Developing correction factors for in-situ fluorescence index (FI) sensors

Dissolved organic matter (DOM) links terrestrial and aquatic carbon pools from headwaters to oceans. The molecular quality of DOM controls residence times, nutrient availability to microbial communities, and the environmental fate of carbon and nutrients. However, DOM chemical composition is complex, and varies dramatically as a function of source and transformation. Fluorescence has been widely adopted in lab as a simple yet effective way to measure general compositional characteristics of fluorescent dissolved organic matter (fDOM) as a proxy for DOM quality. More recently, in-situ fDOM sensors have been used to constrain the spatiotemporal behavior and drivers of DOM in natural systems. To obtain high-quality in-situ fDOM data, corrections for fluorescence interferences (dissolved and suspended solids and temperature) are required. Such corrections have been developed for sensors measuring a single fluorescence region as a proxy for DOM quantity, but not for sensors providing a proxy for DOM quality. Here, we present correction procedures for a custom multi-region fluorometer measuring the ratio of two regions as a proxy for the fluorescence index (FI), a common indicator of DOM quality, and compare results to equivalent corrections for single-region fDOM sensors. Correction procedures for temperature, absorbance, turbidity, and pH were established for different source waters spanning ranges of DOM concentration and composition. While single-region temperature corrections varied predictably with DOM concentration, FI corrections were less predictable. Different responses between single-region and FI measurements were also observed for absorbance and turbidity interferences, varying by DOM source. Finally, pH appeared to more strongly affect multi-region FI estimations than single-region fDOM measurements. Our findings highlight the importance of site-specific fluorescence corrections, particularly for fDOM quality sensors.