Storage and redistribution of anthropogenic CO2 in the western North Pacific: The role of subtropical mode water transportation.

Oceanic uptake and storage of anthropogenic CO2 (CANT) are regulated by ocean circulation and ventilation. To decipher the storage and redistribution of CANT in the western North Pacific, where a major CANT sink develops, we investigated the water column carbonate system, dissolved inorganic radiocarbon and ancillary parameters in May and August 2018, spanning the Kuroshio Extension (KE, 35-39 °N), Kuroshio Recirculation (KR, 27-35 °N) and subtropical (21-27 °N) zones. Water column CANT inventories were estimated to be 40.5 ± 1.1 mol m-2 in the KR zone and 37.2 ± 0.9 mol m-2 in the subtropical zone. In comparison with historical data obtained in 2005, relatively high rates of increase of the CANT inventory of 1.05 ± 0.20 and 1.03 ± 0.12 mol m-2 yr-1 in the recent decade were obtained in the KR and subtropical zones, respectively. Our water-mass-based analyses suggest that formation and transport of subtropical mode water dominate the deep penetration, storage, and redistribution of CANT in those two regions. In the KE zone, however, both the water column CANT inventory and the decadal CANT accumulation rate were small and uncertain owing to the dynamic hydrology, where the naturally uplifting isopycnal surfaces make CANT penetration relatively shallow. The findings of this study improve the understanding of the spatiotemporal variations of CANT distribution, storage, and transport in the western North Pacific.

Exploring contribution of phytoplankton cell death to settleable particulate organic carbon in the East China Sea in spring.

Phytoplankton's death contributes to marine settleable particulate organic matter (POM). In this study, we used laboratory cultivation of different algal species to identify a positive correlation between the cumulative number of dead algal cells and POC>75 (carbon content of the settleable POM). The contribution coefficient of cell death to POC>75 varied among different algal species. Additionally, the field survey and incubation experiment were conducted in the East China Sea (ECS) to explore the spatial-temporal correlation between phytoplankton death and POC>75. The results concluded that phytoplankton death was the main factor controlling POC>75. In the ECS, the relationship between the surface cumulative mass of POC>75 and the cumulative number of dead cells followed: Cumulative mass of POC>75(mg) = 0.487 × Cumulative number of dead cells (/104) + 0.069. This study provided a methodology to quantitatively explain the relationship between phytoplankton death and settleable POM.

Nutrient dynamics in the Bohai and North Yellow seas from seasonal to decadal scales: Unveiling Bohai Sea eutrophication mitigation in the 2010s.

The eutrophication status in the central Bohai Sea tends to be mitigated in recent years. To explore the recent nutrient status, seasonal surveys were carried out from 2018 to 2021, covering both the Bohai Sea and the adjacent North Yellow Sea. In recent cold seasons, both dissolved inorganic nitrogen concentration (DIN) and the ratio of DIN to soluble reactive phosphorus were lower than those in 2016. In warm seasons, the variations in nutrients and apparent oxygen utilization were correlated with each other, roughly following the traditional Redfield ratio of N:P:O2 of approximately 16:1:(-138). When historical data for N*, which is the excess DIN related to soluble reactive phosphorus, was collated, the Bohai Sea showed a decreasing trend for N* at a rate of -0.64 ± 0.12 µmol N* kg-1 a-1 between 2011 and 2021. During the same period, the North Yellow Sea N* concentrations (i.e., the oceanic end-member of the Bohai Sea N* dynamics) and the local atmospheric nitrogen (N) deposition (atmospheric end-member) were estimated to decline at rates of -0.22 ± 0.04 µmol N* kg-1 a-1 and - 0.93 ± 0.34 kg N ha-1 a-2, respectively. Consequently, the oceanic and atmospheric changes accounted for 25.7 % ± 28.4 % and 69.0 % ± 42.6 %, respectively, of the Bohai Sea eutrophication mitigation in 2011-2021. On the long-term changes of the Bohai Sea eutrophication, the terrestrial nutrient source has only minor (likely <10 %) impacts, although it certainly affects the spatial distribution of nutrients. This study has implied that coastal eutrophication is a dynamic process that is subject to sea-land-air interactions, and its mitigation needs both local pollution controls and regional environment management. The latter contains the understanding of oceanic changes and external effects of the air pollution control.

Algal cell viability assessment: The role of environmental factors in phytoplankton population dynamics.

The viability of algal cells is one of the most fundamental issues in marine ecological research. In this work, a method was designed to identify algal cell viability based on digital holography and deep learning, which divided algal cells into three categories: active, weak, and dead cells. This method was applied to measure algal cells in surface waters of the East China Sea in spring, revealing about 4.34 %-23.29 % weak cells and 3.98 %-19.47 % dead cells. Levels of nitrate and chlorophyll a were the main factors affecting the viability of algal cells. Furthermore, algal viability changes during the heating and cooling were observed in laboratory experiments: high temperatures led to an increase in weak algal cells. This may provide an explanation for why most harmful algal blooms occur in warming months. This study provided a novel insight into how to identify the viability of algal cells and understand their significance in the ocean.

Legacy Effects of Late Macroalgal Blooms on Dissolved Inorganic Carbon Pool through Alkalinity Enhancement in Coastal Ocean.

Taking the world's largest green tide caused by the macroalga Ulva prolifera in the South Yellow Sea as a natural case, it is studied here if macroalgae can perform inorganic carbon sequestration in the ocean. Massive macroalgae released large amounts of organic carbon, most of which were transformed by microorganisms into dissolved inorganic carbon (DIC). Nearshore field investigations showed that, along with seawater deoxygenation and acidification, both DIC and total alkalinity (TAlk) increased significantly (both >50%) in the areas covered by dense U. prolifera at the late-bloom stage. Offshore mapping cruises revealed that DIC and TAlk were relatively higher at the late-bloom stage than at the before-bloom stage. Laboratory cultivation of U. prolifera at the late-bloom stage further manifested a significant enhancement effect on DIC and TAlk in seawater. Sulfate reduction and/or denitrification likely dominated the production of TAlk. Notably, half of the generated DIC and almost all the TAlk could persist in seawater under varying conditions, from hypoxia to normoxia and from air-water CO2 disequilibrium to re-equilibrium. The enhancement of TAlk allowed more DIC to remain in the seawater rather than escape into the atmosphere, thus having the long-term legacy effect of increasing DIC pool in the ocean.

Exploring origin of oxygen-consuming organic matter in a newly developed quasi-hypoxic coastal ocean, the Bohai Sea (China): A stable carbon isotope perspective.

The Bohai Sea, adjacent to the Northwest Pacific, is a semi-enclosed shallow-water marginal sea that was considered on a critical path of eutrophication and environmental degradation. To better understand the Bohai Sea metabolism-induced summertime dissolved oxygen (DO) decline, five field surveys were conducted between July 2019 and July 2021 to investigate the seasonal/interannual and spatial variations in DO, dissolved inorganic carbon (DIC), and stable isotopic composition of DIC (δ13CDIC). Although the water-mixing scheme was subject to spatial variation, a uniform apparent ratio of δ13CDIC versus apparent oxygen utilization was estimated at -0.0122‰ per µmol O2 kg-1 in the Bohai Sea in summer. Based on a three-endmember water-mixing model and the mass balance of DIC and its stable isotopic composition, the assumed uniform δ13C values of oxygen-consuming organic matter in the Bohai Sea DO-deficient areas was estimated to be -19.47 ± 1.85‰ in 2020 and between -20.6‰ and - 18.1‰ in 2021. This isotopic composition is very similar to the δ13C value of organic matter from marine diatoms, but different to that of terrestrial organic matter sources surrounding the Bohai Sea. Our results indicate that nearly all the organic matter consumed by community respiration in the Bohai Sea is produced in situ by marine plankton. To mitigate the seasonal DO shortage in the Bohai Sea, reduction of allochthonous nutrients is crucial.

Effects of warming, eutrophication and climate variability on acidification of the seasonally stratified North Yellow Sea over the past 40 years.

The North Yellow Sea (NYS) is a productive marginal sea of the western North Pacific. In summer and autumn, CaCO3 saturation states beneath the seasonal thermocline in the NYS have frequently fallen below critical levels, indicating that marine calcifying organisms are under threat. To explore the long-term evolution of the acidification of the NYS, we reconstructed seasonal variations in subsurface aragonite saturation state (Ωarag) and pH during 1976-2017, using wintertime and summertime temperature, salinity, dissolved oxygen and pH data mainly from a quality-controlled oceanographic database. Over the past 40 years, the wintertime warming rate in the NYS was twice the rate of global ocean surface warming. Warming-induced decrease in CO2 solubility canceled out a part of the wintertime Ωarag decrease caused by atmospheric CO2 increase, and also had minor effect on pH changes in winter. Although the NYS is a semi-enclosed marginal sea, its interannual variations of wintertime temperature, salinity, pH and Ωarag were correlated to Pacific Decadal Oscillation with a lag of 2-3 years. Due to the eutrophication-induced enhancement of net community respiration beneath the seasonal thermocline, long-term declines of bottom-water Ωarag and pH in summer were substantially faster than the declines of assumed air-equilibrated Ωarag and pH in spring. Over the past 40 years, the amplitudes of seasonal variations of bottom-water Ωarag and pH from spring to summer/autumn have increased by 4-7 times. This amplification has pushed the NYS towards the critical threshold of net community CaCO3 dissolution at a pace faster than that forecast under scenarios of atmospheric CO2 increase. In summary, our results provide insights into the combined effects of ocean warming, eutrophication, atmospheric CO2 rise and climate variability on coastal hydrochemistry, explaining how the environmental stresses on local marine calcifying organisms and the benthic ecosystem increased over the past 40 years.

Excess nitrogen in the Bohai and Yellow seas, China: Distribution, trends, and source apportionment.

The Bohai and Yellow seas are marginal seas of the western North Pacific, characterized by coastal eutrophication and populated coastlines. In this work, six survey datasets collected between 2011 and 2018 were used to investigate the excess of dissolved inorganic nitrogen (DIN) related to soluble reactive phosphorus (SRP), referred to as N*, in the Bohai and Yellow seas. High N* of more than 5 µmol kg-1 occurred mostly in the Changjiang and Yellow River plumes and/or near the Jiangsu coast. Away from these river plumes and the Jiangsu coast, however, N* usually ranged from -2.5 to 1.0 µmol kg-1. Combining our field data and previously published data, we found that N* in the Bohai and Yellow seas increased in the 1990s and 2000s, likely caused by the combined effect of atmospheric nitrogen deposition increase and the Kuroshio N* rise. In the 2010s, however, the coastal N* increases stopped. Based on a N*-budgeting approach, marine N (either from in situ decomposition of marine organic matters or from the open seas via current inputs) and non-marine N (either from riverine inputs or from local atmospheric nitrogen deposition) were distinguished. Marine N accounted for 51% ± 38% of DIN in the Bohai Sea and 67% ± 37% of DIN in the Yellow Sea. Although this is a regional study, we suggest that accumulation of atmospheric nitrogen along oceanic circulation pathways dominates the decadal evolution of coastal eutrophication. These findings and new insights may improve management of eutrophication in these two important marginal seas, and will also improve our understanding of nutrient dynamics in other marine systems.

Improving the understanding of central Bohai Sea eutrophication based on wintertime dissolved inorganic nutrient budgets: Roles of north Yellow Sea water intrusion and atmospheric nitrogen deposition.

The Bohai Sea is a shallow-water, semi-enclosed marginal sea of the Northwest Pacific. Since the late 1990s, it has suffered from nutrient over-enrichment. To better understand the eutrophication characteristics of this important coastal sea, we examined four survey datasets from summer (June 2011), late autumn (November 2011), winter (January 2016), and early spring (April 2018). Nutrient conditions in the Bohai Sea were subject to seasonal and regional variations. Survey-averaged N/P ratios in estuarine and nearshore areas were 20-133. In contrast, the central Bohai Sea had mean N/P ratios of 16.9 ± 3.4 in late autumn, 16.1 ± 3.0 in winter and 13.5 ± 5.8 in early spring, which are close to the traditional N:P Redfield ratio of 16. In summer, both dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP) were used up in the surface waters of the central Bohai Sea, suggesting that the biological consumption of DIN and DIP may also follow the Redfield ratio. Wintertime nutrient budgets of the central Bohai Sea water were then established based on a mass balance study. Our results suggest that the adjacent North Yellow Sea supplied additional DIP to the central Bohai Sea via wintertime water intrusion, balancing terrigenous excess DIN that was introduced in summer. A water-mixing simulation combining these two nutrient sources with atmospheric nitrogen deposition suggests that eutrophication in the central Bohai Sea will likely be enhanced by the large-scale accumulation of anthropogenic nitrogen in adjacent open oceans. Such changes in nutrients may have fundamentally contributed to the recent development of algal blooms and seasonal hypoxia in the central Bohai Sea.

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.