Chromophoric dissolved organic matter (CDOM) in first-year sea ice in the western Canadian Arctic

• CDOM is highly enriched in bottom sea ice with abundant ice algae.
• Sea-ice CDOM dynamics are controlled by both in situ and ex situ inputs.
• Biological CDOM production in sea ice is a minor source to underlying seawater.
• CDOM absorption spectra show signatures of mycosporine-like amino acids.
• Mycosporine-like amino acids are readily photodegradable by solar UV radiation.

We monitored the spatiotemporal progression of chromophoric dissolved organic matter (CDOM) in first-year sea ice in the western Canadian Arctic between mid-March and early July 2008. CDOM abundance in bottom ice, as quantified by absorption coefficient at 325 nm, aCDOM(325), showed a positive, linear relationship with the concentration of chlorophyll a, being low at the start of ice algal accumulation, highly enriched during the peak bloom and early post-bloom, and depleted again during sea ice melting. Vertical profiles of CDOM in early and late spring were typically characterized by slight to moderate elevations at both the surface and bottom and rather constancy within the interior ice. In the ice algae-thriving mid-spring, L-type profiles prevailed due to extremely high CDOM in the lowermost 10-cm layer. Bottom-layer CDOM in landfast ice (aCDOM(325): 15.8 m− 1) more than doubled that in drift ice (aCDOM(325): 6.6 m− 1). CDOM in the ice cover, except the bottom layer, was generally lower than or similar to that in the under-ice surface water. Salinity accounted for 58% of the CDOM variability in drift ice having minimal terrestrial and ice algal signatures. CDOM absorption spectra of algae-rich bottom ice samples exhibited ultraviolet (UV) absorption shoulders attributable to mycosporine-like amino acids (MAAs). These compounds were readily photodegradable by solar UV radiation. Our results suggest that 1) inclusion of organic solutes and in situ biological production are the dominant processes controlling the distribution of CDOM in sea ice in the study area, 2) biological production in bottom ice is a minor source of CDOM to the underlying surface water; 3) CDOM plays a critical role in shielding sympagic organisms in bottom ice against UV radiation; 4) the MAAs are effective photoprotectants only under low-UV conditions.