Spatial-temporal distribution and origin of nutrients in the subarctic lagoon of Hichirippu (Hokkaido, Japan)

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  • 亜寒帯汽水湖の火散布沼（北海道）における栄養塩の時空間分布とその起源
  • アカンタイ キスイコ ノ ヒチリップヌマ ホッカイドウ ニ オケル エイヨウエン ノ ジクウカン ブンプ ト ソノ キゲン

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Abstract

<p>Monthly field observations and a 36-hour survey were conducted in the brackish lagoon of Hichirippu in the eastern part of Hokkaido, Japan (43°02'N, 145°00'E). The lagoon covers an area of 3.58 km2 and has a mean water depth of ca. 70 cm. It is inhabited by many animals and benthic plants (e.g. short-necked clam, swan, and the Japanese red-crowned crane), and is designated as a wetland under the Ramsar Convention. The rich natural environment of the lagoon, with a catch yield of fish, shellfish and seaweeds of about 90 ton/yr, should therefore be preserved as a fishery area. In Hichirippu lagoon, we investigated the spatial and temporal distribution of nutrients and physical properties. The daily volume of freshwater input was 0.9～8.0% of total volume of the lagoon, while the volume of water entering the lagoon on the rising tide per half tidal day was 34～59%. The N:P and Si:N ratios were nearly below 16 and higher than 1, respectively, indicating nitrogen limitation. Plots of nutrients vs. salinity suggested nitrate+nitrite supply from the adjacent sea (Pacific Ocean), while the origin of ammonium was neither the adjacent sea nor freshwater. The results of the 36-hour survey showed that tidal nitrate+nitrite influx and outflux was 4.3 and 3.1 kmol/half tidal day, respectively. It implicates 1.2 kmol/half tidal day was supplied to the lagoon. Tidal ammonium flux values are nearly conserved. This suggests that ammonium is mainly regenerated by clam excretion in summer. Previous studies generally have shown that the freshwater input plays an important role in controlling estuarine primary production. Our results suggest that in Hichirippu lagoon both the nutrient import from the adjacent sea and the processes of nutrient regeneration within the estuary have an impor tant effect on the primary production rather than the freshwater input.</p>

Journal

  • Oceanography in Japan

    Oceanography in Japan 20 (1), 19-36, 2011-01-05

    The Oceanographic Society of Japan

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