In the Nordic regions, climate change in terrestrial environments will still have little known impact on aquatic ecosystems. The melting of permafrost and the change in plant composition will release high concentrations of allochtone organic matter to aquatic ecosystems. This dissolved coloured organic matter (CDOM) plays a major role in mitigating subaquatic illumination. This study looks at the optical effects of the quality and quantity of the CDOM in the Nordic estuarine system. The CDOM of the estuary systems of the Great Baleine River and the Mackenzie River was characterised by absorption and synchronous fluorescence. These analyses revealed fundamental differences between these two systems in terms of quality and quantity of MCDOs. However, a specific fluorescence signature has been identified for the Nordic CDOM, which is more consistent than the absorption properties of the CDOM. The visual importance of CDOM in mitigating solar radiation was observed in the estuary system of the Mackenzie River, a river recognised for the influence of particulate matter on subaquatic illumination. Pigmentary strategies, phytoplankton composition and primary production were also measured. CDOM and particulate matter controlled illumination in the visible (400-700 nm), but only the CDOM affected ultraviolet radiation (< 400 nm). Unlike particulate matter, the optical effect of the river CDOM was measurable even beyond the spread of the Mackenzie River on the Beaufort Sea plateau. The impact of an increase in the concentration of allochtone CDOM on primary production has been measured in the estuary system of the Grande Rivière de la Baleine. Modelling analyses have shown that spectral regime changes affect the amount of illumination available for photosynthesis, the depth of the euphotic area, and the spectral coupling of phytoplankton. Using the experimental approach, photosynthetic parameters were measured under a spectral regime modified by an increased concentration of allochtone CDOM. This demonstrated that high concentrations of CDOM contribute to a significant reduction in primary production in the water column.