Long-term changes in the distribution of <i>Trapa</i> L. in six Japanese lakes observed by satellite imagery

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  • Oyama Yoichi
    Marimo Research Center, Kushiro City Board of Education Faculty of Life and Environmental Sciences, University of Tsukuba
  • Matsushita Bunkei
    Faculty of Life and Environmental Sciences, University of Tsukuba
  • Fukushima Takehiko
    Faculty of Life and Environmental Sciences, University of Tsukuba

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  • 衛星画像から観測した国内6湖沼におけるヒシ属<i>Trapa</i> L. の長期分布変化
  • 衛星画像から観測した国内6湖沼におけるヒシ属Trapa L.の長期分布変化
  • エイセイ ガゾウ カラ カンソク シタ コクナイ 6 コショウ ニ オケル ヒシゾク Trapa L.ノ チョウキ ブンプ ヘンカ

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In this study, we used Landsat images to observe changes in the distribution of Trapa L., which has a tendency to overgrow in Japanese lakes. Six Japanese lakes in different locations were chosen as study areas (Lakes Sirarutoro, Takkobu-numa, North Inba-numa, South Inba-numa, Suwa and Mikata). The extraction algorithm for Trapa L. was developed based on the Normalized Difference Water Index (NDWI), which uses short-wave infrared and green reflectances, instead of the more commonly used Normalized Difference Vegetation Index (NDVI); NDVI cannot distinguish between Trapa L. and cyanobacterial blooms due to their similar spectral characteristics at near infrared and red bands. Satellite-derived and in-situ estimates of Trapa L. distribution corresponded closely (R2=0.82, N=8). Seasonal observation in Lakes Sirarutoro, North Inba-numa and Mikata revealed that Trapa L. was absent from February to May, rapidly increased in July, with subsequent numbers maintained through September, and rapidly decreased in October. Changes in the distribution of Trapa L. over the past 25 years showed a tendency toward expansion in most lakes.

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