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.

Impact of dust deposition on phytoplankton biomass in the Northwestern Pacific: A long-term study from 1998 to 2020

Dust deposition can supply nutrients to the ocean and affect phytoplankton growth. However, the impact of dust deposition on phytoplankton biomass in varying trophic regions remains poorly evaluated. The Northwestern Pacific is located in the downwind area of East Asian dust and includes eutrophic regions (Yellow Sea, YS; East China Sea, ECS), high-nutrient low-chlorophyll waters (subarctic Northwestern Pacific, SNWP) and low-nutrient low-chlorophyll waters (Northwestern Pacific subtropical gyre, NWPSG), which is an ideal region to explore the spatial heterogeneity of the dust fertilization effect. Here, the distribution and variation of dust deposition, high dust deposition events (HDDE) and Chlorophyll-a concentration (Chl-a, mg m^-3) in the Northwestern Pacific during spring from 1998 to 2020 were investigated. The differences in the response of phytoplankton biomass (using Chl-a as a proxy) to HDDE in the YS, the ECS, the SNWP and the NWPSG were explored. Our results indicated that a large amount of dust was deposited into the Northwest Pacific during spring, resulting in numerous HDDE. The HDDE could stimulate the increase of phytoplankton biomass in the whole area of the Northwestern Pacific during spring. The response probabilities of Chl-a to HDDE were most significant (~80%) in the SNWP and the duration of response was the longest, even lasting for up to 40 days. While the response probabilities of Chl-a to HDDE were lowest in the YS and ECS (~65%), increasing from north to south, and most of the responses were less than 20 days. The response of Chl-a to HDDE was also detected in NWPSG, confirming the dust fertilization effect in oligotrophic waters, with response probabilities of 70% and duration less than 30 days. Overall, this study provides a more comprehensive understanding of the differences of phytoplankton response to dust deposition in varying trophic regions.

Sources and dry deposition of carbonaceous aerosols over the coastal East China Sea: Implications for anthropogenic pollutant pathways and deposition

75 paired TSP and PM_2.5 samples were collected over four seasons on Huaniao Island (HNI), an island that lies downwind of continental pollutants emitted from mainland China to the East China Sea (ECS). These samples were analyzed for organic carbon (OC) and elemental carbon (EC), with a special focus on char-EC (char) and soot-EC (soot), to understand their sources, and the scale and extent of pollution and dry deposition over the coastal ECS. The results showed that char concentrations in PM_2.5 and TSP averaged from 0.13 to 1.01 and 0.31-1.44 μg m^-3; while for soot, they were from 0.03 to 0.21 and 0.16-0.56 μg m^-3, respectively. 69.0% of the char and 36.4% of the soot were present in PM_2.5. The char showed apparent seasonal variations, with highest concentrations in winter and lowest in summer; while soot displayed maximum concentrations in fall and minimum in summer. The char/soot ratios in PM_2.5 averaged from 3.29 to 17.22; while for TSP, they were from 1.20 to 7.07. Both of the ratios in PM_2.5 and TSP were highest in winter and lowest in fall. Comparisons of seasonal variations in OC/EC and char/soot ratios confirmed that char/soot may be a more effective indicator of carbonaceous aerosol source identification than OC/EC. Annual average atmospheric dry deposition fluxes of OC and EC into ECS were estimated to be 229 and 107 μg m^-2 d^-1, respectively, and their deposition fluxes significantly increased during episodes. It was estimated that the loadings of OC + EC and EC accounted for 1.3% and 4.1% of the total organic carbon and EC in ECS surface sediments, respectively, implying a relatively small contribution of OC and EC dry deposition to organic carbon burial. This finding also indicates a possibly more important contribution of wet deposition to organic carbon burial in sediments of ECS, and this factor should be considered for future study.

Algae explosive growth mechanism enabling weather-like forecast of harmful algal blooms

As a global problem in coastal environments, harmful algal blooms (HABs) have seriously affected the health of coastal ecosystems and regional economies. Here we report an aerosol-trigger mechanism for the occurrence of HABs based on long-term field data and laboratory experiments. The occurrence times of HABs and aerosol events had a significant correlation from 2005 to 2013 in the East China Sea, indicating that aerosol transport was probably an alternative trigger of HABs. HABs mostly occur in the transition time between winter and summer, during which northwest monsoon transport substantial aerosol (rich in phosphate, iron and other trace metals) to coastal waters, as revealed by chemical measurements, transmission electron microscope and electron microprober results. Such nutrients can stimulate algal growth in our incubation experiments, suggesting that such aerosol transport can be important nutrient sources for the East China Sea where phytoplankton growth is relatively phosphate limited. Air-borne nutrients are available for algal growth by rapid downward air flow, which additional results a clear weather condition, and thus adequate light intensity for algal growth. At last, the transition from northwest monsoon to warm southwest monsoon establishes favorable seawater temperature for algal blooms. Such weather-related aerosol-trigger mechanism suggests possibly forecast of HABs.

Effects of continental anthropogenic sources on organic aerosols in the coastal atmosphere of East China

Although organic compounds in marine atmospheric aerosols have significant effects on climate and marine ecosystems, they have rarely been studied, especially in the coastal regions of East China. To assess the origins of the organic aerosols in the East China coastal atmosphere, PM_2.5 samples were collected from the atmospheres of the Yellow Sea, the East China Sea, and Changdao Island during the CAPTAIN (Campaign of Air PolluTion At INshore Areas of Eastern China) field campaign in the spring of 2011. The marine atmospheric aerosol samples that were collected were grouped based on the backward trajectories of their air masses. The organic carbon concentrations in the PM_2.5 samples from the marine and Changdao Island atmospheres were 5.5 ± 3.1 μgC/m³ and 6.9 ± 2.4 μgC/m³, respectively, which is higher than in other coastal water atmospheres. The concentration of polycyclic aromatic hydrocarbons (PAHs) in the marine atmospheric PM_2.5 samples was 17.0 ± 20.2 ng/m³, indicating significant continental anthropogenic influences. The influences of fossil fuels and biomass burning on the composition of organic aerosols in the coastal atmosphere of East China were found to be highly dependent on the origins of the air masses. Diesel combustion had a strong impact on air masses from the Yangtze River Delta (YRD), and gasoline emissions had a more significant impact on the "North China" marine atmospheric samples. The "Northeast China" marine atmospheric samples were most impacted by biomass burning. Coal combustion contributed significantly to the compositions of all of the atmospheric samples. The proportions of secondary compounds increased as samples aged in the marine atmosphere indicating that photochemical oxidation occured during transport. Our results quantified ecosystem effects on marine atmospheric aerosols and highlighted the uncertainties that arise when modeling marine atmospheric PM_2.5 without considering high spatial resolution source data and meteorological parameters.

Characteristics and impacts of trace elements in atmospheric deposition at a high-elevation site, southern China

To investigate the regional background trace element (TE) level in atmospheric deposition (dry and wet), TEs (Fe, Al, V, Cr, Mn, Ni, Cu, Zn, As, Se, Mo, Cd, Ba, and Pb) in 52 rainwater samples and 73 total suspended particles (TSP) samples collected in Mt. Lushan, Southern China, were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS). The results showed that TEs in wet and dry deposition of the target area were significantly elevated compared within and outside China and the volume weight mean pH of rainwater was 4.43. The relative contributions of wet and dry depositions of TEs vary significantly among elements. The wet deposition fluxes of V, As, Cr, Se, Zn, and Cd exceeded considerably their dry deposition fluxes while dry deposition dominated the removal of pollution elements such as Mo, Cu, Ni, Mn, and Al. The summed dry deposition flux was four times higher than the summed wet deposition flux. Prediction results based on a simple accumulation model found that the content of seven toxic elements (Cr, Ni, Cu, Zn, As, Cd, and Pb) in soils could increase rapidly due to the impact of annual atmospheric deposition, and the increasing amounts of them reached 0.063, 0.012, 0.026, 0.459, 0.076, 0.004, and 0.145 mg kg^-1, respectively. In addition, the annual increasing rates ranged from 0.05% (Cr and Ni) to 2.08% (Cd). It was also predicted that atmospheric deposition induced the accumulation of Cr and Cd in surface soils. Cd was the critical element with the greatest potential ecological risk among all the elements in atmospheric deposition.

Process study of biogeochemical cycling of dissolved inorganic arsenic during spring phytoplankton bloom, southern Yellow Sea

Previous studies in the southern Yellow Sea (SYS) suggest that large spring phytoplankton blooms (SPBs) have occurred in recent decades. Elevated primary production in the water column can lead to the accumulation and transformation of trace elements. Two field study cruises (including two drifting anchor surveys) were conducted on 12-19 February and from 24 March to 15 April 2009, to investigate the impact of different SPB development periods on the concentrations of total dissolved inorganic arsenic (TDIAs: [TDIAs]=[As(V)]+[As(III)]) and As(III) (arsenite) in the SYS. The distribution of TDIAs in the study area was similar between the two field studies, with concentrations increasing from coastal to offshore areas. High arsenite concentrations and As(III)/TDIAs ratios were found in areas having high concentrations of chlorophyll-a, particularly in the subsurface waters of the central SYS during the drifting surveys, where a significant SPB occurred. Results show that the integrated arsenite concentrations increased at an average transformation rate of 0.53±0.24nmol/L/d within the 15days during the bloom, and data from the anchor drifting surveys indicated that approximately 15.1% of the arsenate in the euphotic zone (~30m depth) was converted to arsenite. In addition, 7.1% of TDIAs was scavenged from the water column by phytoplankton forming the blooms (a factor of 5 higher than expected). A preliminary box model was established to estimate the budget for TDIAs in the SYS in early spring (February to April). This showed that biological scavenging is an important sink for TDIAs, which may promote the transformation and migration of inorganic arsenic species, and thus have a substantial impact on the biogeochemical cycling of this element in the SYS. Depletion of arsenate in the upper waters could lead to arsenate stress, potentially damaging fisheries and the ecosystem.

PM2.5 in the Yangtze River Delta, China: Chemical compositions, seasonal variations, and regional pollution events

Fine particle (PM_2.5) samples were collected simultaneously at three urban sites (Shanghai, Nanjing, and Hangzhou) and one rural site near Ningbo in the Yangtze River Delta (YRD) region, China, on a weekly basis from September 2013 to August 2014. In addition, high-frequency daily sampling was conducted in Shanghai and Nanjing for one month during each season. Severe regional PM_2.5 pollution episodes were frequently observed in the YRD, with annual mean concentrations of 94.6 ± 55.9, 97.8 ± 40.5, 134 ± 54.3, and 94.0 ± 57.6 μg m^-3 in Shanghai, Nanjing, Hangzhou, and Ningbo, respectively. The concentrations of PM_2.5 and ambient trace metals at the four sites showed clear seasonal trends, with higher concentrations in winter and lower concentrations in summer. In Shanghai, similar seasonal patterns were found for organic carbon (OC), elemental carbon (EC), and water-soluble inorganic ions (K⁺, NH₄⁺, Cl^-, NO₃^-, and SO₄^2-). Air mass backward trajectory and potential source contribution function (PSCF) analyses implied that areas of central and northern China contributed significantly to the concentration and chemical compositions of PM_2.5 in Shanghai during winter. Three heavy pollution events in Shanghai were observed during autumn and winter. The modelling results of the Nested Air Quality Prediction Modeling System (NAQPMS) showed the sources and transport of PM_2.5 in the YRD during the three pollution processes. The contribution of secondary species (SOC, NH₄⁺, NO₃^-, and SO₄^2-) in pollution event (PE) periods was much higher than in BPE (before pollution event) and APE (after pollution event) periods, suggesting the importance of secondary aerosol formation during the three pollution events. Furthermore, the bioavailability of Cu, and Zn in the wintertime PM_2.5 samples from Shanghai was much higher during the pollution days than during the non-pollution days.

Atmospheric dry deposition of mineral dust to the Gulf of Aqaba, Red Sea: rate and trace elements

Atmospheric dry deposition to the Gulf of Aqaba (GoA) is particularly a significant source of trace elements. Amid desert regions, the Gulf receives high fluxes of mineral dust with an average rate of 34.68 g/m(2)/year measured in 2012. Patterns of dry deposition showed seasonal fluxes with highest rates observed in summer and lowest in winter. The observed variations were attributed to wind direction, timing of deposition and sources of dust. The average dry fluxes of Al, Fe, Mn, Cr, Cd, Cu, Pb and Zn were 551, 440, 10.29, 1.42, 0.04, 0.68, 1.42 and 4.02 mg/m(2)/year, respectively. While the dry deposition fluxes were enriched in Cd, Cu, Pb and Zn indicating their dominant anthropogenic sources, they appeared to be less influenced compared to the neighboring Mediterranean area and other industrial countries, but were similar to or slightly higher than those in remote areas. The enrichment values for Fe and Mn were low, consistent with their crustal origin. The fluxes of all elements suggested the impacts of both crustal (due to climate change) and anthropogenic sources became stronger in this region. The Sahara dust was probably a minor contributor to dry deposition in the GoA.

Effects of Asian dust storms on synechococcus populations in the subtropical Kuroshio Current

Asian dust storms (ADSs) are the major source of dust deposition in the Northwest Pacific Ocean. To gain a better understanding on how ADSs affect the ecology of picophytoplankton in this oligotrophic region, five oceanographic cruises were conducted between March 15 and April 15, 2006 on a segment of the Kuroshio Current near the shelf break of the East China Sea (25.05° N, 123.15° E). During the study period, three ADS events were recorded and increases in nutrient concentrations as well as mixing depths were observed. Most of the ADS events stimulated the growth of Synechococcus, but the abundance of Prochlorococcus either remained unaffected or showed mild declines. A more detailed study was conducted during the ADS event between March 16 and 19. Phylogenetic analysis of 16S ribosomal RNA nucleotide sequences revealed that most of the newly appeared Synechococcus belonged to the clade II lineage. Furthermore, messenger RNA (mRNA) levels of three nutrient deficiency indicators, including idiA (an iron deficiency indicator), ntcA (a nitrogen deficiency indicator), and pstS (a phosphorus deficiency indicator), were analyzed by real-time quantitative reverse-transcription polymerase chain reaction. As this ADS event proceeded, mRNA levels of all these indicators decreased from relatively high to non-detectable values. These results suggest that the contributions of iron, nitrogen, and phosphate by the dust deposition from ADSs promote the growth of Synechococcus in the Kuroshio Current.