Reconstruction of marimo population dynamics over 200 years using molecular markers and fossil plankton remains

Recent efforts have focused on reconstructing the historical abundance of
unfossilized organisms using environmental DNA preserved in sediments
(sedDNA). This information is crucial for understanding long-term changes
in ecosystems. However, because sedDNA is prone to degradation, its
quantification may not always provide accurate estimates of past
abundances. To address this issue, we developed a novel method to correct
for sedDNA degradation by incorporating plankton remains and applied it to
estimate the historical abundance of marimo—large spherical colonies of
the green alga Aegagropila brownie (formerly A. linnaei)—in Lake Akan,
Japan, which is the only known habitat for large marimo. We first
quantified marimo sedDNA in lake sediments dating back over 200 years. We
then used our new method to estimate historical changes in their abundance
from sedDNA. Analyses revealed that marimo were historically 10-100 times
more abundant than they are today, but that their abundance declined in
the early 20th century when influxes of muddy water and water level
fluctuations occurred due to deforestation and the operation of a
hydroelectric power plant. These findings align with historical eyewitness
accounts, indicating that, when corrected for degradation using fossilized
remains, sedDNA can be a powerful tool for reconstructing the past
abundance of unfossilized organisms.