Spatio-temporal mapping of aquatic ecosystems by long-term monitoring and citizen science

International audience

إنجليزي. According to the United Nations Educational, Scientific and Cultural Organization (2024), the quality and quantity of water available for all our uses will become the major challenge of the 21st century. Increased temperatures, problems of oxygenation, (micro)pollutants concentrations are some of the problems to be dealt with, whose consequences are many, not only for livestock farming, fishing and aquatic biodiversity, but also for drinking water production. In Europe the Water Framework Directive (WFD) aims to assess the ecological and chemical status of surface waters at the level of the river basin district (i.e., “an area of land and sea, comprising one or more river basins and associated groundwater and coastal waters, identified as the main unit for the purposes of river basin management”). Small streams, typical of watershed headwaters, are not monitored in the WFD. However, they are estimated to account for up to 80% of the total hydrographic length in a river watershed, making a major contribution to the water supply of downstream ecosystems (MacDonald and Coe 2007). Since 2010, the LTSER Zone Atelier du Bassin de la Moselle has been involved on the long-term monthly monitoring, with the help of forest rangers, of an initial set of 16 pristine headwater streams in the Vosges Mountains (https://acev.otelo.univ-lorraine.fr/ , https://deims.org/22915474-7c50-47c1-8239-6c59fa924a1b). These streams are running on granite or sandstone soils in forests dotted with wetlands of various size. The monitoring stations are upstream of any anthropogenic activity, except forestry and extensive tourism. This initial set has been progressively expanded with other nearby streams. However, all of them belong to the same mountainous typology. With this in mind, a participatory research project, O'CitEaux (https://ociteaux.fr/), has been set up to monitor the quality of small rivers in a wider context, using new low-cost sensors. We believe in the importance of such monitoring in the face of climate change (Whyte et al. 2024, von Gönner et al. 2024). Together with an increase of temperature, longer periods of drought interspersed with episodes of heavy rainfall are expected in the coming decades. The flow of small rivers and the quality of their water are therefore likely to be significantly altered. The O'CitEaux participants are: fishing association members (A), primary and secondary school teachers and their students (B), or just people interested in quality of the aquatic environment (C). fishing association members (A), primary and secondary school teachers and their students (B), or just people interested in quality of the aquatic environment (C). Participants A and B are equipped with a low-cost water case which enables them to measure pH, conductivity and temperature in-situ and in the future dissolved organic matter (Ritson et al. 2014). Participants A measure the water level and the width of the watercourse, which can be used for estimation of the discharge rate after proper calibration. All participants collect water samples (one-shot or on a monthly basis, depending upon their level of implication), filtrate them and send them immediately to the research laboratory. Additional information about the location of the station and its immediate surroundings, as well as on biodiversity (odonata, fish, etc.), is collected. Samples collected either by researchers or by O'CitEaux participants follow the same analysis process: filtration at 0.45 µm, analysis of dissolved organic and inorganic carbon, dissolved total nitrogen and major anions and cations, DOM spectral characteristics by UV-visible spectroscopy (aromaticity and molecular weight scoring) and fluorescence spectroscopy (DOM humification, etc.). To date, 150 streams (mainly in France, UK and Scandinavia) have been sampled (i.e., 400 samples) in addition to the 40 streams monitored directly by the researchers on a monthly basis (Fig. 1). The variety of their typology is shown in Fig. 2: high dissolved inorganic carbon concentrations reflect rivers running on calcareous soils, when high dissolved organic carbon concentrations characterized rivers influenced by forests and peatlands. The presentation will discuss the water quality results in function of geology, land use and season and compare them to WFD data collected at a larger scale. An example of data analysis is shown in Fig. 3 for dissolved nitrogen, with a gradient between the forested Vosges Mountains and the western zone where agriculture is more intensive. Citizen involvement (motivation, effectiveness, fear of doing the wrong thing, etc.) will be discussed as well as the best ways for feedback (database, website, counseling).