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| Titel |
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 |
| VerfasserIn |
E. J. D'Sa, J. I. Goes, H. Gomes, C. Mouw |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 11, no. 12 ; Nr. 11, no. 12 (2014-06-17), S.3225-3244 |
| Datensatznummer |
250117470
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| Publikation (Nr.) |
 copernicus.org/bg-11-3225-2014.pdf |
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| Zusammenfassung |
| 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 (ag355) and its spectral slope (S) in conjunction
with excitation–emission 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
protein-like (a tyrosine-like component three, and a tryptophan-like component four)
components. In the extensive shelf region, average absorption coefficients at
355 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 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 ag355
(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. ag355 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 (r2 = 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 ice and phytoplankton dynamics, and to improved
algorithms of ocean color remote sensing for this region. |
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