Submarine groundwater discharge and its implication for nutrient budgets in the western Bohai Bay, China

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.

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.

Nutrient changes in the Bohai Sea over the past two decades

With the growing problem of eutrophication in the Bohai Sea, actions have been taken to reduce nutrient inputs, but it remains to be seen whether nutrient levels and structure have been ameliorated. In this study, the nutrient trends in the Bohai Sea are re-examined based on observations from 2000 to 2019. The results suggest that dissolved inorganic nitrogen (DIN) concentrations and DIN/DIP (dissolved inorganic phosphate) ratios gradually increased from 2000 to 2013 but dramatically decreased from 2013 to 2019. The increase and decrease rates of DIN concentrations decreased with increasing water depth, indicating that DIN concentrations in nearshore waters responded more rapidly to changes in human activities. However, DIP concentrations responded weakly to nutrient inputs, with their trends uncoupled. The DIN/DIP ratios have declined close to and in some seasons even below the canonical Redfield ratio in areas with water depths >20 m recently, implying that relative nutrient limitation in these areas may be shifting from relative phosphorus (P) limitation to absence of relative nutrient limitation or relative nitrogen (N) limitation. Atmospheric deposition, wastewater discharge, and riverine input were responsible for 66 %, 21 %, and 13 % of the variance in the decline of DIN concentration, respectively. Several environmental indicators responded positively to the decrease in DIN concentrations and DIN/DIP ratios, with varying degrees of recovery recently. Our study proves the phased success of various nutrient reduction measures taken by the Chinese government to improve the environment of the Bohai Sea over the past decade.

Effect of submarine groundwater discharge on nutrient distribution and eutrophication in Liaodong Bay, China

Driven by the anthropogenic activities associated with coastal settlements, eutrophication has become a global issue. Submarine groundwater discharge (SGD) is a significant continuous pathway for transporting nutrients from land to coastal waters, but its influence on eutrophication in Liaodong Bay (LDB) has received limited attention. In this study, radium isotopes and nutrient data from coastal waters were analyzed to evaluate the SGD flux and its implications for potential eutrophication in LDB. We found that the mean concentrations of dissolved inorganic nitrogen (DIN), phosphorous (DIP), and silicate (DSi) in groundwater were higher than those of seawater and river water. Based on 223Ra and 228Ra mass balance models, the SGD fluxes were estimated to be (0.53-2.03) × 109 m3/d, of which the fresh SGD accounted for 4 %-15 %. SGD is a vital invisible source of nutrients, contributing more than 79 % of the total inputs of DIN, DIP, and DSi into LDB. With high DIN/DIP ratios (average=85.8) and large nutrient inputs, SGD may significantly drive the phosphorus limitation and eutrophication in LDB. This study shows that SGD-derived nutrient fluxes should be considered in the assessment of water eutrophication for the formulation of future environmental management protocols in coastal systems.

Using 222Rn temporal and spatial distributions to estimate the groundwater discharge rate and associated nutrient fluxes into high salinity lakes in Badain Jaran Desert, Northwest China

Groundwater is the main source of water and salt recharge for to lakes in arid regions. Quantifying the groundwater discharge and its nutrient input is important in the evolution of lake environments in the Badain Jaran Desert (BJD), Northwest China. In this study, ten BJD lakes were sampled for ²²²Rn in April and September 2021, and the ²²²Rn mass balance model was used to quantify the groundwater discharge rates and derived nutrient fluxes to these lakes. The results showed that the ²²²Rn activity and the groundwater recharge rate of lake water both present a positively correlated with lake water depth. The hot points of high ²²²Rn activity in the lake water were consistent with the locations of groundwater discharge areas. According to the ²²²Rn temporal and spatial distributions, the mean groundwater recharge rates for the ten lakes in April and September were 5.4 ± 0.6 and 7.7 ± 1 mm/d, respectively, and the annual mean groundwater discharge rates varied between 1.1 ± 0.2 and 14.6 ± 1.6 mm/d, with a mean of 7 ± 0.9 mm/d. Given that all the perennial lakes in the BJD have the same groundwater recharge rate as the mean recharge rate of the ten studied lakes, the annual mean groundwater recharge amount received by the lakes in the entire BJD is (5.6 ± 0.7) × 10⁷ m³/a. According to the groundwater recharge amount, the inputs of dissolved inorganic nitrogen, dissolved inorganic phosphorus, dissolved inorganic silicon, total nitrogen, and total phosphorus to the BJD lakes from groundwater were (4.7 ± 0.6) × 10⁵, (3.8 ± 0.5) × 10⁴, (7.9 ± 1) × 10⁵, (7.2 ± 0.9) × 10⁵, (2.5 ± 0.3) × 10⁴ kg/a, respectively. This study provides a reference for quantifying of groundwater discharge rates into salt lakes in other arid regions.

Nutrient budgets for the Bohai Sea: Implication for ratio imbalance of nitrogen to phosphorus input under intense human activities

Eutrophication is a global problem for coastal ecosystems, one that the Bohai Sea (BHS), China, is severely afflicted by due to rapid economic and social development over the last forty years. For sustainable nutrients management in the BHS, comprehensive budgets for Nitrogen (N) and Phosphorus (P) was characterized in 2017, and the relative contributions of river input, submarine fresh groundwater discharge, atmospheric deposition, sediment diffusion, and exchange with the Yellow Sea were quantified. The annual N and P fluxes into the BHS were 362 × 10³ t and 10.4 × 10³ t, respectively. The terrigenous N inputs occupied the highest proportion, while the largest P input was from sediment diffusion. The ratio of N:P was 77 for total external inputs, while that of the Yellow River was 680; both exceeded the Redfield ratio, indicating an imbalance in the nutrient structure and a P limitation in the BHS.

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.

Influences of phosphorus concentration and porewater advection on phosphorus dynamics in carbonate sands around the Weizhou Island, northern South China Sea

A series of flow-through reactor experiments were undertaken to assess the potential effect of porewater advection and dissolved inorganic phosphorus (DIP) concentration on benthic DIP dynamics in permeable sediments collected from the Weizhou Island, northern South China Sea. The flux of DIP ranged from -0.13 to 0.05 mmol m^-2 h^-1, and the reversal from DIP efflux to influx occurred when the DIP concentration reached a threshold. DIP release from the sediment into the seawater peaked at intermediate advection rate, which perhaps provide optimum conditions for DIP release related to CaCO₃ dissolution. Phosphorus limitation in seawater could be relieved by DIP release from the sediment, and CaCO₃-bound P in carbonate sands may play a major role in benthic DIP release and decrease in the molar nitrogen/phosphorus ratio in seawater around the Weizhou Island.

222Rn, 210Pb and 210Po in coastal zone groundwater: Activities, geochemical behaviors, consideration of seawater intrusion effect, and the potential radiation human-health risk

Groundwater quality in human-influenced coastal landscapes is receiving novel attention. Radionuclides have been recognized as another important monitoring indicator in many developed countries due to the discovery of extremely high level of natural ²¹⁰Po (up to 10,000 Bq/m³) and radium and radon isotopes. This study aims to evaluate the groundwater quality in the Beibu Bulf-Guangxi coast from radiological point of view. ²¹⁰Po, ²¹⁰Pb and ²²²Rn activities in 20 wells ranged from 0.24 ± 0.05 to 6.96 ± 1.62 Bq/m³, 2.17 ± 0.12 to 13.08 ± 0.74 Bq/m³ and 1500 ± 200 to 31,800 ± 900 Bq/m³, respectively. Compared with research data of other countries, groundwaters in this area have ²¹⁰Po, ²¹⁰Pb and ²²²Rn activity within low levels. The large deficiencies of ²¹⁰Po and ²¹⁰Pb relative to ²²²Rn in groundwaters implied that ²¹⁰Po and ²¹⁰Pb are strong particle-reactive radionuclides and they might be controlled by similar scavenging processes in groundwaters due to a good positive correlation between ²¹⁰Pb and ²¹⁰Po (R² = 0.67, p < 0.01). The concentrations of ²¹⁰Po and ²¹⁰Pb decreased with increasing pH values and salinity, which indicated that geochemical behaviors of ²¹⁰Po and ²¹⁰Pb in groundwater were influenced by seawater intrusion and pH changing. Groundwater ²²²Rn activity concentrations decreased with increasing salinity in coastal zone, which may be caused by dilution due to seawater intrusion or intensified ²²²Rn escaping from well-developed pores in coastal zone. The estimated annual ingestion doses for infants, children and adults were well below the recommended reference dose level (RDL) of 0.2-0.8 mSv/a, suggesting that consumption of analyzed groundwaters is safe from radiological point of view.