Below-average water level in Lake Izunuma-Uchinuma in Miyagi Pref, Japan, in the winter of 2016/17 induced whooper swan feeding activity, leading to reduced coverage of lotus vegetation and increased dissolved oxygen in the water

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  • 2016/17年の低水位時に生じたオオハクチョウの採食活動によるハス群落の減少とその後の溶存酸素濃度の上昇
  • 2016/17ネン ノ テイスイイジ ニ ショウジタ オオハクチョウ ノ サイショク カツドウ ニ ヨル ハス グンラク ノ ゲンショウ ト ソノゴ ノ ヨウゾンサンソ ノウド ノ ジョウショウ

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Abstract

<p>The water level in Lake Izunuma-Uchinuma in the winter of 2016/17 was 26 cm lower than average. The highest recorded number of whooper swans in the lake during this period came to 6,461 individuals, more than double that of usual years. Analysis of images from monitoring cameras showed that swans were mainly distributed along the shoreline. Where the water depth was shallow enough, the swans dug into the lake bottom in the shallow water, feeding on lotus roots. The following summer, there was no lotus vegetation in areas where the swans had dug. Lotus vegetation disappeared from areas where the water depth was 0.74 m or less, although lotus had occurred in these areas in normal winters, when the water depth was an average of 1.0 m. As a result, 64.4 ha of lotus vegetation were eliminated, leaving open water. The dissolved oxygen of the lake water increased in the new open-water areas created by swans, probably due to enhanced water circulation. Foraging activity of swans thus affected the extent of lotus vegetation, causing a change in the water quality of the lake.</p>

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