296As interest in the aquatic cycle of organic carbon (OC) has increased, the deployment of in situ optical sensors to measure CDOM fluorescence (chromophoric dissolved organic matter) as a proxy for OC concentration has become more common (e.g., Downing et al. 2009;Sandford et al. 2010). CDOM sensors typically use UV light (~350 nm) to excite the emission of blue light (~450 nm) from certain organic fluorophores, allowing investigators to distinguish CDOM from more commonly measured phytoplankton pigments. Given that CDOM may be more labile than previously thought and given that rates of OC mineralization may vary with fluctuating environmental factors, such as temperature and light, these inexpensive sensors could afford a substantial advantage over traditional wet chemistry methods-provided that the artifactual effects of environmental factors on fluorescence efficiency are well constrained (Graneli et al. 1996;Bertilsson and Tranvik 2000;Bastviken et al. 2004;Hanson et al. 2003;Vahatalo 2009).Here, we quantify the effect of temperature on the fluorescence of CDOM from two dystrophic Wisconsin lakes and an aquatic NOM reference material. Based on laboratory experiments over a wide range of OC concentrations, we derive a function that can be used to standardize CDOM measurements to any reference temperature (and, thereby, remove the effect of temperature variation on CDOM fluorescence). Using a reference temperature of 20°C, we then apply the function to field data and show how temperature compensation affects temporal changes in CDOM fluorescence under natural conditions. Methods and proceduresTwo commercial CDOM fluorometers were used: 1) the C3 Submersible Fluorometer from TurnerDesigns, Inc.; and 2) the ), and the subscripts r and m stand for the reference and measured values. (We note that an analogous function is used widely to calculate temperature-specific conductance from the measured conductivity of natural waters.) For the two sensors we tested, the temperature-specific fluorescence coefficients (r) were -0.015 ± 0.001 and -0.008 ± 0.0008 for Wisconsin bog waters at 20°C. When applied to field data, temperature compensation removed the effect of multi-day trends in water temperature, and it also damped the diel CDOM cycle. We conclude that temperature compensation is a necessary and important aspect of CDOM monitoring using in situ fluorescence sensors.