Water pH as a driver of bryophyte species distribution in Moss Balls of Lake Kussharo, Japan

Lake Kussharo is the largest caldera lake in Japan and serves as the habitat for "Marigoke" (moss balls), which was designated as a natural monument of Teshikaga Town on July 11, 1969. Recently, a decline has been observed in both Marigoke and its constituent plant species. This study aimed to clarify the impact of the pH of lake water on Marigoke and its constituent plants. Historically, the constituent species were Drepanocladus fluitans var. kutcharokensis and Jungermannia exsertifolia Steph. However, current observations indicate that Fontinalis antipyretica Hedw. is dominant in neutral environments, while Drepanocladus fluitans (Hedw.) Warnst. prevails in acidic environments. Furthermore, this study revealed the existence of two distinct types of Marigoke in Lake Kussharo, each composed of different species depending on the pH of the environment. The growth process of Marigoke is governed by a complex interplay of multiple factors, including water quality, presence or absence spring water, geology, sediment, topography, and flow characteristics. The formation of Marigoke requires specific environmental conditions, including physical factors, and its occurrence is restricted to extremely limited areas.