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 2013, 2014, and 2015. The CDOM absorption coefficient at 350 nm, aCDOM(350) showed significant interannual variation (T test, p < 0.00001). In 2013, the highest average aCDOM(350) values (aCDOM(350) = 0.30 ± 0.12 m−1) were observed due to the influence of cold and low-salinity water from the Sørkapp Current (SC) in the southern part of the West Spitsbergen Shelf. In 2014, aCDOM(350) values were significantly lower (T test, p < 0.00001) than in 2013 (average aCDOM(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 (average aCDOM(350) = 0.19 ± 0.05 m−1) was observed. In situ measurements of three FDOM components revealed that fluorescence intensity of protein-like FDOM dominated in the surface layer of the Nordic Seas. Concentrations of marine and terrestrial humic-like DOM were very low and distribution of those components was generally vertically homogenous in the upper ocean (0–100 m). Fluorescence of terrestrial and marine humic-like DOM decreased in surface waters (0–15 m) near the sea ice edge due to dilution of oceanic waters by sea ice meltwater. The vertical distribution of protein-like FDOM was characterized by a prominent subsurface maximum that matched the subsurface chlorophyll a maximum and was observed 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 (R2 = 0.65, p < 0.0001, n = 24 490), which suggests that phytoplankton was the primary source of protein-like DOM in the Nordic Seas and West Spitsbergen Shelf waters. Observed variability in selected spectral indices (spectral slope coefficient, S300–600, carbon-specific CDOM absorption coefficient at 254 and 350 nm, SUVA254, a*CDOM(350)) and the nonlinear relationship between CDOM absorption and the spectral slope coefficient also indicate a dominant marine (autochthonous) source of CDOM and FDOM in the study area. Further, our data suggest that aCDOM(350) cannot be used to predict dissolved organic carbon (DOC) concentrations in the study region; however the slope coefficient (S300–600) shows some promise in being used