Biogeochemical control on fluorescent dissolved organic matter dynamics in a large freshwater lake (Lake Biwa, Japan)

The dynamics of fluorescent dissolved organic matter (FDOM) in the large monomictic freshwater Lake Biwa (surface area 675 km², maximum depth 104 m) was studied from December 2010 to December 2011. The protein-like FDOM (FDOM_T) and dissolved organic carbon (DOC) showed epilimnetic accumulation (FDOM_T from 4.42 ± 0.22 quinine sulfate units [QSU] to 6.30 ± 0.04 QSU; DOC from 80.8 ± 2.7 μmol L^-1 to 102.7 ± 3.5 μmol L^-1) between nutrient-replete winter mixing to nutrient-depleted stratified periods. This accumulation is attributed to the reduced heterotrophic activity following severe P-limitation. The positive correlation between accumulated DOC and FDOM_T in the epilimnion and their uniform reduction in the hypolimnion (~ 9%) suggest FDOM_T as a proxy for semi-labile DOM. The humic-like FDOM (FDOM_M) generally increased with depth, a pattern similar to nutrients and total carbon dioxide (TCO₂), but adverse to dissolved oxygen. The significant positive correlations of FDOM_M with apparent oxygen utilization (r = 0.86, p < 0.001), TCO₂ (r = 0.91, p < 0.001), nitrate (r = 0.83, p < 0.001), and phosphate (r = 0.76, p < 0.001) in the deeper layers suggest that FDOM_M is formed during hypolimnetic mineralization. We estimated that ~ 8% of the organic carbon degraded in the hypolimnion is transferred into humic substances. The minor contribution of DOC (6.4%) to hypolimnetic mineralization suggests that production of humic substances is mainly fueled by the mineralization of sinking biogenic particles. The production and consumption of FDOM in freshwater lakes may influence the quality and bioavailability of carbon exported from these systems.