Nutrient changes in the Bohai Sea over the past two decades

Spatial distribution characteristics and the environmental regulation mechanisms of phytoplankton chlorophyll a in the Bohai Sea and the Yellow Sea

To comprehensively analyze the relation between the environmental factors and chlorophyll a (Chl-a) in the Bohai Sea (BS) and the Yellow Sea (YS), three investigations were conducted during April and October of 2019, and July of 2020. The distribution of Chl-a demonstrated pronounced regional and seasonal variability. The horizontal distribution of Chl-a was notably heterogeneous, generally exhibiting higher concentrations near the shoreline and diminishing values towards the central regions of the sea. Meanwhile, the vertical distribution of Chl-a concentrations also illustrated significant variations with depth and regional differences. The relationship between Chl-a concentration and environmental factors varied significantly across seasons. In the BS and the YS, seasonal variations in temperature and salinity were the principal drivers of the seasonal fluctuations in Chl-a concentrations. Additionally, variations in nutrient concentrations, which were influenced by temperature, contributed significantly to the seasonal variability of Chl-a concentrations.

Assessment of marine eutrophication: Challenges and solutions ahead

Marine eutrophication remains a pressing global environmental challenge, demanding urgent advances in science-based assessment frameworks to mitigate its ecological and socio-economic impacts. Current methodologies, however, face critical limitations, including overly complex indicator systems, high spatiotemporal variability in coastal and marine environments, challenges in distinguishing natural and anthropogenic drivers, and pervasive data scarcity. These issues often result in discrepancies between assessment outcomes and observed eutrophication dynamics, undermining management efficacies. This study proposes a refined assessment framework integrating emerging technologies to enhance diagnostic resolution and accuracy. Strategic priorities include reefing feasible indicators, harmonizing multi-source data, establishing ecosystem-specific thresholds, and leveraging innovations in machine learning and remote sensing. The proposed framework bridges the gap between eutrophication science and actionable governance solutions, providing robust evidence for policymakers to safeguard marine biodiversity and ecosystem sustainability.

The individual and combined effects of coastline changes and riverine input on water quality: A multi-scenario simulation perspective

The Bohai Sea Rim is densely populated and highly industrialized, facing threats from large-scale land reclamation, riverine pollutant discharge, and other anthropogenic activities. Aiming to improve seawater quality, the optimal allocation of land-based total nitrogen (TN) loads among four major rivers around the Bohai Sea has been studied before. However, the individual and combined impacts of coastline changes and riverine input were not previously quantified and analyzed. Therefore, a coupled hydrodynamics-water quality model was used to simulate the variation of dissolved inorganic nitrogen (DIN) and TN caused by reclamation and sedimentation. The results show that coastline changes significantly influence tidal dynamics, tidal prisms, residual currents, and the degree of openness, thereby altering the diffusion pathway of pollutants. Specifically, land reclamation activities and Yellow River mouth movements have led to increased concentrations of DIN and TN in estuarine areas (e.g., Haihe River Estuary, Ziyaxin River Estuary, and Wei River Estuary). Additionally, the combined impacts of coastline changes and TN input variations were found to amplify or diminish their individual impacts on water quality. Considering the individual factors, the impact of coastline change (15-year basis) was much greater than that of land-based TN input (a 10 % and 20 % random error, respectively). This study provides valuable insights into the complex interactions between coastline dynamics and marine water quality, emphasizing the importance of considering both natural and anthropogenic factors in coastal environmental management.

Dissolved oxygen depletion in Chinese coastal waters

Estuarine and coastal environments have experienced dissolved oxygen (DO hereafter) depression and hypoxia due to increasingly intensified anthropogenic eutrophication and climate warming. This review compared diverse systems in Chinese coastal waters that experience DO depletion or hypoxia, aiming to identify essential aspects in advancing the abilities in comprehensively understanding DO dynamics across systems that span wide ranges of physical and biogeochemical environments. The coastal DO depression and relevant ecological consequences around the world are generally overviewed. DO depression in specific systems around Chinese coastal waters, ranging from large estuarine-coastal system to small embayment, are selected to synthetically understand the environment, the controlling processes, the evolution of eutrophication level, and the potential environmental changes under warming trend. Stressed ecosystems would be put at higher risks with high confidence due to increased complexity and uncertainty caused by future socioeconomic transformation and climate warming. This review proposes key aspects to advancing the abilities in predicting, managing, and mitigating DO stress for marine ecosystems in Chinese coastal waters, potentially providing a framework to discuss future DO changes in the coastal waters worldwide.

Changes and influencing factors of phytoplankton in the Tianjin coastal waters of Bohai Bay at the early stage of COVID-19 outbreak

We studied the characteristics of plankton community and its response to water environmental factors at the early stage of COVID-19 outbreak (2019-2020) in the Tianjin coastal waters of Bohai Bay. The water quality showed a good trend during this period, due to the reduction of pollution brought by the runoff rivers and water exchange driven by the circulation. In the survey area, 68 species of diatomata and dinoflagellata phytoplankton were found, where diatomata was the dominant population. The number of phytoplankton increased from 636,000 in 2019 to 1,642,600 in 2020. Compared with 2019, the impact factors of water environment in the surveyed area changed greatly, where nitrogen was the main impact factor in 2020. The adjustment of production and lifestyle in the early stage of COVID-19 outbreak suddenly slowed down the growth of the secondary industry, which contributed a lot to the improvement of water quality in 2020.'

Underestimation of calcium carbonate saturation state in marginal seas due to the disregard of calcium ion addition: A case study of the Bohai sea, China

Seawater calcium ion (Ca2+) concentration was investigated based on the potentiometric titration method during the summer of 2018 in the Bohai Sea, China. The measured Ca2+ concentration ranged from 7760 to 9739 μmol kg-1 and deviated from the theoretical Ca2+ values, which were estimated from the calcium/salinity ratio. The excess calcium (Ca2+excess) ranged from 186 to 1229 μmol kg-1, showing a decreasing trend from the estuary to the nearshore, and then the offshore areas. Riverine input was an important source of seawater Ca2+excess in the Bohai Sea. Biological activity was another factor in regulating seawater Ca2+excess by precipitation in the Yellow River estuary and dissolution in other area of the Bohai Sea. Furthermore, the aragonite saturation state (Ωarag) values calculated from the measured Ca2+ concentrations showed a significant deviation from the values calculated from the theoretical Ca2+ concentrations, especially in the estuarine area with a maximum difference of 18.5%. Therefore, the disregard of the calcium addition would lead to an underestimation of the calcium carbonate saturation state and a deviation in the assessment of ocean acidification in marginal seas.

Unveiling the dominance of submarine groundwater discharge on nutrient sources in the Eastern China Marginal Seas

River and atmosphere are traditionally recognized as the primary nutrient sources impacting coastal ecosystems. Despite the increasing attention towards the often-neglected submarine groundwater discharge (SGD), its understanding and significance in highly human-impacted marginal seas remain limited. This study utilizes unprecedented high-resolution data (561 seawater and 282 groundwater radium samples) to provide precise estimates of 226Ra and 228Ra sources and sinks in the Eastern China Marginal Seas. A coupled 226Ra and 228Ra mass balance model enable an integrated SGD flux of (3.7 ± 2.4) × 1012 m3 yr-1, surpassing rivers by 3.4 times. Furthermore, nutrient delivery from SGD exceeds riverine and atmospheric inputs, potentially inducing substantial changes in coastal nutrient cycles. These alterations have profound implications for primary production and biological communities, deviating significantly from the Redfield ratio. Therefore, comprehending the significance of SGD in nutrient budgets is vital for a comprehensive understanding of biogeochemical dynamics and functionality of marginal sea ecosystems.

Key biogeochemical processes and source apportionment of nitrate in the Bohai Sea based on nitrate stable isotopes

Excessive nitrate input is one of the primary factors causing nearshore eutrophication. This study applied the nitrate stable isotope techniques to analyse the biogeochemical processes and sources of nitrate in the Bohai Sea (BHS). The results showed that intensive NO3- assimilation probably occurred at surface in summer, while nitrification primarily occurred in the Yellow River diluted water. In autumn, regional assimilation and nitrification were still identified. For avoiding the interference from assimilation, the isotopic fractionations were further calculated as correction data for the quantitative analysis of nitrate sources. The river inputs were identified as the primary source of nitrate in the BHS in summer and autumn, accounting for >50 %, and the atmospheric deposition was the secondary source. This study provides quantitative data for evaluating the significance of river inputs to the nearshore nitrate, which will be beneficial to policy formulation on the BHS eutrophication control.

Phosphorus depletion is exacerbated by increasing nitrogen loading in the Bohai sea

Phosphorus (P) is an essential nutrient for algal growth in nearshore ecosystems. In recent years, there has been a shift in nutrient dynamics in nearshore areas, leading to an exacerbation of P limitation, although the underlying mechanisms remain unclear. This study analyzed the P species and budget in the Bohai Sea (BS) from 2011 to 2020, aiming to explore the intrinsic mechanisms of P limitation in the BS. The results show that the main external source of P in the BS was river transport (89%), and the primary fate of P was burial (96%) into the sediment. Due to excessive nitrogen (N) input and biological processes in the BS, the P budget in the BS is unbalanced, resulting in an increase in the N/P ratio, particularly in nearshore areas. Nearshore areas typically have lower concentrations of dissolved inorganic P (DIP) in the water and higher concentrations of reactive P (Reac-P) in the sediments. This pattern is particularly evident in Bohai Bay and the northwest nearshore region, where harmful algal blooms occur frequently. To cope with enhanced P limitation, the biologically driven P regeneration and cycling processes within the BS are accelerated. From 2011 to 2020, the concentration of DIP in the BS during autumn increased, while the content of Reac-P in sediments slightly decreased. Historical data indicate that P depletion in the BS is intensifying and expanding, primarily due to N enrichment and algal production. N enrichment alters the structure and composition of primary production, potentially exacerbating P depletion in the BS. Excessive N may have significant impacts on the P pool, potentially influencing the stability of future coastal ecosystems.

A comprehensive analysis of submarine groundwater discharge and nutrient fluxes in the Bohai Sea, China

Groundwater discharge and associated nutrient fluxes in the Bohai Sea, China has attracted great attention, but most studies lacked high spatial resolution for the whole sea. As the largest semi-enclosed sea in China, the Bohai Sea is confronted with strong environmental pollution problems such as eutrophication induced by terrestrial nutrient inputs. However, the role of SGD has not been evaluated well for the whole Bohai Sea. In this study, stable isotopes (hydrogen and oxygen), radioactive isotope (228Ra), salinity, and temperature were combined to trace the diluted seawater. Mass balances of 228Ra, oxygen isotope, and salinity were used to quantify SGD and nutrient fluxes to the Bohai Sea. The estimated submarine fresh groundwater discharge (SFGD) and SGD to the Bohai Sea were (6.0 ± 0.5) × 109 and (2.7 ± 1.6) × 1011 m3 a-1, respectively. SFGD represents 10 % to 11 % of the total river discharge and SGD is about 2 to 8 folds of the total river discharge to the sea. Moreover, SGD derived dissolved nutrients to the Bohai Sea were (4.8 ± 4.0) × 1010 mol a-1 for dissolved inorganic nitrogen, (1.9 ± 1.7) × 1010 mol a-1 for dissolved inorganic phosphorus, and (6.7 ± 5.5) × 1010 mol a-1 for silicon. These nutrient inputs were about 10 to 20 folds of the total riverine inputs. Overall, this study underscores the importance of evaluating SGD to better understand the terrestrial imported nutrients in regional scale.

Comparison of nutrients status in Liaodong Bay and Northern Yellow Sea, China: Controlling factors and nutrient budgets

Under the dual stress of global warming and human interaction, Liaodong Bay (LDB) and northern Yellow Sea (NYS) are undergoing significant ecological changes. Little is known about the driving nutrients characteristics supporting fishery resource output in these areas. We carried out three field observations in 2019 to investigate nutrient status. Results showed that dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silica (DSi) concentrations changed seasonally, with lowest values in spring, and highest values in autumn. High DIN, DIP, and DSi concentrations were detected in LDB and NYS's estuary areas. The Yellow Sea Cold Water Mass plays a role in the distribution and seasonal variation of nutrients. Exchanges across the sediment-water interface, SFGD, atmospheric deposition, and the adjacent sea input dominated DIN dynamics of these areas. DIP primarily came from the adjacent sea input and DSi mainly originated from sediment release and the adjacent sea input. NYS seawater invasion accounted for 13.8% of DIN, 63.4% of DIP, and 35.1% of DSi in LDB. These results provide new insights to better facilitate the formulation of nitrogen and phosphorus reduction and control policies in these marginal seas.

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.