Abstract
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/km2), 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 CO2 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.
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Acknowledgements
We are grateful to Kota Nojima, Shota Kambayashi, Hironori Tashiro, and Aoto Matsumoto for gathering data. We would also like to thank two anonymous reviewers. We acknowledge constructive comments from Qian Liu (Ocean University of China), Bing Zhang (Tianjin Normal University), and Yasuhiko Tago (Fisheries Research Institute, Toyama Prefectural Agricultural, Forestry & Fisheries Research Center).
Funding
This work was supported by the Environment Research and Technology Development Fund (JPMEERF20212001 and JPMEERF14S1130) of the Environmental Restoration and Conservation Agency of Japan, JSPS KAKENHI Grant Number 20H04319, a Sasakawa Scientific Research Grant from The Japan Science Society (2018–7040), the Support Project for Japan Seaology Promotion Organization in 2016–2018, ERAN’s Collaborative Research Project (Y-19–24 and Y-20–27), the cooperative research program of the Institute of Nature and Environmental Technology, Kanazawa University (18043), Toyama Daiichi Bank Scholarship Foundation in 2020, and Joint Research Grant for the Environmental Isotope Study of Research Institute for Humanity and Nature.
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All authors contributed to the study conception and design. Jing Zhang and Saki Katazakai performed material preparation, data collection, analysis, and funding acquisition. The first draft of the manuscript was written by Saki Katazakai, and Jing Zhang reviewed and edited previous versions of the manuscript. All authors read and approved the final manuscript.
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Katazakai, S., Zhang, J. A quarter-century of nutrient load reduction leads to halving river nutrient fluxes and increasing nutrient limitation in coastal waters of central Japan. Environ Monit Assess 193, 573 (2021). https://doi.org/10.1007/s10661-021-09279-5
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DOI: https://doi.org/10.1007/s10661-021-09279-5