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.

Submarine groundwater discharge and associated metal elements into an urbanized bay.

In this study, geochemical tracers (radium isotopes) and heavy metals (Pb, Zn, Cd, Cr and As) were analyzed to derive the submarine groundwater discharge (SGD) and associated metal fluxes during four seasons in an urbanized bay (Daya Bay, China). Results showed that Pb and Zn were the main pollutants in bay water. SGD was found to exhibit an obvious seasonal trend (autumn > summer > spring > winter). Such seasonal patterns may be related to the hydraulic gradient between groundwater level and sea level, storm surges and tidal range. SGD was a dominant source of marine metal elements, contributing 19 %-51 % of the total inputs of metals into Daya Bay. The bay water was classified as slight pollution to heavy pollution, which could be linked to SGD-derived metal fluxes. This study provides a better understanding of the important role that SGD plays in metal budgets and ecological environments of coastal waters.

Microbial community assembly and co-occurrence relationship in sediments of the river-dominated estuary and the adjacent shelf in the wet season.

In the estuarine ecosystem, microbial community plays a vital role in controlling biogeochemical processes. However, there is currently limited comprehensive study on the deterministic and stochastic processes that drive the microbial community assembly in the estuaries and adjacent shelves. In this study, we systematically investigated the co-occurrence relationship and microbial community assembly in the sediments along a large river-dominated estuary to shelf in the northern South China Sea during the wet season. The sampling sites were divided into estuary, transection, and shelf sections based on their salinity values. The microbial co-occurrence networks, hierarchical partitioning-based canonical analysis, null model, neutral community model, and the Mantel test were used to investigate the community assembly. Results suggested that microbial community in the estuary section exhibited more interactions and a higher positive interaction ratio than those in the transition and shelf sections. Stochastic processes dominated community assembly in the study, with homogenizing dispersal contributing the most. The estuary exhibited a higher degree of heterogeneous selection than the transition and shelf sections, whereas homogeneous selection showed an opposite trend. Only the estuary section showed dispersal limitation and undominated processes. The river inflow and the resulting environmental heterogeneity were believed to be the key regulators of the community assembly in the studied area. Our study improved the understanding of how microbial community assembly in estuaries and adjacent shelves.

Pollution assessment and sources of dissolved heavy metals in coastal water of a highly urbanized coastal area: The role of groundwater discharge.

Heavy metal concentrations and physicochemical parameters in coastal waters were measured to analyze the spatial distribution characteristics, pollution degrees, and sources of heavy metals in the heavily urbanized Guangdong-Hong Kong-Macao Greater Bay Area (GBA) in China. Heavy metal concentrations in the eastern GBA were higher than those in the west, and the levels of Pb and Zn in seawater were higher than those in groundwater and river water. Both the pollution factors and comprehensive water quality index demonstrated that seawater was not contaminated with As, Cd, Cr, and Ni, whereas low to considerable levels of contamination of Pb and Zn were observed in the central and eastern sections of the GBA. Multiple statistical analyses suggested that the Pb and Zn contaminations in seawater were probably derived from atmospheric deposition and human activities, and the excess amounts of As, Cd, Cu, Ni, and Zn in groundwater were attributed to anthropogenic activities. The heavy metal fluxes from submarine groundwater discharge (SGD) were comparable to, or even greater than, those from local rivers. Therefore, SGD is a significant invisible contributor of heavy metals into the coastal ocean that has often been overlooked in comparison to other visible pollution sources. This study suggests that SGD should be considered in the assessment of heavy metal pollution and future water quality management protocols in marine ecosystems.

Radium and nitrogen isotopes tracing fluxes and sources of submarine groundwater discharge driven nitrate in an urbanized coastal area.

The quantitative evaluations of nutrients delivered by submarine groundwater discharge (SGD) have been widely conducted worldwide, but sources of nutrients in the discharged submarine groundwater remain unclear. Identifying these sources of nutrients is essential to the protection and management of marine ecological environments. This study aims to evaluate the magnitudes of SGD and the associated nitrate in the Guangdong-Hongkong-Macao Greater Bay Area (GHM Greater Bay Area), China, and identify the sources of SGD-driven nitrate in this region using radioactive radium (Ra) isotopes (223Ra, 224Ra, and 228Ra) and stable nitrogen (N) and oxygen (O) isotope composition of nitrate (δ15N-NO3- and δ18O-NO3-). The results of the Ra mixing model show that the estimated SGD and the associated nitrate fluxes into the Greater Bay Area are (9.15 ± 1.26) × 108 m3/d and (3.77 ± 0.52) × 107 mol/d, respectively, both of which are comparable to the contributions from the Pearl River. Combing NO3- dual isotopic signatures of sampled coastal groundwater and five kinds of potential nitrate sources, we found that ammonium (NH4+) fertilizer and natural soil N are the two main sources of nitrate in discharged submarine groundwater and rivers. No anthropogenic inputs from manure or sewage waste were identified. This study provides significant insights into the establishment of effective management strategies for controlling SGD-nutrients into the bay and protecting the marine ecological environment.

Distributions, quality assessments and fluxes of heavy metals carried by submarine groundwater discharge in different types of wetlands in Jiaozhou Bay, China.

Intertidal groundwater and seawater were sampled to analyze the distribution characteristics, the contamination status and the submarine groundwater discharge (SGD)-associated fluxes of heavy metals Cu, Pb, Zn, Cd, Cr, and Hg as well as the metalloid As at four typical intertidal wetlands (including a sandy beach, a mud flat, a tidal marsh and an estuarine intertidal zone) of Jiaozhou Bay, China. Results show that the surface water near the Dagu River estuary suffers from a severe Cu pollution. The groundwater in the sandy beach and mud flat has stronger enrichment abilities of heavy metals than those at the other two sites. The contents of Pb and Zn in groundwater are mainly controlled by the sulfate reduction. At the mud flat, human activities may cause potential Pb contamination to groundwater. The heavy metal effluxes in the sandy beach are the largest of all the four wetlands.