Abstract
Abstract. 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, aCDOM*(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.
Highlights
The rapid reduction of summer sea ice in the Arctic Ocean in the past decades has various repercussions on the structure and functioning of the Arctic marine system, forcing changes in physics, biogeochemistry, and ecology of this complex oceanic system (Meier et al, 2014)
The lowest CDOM absorption in 2013 was observed at the northernmost and northeasternmost stations influenced by lowsalinity Polar Surface Water (PSW) affected by sea ice meltwater
It is likely that these yearto-year changes in CDOM absorption coefficient and spectral slope coefficient were related to the intensity of Atlantic Water (AW) inflow to the Nordic Seas
Summary
The rapid reduction of summer sea ice in the Arctic Ocean in the past decades has various repercussions on the structure and functioning of the Arctic marine system, forcing changes in physics, biogeochemistry, and ecology of this complex oceanic system (Meier et al, 2014). One of the most significant consequences of observed rapid Arctic Ocean transition is an increase in the primary productivity of the Arctic Ocean (Arrigo et al, 2008), which could potentially contribute to increased production of autochthonous (marine) dissolved organic matter (DOM) in ice-free and under-ice waters. Dissolved organic carbon (DOC) produced by ice algae has a limited effect on overall organic carbon mass balance in the Arctic Ocean, as melting of 1 m of sea ice would negligibly change DOC concentration in the top 50 m of the water column, assuming an averaged DOC content in the ice of 100 μMol C (Anderson and Amon, 2015). Terrestrial DOM plays a considerable role in the carbon budget of the Arctic Ocean (Findlay et al, 2015; Stein and Macdonald, 2004), especially in coastal waters and the continental shelf with a large inflow of terrestrial DOM, which constitutes 80 % of total organic carbon delivered by Arctic rivers (Stedmon et al, 2011)
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