2:15 PM - 2:30 PM
[MIS22-03] Factors influencing microbial carbon pump efficiency in the hypolimnion of a large lake: Insights from fluorescent dissolved organic matter
Keywords:Aquatic environments, Dissolved Organic Matter, Molecular size, Lake Biwa, Microbial Carbon Pump
Dissolved organic matter (DOM) is a fundamental component of biogeochemical cycles in aquatic environments. The microbial production of refractory dissolved organic matter (RDOM), i.e., the microbial carbon pump (MCP), has received significant attention as it facilitates organic carbon sequestration. Fluorescent dissolved organic matter (FDOM) has emerged as a powerful tool for investigating the dynamics of microbially derived DOM. The fluorescence intensity of humic-like FDOM (FDOM-H) in the deep oceanic and lacustrine waters exhibits a strong positive linear correlation with apparent oxygen utilization (AOU). It is hypothesized that as aquatic microorganisms metabolize labile organic matter, such as sinking particles, they release RDOM, including FDOM-H, as a metabolic by-product. The slope of correlation between FDOM-H and AOU is considered indicative of MCP efficiency. Therefore, elucidating the factors governing the FDOM-H–AOU slope is critical for understanding how MCP responds to environmental changes. The variability in the FDOM-H–AOU slope is believed to result from differences in water temperature and lability of organic substrates; however, direct empirical evidence remains limited. Additionally, sediment-derived FDOM-H may influence the FDOM-H–AOU slopes in marginal seas, further complicating its interpretation.
This research aimed to elucidate the key factors influencing the FDOM-H–AOU slopes. Initially, we performed long-term biodegradation experiments on lake water organic matter from various locations, depths, and seasons within Lake Biwa (14 samples), spanning over 400 days under constant temperature conditions (20°C). Subsequently, to evaluate the influence of water temperature, we conducted additional biodegradation experiments at multiple temperature levels (ranging from 8 to 30°C) on lake water organic matter from offshore Site 17B in Lake Biwa (bottom water depth = ca. 89 m). The field survey involved the collection of monthly water samples during the stratified period (April to December) at Site 17B between 2019 and 2024.
DOM fluorescence intensity was measured using a spectrofluorometer, and FDOM-H was calculated by Peak M intensity. Total organic carbon (TOC), dissolved organic carbon (DOC), and particulate organic carbon (POC) were quantified using a TOC analyzer. DOC concentrations across molecular size fractions were determined via size exclusion chromatography coupled with a TOC analyzer. AOU in natural lake water was calculated based on water temperature and dissolved oxygen concentration. Oxygen consumption rates during the biodegradation experiments were derived from the reduction in TOC concentration, assuming a respiratory quotient of 0.7.
In the 20°C biodegradation experiments, FDOM-H–AOU slopes exhibited variations of up to fourfold depending on the sample origin. A subsequent analysis of the factors influencing the slope variations revealed a significant negative correlation with the concentrations of semi-labile low molecular weight DOC or semi-labile POC. The FDOM-H–AOU slopes tended to increase at higher water temperatures. These findings imply a correlation between the degradability of organic matter and the release of FDOM-H.
For the lake water samples collected during the field survey, the FDOM-H–AOU correlation was examined by water depth. In the bottom layer (depths of 85 or 88 m), where the majority of oxygen consumption originates from sedimentary processes, a strong linear correlation between FDOM-H and AOU was observed. In contrast, the correlation was not significant in the lower thermocline (depth of ca. 15 to 20 m), where the contribution of sediment was minimal. A moderate linear correlation was observed at a depth of 60 m, where both the water column and sediment played significant roles. These findings would indicate a critical role of sedimentary processes in the production of FDOM-H in the hypolimnion of Lake Biwa.
This research aimed to elucidate the key factors influencing the FDOM-H–AOU slopes. Initially, we performed long-term biodegradation experiments on lake water organic matter from various locations, depths, and seasons within Lake Biwa (14 samples), spanning over 400 days under constant temperature conditions (20°C). Subsequently, to evaluate the influence of water temperature, we conducted additional biodegradation experiments at multiple temperature levels (ranging from 8 to 30°C) on lake water organic matter from offshore Site 17B in Lake Biwa (bottom water depth = ca. 89 m). The field survey involved the collection of monthly water samples during the stratified period (April to December) at Site 17B between 2019 and 2024.
DOM fluorescence intensity was measured using a spectrofluorometer, and FDOM-H was calculated by Peak M intensity. Total organic carbon (TOC), dissolved organic carbon (DOC), and particulate organic carbon (POC) were quantified using a TOC analyzer. DOC concentrations across molecular size fractions were determined via size exclusion chromatography coupled with a TOC analyzer. AOU in natural lake water was calculated based on water temperature and dissolved oxygen concentration. Oxygen consumption rates during the biodegradation experiments were derived from the reduction in TOC concentration, assuming a respiratory quotient of 0.7.
In the 20°C biodegradation experiments, FDOM-H–AOU slopes exhibited variations of up to fourfold depending on the sample origin. A subsequent analysis of the factors influencing the slope variations revealed a significant negative correlation with the concentrations of semi-labile low molecular weight DOC or semi-labile POC. The FDOM-H–AOU slopes tended to increase at higher water temperatures. These findings imply a correlation between the degradability of organic matter and the release of FDOM-H.
For the lake water samples collected during the field survey, the FDOM-H–AOU correlation was examined by water depth. In the bottom layer (depths of 85 or 88 m), where the majority of oxygen consumption originates from sedimentary processes, a strong linear correlation between FDOM-H and AOU was observed. In contrast, the correlation was not significant in the lower thermocline (depth of ca. 15 to 20 m), where the contribution of sediment was minimal. A moderate linear correlation was observed at a depth of 60 m, where both the water column and sediment played significant roles. These findings would indicate a critical role of sedimentary processes in the production of FDOM-H in the hypolimnion of Lake Biwa.