Nitrate dynamics in coastal groundwater and rivers: Insights into oligotrophication management

Coastal ecosystems rely significantly on terrestrial-derived essential nutrients for primary productivity. However, efforts to reduce anthropogenic nutrient inputs to mitigate eutrophication led to coastal oligotrophication when tipping points were exceeded. In Toyama Bay, land-derived nutrients significantly enrich the euphotic zone. However, nutrient reduction policies since the 1990s have raised concerns about declining marine productivity. To investigate these trends, we studied the Kurobe River Alluvial Fan (KRAF), Japan's largest fan, focusing on nitrate (NO₃⁻) dynamics. The KRAF was selected due to its multi-water system and diverse land use influenced by the Asian Monsoon Climate, which controls its hydrological and nutrient dynamics, serving as a model for the broader Asian region. Using isotopic analyses (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ²H-H₂O, δ¹⁸O-H₂O), hydrochemical parameters, and mixing models, we examined hydrological dynamics, NO₃⁻ sources, transformation processes, and temporary storage from 35 seasonally collected water samples. Results showed low NO₃⁻-N concentrations (0.05–1.89 mg/L), over 80 % below environmental standards, influenced by hydrological dynamics, land-use changes, and management policies. Soil and fertilisers were primary NO₃⁻ sources, highlighting the impact of water management strategies on nutrient reduction. Nitrification was dominant in lowland and riverine areas, while denitrification was primarily limited to highland paddy fields. Fresh submarine groundwater discharge (FSGD) contributed 52 %–70 % of the total NO₃⁻ flux to the sea, surpassing river contributions. Total flux of terrestrial NO₃⁻ was estimated at 347.4 ± 121.1 kg/year, a 31 % decrease compared to the 2005 estimate. Promoting coastal agriculture can mitigate oligotrophication by slowing water flow and enhancing nutrient supply through the use of FGSD. This research employs a science-based approach to recommend sustainable nutrient management strategies. More broadly, the method can be adapted for use in other water bodies to identify NO₃⁻ sources and dynamics, thereby promoting comprehensive nutrient management and supporting the health and productivity of coastal ecosystems.