Deciphering Multi‐Scale Submarine Groundwater Discharge in a Typical Eutrophic Bay

• Published in: Journal of geophysical research. Oceans 2024-06, Vol.129 (6), p.n/a
• Main Authors: Xu, Haowei, Xu, Bochao, Yu, Huaming, Zhao, Shibin, Burnett, William C., Yao, Qinzhen, Dimova, Natasha T., Song, Shasha, Guo, Xiaoyi, Chen, Xiaogang, Zhang, Honghai, Yu, Zhigang
• Format: Article
• Language: English
• Subjects: Benthos; Biogeochemical cycle; Biogeochemical cycles; Estuaries; Estuarine dynamics; Eutrophication; Exchanging; Fluxes; Green tides; Groundwater; Groundwater discharge; Isotopes; Marginal seas; Marine ecosystems; Mass balance; Nutrient cycles; Nutrient sources; Nutrients; Oceans; Phosphorus; Pore water; Radium; Radium 226; Radium isotopes; Radium radioisotopes; Radon; Radon isotopes; Residence time; River discharge; River flow; Rivers; Seawater; Silicates; Tides; Water discharge; Water flow
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2024JC021042

As a significant source of nutrients and other components into the coastal ocean, submarine groundwater discharge (SGD) is a combination of multiple spatial‐temporal scale processes, including fresh terrestrial groundwater (FSGD), recirculating seawater (RSGD), and porewater exchange (PEX). Quantifying the different types of SGD is extremely important for understanding the coastal biogeochemical cycles of various materials. The green tide bloom in the southern Yellow Sea (China) has attracted increasing attention during the past decade. Haizhou Bay is thought to be an important green tide proliferating area, but the source of an apparent high supply of nutrients has not been identified yet. We report here on our investigations of the distribution patterns of Ra and Rn isotopes in groundwater and seawater in Haizhou Bay. By solving a combined mass balance for 224Ra, 223Ra, 226Ra, and 222Rn, we estimated that the bay's water residence time is 28.8 days, FSGD is highest at 3.6 cm d−1, RSGD is 2.7 cm d−1, and PEX is lowest at 0.6 cm d−1. The total SGD into Haizhou Bay is estimated at 9.40 × 107 m3 d−1 (6.9 cm d−1), about 12 times that of the local river discharge into the bay. SGD‐derived nutrients are shown to play an important role when considered among all known nutrient sources. Our results suggest that dissolved inorganic nitrogen (DIN) and silicate (DSi) are transported by SGD mainly via FSGD. We also note that the phosphorus (DIP) budget is heavily influenced by RSGD. Plain Language Summary Submarine groundwater discharge (SGD) has been drawing attention as an important benthic boundary exchange pathway in the global marginal seas. As any water flow entering the sea, SGD may happen on small scales (centimeters, minutes to hours) as porewater exchange (PEX), and/or large scales (>meters, days to seasons) as fresh groundwater discharge (FSGD) and seawater recirculation (RSGD) in the subterranean estuary. To better understand its significance, it is necessary to interpret multi‐scale SGD processes and quantify associated nutrients fluxes accordingly. In this study, different types of SGD and associated nutrient fluxes were quantified based on multiple radium and radon isotopes in Haizhou Bay, China, which is one of the world's largest green tide proliferating areas. The FSGD and RSGD were found to be the main processes, accounting for more than 90% of the total SGD fluxes. SGD was recognized as a key nutrient source in Haizhou Bay, which is more than one o