The hypolimnion of deep lakes is the site where essential ecosystem functions such as O₂ consumption and nutrient regeneration occur. We conducted long-term biodegradation experiments and field surveys on a large, deep lake (Lake Biwa, Japan) to elucidate the dynamics of dissolved organic matter (DOM) in the hypolimnion. The biodegradation experiments (>250 days) were conducted with hypolimnetic water collected in March 2020 (the mixing period). The water samples were incubated in the dark at 9°C (the water temperature of the hypolimnion) or 20°C. The hypolimnetic waters at the same site were collected monthly from March to December 2020 (through the stratified period). The molecular size distribution of DOM was quantified using a size-exclusion chromatograph coupled with a total organic carbon detector (SEC-TOC). The net degradation rates of high molecular weight DOM (HMWDOM: M_w ~ 100kDa) were estimated for each concentration time series. The rates were faster in the order of the 20°C experiment > the 9°C experiment > and the monthly time series of hypolimnetic waters. This result indicates that the HMWDOM production fueled by sinking particles would be considered to explain the apparent persistence of HMWDOM in the hypolimnion. We also estimated the gross degradation flux (= the net degradation flux + the production flux) of HMWDOM in the hypolimnion, which was 4 times higher than the net degradation flux. The cryptic cycling of HMWDOM was linked to O₂ consumption in the hypolimnion, suggesting that it should be a critical biogeochemical process in deep lakes.

Primary Presenter: Yasuhiko Yamaguchi, Lake Biwa Environmental Research Institute (yamaguchi-y@lberi.jp)

Authors:

Yusuke Okazaki, Institute for Chemical Research, Kyoto University  ()

Koichi Shimotori, NIES Lake Biwa Branch Office, National Institute for Environmental Studies  ()

Akio Imai, NIES Lake Biwa Branch Office, National Institute for Environmental Studies  ()

Kazuhide Hayakawa, Lake Biwa Environmental Research Institute  ()