Coastal seas are the hotspots in global marine carbon fluxes (Dai et al 2022). They receive considerable amounts of organic carbon originating from terrestrial and freshwater systems through rivers and estuaries. Compared to the open ocean, coastal seas often have high biological activity, both in producing new organic matter through photosynthesis and in mineralising the existing organic matter pools. In coastal waters the increase of dissolved organic matter (DOM) causes brownification, a change in water colour and increase of light attenuation, affecting light availability for primary productivity. DOM also contains nutrients that may boost primary production or bacterial processes, and the labile carbon in DOM will induce mineralisation processes. Thus, DOM fluxes affect the aquatic metabolism and determine if the system is a source or sink of oxygen and carbon. Terrestrial carbon is also a key determinant of carbon budgets in many coastal systems and riverine inputs have affects the carbonate system (alkalinity, DIC, pH, pCO2). A high-quality pan-European monitoring network to study coastal carbon dynamics and transport of carbon from terrestrial systems to coastal seas and further to open ocean has been proposed by ICOS, Euro ARGO, and JERICO communities (Felgate et al 2020, Frigstad et al 2024).Optimally, such a monitoring system includes measurements of various carbon fractions, related carbonate system variables (TA, pH) and optical proxies for dissolved organic matter  – coloured dissolved organic matter (CDOM), and the fraction of CDOM that fluoresces (fDOM) (Felgate et al 2020). The user group, including six partners doing Carbon-related research and monitoring in the Baltic Sea and North Sea, have the goal to harmonize and standardize fDOM measurements in order for harmonisation of methods and adoption of the best practices that is required to yield consistent data across the studied systems.