Hydrogeochemistry and Acidic Property of Submarine Groundwater Discharge Around Two Coral Islands in the Northern South China Sea

Hon-Kit Lui,Feng-Yu Wang,Li-Lian Liu,Hsiao-Chun Tseng,Rae Chang,Chen-Tung Arthur Chen,Min-Yun Liu,Wei-Ping Hou,Hsiu-Chin Lin

doi:10.3389/feart.2021.697388

Abstract

Submarine groundwater discharge (SGD) is an important source of nutrients in many coastal regions, yet little information is available on its carbonate chemistry and controlling factors. This study examined the processes and factors controlling the hydrogeochemistry and acidic property of the groundwaters and SGD waters of two isolated coral islands, Liuqiu Island (13 km off southwestern Taiwan) and Dongsha Island (located in the northern South China Sea, 420 km away from Liuqiu Island). Our results showed that the total alkalinity and dissolved inorganic carbon (DIC) of the fresh SGD waters were controlled mainly by the chemical weathering of carbonate minerals. Part of the DIC came from the organic matter decomposition or soil CO2, reducing the pH and CO32− concentration. Distributions of the carbonate chemistry and nutrients of the SGD waters were controlled mainly by physical mixing between the groundwater and the ambient seawater under the seabed, the so-called subterranean estuary. The Ca2+ released through weathering significantly increased the saturation state of aragonite or calcite, reducing the corrosiveness of the SGD waters on the carbonate rocks. This study is likely the first to examine the effects of the acidic property of SGD waters on the biogenic carbonate spine of a sea urchin and a pteropod shell. The spring water with similar carbonate chemistry to that of the freshwater SGD endmember from Liuqiu Island with a saturation state of aragonite of 0.96 caused observable dissolution on the spine of a sea urchin and a pteropod shell, but the spine dissolved more readily. This was because the spine is made of high-Mg calcite, which has higher solubility than that of aragonite or calcite. Such a result implies that some marine organisms with carbonate skeletons or shells containing high Mg:Ca ratios may suffer the impact of ocean acidification earlier. Although the SGD may contribute less than 10% of freshwater discharge by rivers to the coastal area, its impact on coastal biogeochemical cycles and ecosystems due to its acidic property and continual effect on the coast all year round deserves further investigation.

Highlights

River inputs play an essential role in global water cycles and biogeochemical cycles
This study aims to investigate the processes and factors controlling the hydrogeochemistry and the acidic property of the submarine groundwater discharge (SGD) water and groundwater of two isolated coral islands, Liuqiu Island (13 km off southwestern Taiwan) and Dongsha Island (Figure 1)
Major ions in the spring waters of Liuqiu Island are contributed mainly from the chemical weathering of rocks, as the major cations (Na+, K+, Ca2+, Mg2+) and anions (Cl−, SO42−, HCO3−) in rainwater collected on 2020/8/26 were negligibly small when compared with the spring water in Liuqiu Island collected on 2020/12/3 (Table 1)

Summary

Introduction

River inputs play an essential role in global water cycles and biogeochemical cycles. Taniguchi et al (2019) defined SGD as “the flow of water through continental and insular margins from the seabed to the coastal ocean, regardless of fluid composition or driving force”, stating that islands typically have higher SGD, in terms of fluxes per unit area of landmass, than continents. According to this definition, the SGD includes both the freshwater and recirculated (or recharged) seawater

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