# A low-cost optical probe for dissolved organic matter monitoring in aquatic ecosystems

Marie-Noëlle Pons, David Brunello, Blandine Caquet, Anne Poszwa

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Marie-Noëlle Pons, David Brunello, Blandine Caquet, Anne Poszwa. A low-cost optical probe for dissolved organic matter monitoring in aquatic ecosystems. 3ème congrès TERENO-OZCAR, Sep 2025, Paris, France. hal-05342447

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Submitted on 2 Nov 2025

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# A low-cost optical probe for dissolved organic matter monitoring in aquatic ecosystems

Marie-Noëlle Pons¹,², David Brunello¹, Blandine Caquet³, Anne Poszwa⁴,²

1 Université de Lorraine, CNRS, LRGP, 54000, Nancy, France

2 LTSER Zone Atelier du Bassin de la Moselle, Université de Lorraine, 54000, Nancy, France

3 Vigie de l'Eau, Galerie thermale, 88800, Vittel, France

4 Université de Lorraine, CNRS, LIEC, Vandœuvre-lès-Nancy, France

Dissolved organic matter (DOM) is a complex, heterogeneous mixture of carbonaceous substances derived either from the natural processes of decomposition and transformation of organic residues (plants, animals, microorganisms) or from the excretions of living organisms (humans, animals) in particular via ill-treated municipal wastewater. DOM plays a crucial role in the sequestration of atmospheric carbon and in the transport of metals. Climate change, decrease in the acidification of rivers and lakes, land-use change and discharge of anthropogenic organic matter are among the factors leading to a continuous evolution of DOM in water bodies and thus modifying the global cycle of carbon and directly impacting water resources such as those used for drinking water production.

DOM cannot be defined through specific chemical compounds and is considered as a supramolecular assembly of small molecules such as sugars, tannins, proteins, lipids, etc. Optical methods such as UV-visible spectroscopy and fluorescence spectroscopy are commonly used to investigate DOM molecular and structural properties in laboratories using benchtop devices. Our goal within the Terra Forma project (https://terra-forma-web.osug.fr/) has been to go further by developing an in-situ and low-cost spectrometer that combines UV-Visible light absorption and fluorescence measurements for the analysis of DOM, focusing on protein-like (a potential marker for fecal contamination) and humic-like characteristics. The system should be efficient and easy to use for researchers as well as for participants in citizen science programs.

The development has been optimized in terms of the selection of the LEDs and photodiodes (wavelength range and power), the light pulse regime and the sensitivity via a test module. The optical device is driven with an Arduino Nano. The final probe is fitted with quartz windows and protected by a copper-based wire mesh for long-term deployment. It can be used in a stand-alone mode (for long-term deployment) or in a nomadic mode (connected to a case fitted with pH, conductivity and temperature probes).

The systems (test module and probe) are calibrated with tryptophan and quinine sulfate solutions. Samples from different aquatic ecosystems (surface water impacted by natural and anthropogenic DOM sources, effluents from wastewater treatment plants and constructed wetlands, etc.) have been tested and the responses are compared to data provided on the same samples by benchtop devices. The samples were collected during various research projects and also by citizens in the framework of the O'CitEaux project (https://ociteaux.fr/), displaying a large range of DOM specificities.