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

Abstract

Abstract. The absorption and fluorescence properties of chromophoric dissolved organic matter (CDOM) are reported for the inner shelf, slope waters and outer shelf regions 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 absorption at 355 nm (a g 355) and its spectral slope (S) in conjunction with excitationemission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC) revealed large variability in the characteristics of CDOM in different regions of the Bering Sea. PARAFAC analysis aided in the identification of three humic-like (components one, two and five) and two proteinlike (a tyrosine-like component three, and a tryptophanlike component four) components. In the extensive shelf region, average absorption coefficients at 355 nm (a g 355, 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 spectral slopes 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 of photodegradation in the highly stratified surface mixed layer, which intensified northwards in the northern middle shelf likely contributing to greater light penetration and to phytoplankton blooms at deeper depths. The fluorescent humic-like components one, two, and five were most elevated in the inner shelf most likely from riverine inputs. Along the productive "green belt" in the outer shelf/slope region, absorption and fluorescence properties indicated the presence of fresh and degraded autochthonous DOM. Near the Unimak Pass region of the Aleutian Islands, low DOC and a g 355 (mean 66.99 ± 7.94 µM; 0.182 ± 0.05 m −1 ) and a high S (mean 25.95 ± 1.58 µm −1 ) suggested substantial photobleaching of the Alaska Coastal Water, but high intensities of humic-like and protein-like fluorescence suggested sources of fluorescent DOM from coastal runoff and glacier meltwaters during the summer. The spectral slope S vs. a g 355 relationship revealed terrestrial and oceanic end members along with intermediate water masses that were modeled using nonlinear regression equations that could allow water mass differentiation based on CDOM optical properties. Spectral slope S was negatively correlated (r 2 = 0.79) with apparent oxygen utilization (AOU) for waters extending from the middle shelf into the deep Bering Sea indicating increasing microbial alteration of CDOM with depth. Although our data show that the CDOM photochemical environment of the Bering Sea is complex, our current information on its optical properties will aid in better understanding of the biogeochemical role of CDOM in carbon budgets in relation to the annual sea ic...

Characterization of the Water Masses in the Shelf Region of the Bering and Chukchi Seas With Fluorescent Organic Matter

“…The factors shaping the distribution of FDOM H , the major fraction of FOM H , in the shelf of the Bering and Chukchi Seas have been studied (e.g., D'Sa et al, 2014;Gonsior et al, 2017;Guéguen et al, 2007Guéguen et al, , 2012Hioki et al, 2014;Logvinova et al, 2016;Nishimura et al, 2012;Tanaka et al, 2016). These studies clearly indicated that FDOM H in the region is mainly derived from riverine and sedimentary inputs with minor contributions of autochthonous inputs.…”

“…Journal of Geophysical Research: Oceans residence time at the surface (D'Sa et al, 2014). Therefore, the SBBW, in which FDOM H could be characterized as historically highly photobleached at the slope/basin, flows to the shelf of the Bering Sea by eddies (Ladd et al, 2012;Mizobata et al, 2002Mizobata et al, , 2006.…”

Drivers of fluorescent dissolved organic matter in the global epipelagic ocean

“…waters have been recently published. Jørgensen et al (2011) presented a unique global dataset of the main FDOM components, whereas other studies covered partial oceanic regions of the Atlantic (Heller et al 2013;Kowalczuk et al 2013;De La Fuente et al 2014;Lønborg et al 2015), Pacific (Kaiser and Benner 2009;Yamashita et al 2010;Omori et al 2011;Dainard and Gu eguen 2013;Tanaka et al 2014;Kim and Kim 2015), Indian (Coble et al 1998), Arctic (Gu eguen et al 2007, 2014D'Sa et al 2014;Jørgensen et al 2014) and Southern oceans (Yamashita et al 2007). This study complements previous work focused on the distribution of PAR-AFAC components in the main water masses of the dark global ocean during the Spanish Circumnavigation Malaspina 2010 (Catal a et al 2015a).…”

Factors controlling the geographical distribution of fluorescent dissolved organic matter in the surface waters of the Pacific Ocean

“…4a). The level of humic-like components was highest at high latitudes in the Northern Hemisphere, i.e., the BPLR and BERS, where substantial contributions of terrigenous humic-like fluorophores from Alaskan rivers and high biological production occur (Grebmeier et al 2006;D'Sa et al 2014;Tanaka et al 2016). This level gradually decreased toward the south and reached its lowest value in the North Pacific tropical gyre (NPTG).…”

“…A linear relationship was evident between C3 and C1 for the surface waters (Fig. 3a), including the Chukchi Sea (BPLR) and the Bering shelf (eastern BERS), where terrigenous humic-like fluorophores were possibly present (D'Sa et al 2014;Tanaka et al 2016). The geographical distribution of the ratio of the two humic-like components (C3/C1) was determined to further evaluate the factors that control not only the level but also the composition of humic-like components (Figs.…”