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<p>Coastal areas located in the interfacial regions between terrestrial and marine environments are biologically and geochemically active. They provide valuable ecosystem services but suffer from oxygen depletion. To understand the nutrient distribution in a saline lake that experiences complete circulation and temporarily forms an anoxic-water mass but has limited freshwater inflow, we monitored the vertical profiles of inorganic nutrients and physicochemical parameters in Lake Hiruga during various intervals from January 2019 to July 2021. We found that dissolved oxygen (DO) was depleted below 20-m depth from April to December in both 2019 and 2020. However, DO was supplied to the lake bottom during the winter period (January–March), except for 2021, when oxygen-depleted water remained below the 35-m depth. The concentrations of ammonium, phosphate, and silicates remained high in the oxygen-depleted water above the lakebed and increased throughout the water column during the winter blast period. Nitrates and nitrites were distributed within the borderline region between the anoxic and oxic waters during the stratification period from spring to early winter. The oxygen-depleted water mass formed high nitrate and low nitrite concentrations during the winter of 2020, indicating that DO was supplied to the lake bottom but was consumed immediately. This suggested that inorganic nutrients in Lake Hiruga originated mainly from lakebed sediments and were eluted into the anoxic-water mass and then distributed throughout the water column via winter mixing. Furthermore, DO supplied through lake circulation processes played a role in changing the chemical forms of nutrients.</p>

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Spatiotemporal distributions of inorganic nutrients in a hypoxia forming saline lake, Lake Hiruga in Japan

  • 期刊论文

  • 发表日期：

    2025年2月8日

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  • Takafumi Kataoka,
  • Takehiro Nakamura,
  • Ippei Miki,
  • Rei Somiya

摘要

Coastal areas located in the interfacial regions between terrestrial and marine environments are biologically and geochemically active. They provide valuable ecosystem services but suffer from oxygen depletion. To understand the nutrient distribution in a saline lake that experiences complete circulation and temporarily forms an anoxic-water mass but has limited freshwater inflow, we monitored the vertical profiles of inorganic nutrients and physicochemical parameters in Lake Hiruga during various intervals from January 2019 to July 2021. We found that dissolved oxygen (DO) was depleted below 20-m depth from April to December in both 2019 and 2020. However, DO was supplied to the lake bottom during the winter period (January–March), except for 2021, when oxygen-depleted water remained below the 35-m depth. The concentrations of ammonium, phosphate, and silicates remained high in the oxygen-depleted water above the lakebed and increased throughout the water column during the winter blast period. Nitrates and nitrites were distributed within the borderline region between the anoxic and oxic waters during the stratification period from spring to early winter. The oxygen-depleted water mass formed high nitrate and low nitrite concentrations during the winter of 2020, indicating that DO was supplied to the lake bottom but was consumed immediately. This suggested that inorganic nutrients in Lake Hiruga originated mainly from lakebed sediments and were eluted into the anoxic-water mass and then distributed throughout the water column via winter mixing. Furthermore, DO supplied through lake circulation processes played a role in changing the chemical forms of nutrients.