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

[ACG40-02] Influence of groundwater-surface water exchange on riverine nutrient export to coastal seas: a comparative study at the six tide-less rivers in Wakasa Bay

*Toshimi Nakajima¹, Ryo Sugimoto¹ (1.Fukui Prefectural University)

Keywords:Groundwater-surface water exchange, Riverine nutrient export, Nitrogen, Phosphorus, Coastal sea, Wakasa Bay

Groundwater-surface water exchange (GSE) has been increasingly recognized for its role in transferring water, heat, and chemicals across the land-ocean continuum. Although the magnitude of nutrient transport from surface rivers to coastal seas is crucial for the sustainability of coastal ecosystems, the effects of GSE in most downstream rivers and/or estuarine areas on nutrient flux have not still been clarified yet. Here, we assessed the quantitative effects on groundwater-surface water exchange on nitrogen and phosphorus transports at six tide-less rivers flowing into Wakasa Bay. We took river waters from five sites throughout the river continuum from the upstream site (< 3.7 km from the river mouth) to the most downstream site (river mouth) in each river and constructed the mass balance model for water, salt, and ²²²Rn coupled with the direct measurement of river discharge. The water fluxes at the most downstream sites compared to the most upstream sites changed from 0.1 to 1.3 folds depending on surface water recharge into groundwaters or groundwater discharge to surface water. Export fluxes of total nitrogen (TN) and phosphorus (TP) at the most downstream sites were 0.1 – 1.5 and 0.1 – 3.0 folds compared to those at the most upstream sites, respectively. These changes of TN and TP fluxes between at the upstream sites and downstream sites were determined by those of dissolved inorganic nitrogen (DIN) and phosphorus (DIP), respectively. The higher contribution rates of GSE on the changes in DIN export fluxes indicated that GSE was crucial for riverine N export to coastal seas. A significant relationship for DIP was also found except for one river. We conclude that the surface water-groundwater exchange can alter net export rates of riverine nutrients to coastal seas, resulting in under- or overestimates of riverine nutrient fluxes.