Marimo in Lake Akan, Hokkaido, was discovered in 1898 by botanist Takiya Kawakami. It was designated as a natural monument in 1921 due to its rarity and unique morphology, and later as a special natural monument in 1952. Yoshiji Yoshii, an ecology professor at Tohoku Imperial University and the first president of the Ecological Society of Japan, who served as an examiner for the Ministry of Home Affairs' Historic Sites, Places of Scenic Beauty, and Natural Monuments, inspected Lake Akan and documented that while marimo was abundant in the early 1900s, it had significantly decreased by around 1950. However, there was no quantitative information about marimo's past biomass, and it remained unclear how abundant marimo once was.
A joint research team consisting of Professor Emeritus Jotaro Urabe, Academic Researcher Hajime Ohtsuki, Assistant Professor Yurie Otake, and Doctoral Student Ryotaro Ichige from the Graduate School of Life Sciences, Tohoku University; Dr. Isamu Wakana from the Kushiro International Wetland Centre; Professor Toshifumi Minamoto and Academic Researcher Masayuki K. Sakata from Kobe University's Graduate School of Human Development and Environment; and Professor Michinobu Kuwae from Ehime University's Center for Marine Environmental Studies used marimo DNA (environmental DNA) preserved in Lake Akan's bottom sediments to reveal changes in marimo biomass from 200 years ago to the present. Their findings were published in the journal Environmental DNA.
The group developed a method to correct for DNA degradation rate over time using Daphnia remains and DNA. The analysis revealed that the marimo biomass in the early 1900s was 10-100 times greater than it is today and declined significantly over subsequent decades. The research showed that sediment influx due to deforestation around Lake Akan and water level fluctuations caused by hydroelectric power operations had a major impact on the marimo's growing environment. The study also indicated that after 1950, eutrophication of Lake Akan due to tourism development may have hindered the recovery of growth conditions.
In recent years, factors contributing to decline have further diversified, including global warming, invasion of aquatic plants, and beaching due to wind. As a special natural monument, marimo is also an important tourism resource, and its conservation is essential for revitalizing the local economy. This research is expected to drive further development of conservation measures for marimo. Additionally, this study demonstrates that it is now possible to estimate past habitats and growth conditions of organisms that do not leave fossils by correcting the amount of DNA remaining in sediments using plankton remains. For biodiversity restoration and conservation, it is essential to understand the species that lived and grew in the past and their biomass, and to clarify the changes in their habitat and growth conditions leading to the present, as well as the factors causing their decline. The method used in this study enables the reconstruction of the past for many organisms that do not leave fossils and is expected to contribute to better target-setting and conservation of natural environments and biodiversity.
Journal Information
Publication: Environmental DNA
Title: Reconstruction of Marimo Population Dynamics Over 200 Years Using Molecular Markers and Fossil Plankton Remains
DOI: 10.1002/edn3.70085
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.