[M-IS22] Biogeochemistry

[MIS22-04] Tracing sources of organic matter driving hypolimnetic oxygen consumption in Lake Biwa using stable carbon and radiocarbon isotopes

*Wataru Nakamura^1,2, Yasuhiko T. Yamaguchi¹, Kazuhide Hayakawa¹, Sho Ogasawara¹, Eiso Inoue¹, Shish Muhammad Soyaib², Yosuke Miyairi², Yusuke Yokoyama² (1.Lake Biwa Environmental Research Institute, 2.The University of Tokyo)

Keywords:Lake Biwa, Hypoxia, Hypolimnion, Sinking particles, Radiocarbon

Stronger thermal stratification resulting from global warming has led to a lower level of dissolved oxygen (DO) in the hypolimnion during the stratification period in lacustrine ecosystems. In Lake Biwa, hypoxia in the hypolimnion has been progressively intensifying, raising concerns about its impact on the ecosystem. In this study, we sought to estimate the sources of organic matter responsible for DO consumption in the hypolimnion of Lake Biwa. We employed stable carbon isotope (δ¹³C) and radiocarbon isotope (Δ¹⁴C) as tracers to discern the sources of organic matter (e.g., autochthonous vs allochthonous). Concentrations of dissolved inorganic carbon (DIC) and carbon isotope ratios of DIC (δ¹³C_DIC and Δ¹⁴C_DIC) were measured monthly at eight depths at a pelagic site (center of Imazu offshore (17B): maximum water depth = ca. 89 m) from May 2024 to March 2025 to investigate temporal variations in carbon isotope ratios caused by DO consumption. Furthermore, incubation experiments of sediment cores were conducted in each season to estimate sources of the organic matter used for DO consumption by sediments. The estimation process involved solving a mass balance model based on the changes in DO, DIC, δ¹³C_DIC, and Δ¹⁴C_DIC in the water directly above the sediment cores before and after the experiments. Carbon isotope ratios of particulate organic carbon (δ¹³C_POC and Δ¹⁴C_POC) were analyzed for sinking particles to ascertain the seasonal fluctuations in organic matter within Lake Biwa. The sinking particles were collected at a depth of 60 m in Site 17B using a time-series sediment trap and subsequently divided into three distinct size fractions. Depth profiles of δ¹³C_POC and Δ¹⁴C_POC in sediment core samples were analyzed to test the possibility of DO consumption by the organic matters accumulated in the past (a legacy effect). The estimated sources of organic matter utilized for DO consumption exhibited seasonal fluctuations, with the observed DO consumption in the spring attributable to fresh autochthonous carbon produced in the lake. Conversely, it was postulated that the DO consumption observed from the summer to fall was attributable to old allochthonous carbon of terrestrial sources. Furthermore, the carbon isotope ratios of organic matter presumed to consume DO were similar to those of the sinking particles, suggesting that these particulate organic matters sinking during the same season consume DO in the hypolimnion of Lake Biwa.