117 応用生態工学 22 (2), 117124, 2020 はじめに 生物多様性の損失は深刻な問題であり,特に湖沼では 緊急を要する課題である.日本の湖沼における生物多様 性については,松崎ほか(2016)が湖沼生態系の中で重 要な生態系機能を担っている水生植物と純淡水魚を指標 分類群として評価している.ここでは,過去(1999 年 以前)と現在(2000 年以降)の比較において,在来種 数が水生植物では調査対象とした湖沼全体で平均 48%, 純淡水魚では平均 25%減少していることに加えて,い ずれの生物群も国外外来種の侵入が広域で確認されてい るなど,生態系が劣化していることを指摘している. これまで失われた水生植物を再生させる試みとして, 種子や殖芽などを含む散布体バンクを用いた撒き出し実 験が各地で行われており,過去に生育していた種が再生 した事例が報告されている(久城ほか 2009;藤本ほか 2019 年 3 月 23 日受付,2019 年 10 月 9 日受理 * e-mail: ┱┳┶┭┥┯┄┴┶┩┪⓲┸┳┸┸┳┶┭⓲┰┫⓲┮┴ ORIGINAL PAPER 汽水湖東郷池における淡水性水生植物セキショウモ Vallisneria asiatica Miki の生育環境 森 明寛 1)* ・前田 晃宏 2) ・日置 佳之 3) 1) 鳥取県水環境保全課 〒680 8570 鳥取市東町 1 丁目 220 番地 2) 鳥取県衛生環境研究所 〒682 0704 鳥取県東伯郡湯梨浜町南谷 526 1 3) 鳥取大学農学部 〒680 8553 鳥取市湖山町南 4 丁目 101 Akihiro MORI 1)* , Akihiro MAEDA 2) , Yoshiyuki HIOKI 3) : Growing Environment of Vallisneria asiatica Miki, a Freshwater Plant in Brackish Lake Togo-ike. Ecol. Civil Eng. 22 (2) , 117-124, 2020 1) ⓤ┛┥┸┩┶ⓤ┉┲┺┭┶┳┲┱┩┲┸ⓤ┑┥┲┥┫┩┱┩┲┸ⓤ┈┭┺┭┷┭┳┲⓰ⓤ┘┳┸┸┳┶┭ⓤ└┶┩┪┩┧┸┹┶┩⓰ⓤ1220ⓤ┌┭┫┥┷┬┭⓱┱┥┧┬┭⓰ⓤ ┘┳┸┸┳┶┭⓰ⓤ┘┳┸┸┳┶┭ⓤ6808570⓰ⓤ┎┥┴┥┲ 2) ⓤ┘┳┸┸┳┶┭ⓤ└┶┩┪┩┧┸┹┶┥┰ⓤ┍┲┷┸┭┸┹┸┩ⓤ┳┪ⓤ└┹┦┰┭┧ⓤ┌┩┥┰┸┬ⓤ┥┲┨ⓤ┉┲┺┭┶┳┲┱┩┲┸┥┰ⓤ┗┧┭┩┲┧┩┷⓰ⓤ5261ⓤ ┑┭┲┥┱┭┨┥┲┭⓰ⓤ┝┹┶┭┬┥┱┥⓰ⓤ┘┳┬┥┯┹⓰ⓤ┘┳┸┸┳┶┭ⓤ6820704⓰ⓤ┎┥┴┥┲ 3) ⓤ┊┥┧┹┰┸┽ⓤ┳┪ⓤ┅┫┶┭┧┹┰┸┹┶┩⓰ⓤ┘┳┸┸┳┶┭ⓤ┙┲┭┺┩┶┷┭┸┽⓰ⓤ4101ⓤ┏┳┽┥┱┥⓱┧┬┳ⓤ┑┭┲┥┱┭⓰ⓤ┘┳┸┸┳┶┭⓰ⓤ ┘┳┸┸┳┶┭ⓤ6808553⓰ⓤ┎┥┴┥┲ Abstract: We investigated the growth potential of the freshwater plant ┚┥┰┰┭┷┲┩┶┭┥ⓤ┥┷┭┥┸┭┧┥ Miki in Lake Togo-ike, a brackish lake that was shown to have reduced biodiversity in a study focusing on salinity and light, which are important aspects of the growing environ- ment for aquatic plants. Based on the results of a one-year salinity study that began in April 2016, it was revealed that growth was possible at 698 PSU, which exceeds the previ- ously reported upper limit of salinity. Furthermore, based on the average annual transpar- ency in Lake Togo-ike in 2016, it was estimated that ┚┥┰┰┭┷┲┩┶┭┥ⓤ┥┷┭┥┸┭┧┥ Miki can grow at water depths of up to 15 m. With these conditions, areas of Lake Togo-ike that are suitable for the growth of ┚┥┰┰┭┷┲┩┶┭┥ⓤ┥┷┭┥┸┭┧┥ Miki were extracted from a salinity distribution map and a topographic map of the lake bottom. The northeastern and western inlets, which sat- isfied both conditions, were selected as areas with high growth potential. It was suggested that these areas, which have both low salinity and a shallow water depth throughout the year, would be important sites for the conservation of this species. Key words: biodiversity, brackish, salinity, transparency, ┚┥┰┰┭┷┲┩┶┭┥ⓤ┥┷┭┥┸┭┧┥ Miki 原著論文
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Akihiro MORI1)*, Akihiro MAEDA2), Yoshiyuki HIOKI3): Growing Environment of Vallisneria asiatica Miki, a Freshwater Plant in Brackish Lake Togo-ike. Ecol. Civil Eng. 22(2), 117-124, 20201) Water Environment Management Division, Tottori Prefecture, 1-220 Higashi-machi, Tottori, Tottori 680-8570, Japan2) Tottori Prefectural Institute of Public Health and Environmental Sciences, 526-1 Minamidani, Yurihama, Tohaku, Tottori 682-0704, Japan3) Faculty of Agriculture, Tottori University, 4-101 Koyama-cho Minami, Tottori, Tottori 680-8553, Japan
Abstract: We investigated the growth potential of the freshwater plant Vallisneria asiatica Miki in Lake Togo-ike, a brackish lake that was shown to have reduced biodiversity in a study focusing on salinity and light, which are important aspects of the growing environ-ment for aquatic plants. Based on the results of a one-year salinity study that began in April 2016, it was revealed that growth was possible at 6.98 PSU, which exceeds the previ-ously reported upper limit of salinity. Furthermore, based on the average annual transpar-ency in Lake Togo-ike in 2016, it was estimated that Vallisneria asiatica Miki can grow at water depths of up to 1.5 m. With these conditions, areas of Lake Togo-ike that are suitable for the growth of Vallisneria asiatica Miki were extracted from a salinity distribution map and a topographic map of the lake bottom. The northeastern and western inlets, which sat-isfied both conditions, were selected as areas with high growth potential. It was suggested that these areas, which have both low salinity and a shallow water depth throughout the year, would be important sites for the conservation of this species.
Fig. 1. Location of the study area and observation points. A‒F show the points at which annual salinity was measured. Vallisneria asiatica Miki inhabits points A‒E, but not point F. The small white circles indicate the 22 points at which salinity was measured on July 26, 2016.
Fig. 2. The annual salinity range at observation points A‒F in Lake Togo-ike. Salinity at each point was ob-served every hour from April 6, 2016 to April 5, 2017. The sal inity of the lake bottom remained at ≤6.98 PSU at points A‒E, whereas the maximum sa-linity at point F reached 30.12 PSU.
A B C D E F
Salin
ity (P
SU)
0
10
20
30
*1 : outlier *2 : maximum *3 : 3rd quartile*4 : median *5 : 1st quartile *6 : minimum
*1
*2*3*4*5*6
Fig. 3. Distribution of salinity and suitable habitats for Vallisneria asiatica Miki in Lake Togo-ike. The contour map shows the distribution of salinity estimated from the salinity at the bottom of the lake on July 26, 2016. The hollow areas are areas in which the salinity at the lake bottom was ≤6.98 PSU, which is suitable for the growth of Vallisneria asiatica Miki. The gray-colored areas are areas in which the salinity was >6.98 PSU, which may not be suitable for the growth of Vallisner-ia asiatica Miki.
Fig. 4. Topographic map of Lake Togo-ike. A topographic map of Lake Togo-ike created based on side scan so-nar observation in 2016. The contour lines inside the lake show the water depth at 0.25-m intervals.
0 10.5Km
Depth (m)
0-11-22-33-4
Fig. 5. Suitable habitat for Vallisneria asiatica Miki based on the light environment of Lake Togo-ike. The hollow area indicates the area in which the water depth was ≤1.5 m. In this area, the light environment is suitable for the growth of Vallisneria asiatica Miki. The gray-colored area shows where the water depth was >1.5 m, which may not be suitable for the growth of Vallisneria asiatica Miki.
0 10.5KmDepth (m)
> 1.50 - 1.5
Fig. 6. Suitable habitat for Vallisneria asiatica Miki ac-cording to both salinity and light in Lake Togo-ike. Suitable habitat in Lake Togo-ike for the growth of Vallisneria asiatica Miki was extracted from the salini-ty distribution map and the lake bottom topographic map in the lake. The hollow area is suitable for the growth of Vallisneria asiatica Miki. The gray-colored area is not suitable the growth of Vallisneria asiatica Miki.
Fig. 7. Changes in the salinity of Lake Togo-ike since 1975 to 2017. The data shows the annual salinity range of the surface water at the center of the lake corresponding to each year since 1975 to 2017. The salinity is converted from the chloride ion level, which is measured every month.
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