A temperature compensation method for CDOM fluorescence sensors in freshwater

The effect of temperature on CDOM fluorescence was investigated in dystrophic freshwaters of Wisconsin and in aqueous standards. Laboratory experiments with two commercial in situ fluorometers showed that CDOM fluorescence intensity decreased as ambient water temperature increased. A temperature compensation equation was derived to standardize CDOM fluorescence measurements to a specific reference temperature. The form of the equation is: CDOM(r) = CDOM(m)/[1 + rho(T(m) - T(r))], where T is temperature (degrees C), rho is the temperature-specific coefficient of fluorescence (degrees C(-1)), 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 (rho) were -0.015 +/- 0.001 and -0.008 +/- 0.0008 for Wisconsin bog waters at 20 degrees 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.