Coloured dissolved organic matter (CDOM), also know as yellow substances, dissolved in Baltic Sea waters is a dominant constituent, absorbing light in the blue and green parts of the spectrum. The relative share of CDOM absorption in the total absorption of blue light can reach 80%. This causes a high correlation between the CDOM absorption coefficient a _y(400) and irradiance diffuse attenuation coefficient K _d(412). The seasonal variations of CDOM and particulate absorption have a combined effect on the magnitude and shape of the irradiance diffuse attenuation coefficient spectra, which is represented by a shift of the maximum transmission wavelength towards the red part of the spectrum. Such an effect has been observed in the Gulf of Gdansk during recent experiments. CDOM absorption also influences the shape of remote sensing reflectance spectra. Appropriate spectral bands for the construction of a local, empirical, remote sensing algorithm for quantification of CDOM absorption are proposed, based on the seasonal variability of remote sensing reflectance spectra. The empirical relationships between CDOM absorption and downwelling irradiance diffuse attenuation coefficient and spectral reflectance are presented and their potential for quantifying CDOM absorption is explored. The CDOM absorption coefficient is highly correlated with downwelling irradiance diffuse attenuation coefficient in the blue. The estimation of CDOM absorption using the downwelling irradiance measurements can be achieved with low errors: −4.6% systematic and 25.3% random. The estimation of CDOM absorption from empirical relationships between absorption and remote sensing reflectance suffers lower accuracy: 4% for systematic and 32% for random errors of estimates.

Global warming is not often discussed in the context of light availability within the water column. However, the light regimes in the Arctic Ocean are controlled by three factors that are influenced by climate changes which are the sea icealbedofeedback, glacial meltwater runoff and marine primary production. Based on a satellite data series acquired during time period 1997 – 2019, we inspected long-term changes of the three commonly used optical characteristicsChla,a_tot(443), andK_d(490) within the Svalbard fjords and on the West Spitsbergen Shelf. We revealed evident positive trends in summer (July – September) in the Isfjorden, the Kongsfjorden, the Hornsund fjord, and the Bellsund. Moreover, between 1997 and 2019 at the majority of locations we observed a regime shift toward darker waters. We showed that light availability has changed in the Svalbard fjords since 1997, which may be a symptom of transformations in the environment that should be further investigated and monitored.