Characteristics of chromophoric and fluorescent dissolved organic matter in the Nordic Seas

Optical properties of Chromophoric (CDOM) and Fluorescent Dissolved Organic Matter (FDOM) were characterized in the Nordic Seas including the West Spitsbergen Shelf during June–July of 2013, 2014 and 2015. The CDOM absorption coefficient at 350 nm, a CDOM (350) showed significant interannual variation. In 2013, the highest average a CDOM (350) values ( a CDOM  = 0.30 ± 0.12 m −1 ) were observed due to the influence of cold and low–saline water from the Sorkapp Current in the southern part of West Spitsbergen Shelf. In 2014, a CDOM (350) values were significantly lower than in 2013 (av. a CDOM (350) = 0.14 ± 0.06 m −1 ), which was associated with the dominance of warm and saline Atlantic Water (AW) in the region, while in 2015 intermediate CDOM absorption (av. a CDOM (350) = 0.19 ± 0.05 m −1 ) was observed. In situ measurement of three FDOM components revealed that protein–like FDOM dominated and concentration of marine and terrestrial humic–like DOM were very low and its distribution were generally vertically homogenous in the upper ocean (0–100 m). Fluorescence of terrestrial and marine humic–like FDOM decreased in surface waters (0–15 m) near the sea–ice edge by dilution of oceanic waters by sea–ice melt water. The vertical distribution of protein–like FDOM was characterized by a prominent sub–surface maximum that matched the subsurface chlorophyll a maximum and was observed all across the study area. The highest protein–like FDOM fluorescence was observed in the Norwegian Sea in the core of warm AW. There was a significant relationship between the protein–like fluorescence and chlorophyll a fluorescence (R 2  = 0.65, p  2  = 0.36, p  S 300–600 , carbon specific CDOM absorption coefficient at 254 and 350 nm, SUVA254, a CDOM *(350)) and non–linear relationship between CDOM absorption and spectral slope coefficient indicate a dominant marine (autochthonous) source of CDOM and FDOM in the study area. Further, our data suggest that while a CDOM (350) cannot be used to predict dissolved organic carbon (DOC) concentrations in the study region, and the slope coefficient ( S 300–600 ) shows some promise to be used.