The biogeochemistry of inorganic carbon and nutrients in the Pearl River estuary and the adjacent Northern South China Sea

Seasonal variations in the inorganic carbon system in the Pearl River (Zhujiang) estuary

We examined the dynamics of dissolved inorganic carbon (DIC) and pH in the Pearl River Estuary (PRE) and the adjacent northern South China Sea (NSCS) shelf in summer, aiming for a better understanding of the interaction between eutrophication, hypoxia, and ocean acidification. Using a semi‐analytical diagnostic approach based on validated multiple end‐member water mass mixing models, we showed a −191 ± 54 μmol kg−1 deficit in DIC concentrations in an extensive surface plume bulge, corresponding to a significant pH increase of ∼ 0.57 ± 0.19 units relative to conservative mixing. In contrast, DIC additions in the bottom hypoxic zone reached ∼ 139 ± 21 μmol kg−1, accompanied by a decrease in pH of −0.30 ± 0.04 units. In combination with stable carbon isotopic compositions, we found biological production and CO2 outgassing to be responsible for DIC deficits in surface waters, while degradation of organic matter (OM) accounted for DIC additions in bottom waters. The PRE‐NSCS plume system as a whole served as a net source of atmospheric CO2 from the perspective of Lagrangian observations, because strong CO2 outgassing in the inner estuary overwhelmed the CO2 uptake in the plume despite strong phytoplankton blooms. Using a two‐layer box model, we further estimated that at least ∼ 45 ± 13% of eutrophication‐driven OM production in the surface plume accounted for 67 ± 18% of the DIC addition and oxygen consumption in bottom waters. Eutrophication also buffered ocean acidification in surface waters while hypoxia enhanced it in bottom waters, but their effects on acid‐base buffering capacity were secondary to the amplification of coastal ocean acidification caused by freshwater inputs.

The Pearl River delivers a large amount of plastic waste to the Pearl River Estuary (PRE) and adjacent Northern South China Sea (NSCS) region each year. However, the transport of floating litter after release is difficult to predict due to the complex hydrodynamic conditions caused by the climate variability. A regional ocean circulation model coupled with a Lagrangian particle tracking model is utilized in this study to simulate the distribution and fate of floating litter particles in the Pearl River Estuary (PRE) and Northern South China Sea (NSCS) under the influence of El Niño - Southern Oscillation (ENSO) event. Simulations are conducted during all four seasons (spring, summer, fall, and winter) in typical El Niño, La Niña, and ENSO-neutral year. The model reveals that most floating litter remains within Lingding Bay before being transported westward by the counterclockwise circulation over the NSCS and arriving at the Qiongzhou Strait. After crossing the Strait, the debris is carried by the counterclockwise circulation of the Beibu Gulf, and eventually arriving at the coasts of Vietnam and Laos. The ENSO warm (El Niño) and cold (La Niña) phases disrupt circulation patterns and modulate the amount of Pearl River runoff, thereby altering the transport pathways and grounding probabilities of floating litter. During La Niña years, floating litter particles spread over a wider area, travel longer distances, and have lower beaching probabilities. Conversely, during El Niño year, floating litter particles tend to remain within Lingding Bay for longer durations, with some debris entrained towards the Hong Kong region. This study underscores the impact of climate mode of variability in influencing the litter sources, fate and transport and accumulation at estuarine-coastal oceans, which will provide critical scientific insights for plastic pollution management in the PRE - NSCS region, which is a newly identified hotspot for floating litter and microplastic pollution in global oceans.

Spatial and temporal variation of organic carbon in the northern South China Sea revealed by sedimentary records

Coastal estuaries and adjacent continental shelf seas constitute vital global carbon reservoirs, and the sources and transformations of organic carbon in these regions are crucial to global biogeochemical cycles and climate change. This study investigated the total organic carbon (TOC), total nitrogen (TN), black carbon (BC), and their stable carbon and nitrogen isotopes (δ15NTN, δ13CTOC, δ13CBC) in the surface sediments of the Pearl River Estuary (PRE) and its adjacent northern South China Sea (NSCS) aiming to assess the impact of human activities on organic carbon dynamics in these areas. Results showed that the highest TOC concentrations occurred in the inner PRE due to intense human activities, and decreased seaward. The west side of the PRE exhibited higher TOC levels than the east side, which was attributed to differences in hydrodynamic processes and human activities. The westward flow of the Pearl River diluted water, which carried terrestrial organic matter inputs due to the influence of the Coriolis effect and intense local human activities, was a primary contributor to higher TOC levels on the west side (terrestrial source). In contrast, increased productivity and intensive mariculture activities on the east side predominated as sources of organic matter (marine source). Similar to the TOC, BC and TN sources were mainly influenced by human activities. δ15NTN distribution shows that TN in the east side of PRE mainly originated from industrial wastewater input from the Pearl River, while in the east side TN was mainly from domestic sewage discharge. Additionally, BC sources have shifted from primarily biomass combustion in the 1990s to fossil fuel emissions presently. Isotopic analysis revealed that over 70% of BC originated from fossil fuel inputs and C3 plant combustion, highlighting the significant influence of human activities in the PRE and adjacent NSCS, and underscoring the need for effective management and protection of the eco-environment in these regions.

Temporal trends of hydrocarbons in sediment cores from the Pearl River Estuary and the northern South China Sea

Surface sediment (0-5 cm) samples were collected from the Pearl River Estuary (PRE) and the adjacent northern South China Sea (SCS) in July 2002 and analyzed for 25 polycyclic aromatic hydrocarbons (PAHs) and 8 organochlorine pesticides (OCPs) including dichlorodiphenyltrichloroethanes (DDTs), hexachlorocyclohexanes (HCHs), and heptachlor. The total PAHs and OCPs concentrations were 138-1100 and 0.18-3.57 ng/g dry weight, respectively. Compositional pattern analysis suggested that PAHs in the PRE were derived from both pyrogenic and petrogenic sources, whereas most PAHs in the northern SCS were pyrogenically originated. The concentrations of both PAHs and OCPs were higher in the PRE than in the northern SCS, and a decreasing trend with the distance from the estuary to the open sea was observed. In addition, perylene was a predominant component in all samples and clustered with PAH compounds with high log Kow values (from phenanthrene). These findings indicated that river outflows were the major source of contamination in the offshore sediments. A preliminary assessment suggested that atmospheric deposition contributed only a minor portion of PAHs or OCPs in the sediments of the northern SCS. The sediment (0-5 cm) mass inventories were 126 and 423 metric tons for PAHs and were 0.4 and 1.4 metric tons for OCPs in the PRE and the northern SCS, respectively. Clearly, contaminated sediments of the northern SCS may be a potential source of PAHs and OCPs to the global oceans.

Sedimentary record of black carbon in the Pearl River estuary and adjacent northern South China Sea

Dual isotope measurements were performed on nitrate (δ15N‐NO3− and δ18O‐NO3−) in the Pearl River Estuary (PRE) and the adjacent northern South China Sea (NSCS) to investigate nitrate sources and its biogeochemical processes during rainy season. Our results indicated that in the PRE, high nitrate levels were associated with the intense human activities in PRE delta, the sources of which included the reduced nitrogen (N) fertilizer (29%), SN (nitrate derived from soil N) (18%), manure and sewage (10%), and atmospheric deposition (7%). In the nearshore area, nitrate was characterized by relatively high δ15N‐NO3− and δ18O‐NO3− values suggestive of incomplete nitrate assimilation. Besides, nitrification might be significant in the intermediate and bottom waters due to ammonia release from remineralization of sinking and sedimented organic matter (OM). In the offshore area, nitrate was depleted due to nearly complete utilization, and δ15N‐NO3− and δ18O‐NO3− showed moderate values. In addition, δ18O‐NO3− exhibited a clear vertical gradient with a decrease from the surface to the bottom. A coupled nitrification‐denitrification process in sediment porewater was supposed, which left isotope imprints in bottom waters of the offshore area. In surface waters from the nearshore to the offshore area, although assimilation was the prominent process, the δ15N‐NO3− and δ18O‐NO3− relationship deviated from the 1:1 line, suggesting significant contribution of atmospheric deposition that increased offshore. Our study suggests the external source of nitrate shifts from anthropogenic nitrogen to atmospheric source in the continuous river–estuary–ocean system, and nitrification‐denitrification become more active as it goes seaward.

Seawater, atmospheric dimethylsulfide and aerosol ions in the Pearl River Estuary and the adjacent northern South China Sea

Spatial distributions of polyunsaturated aldehydes and their biogeochemical implications in the Pearl River Estuary and the adjacent northern South China Sea

The mixing of different water masses is important for local physical and biogeochemical processes as well as for ecosystems in the ocean. In this study, a new dataset of stable water isotopes (δD and δ18O) combined with temperature–salinity profiles was used to quantitatively understand the mixing of water masses in the Pearl River Estuary (PRE) and the adjacent northern South China Sea (SCS). Based on hydrographic characteristics and the isotope–salinity relationships in the water column, three water masses, namely, low isotopic values (<1.5‰ for δD and <0‰ for δ18O) with a salinity of <33.20 for PRE water (PREW), high isotopic values (>2.0‰ for δD and >0.6‰ for δ18O) with a salinity of >34.60 for SCS Kuroshio Branch (SCSKB), and higher isotopic values (>3.0‰ for δD and <0.4‰ for δ18O) with a salinity of >33.30 for SCS water (SCSW), were identified in the PRE and the adjacent SCS. The mixing of the three water masses in the PRE and the adjacent SCS was mainly from SCSW (71%), followed by the SCSKB (23%), and the proportion of PREW only accounted for 6%. However, different water layers and regions are affected differently by these three water masses. The surface water is mainly influenced by the PREW, whereas the subsurface water is mainly influenced by the intrusion of SCSKB (100–300 m). The mixing process of water masses in the west side of the study area (<115°E) is mainly contributed by the SCSW (86%), whereas the contributions of PREW and SCSKB are only 4% and 10%, respectively. By contrast, the mixing of water masses in the east side (>115°E) is mainly influenced by the Kuroshio intrusion (50%). This study reveals that dual water isotopes are exquisitely sensitive to determine the complex hydrological process in the PRE and the adjacent SCS, and water masses on marine environment should deserve more attention.

Dispersal and aging of terrigenous organic matter in the Pearl River Estuary and the northern South China Sea Shelf

The Western Pacific Coasts (WPC) are the outlets of many large Asian rivers. In recent years, the interplay of climate changes and human activities has persistently altered the suspended sediment concentrations (SSC) in the WPC, triggering substantial shifts in coastal ecosystems. However, the scarcity of coastal observation stations hampered comprehensive investigations at large scales. This study employed three representative SSC retrieval models and utilized Landsat images acquired from 1990 to 2020 to estimate the SSC in the WPC with a focused endeavor to dissect the intricate spatial and temporal variability of SSC in the region. The findings revealed the following insights: (1) The outcomes derived from the three distinct SSC models consistently manifested a discernible decreasing pattern in SSC changes over the past three decades across all six major estuaries (Liao River Estuary, Yellow River Estuary, Yangtze River Estuary, Hangzhou Bay, Pearl River Estuary, and Mekong River Estuary). (2) The seasonal attributes of the six major estuaries differed, primarily due to distinct dominant influencing factors like precipitation, upstream sediment load, wind, and tides. (3) Collectively, SSC tends to be relatively higher in the Yangtze River Estuary, Hangzhou Bay, and Yellow River Estuary, while the Pearl River and Mekong River Estuaries exhibit relatively lower levels. Notably, the SSC exhibited distinct spatial traits along the coastlines of different estuaries. (4) SSC in the non-estuarine regions along the WPC, a similar significant declining trend in SSC is observed as in the estuaries, albeit the rate of decline generally appeared to be less pronounced. Furthermore, regions with faster rates of SSC reduction are typically concentrated near major estuaries in the northern part of the Coasts. The decline in estuarine SSC plays an important role in the overall decrease in SSC across the WPC. These study outcomes held substantial significance for advancing the stability and sustainable evolution of the WPC.

Declining particulate organic carbon flux to estuary yet rising oceanic flux over the past 20 years: A case study of the Pearl River Estuary.