EXCLUSIVE ONLINE REPORT

Is Lake Biwa Becoming Anoxic?

Dr. Kumagai Michio and Brian Williams

Lake Biwa is far and away Japan’s largest lake, stretching more than 63 kilometers from end to end, with a maximum depth of just over 100 meters. While the lake’s deep area is not rich in biodiversity,  most of the benthic (bottom-dwelling) animals there are endemic and precious. They are also in danger, and their peril points to our own.

The history of Lake Biwa is relatively long. Although the present lake basin was formed about 400,000 years ago by a prolonged and still continuing combined uplift-subsidence tectonic event, its original position was roughly100 kilometers to the south. Thus, the lake is approximately four million years old. The bottom of the lake is still tectonically active, sinking very slowly at a rate basically equal to sediment accumulation, creating relatively stable depths over geologic time. This has become the mother basin for many endemic organisms.

At the lake bottom, oxygen is usually supplied by oxygen-rich surface water that chills and descends in wintertime, a phenomenon called vertical circulation which includes forced mixing and cold water intrusion. If vertical circulation becomes weaker, less oxygen is supplied to the bottom. We have recently found that vertical circulation has been dropping over the last two decades. This has resulted in a critical decrease in the concentration of dissolved oxygen near the lake bottom, which has reached a tipping point for changing the ecosystem in this lake.

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credits

Brian Williams, an artist, has resided in a restored farmhouse in a satoyama-type village near Kyoto for a quarter century, and has lived and painted in Japan for 38 years. His conviction that a landscape painter should also work to preserve and restore that landscape complicates and enriches his life.

brianwilliamsart.com

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