Industrialization developments decimated the population of “marimo” algae balls, a government-designated “special natural monument,” in Lake Akanko in Hokkaido by about 75 years ago, a research team said,
The scientists from Tohoku University and other institutions said the lake had 10 to 100 times the current amount of the ball-forming algae until about 120 years ago.
But during the first half of the 20th century, influxes of muddy water due to deforestation and the use of water for hydroelectric power generation posed as serious threat to the marimo population in the lake.
The green alga species forms fuzzy spheres that can grow to a maximum diameter of nearly 30 centimeters. Smaller ones have been placed in jars and sold as cute “pets.”
The research team, led by Jotaro Urabe, a Tohoku University professor emeritus of ecology, collected 1-meter-deep core samples from the bottom of Lake Akanko in Kushiro, eastern Hokkaido.
They used DNA contained in lacustrine sediments to estimate temporal changes in marimo’s population abundance.
The DNA, however, degrades and decreases in abundance with time. So, the researchers used plankton of the Daphnia dentifera species from the same sediments to calculate the rate of DNA degradation on Akanko’s bottom.
Daphnia dentifera has a hard body tissue called the postabdominal claw, which resists decomposition and survives for a very long time.
By counting the numbers of daphnia DNA copies and daphnia postabdominal claws preserved in the sediments, the researchers could calculate the relationship between the past population sizes of organisms and their DNA copy numbers in the sediments.
They obtained a temporal decay curve for DNA copy numbers, which they used to estimate the marimo population abundance.
Their study showed that, until about 120 years ago, marimo were between 10 times and 100 times more abundant than they are now.
The marimo population in Lake Akanko dropped sharply by around 1950.
That coincides with a period when deforestation led to muddy water entering Lake Akanko and a hydroelectric power plant built downstream lowered the water level in the lake, the scientists said.
Isamu Wakana, a member of the research team who serves as “large spherical marimo restoration officer” with the Kushiro City World Natural Heritage Promotion Office, said hornwort, an aquatic plant species, has propagated so much in Akanko in recent years that it is obstructing the growth of marimo.
Marimo balls rotate as they grow. Excessive propagation of hornwort is hampering water currents in the lake, making it difficult for marimo to revolve and photosynthesize evenly, Wakana explained.
Wakana and his colleagues removed hornwort from an area measuring 40 meters wide and 200 meters long off a marimo habitat between late June and early July and have since been observing developments.
“I hope our research results will be used to set a future preservation goal for Lake Akanko’s marimo,” Urabe said. “I think we should strive to increase its abundance at least 10-fold from now.”
The research results were published in an international science journal.
AloJapan.com