Distribution of Humic Fluorescent Dissolved Organic Matter in Lake Shihwa: the Role of the Redox Condition

Hypoxia has occurred worldwide in coastal and marginal oceans. The redox condition has recently garnered major interest as a favorable condition for delivering sedimentary organic matter to the water column. In this study, we measured the fluorescence of fluorescent dissolved organic matter (FDOM) in brackish Lake Shihwa, Korea, in which hypoxic conditions are typically observed during summer. Especially, fluorescent intensities of the humic FDOM (FDOMH) were relatively high during summer, with a relatively lower dissolved oxygen (DO) level, and exponentially increased with decreasing DO concentrations. The results indicated that the production of FDOMH is associated with anaerobic processes. It was further supported by a significantly positive relationship between FDOMH and ammonium. Based on the relatively low values of redox potential (up to 60.0 mV) and high concentration of phosphate with the low DO level, this enrichment of FDOMH seems to be linked to the input of anoxic porewater. Using a simple schematic diagram, the contribution of FDOMH from reducing environments is comparable to that from stream water, which is known to be a major source in coastal regions. This study highlights that the redox condition is a key factor contributing to the production of FDOMH in coastal marine environments.