Aegagropila linnaei is a freshwater green alga, which at one time was distributed widely in the northern hemisphere. The aggregate often forms beautiful spherical shapes known as “lake balls” or “Marimo”. The population of Marimo has been rapidly decreasing worldwide, and today the large Marimo, with a diameter of more than 12 cm, exit only in Lake Akan in Japan. However, how Marimo grow and maintain their unique spherical shape in natural habitats remains unsolved. Here we show that Marimo are “polished” into spheres by the rotation induced by wind waves. Such a process enhances the water exchange between the interior and exterior of the Marimo, thereby recycling nutrients for growth. Our results provide an intriguing model of a physical environment interacting with biological processes in a self-sustaining ecosystem. We also demonstrate that Marimo have a spherical annual ring structure, and their growth rate is associated with ice cover. The balance between the ecology of Marimo and the water environment in Lake Akan is highly vulnerable and at risk of irreversible degradation. We must endeavor to rescue Marimo from the fate of a "canary in the coal mine" of global climate change.
正会員 博(工) 北見工業大学工学部 社会環境工学科(〒090-8507 北海道北見市公園町165番地)2 正会員 修(工) (株) アルファ水工コンサルタンツ (〒063-0829 北海道札幌市西区発寒9条14丁目516-336)This study describes the development of a three-layer model based on a fully-nonlinear and strongly-dispersive internal wave model and its application to a stratified lake response. The efficient numerical model for three-layer system is needed to analyze non-linear and non-hydrostatic lake response as a two-layer model often fails to illustrate realistic stratified lake responses. Therefore, this study aims to develop a three-layer model based on the fully-nonlinear and strongly-dispersive internal wave model (the three-layer FSI model). The three-layer FSI model was verified by simulating the belt-driven three-layer laboratory experiments by Stevens & Imberger (1996). The present model well predicts the measured results of the experiments and supports the classification by the Wedderburn number and Csanady's theory. The computed results also show the importance of non-hydrostatic pressure on the generation of the internal solitary wave during upwelling process.
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