[A-CG40] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG40-03] Quantifying flux of groundwater-derived nutrients and phytoplankton assimilation rate of nutrients in a volcanic sandy beach

*Ryo Sugimoto¹, Ryoma Yagi¹, Toshimi Nakajima¹, Tatsuhiro Ishida¹, Makoto Yamada² (1.Faculty of Marine Biosciences, Fukui Prefectural University, 2.Faculty of Economics, Ryukoku University)

Keywords:Submarine groundwater discharge, Radioisotopes, Primary production

Volcanic coast is a hotspot occurring meteoric fresh groundwater discharge. Although groundwater supplies nutrients into the water column and contributes to primary productivity in nearshore coasts, there is little quantitative evidence concerning nutrient supply via groundwater discharge and nutrient assimilation by phytoplankton. In the study, we conducted the field survey at the sandy beach along the Chokai volcano coast in August 2020. Here we assessed (1) allochthonous nutrient fluxes via meteoric fresh groundwater (FGW), marine recirculated groundwater (RGW), and offshore seawater, and (2) the net primary productivity (NPP) of phytoplankton. The mass balance models of salt, ²²²Rn, and ²²⁴Ra showed that the total meteoric fresh groundwater discharge was 248 m³ d^-1 composed of two pathways: springs (~90%) and seeps (~10%). Recirculated marine groundwater discharge reached 2881 m³ d^-1. Total allochthonous nutrient fluxes of dissolved inorganic nitrogen (DIN), phosphorous (DIP), and silica (DSi) were 25.0 mol d^-1, 1.4 mol d^-1, and 642.0 mol d^-1, respectively. RGW occupied 62% of total DIP flux and 78% of total DSi flux, while FGW occupied 49% of total DIN flux. The average NPP measured at six sites was 0.35±0.06 mg C L^-1 d^-1, which equals 182.7 mol C d^-1 on the beach scale. Based on the Redfield stoichiometric ratio, the assimilation rates of DIN, DIP, and DSi were 27.6 mol d^-1, 1.7 mol d^-1, and 25.9 mol d^-1, respectively. These results revealed that groundwaters could sustain a maximum of 74% (FGW=44%, RGW=30%) of DIN and 71% (FGW=22%, RGW=49%) of DIP assimilated by phytoplankton.