The application of fDOM sensors in freshwater systems: Limitations, knowledge gaps and recommendations for future enhancement and novel development

Commercially-available, field deployable sensors designed to measure colored or chromophoric dissolved organic matter fluorescence (fDOM; ex370/em460 nm) in-situ are of considerable interest to researchers owing to their potential to serve as a proxy for continuous DOC concentrations in many freshwater environments. The major challenge with these sensors stems from non-linearity effects between measured fDOM and DOC concentration over the range of DOC encountered as well as interferences from organic and inorganic particles and temperature effects. A series of laboratory based evaluations were designed to compare inherent sensor optical geometries as well as reveal non-linearity effects due to (1) high fDOM concentrations (inner filter effects), (2), particle interference, and (3) temperature effects. In addition to evaluating the correlation between DOC concentration and fDOM, we considered sensor dynamic range and gain settings as well as data output protocol (e.g. analog or digital). Previous studies have illustrated the role of particle interference, which results in underestimates of fDOM greater than 50% at turbidities observed in many rivers and streams. Here, we present the results from the array of laboratory evaluations and discuss the practical limitations, propose correction methods and highlight knowledge gaps.