Fjords in Norway often exhibit striking green or turquoise color during spring and summer, driven by highly scattering particles of glacial meltwater or blooms of the coccolithophore Gephyrocapsa huxleyi (G. huxleyi) (formerly known as Emiliania huxleyi ). Despite their distinct origins, mineral particles versus biogenic calcite platelets, both water types produce similar apparent optical properties, including remote sensing reflectance spectra and perceived color. We conducted comprehensive measurements of inherent and apparent optical properties in two contrasting fjords, Gaupnefjorden and Hardangerfjorden. In both fjords, the total absorption spectra show minima near 532 nm, corresponding to the dominant green color observed in satellite imagery. Remote sensing reflectance values and RGB chromaticity coordinates derived from radiometric measurements are nearly indistinguishable between the two fjords. However, inherent optical properties reveal clear differences. Scattering measurements further distinguished the two water types. Both fjords exhibited high backscattering ratios (0.02–0.04) typical for optically complex coastal waters. In Hardangerfjorden, volume scattering functions (VSFs) showed pronounced forward scattering, while Gaupnefjorden displayed broader angular scattering patterns. This is consistent with the wider size distribution of mineral particles from glacial meltwater, and that larger particles sink out faster. Our results imply that it is not possible to discriminate between glacial and biogenic particle regimes in fjord systems with remote sensing reflectance using standard algorithms without a priori knowledge. However, differences in inherent optical properties indicate that autonomous observations may play a key role in quantifying primary production in high-latitude coastal systems. • Glacial meltwater and coccolithophore blooms produce near-identical Rrs spectra. • Both water types show green/turquoise color with absorption minima near 532 nm. • Backscattering ratios (0.02–0.04) are similar, explaining the overlapping Rrs spectra. • Volume scattering functions and particulate absorption distinguish two water types. • Standard satellite algorithms cannot separate mineral from biogenic particle regimes.
Kristoffersen et al. (Wed,) studied this question.