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| Titel |
Synoptic evaluation of carbon cycling in the Beaufort Sea during summer: contrasting river inputs, ecosystem metabolism and air–sea CO2 fluxes |
| VerfasserIn |
A. Forest, P. Coupel, B. Else, S. Nahavandian, B. Lansard, P. Raimbault, T. Papakyriakou, Y. Gratton, L. Fortier, J.-É. Tremblay, M. Babin |
| 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. 10 ; Nr. 11, no. 10 (2014-05-27), S.2827-2856 |
| Datensatznummer |
250117426
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| Publikation (Nr.) |
 copernicus.org/bg-11-2827-2014.pdf |
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| Zusammenfassung |
| The accelerated decline in Arctic sea ice and an ongoing trend toward more
energetic atmospheric and oceanic forcings are modifying carbon cycling in
the Arctic Ocean. A critical issue is to understand how net community
production (NCP; the balance between gross primary production and community
respiration) responds to changes and modulates air–sea CO2 fluxes.
Using data collected as part of the ArcticNet–Malina 2009 expedition in the
southeastern Beaufort Sea (Arctic Ocean), we synthesize information on sea
ice, wind, river, water column properties, metabolism of the planktonic food
web, organic carbon fluxes and pools, as well as air–sea CO2 exchange,
with the aim of documenting the ecosystem response to environmental changes.
Data were analyzed to develop a non-steady-state carbon budget and an
assessment of NCP against air–sea CO2 fluxes. During the field
campaign, the mean wind field was a mild upwelling-favorable wind
(~ 5 km h−1) from the NE. A decaying ice cover (< 80%
concentration) was observed beyond the shelf, the latter being fully exposed
to the atmosphere. We detected some areas where the surface mixed layer was
net autotrophic owing to high rates of primary production (PP), but the
ecosystem was overall net heterotrophic. The region acted nonetheless as a
sink for atmospheric CO2, with an uptake rate of
−2.0 ± 3.3 mmol C m−2 d−1 (mean ± standard
deviation associated with spatial variability). We attribute this discrepancy
to (1) elevated PP rates (> 600 mg C m−2 d−1) over the shelf
prior to our survey, (2) freshwater dilution by river runoff and ice melt,
and (3) the presence of cold surface waters offshore. Only the Mackenzie
River delta and localized shelf areas directly affected by upwelling were
identified as substantial sources of CO2 to the atmosphere
(> 10 mmol C m−2 d−1). Daily PP rates were generally
< 100 mg C m−2 d−1 and cumulated to a total PP of
~ 437.6 × 103 t C for the region over a 35-day period.
This amount was about twice the organic carbon delivery by river inputs
(~ 241.2 × 103 t C). Subsurface PP represented
37.4% of total PP for the whole area and as much as ~ 72.0%
seaward of the shelf break. In the upper 100 m, bacteria dominated
(54%) total community respiration
(~ 250 mg C m−2 d−1), whereas protozoans, metazoans, and
benthos, contributed to 24, 10, and 12%, respectively. The range of
production-to-biomass ratios of bacteria was wide (1–27% d−1),
while we estimated a narrower range for protozoans (6–11% d−1)
and metazoans (1–3% d−1). Over the shelf, benthic biomass was
twofold (~ 5.9 g C m−2) the biomass of pelagic heterotrophs
(~ 2.4 g C m−2), in accord with high vertical carbon fluxes on
the shelf (956 ± 129 mg C m−2 d−1). Threshold PP (PP at
which NCP becomes positive) in the surface layer oscillated from 20 to
152 mg C m−2 d−1, with a pattern from low-to-high values as the
distance from the Mackenzie River decreased. We conclude that (1) climate
change is exacerbating the already extreme biological gradient across the
Beaufort shelf–basin system; (2) the Mackenzie Shelf acts as a weak sink for
atmospheric CO2, suggesting that PP might exceed the respiration of
terrigenous and marine organic matter in the surface layer; and (3) shelf
break upwelling can transfer CO2 to the atmosphere, but CO2
outgassing can be attenuated if nutrients brought also by upwelling support
diatom production. Our study underscores that cross-shelf exchange of waters,
nutrients and particles is a key mechanism that needs to be properly
monitored as the Arctic transits to a new state. |
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