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<p>Oligotrophication and climate change affect coastal ecosystems in a variety of ways. In Chita Bay, a semi-enclosed area in Japan, discoloration of nori seaweed has occurred due to the interannual decrease in nutrient concentration in autumn-winter. This study demonstrates the interannual change in the seasonality of riverine nutrient discharge and the coastal nutrient environment during the 2000s and 2010s based on in situ data in Chita Bay and examines how the change in riverine nutrient discharge contributes to the coastal nutrient dynamics using a one-dimensional coupled physical and biochemical model. The results show that the seasonal peak river discharge is delayed, from summer before 2012 to early autumn after 2013. Despite the interannual increase in riverine total phosphorous (TP) discharge in early–mid-autumn, however, the autumn–winter dissolved inorganic phosphorous (DIP) concentration in Chita Bay decreased. DIP during late autumn and mid-winter was found to correlate positively with the summer sediment TP, which correlated positively with spring riverine TP discharge. The hindcast generated by our model predicted the interannual decrease in the bottom DIP during summer and DIP during autumn–winter under the effects of suppressed phytoplankton growth. We propose that the winter DIP concentration was controlled by the spring river TP discharge rather than the autumn river discharge in Chita Bay.</p>

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Mismatch between interannual change of riverine phosphorus discharge and coastal DIP in autumn-winter in a semi-enclosed sea area (Chita Bay, Japan)

  • 期刊论文

  • 发表日期：

    2025年5月23日

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  • Kazuhiro Aoki,
  • Shigeho Kakehi,
  • Sayaka Sogawa,
  • Asatarou Tsuge

摘要

Oligotrophication and climate change affect coastal ecosystems in a variety of ways. In Chita Bay, a semi-enclosed area in Japan, discoloration of nori seaweed has occurred due to the interannual decrease in nutrient concentration in autumn-winter. This study demonstrates the interannual change in the seasonality of riverine nutrient discharge and the coastal nutrient environment during the 2000s and 2010s based on in situ data in Chita Bay and examines how the change in riverine nutrient discharge contributes to the coastal nutrient dynamics using a one-dimensional coupled physical and biochemical model. The results show that the seasonal peak river discharge is delayed, from summer before 2012 to early autumn after 2013. Despite the interannual increase in riverine total phosphorous (TP) discharge in early–mid-autumn, however, the autumn–winter dissolved inorganic phosphorous (DIP) concentration in Chita Bay decreased. DIP during late autumn and mid-winter was found to correlate positively with the summer sediment TP, which correlated positively with spring riverine TP discharge. The hindcast generated by our model predicted the interannual decrease in the bottom DIP during summer and DIP during autumn–winter under the effects of suppressed phytoplankton growth. We propose that the winter DIP concentration was controlled by the spring river TP discharge rather than the autumn river discharge in Chita Bay.