Recommendations for Improving In situ Dissolved Organic Carbon Fluorescence Sensors Based on Analysis of Nationwide Excitation Emission Matrix (EEM) Data

In situ single-wavelength fluorometers are increasingly used to estimate dissolved organic carbon (DOC) concentrations in aquatic systems. For historic reasons, these instruments measure fluorescence at an excitation center wavelength near 370 nm and an emission center wavelength near 460 nm, with varying bandpass filters. This excitation-emission pair has been shown to be an excellent proxy for DOC concentration in many settings. However, developing robust correlations with DOC and making comparisons among instruments across space and time is difficult because this region of the EEM spectrum is on the sloping edge of a commonly identified fluorophore (peak C), which makes it particularly sensitive to small differences in excitation and emission bandpasses among instruments. Moreover, natural shifts in the position of the peak occur due to differences in DOC source (waste water, algal, soil, urban) as well as due to changes in pH, salinity, and metal concentrations. We examined several thousand EEMs collected from a variety of fresh, estuarine, and marine settings to identify regions within the fluorescent spectrum where in situ measurements may be less susceptible to these interferences and confounding factors. We identified the most stable regions of the EEM landscape that are strongly related to DOC concentration. Our results demonstrate that changing the center wavelength pair of in situ DOC fluorescence sensors will improve the stability and comparability of these measurements across instrument types and aquatic systems. We also identified cases where water receiving different sources of DOC, such as wastewater and urban runoff, may require measurment of additional wavelength pairs to adequately determine DOC concentration.