrient conditions were coupled and analyzed; the intra-seasonal variations in the upwelling in the front of the Yellow Sea Cold Water Mass (YSCWM) and impacts on nutrient transport were explored. The coastal area was controlled by the low-salinity high-nutrient Lubei Coastal Current, Subei Coastal Current, and Yangtze River Diluted Water from north to south; at bottom, the northeastern SWYS was controlled by the low-temperature high-salinity high-nutrient YSCWM. Temperature, salinity and nutrient fronts formed around YSCWM. The upwelling velocity in the front increased during April to late June and decreased in early September; the upwelled fluxes of dissolved inorganic nitrogen (0.29×103-7.77×103 µmol·m-2d-1), phosphate (0.02×103-0.27×103 µmol·m-2d-1) and silicate (0.98×103-8.75×103 µmol·m-2d-1) showed similar variations during April-September. The upwelled nutrients could potentially contribute to local green tide development and phytoplankton growth during spring-summer.

Does submarine groundwater discharge contribute to summer hypoxia in the Changjiang (Yangtze) River Estuary?

The Changjiang (Yangtze) River Estuary (CJE) is one of the largest and most intense seasonal hypoxic zones in the world. Here we examine the possibility that submarine groundwater discharge (SGD) may contribute to the summer hypoxia. Spatial distributions of bottom water 222Rn suggest a hotspot discharge area in the northern section of the CJE. SGD fluxes were estimated based on a 222Rn mass balance model and were found to range from 0.002 ± 0.004 to 0.022 ± 0.011 m3/m2/day. Higher SGD fluxes were observed during summer hypoxia period. The well-developed overlap of the distribution patterns for SGD flux and dissolved oxygen (DO) implies that SGD could be an important contributor to summer hypoxia in the region off the CJE. We suggest that SGD contributes to the seasonal hypoxia either: (1) directly via discharge of anoxic groundwaters together with reducing substances; and/or (2) indirectly by delivering excess nutrients that stimulate primary productivity with consequent consumption of DO during organic matter decomposition.

Influences of Seasonal Monsoons on the Taxonomic Composition and Diversity of Bacterial Community in the Eastern Tropical Indian Ocean.

The Indian Ocean is characterized by its complex physical systems and strong seasonal monsoons. To better understand effects of seasonal monsoon-driven circulation on the bacterioplanktonic community structure in surface waters and the bacterial distribution response to vertical stratification, patterns of seasonal, and vertical distribution of bacterial communities in the Eastern Tropical Indian Ocean were investigated using 16S rRNA gene profiling. Water samples were collected during the Southwest monsoon (from June to August), the fall inter-monsoon (from October and November) and the Northeast monsoon (from December to January), respectively, onboard during three cruises from July 2016 to January 2018. Surface bacterioplankton communities in these three seasons and in the upper water (3-300 m with six depths) during the Northeast monsoon contained a diverse group of taxa, mainly Proteobacteria, Cyanobacteria, Actinobacteria, Bacteroidetes, and Chloroflexi. Redundancy discriminant analysis (RDA) uncovered that temperature, salinity, and dissolved oxygen (DO) were crucial environmental parameters that affected the structure of bacterial community in overall surface samples. However, significant differences in the composition of the bacterial community are likely due to changes in concentrations of salinity during the fall inter-monsoon, while phosphate for both the Southwest monsoon and the Northeast monsoon. Pearson's analysis revealed that the seasonal variation rather than the vertical variation of environmental factors had a more significant impact on the composition of bacterial community. In addition, a clear seasonal pattern of bacterial co-occurrence showed that inter-taxa associations during the fall inter-monsoon were closer than during the Northeast monsoon and the Southwest monsoon. Overall, our results implied clear differences in the composition of bacterial community, with more pronounced seasonal variation compared to the vertical variation in response to environmental changes.

Numerical study on the summertime patches of red tide in the adjacent sea of the Changjiang (Yangtze) River Estuary, China.

High incident rates of red tide have occurred off the coast of the Changjiang (Yangtze) River Estuary in summer, resulting from a magnified population growth discharging substantial nutrient loads into this vicinity. The presence of elevated Chlorophyll-a concentrations (≥36.3 µg/l), low suspended sediment concentrations in surface and mid-layers (<10 mg/l), a strong saline front and surface water temperature gradient, veering surface winds, and a bimodal shape to the Changjiang Diluted Water (CDW) revealed two red tide patches appearing between August 6-13, 2010. Two distinguishable hydrodynamic driving mechanisms, connected to these incidents, are diagnosed. Field observations and numerical simulations determined a red tide patch in the northeastern offshore area is caused by a northeast transport of the CDW truncated by a northwest intrusion of the Taiwan Warm Current (TWC) as winds deviated. Red tide occurrence in the southern nearshore area originated from a plume front where CDW interfaces with the TWC.

Long-term changes in nutrient regimes and their ecological effects in the Bohai Sea, China.

The nutrient regime has changed significantly in the Bohai Sea (BS) during the past six decades because of anthropogenic perturbations. Specifically, the concentration of DIN increased by about 7-fold from the end of the 1950s to the mid-2010s, while DIP and DSi concentrations decreased from the end of the 1950s to the beginning of the 1990s, and have since increased again. Unsynchronized changes in nutrient levels have led to changes in the nutrients structure, which has caused a series of ecological effects. Phytoplankton biomass increased by 6-fold from the 1960s to the mid-2010s. Additionally, phytoplankton composition shifted from a diatom-dominated to a dinoflagellate-dominated system, and the dominant species of macrozoobenthos changed. Red tides rarely occurred before the 1980s, but have occurred periodically and frequently since the 1990s. Finally, the BS ecosystem has shifted from an N-limited oligotrophic state before the 1990s to a potentially P-limited eutrophic state.

Spatiotemporal variations in the summer hypoxia in the Bohai Sea (China) and controlling mechanisms.

Based on field observations in the summer of 2006 and long-term series data, this paper investigates the spatiotemporal variations of hypoxia and associated physical-biogeochemical driving mechanisms in the Bohai Sea (BS), China. Results show that the benthic hypoxic zone is mainly distributed in the "V"-shaped trough region in the western BS, and it tends to form two hypoxic centers which generally correspond to the bottom cold-water core. The regional difference in the intensity of stratification has a significant impact on the spatial distribution of hypoxia. The relatively weak stratification and the mesoscale anticyclonic eddy in the central shoal of the BS weaken the connectivity between the southern and northern hypoxic zones. Organic matter decomposition contributes to hypoxia and results in corresponding nutrient pool with a "dual (southern and northern)-core" structure. Intensified eutrophication is the main drive for decreasing in bottom dissolved oxygen and expansion of hypoxic zone in the BS.

Hydrochemical properties and chemocline of the Sansha Yongle Blue Hole in the South China Sea.

Blue holes can provide valuable information regarding paleoclimate, climate change, karst processes, marine ecology, and carbonate geochemistry. The Sansha Yongle Blue Hole, located on Yongle Atoll in the Paracel Islands in the South China Sea, is the deepest blue hole in the world. A comprehensive investigation of the blue hole was conducted to determine the hydrochemical properties and associated redox processes active in the water column. Results indicate the presence of two thermoclines, one at 13-20 m and a second at 70-150 m, dividing the water column into five stratified water layers. Based on redox state, the water column can be divided into three layers: an oxic layer in the top 70 m, a chemocline at 70-100 m, which acts as a redox boundary, and an anoxic deep layer. In the oxic layer, photosynthesis in the oxic layer above the seasonal thermocline, results in nutrient uptake, transformation of inorganic carbon to organic carbon in the top mixed layer above the seasonal thermocline; Below the seasonal thermocline, organic matter degradation and nitrification, which are the main biological process at depths around 30 m and 50-70 m, lead to the accumulation of nitrate and a decrease in dissolved oxygen and pH; whereas photosynthesis is dominant at depths of 30-50 m, leading to increase in dissolved oxygen and pH. Within the chemocline, organic matter decays via a variety of reactions (e.g. aerobic mineralization, denitrification and anammox), leading to sharp decreases in the oxidizing chemical species (e.g., dissolved oxygen and nitrate) and corresponding increases in the reduced species (e.g., ammonium and sulfide). Within this layer, about 60% of the nitrogen is lost and chemoautotrophic/photoautotrophic production may contribute significantly to particulate organic carbon. Within the deep anoxic layer, sulfate reduction and degradation of organic matter result in accumulations of sulfide, dissolved inorganic carbon, and nutrients.

A historical overview of coastal eutrophication in the China Seas.

China's rapid economic and social development has led to an acceleration in nutrient inputs to coastal waters, which, in turn, has resulted in severe coastal eutrophication. On the occasion of the 40th anniversary of China's reform and opening up, the evolution of the causative factors and the state as well as future prospects for coastal eutrophication in the China Seas are analyzed and summarized. Results showed that the coastal eutrophication situation was not so serious at the beginning of reform and opening up, but it worsened rapidly from the end of the 1980s to the mid-2000s. In the last decade, the worsening trend has been curbed but the status of coastal eutrophication has not been substantially improved. Much work is still needed to be able control the total amount of nutrients entering coastal waters and enable comprehensive treatment of coastal eutrophication in the China Seas.

Long-term variations and influence factors of nutrients in the western North Yellow Sea, China.

This study investigated the long-term variations and compositions of nutrients and the associated controlling factors in the western North Yellow Sea on the basis of historical data. The NO3-N and DIN concentrations and N/P showed continuous increases over the past two decades, which were dominantly affected by riverine inputs, such as inputs from the Yellow River, Yalujiang River and Jia River and atmospheric deposition. However, due to human activities, such as dam construction in rivers and climate change, the SiO3-Si concentrations and Si/P ratios decreased before the early 1990s and then gradually increased. The vertical distributions of nutrients displayed higher concentrations at the bottom than those at the surface in summer, which was attributed to the combined influence of the thermocline, the Yellow Sea Cold Water Mass, the Yellow Sea Warm Current and biological activities.

Hydro-biogeochemical processes and their implications for Ulva prolifera blooms and expansion in the world's largest green tide occurrence region (Yellow Sea, China).

The hydro-biogeochemical processes in the world's largest area of green tide occurrence, off the Jiangsu coast in the Yellow Sea, are investigated in the summer, and their implications for Ulva prolifera blooms are discussed. The results show that the offshore transport of coastal water and the inshore upwelling of offshore bottom water both occur off the Jiangsu coast, and the upwelling position is consistent with the 20- to 30-m isobath off the Subei Shoal. The upwelling results in nutrient supplementation off the shoal, where a rapid decrease in the suspended particulate matter content contributes to good light conditions. As a result, a high-value area of phytoplankton is formed within the 20- to 30-m isobath. Eutrophication in the shoal has provided nutrients for the frequent occurrence of Ulva prolifera in recent years, whereas the upwelling area off the shoal has served as a "service station" or "courier station" for floating Ulva prolifera and promoted the species' propagation. The propagation of Ulva prolifera in the upwelling area and its blooms within the Subei Shoal can have a spatially synergistic effect, leading to its large-scale development. Our findings reveal the mechanisms that trigger the world's largest green tides from the perspective of physical-biogeochemical interactions.