[A-HW27] Biodiversity, nutrients and other materials in ecosystems from headwaters to coasts

[AHW27-22] Topographic control of submarine groundwater discharge (SGD) and SGD-derived nutrient dynamics on a tidal flat scale

*GAO QIDI¹, Shin-ichi Onodera¹, Mitsuyo Saito¹, Sharon Bih Kimbi¹ (1.Graduate School of Advanced Science and Engineering, Hiroshima University)

Submarine groundwater discharge (SGD) refers to groundwater from coastal aquifers flowing into the ocean under tidal influences. It includes freshwater submarine groundwater discharge (FSGD) and recirculated seawater groundwater discharge (RSGD). Nutrient exchange occurs during this hydrological process, with SGD serving as a vital nutrient source for coastal ‘blue carbon’ ecosystems. However, the previous research has not clearly examined how different topography and substrates affect SGD and SGD-derived nutrient dynamics in a tidal flat scale. This study aims to evaluate the topographic control of SGD on a tidal flat scale. This study has three target research areas, namely Gohonmatsu, Miyabara, and Hayashi tidal flats, and different numbers of piezometers were installed. These tidal flats have different characteristics, Gohonmatsu tidal flats exhibit significant heterogeneity in east and west sides, and almost no river runoff, nutrients primarily enter the ocean along with groundwater flow. Miyabara tidal flat has a small river runoff than Gohonmatsu. While Hayashi tidal flat has great river runoff. They are all located on Ikuchijima Island in the Seto Inland Sea in western Japan. The results are summarized as follows: 1) Landward areas exhibit greater hydraulic conductivity compared to offshore. FSGD predominates on the west side, while RSGD is more prominent on the east side. The composition of SGD is influenced by substrate and topography. 2) Temporally, SGD, FSGD and RSGD values are higher during spring tide. The unsaturated zone significantly contributes to RSGD.The unsaturated zone significantly contributes to RSGD on the east side with sand dune, leading to greater SGD compared to the west side without sand dune. 3) FSGD dominated areas exhibit higher nitrate concentration in SGD. However, different type of RSGD has different major nutrient. RSGD derived from the unsaturated zone is primarily rich in nitrate, while offshore RSGD contains more ammonium. In general, the three tidal flats on Ikuchijima exhibit DIP:DIN and DSi:DIN ratios below the Redfield ratio, suggesting that SGD is relatively enriched in phosphorus and silica.

Acknowledgement This research was supported by JSPS Grant-in-Aid for Scientific Research (B) (No. 21H03650, PI: Mitsuyo Saito), Fostering Joint International Research (A) (No. 20KK0262, PI: Mitsuyo Saito) and JSPS Grant-in-Aid for Challenging Research (Exploratory), PI: Mitsuyo Saito, No. 22K19869