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Fresh submarine groundwater discharge (FSGD), a well-established land-to-sea pathway for water and materials, has increased (∼30%) along the midlatitude coast of Japan with the shift from snowfall to rainfall since the 1980s. The terrestrial-derived FSGD in this area has a residence time of 10-20 years, and its material flux may respond quickly to the effects of recent warming. In this study, FSGD collected directly from the seafloor (∼10 m water depth) during two different periods (2017-2021 and 2001-2003) was observed using the same consistent method to evaluate the warming-related effects on FSGD material fluxes. In the FSGD system, dissolved constituents and the isotopic composition decreased by 10-30% and residence time by one-third compared to the previous data, even though homogeneous water quality was maintained among the FSGD-connected aquifers for each period. Most concerning is that these changes resulted in a drop in pH and an increase in FSGD-derived carbon flux. Total nutrient fluxes from land have roughly halved, resulting in lower primary productivity and an estimated doubling of excess dissolved inorganic carbon in coastal waters. Our findings present direct evidence of the sensitivity of carbon flux to climate change and the urgency for carbon-related FSGD research worldwide.

Nutrient load reduction is widely used to improve coastal water quality, but it can lead to oligotrophication. This paper evaluates the current status of river water origin and the water recharge system based on isotope values and dissolved compositions recorded in 2018, and it also assesses the impact of nutrient load reduction efforts on river nutrient fluxes and coastal water quality using 25 years of monitoring data. This study focuses on the coast of Toyama Bay as a model area because (1) up to 20% of terrestrially derived nutrient support the growth of coastal primary productivity, and (2) the adjacent land is a typical city in a population-dense area (~ 500 persons/km 2 ), a demographic characteristic that exists in 88% of Japan's total land area and 96% of the total length of the country's coastline. Since the government adopted new wastewater treatment systems in 1993, river nutrient supplies in the study area have been halved, while the total river flow and annual precipitation have remained almost unchanged. The reduction in riverine nutrient supply has increased phosphorus deficiencies in the coastal waters. Most notably, the decline in nutrient concentrations in coastal surface waters and the enlarged nutrient-restricted areas are facing most parts of Japan, suggesting a resulting 25-50% decrease in CO 2 uptake by primary production. This study is in agreement with previous studies from various countries in emphasizing the importance of setting appropriate nutrient-management goals for maintaining a sustainable marine environment. This paper recommends the need to accumulate various case studies of different areas and to share in a timely manner scientific evidence on a regional and global scale., (© 2021. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)