Comparison of UV/Vis and FDOM sensors for in situ measurement of stream DOC concentrations

Abstract

Optical sensors such as ultra-violet/visible (UV/Vis) spectrophotometric sensors and fluorescent dissolved organic matter (FDOM) sensors have been frequently used to estimate dissolved organic carbon (DOC) concentrations of streams and rivers at high temporal resolution. Despite of the merits of both sensors, temperature changes and particulate matter in water can interfere the sensor readings, over- or under-estimating DOC concentrations. There is little information on how differently the two types of the sensors perform in a natural condition. We conducted both laboratory experiments and in-situ monitoring with UV/Vis and FDOM sensors during the three storm events in the fall of 2012 and the spring of 2013 in a small forest stream in Korea in order to compare and contrast their performance. Laboratory experiments using extracted DOC from the forest soils and the reference materials including the Suwannee River (SR) natural organic matter, SR fulvic acid, and SR humic acid showed that both sensors can be successfully used as a proxy for stream DOC concentrations in clear water. However, the UV/Vis sensor overestimated DOC concentrations by about 400% at turbidity of about 1100 NTU while about 60% of FDOM signals were attenuated under the same turbidity. Temperature effect on the UV/Vis sensor was minimal while FDOM signals decreased by about 1.5-1.8 % per 1℃ increase. In-situ monitoring of the UV/Vis and FDOM signals during storms also demonstrated that both types of sensors could be used for a proxy for DOC concentrations after compensation for turbidity and temperature. The correlation between the sensor outputs in field and measured DOC concentrations of filtered stream water in laboratory suggests that the FDOM sensor has an advantage over the UV/Vis sensor in the forest stream due to its relatively low sensitivity to turbidity.