Japan Geoscience Union Meeting 2023

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[J] Oral

M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS09] Biogeochemistry

Tue. May 23, 2023 1:45 PM - 3:00 PM 105 (International Conference Hall, Makuhari Messe)

convener:Keitaro Fukushima(Faculty of Food and Agricultural Sciences, Fukushima University), Keisuke Koba(Center for Ecological Research, Kyoto University), Naohiko Ohkouchi(Japan Agency for Marine-Earth Science and Technology), Youhei Yamashita(Faculty of Environmental Earth Science, Hokkaido University), Chairperson:Keisuke Koba(Center for Ecological Research, Kyoto University), Karibu Fukuzawa(Field Science Center for Northern Biosphere, Hokkaido University)

2:15 PM - 2:30 PM

[MIS09-03] Cryptic cycling of high molecular weight dissolved organic matter in the hypolimnion of a deep lake

*Yasuhiko T. Yamaguchi1, Yusuke Okazaki2, Koichi Shimotori3, Akio Imai3, Kazuhide Hayakawa1 (1.Lake Biwa Environmental Research Institute, 2.Institute for Chemical Research, Kyoto University, 3.NIES Lake Biwa Branch Office, National Institute for Environmental Studies)

Keywords:Aquatic environments, Dissolved Organic Matter, Molecular size, Lake Biwa, Microbes

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: Mw ~ 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 almost 7 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.