Colored dissolved organic matter absorption (aCDOM) and spectal slopes (S) in the surface water during 4 expeditions from spring to fall in 2019, Mackenzie Delta Region (CA)

Measurement of CDOM absorption was conducted from a water sample within 12 hours of collection using an UltraPath liquid waveguide system (World Precision Instruments, Inc.) over the wavelengths ranging from 200 to 722 nm (see also Matsuoka et al. (2012; doi:10.5194/bg-9-925-2012) for details). To minimize temperature effects, both the sample and the reference water were kept at 4 °C for at least 30 minutes prior to analysis. We followed the International Ocean Colour Coordinating Group (IOCCG) Ocean Optics and Biogeochemistry CDOM protocols (Mannino et al., 2019 (see further details)) with a few modifications: 1) reference water with salinity ±2 relative to the sample was prepared on site a few hours before sample analysis to minimize the effect of difference in refractive index between sample and reference; 2) aCDOM(λ) was measured in flow mode, meaning, a measurement was made while water was running using a peristaltic pump (Lefering et al., 2017; doi:10.1364/AO.56.006357). While the use of a long optical cell provides a good better signal particularly withinin the visible spectral domain essential to SOCRS, it necessarily suffers from light saturation in the UV domain. To overcome this issue, an optimal length of a cell (i.e. 10 cm or 200 cm) was selected following an empirical relationship between optical density observed at 350 and 443 nm based on Matsuoka et al. (2012; doi:10.5194/bg-9-925-2012). For each sample, measurements were done in triplicates of which each was visually inspected for quality control. CDOM measurements were fitted using following equation: a_CDOM (λ)=a_CDOM (λ_0 )*e^(-S(λ-λ_0)), where S is the spectral slope of aCDOM(λ) between 350 and 500 nm (Babin et al., 2003; doi:10.1029/2001JC000882 and Matsuoka et al., 2012; doi:10.5194/bg-9-925-2012).