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

[ACG40-P01] Characteristics of radium distributions in surface water in Ise Bay

*Yuitsu Tochio¹, Tomohiko Kobayashi², Ryo Sugimoto¹ (1.Fukui prefectural university, 2.Mie Prefecture Fisheries Research Institute)

Keywords:Ise Bay, Radium isotopes, Radium age

Ise Bay is one of the largest embayments in Japan. A large amount of river water flows into the bay and affects physical processes and biogeochemical cycles. However, little is known about the inputs of terrestrial and coastal groundwaters into the bay. Radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra, ²²⁸Ra) can be a powerful tool for assessing terrestrial impacts, especially in groundwaters. In seawaters, dissolved radium exhibits conservative behavior unaffected by biological or chemical actions. The activity of radium isotopes decreases depending on the half-life of each isotope, and the ratio of these isotopes can be used to calculate the radium age (≒ water mass age). In this study, we collected surface seawater, river water, and groundwater on a sandy beach from October to December 2022 to assess the usefulness of radium isotopes in tracing terrestrial impacts on the ecosystem in Ise Bay. The mean activity of ²²⁴Ra (t_1/2 = 3.66 d) in river waters (Kiso, Nagara, and Ibi Rivers) was 5.29 dpm/100L, and increased to 43.76 dpm/100L at the river mouth. The activity in seawater showed a decreasing trend from the river mouth to the bay mouth. A similar trend was observed for ²²³Ra (t_1/2 = 11.4 d). The mixing diagram between salinity and ²²⁴Ra showed the notable contribution of groundwater (mainly in saline). Using the Bayesian mixing model, we assessed the contribution of river water and groundwater to surface seawater. As a result, river water and groundwater contributed 18.4% and 12.0% on average, respectively. Considering that groundwater accompanied 40% of the total terrestrial contributions, the influence of groundwater in Ise Bay is not negligible. The water mass age calculated from ²²⁴Ra/²²³Ra was 0.5 days at the river mouth and about 6 days at the bay mouth. The relationship between nutrient concentration and water mass age showed that dissolved inorganic nitrogen and phosphorous depleted over 2 days, possibly due to the assimilation by phytoplankton based on the relationship with chlorophyll-a.