Assessing Fluorescent Organic Matter in Natural Waters: Towards In Situ Excitation–Emission Matrix Spectroscopy

Abstract

Natural organic matter (NOM) is a key parameter in aquatic biogeochemical processes. Part of the NOM pool exhibits optical properties, namely absorption and fluorescence. The latter is frequently utilized in laboratory measurements of its dissolved fraction (fluorescent dissolved organic matter, FDOM) through excitation–emission matrix spectroscopy (EEMS). We present the design and field application of a novel EEMS sensor system applicable in situ, the ‘Kallemeter’. Observations are based on a field campaign, starting in Norwegian coastal waters entering the Trondheimsfjord. Comparison against the bulk fluorescence of two commercial FDOM sensors exhibited a good correspondence of the different methods and the ability to resolve gradients and dynamics along the transect. Complementary laboratory EEM spectra measurements of surface water samples and their subsequent PARAFAC analysis revealed three dominant components while the ‘Kallemeter’ EEMS sensor system was able to produce reasonable EEM spectra in high DOM concentrated water bodies, yet high noise levels must be addressed in order to provide comparable PARAFAC components. Achievements and limitations of this proof-of-concept are discussed providing guidance towards full in situ EEMS measurements to resolve rapid changes and processes in natural waters based on the assessment of spectral properties. Their combination with multiwavelength FDOM sensors onboard autonomous platforms will enhance our capacities in observing biogeochemical processes in the marine environment in spatiotemporal and spectral dimensions.

In situ fluorescence measurements of dissolved organic matter: A review

“…One direction of sensor development was oriented towards maximizing the information extracted from the device. For example, Zielinski et al (2018) have developed a submersible sensor system that is able to record full EEMs, within a high spectral range ((excitation/emission = 220-750 nm / 200-950 nm). Another example of novel devices is the "one LED & three signals" system developed by Li et al (2016a) for in situ measurements.…”

“…One example is the next generation devices, which are sufficiently stable to be installed on gliders, buoys and surface platforms (Cyr et al, 2019;Ferdinand et al, 2017). Also, the novel device constructed by Zielinski et al (2018) can be installed in a moonpool in a vessel or integrated into an underwater platform.…”

“…Despite recent developments in field fluorimeters, there are still some limitations in terms of sensitivity, compared with benchtop spectrofluorimeters. For example, the Kallemeter sensor, developed by Zielinski et al (2018), has a sensitivity of 34 signal-to-noise ratio (SNR) measured for the water Raman peak, while a typical benchtop spectrofluorimeter (Aqualog, Horiba, Japan) has a Water-Raman SNR of > 20,000. It is not known, though, if the water Raman peak was measured at similar wavelengths and integration times.…”

“…(2010): a) a pre-calibration to test the precision, pressure and mechanical and electrical stability of the device; b) calibration of signal output by measuring dark and saturation counts (Zielinski et al, 2018); c) calibration of internal temperature to test its impact on sensor optical components (Cyr et al, 2017;Yamashita et al, 2015); d) calibration with distilled water at a certain temperature; e) absolute calibration with known substances.…”

“…Nevertheless, Tedetti et al (2013) showed that the high fluorescence intensity of the phenanthrene-like peak was actually caused by fluorene, another PAH. Potential solutions to this problem are to use a combination of field fluorimeters, operating at wavelengths specific for each fluorophore, or to use in situ devices that measure at multiple wavelengths, like the ones used by Cyr et al (2017), Puiu et al (2015), Tedetti et al (2010 and Zielinski et al (2018). In addition, Tedetti et al (2010) showed that the different quantum yield of fluorophores may help at separating between compounds.…”