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The absorption and fluorescence properties of chromophoric dissolved organicmatter (CDOM) are reported for the inner shelf, slope waters and outer shelfregions of the eastern Bering Sea during the summer of 2008, when a warm,thermally stratified surface mixed layer lay over a cold pool(< 2 °C) that occupied the entire middle shelf. CDOM absorptionat 355 nm (ag355) and its spectral slope (S) in conjunctionwith excitation–emission matrix (EEM) fluorescence and parallel factoranalysis (PARAFAC) revealed large variability in the characteristics of CDOMin different regions of the Bering Sea. PARAFAC analysis aided in theidentification of three humic-like (components one, two and five) and twoprotein-like (a tyrosine-like component three, and a tryptophan-like component four)components. In the extensive shelf region, average absorption coefficients at355 nm (ag355, m−1) and DOC concentrations (μM)were highest in the inner shelf (0.342 ± 0.11 m−1,92.67 ± 14.60 μM) and lower in the middle(0.226 ± 0.05 m−1, 78.38 ± 10.64 μM) and outer(0.185 ± 0.05 m−1, 79.24 ± 18.01 μM) shelves,respectively. DOC concentrations, however were not significantly different,suggesting CDOM sources and sinks to be uncoupled from DOC. Mean spectralslopes S were elevated in the middle shelf(24.38 ± 2.25 μm−1) especially in the surface waters(26.87 ± 2.39 μm−1) indicating high rates ofphotodegradation in the highly stratified surface mixed layer, whichintensified northwards in the northern middle shelf likely contributing togreater light penetration and to phytoplankton blooms at deeper depths. Thefluorescent humic-like components one, two, and five were most elevated in the innershelf most likely from riverine inputs. Along the productive "green belt"in the outer shelf/slope region, absorption and fluorescence propertiesindicated the presence of fresh and degraded autochthonous DOM. Near theUnimak Pass region of the Aleutian Islands, low DOC and ag355(mean 66.99 ± 7.94 μM; 0.182 ± 0.05 m−1) and ahigh S (mean 25.95 ± 1.58 μm−1) suggested substantialphotobleaching of the Alaska Coastal Water, but high intensities ofhumic-like and protein-like fluorescence suggested sources of fluorescent DOMfrom coastal runoff and glacier meltwaters during the summer. The spectralslope S vs. ag355 relationship revealed terrestrial and oceanicend members along with intermediate water masses that were modeled usingnonlinear regression equations that could allow water mass differentiationbased on CDOM optical properties. Spectral slope S was negativelycorrelated (r2 = 0.79) with apparent oxygen utilization (AOU) for watersextending from the middle shelf into the deep Bering Sea indicatingincreasing microbial alteration of CDOM with depth. Although our data showthat the CDOM photochemical environment of the Bering Sea is complex, ourcurrent information on its optical properties will aid in betterunderstanding of the biogeochemical role of CDOM in carbon budgets inrelation to the annual sea ice and phytoplankton dynamics, and to improvedalgorithms of ocean color remote sensing for this region

biogeosciences

Absorption and fluorescence properties of chromophoric dissolved organic matter of the eastern Bering Sea in the summer with special reference to the influence of a cold pool