2 research outputs found
Synoptic evaluation of carbon cycling in the Beaufort Sea during summer: contrasting river inputs, ecosystem metabolism and airâsea CO2 fluxes
International audienceThe 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 airâsea CO2 fluxes. Using data collected as part of the ArcticNetâMalina 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 airâsea 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 airâsea CO2 fluxes. During the field campaign, the mean wind field was a mild upwelling-favorable wind (⌠5 km hâ1) from the NE. A decaying ice cover ( 600 mg C m â2 d â1) 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 mâ2 dâ1). Daily PP rates were generally < 100 mg C mâ2 dâ1 and cumu-lated to a total PP of ⌠437.6 Ă 10 3 t C for the region over a 35-day period. This amount was about twice the organic carbon delivery by river inputs (⌠241.2 Ă 10 3 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 mâ2 dâ1), whereas protozoans, metazoans, and benthos, contributed to 24, 10, and 12 %, respectively. The range of production-to-biomass Published by Copernicus Publications on behalf of the European Geosciences Union. 2828 A. Forest et al.: Synoptic evaluation of carbon cycling ratios of bacteria was wide (1â27 % dâ1), while we estimated a narrower range for protozoans (6â11 % d â1) and metazoans (1â3 % dâ1). Over the shelf, benthic biomass was twofold (⌠5.9 g C mâ2) the biomass of pelagic heterotrophs (⌠2.4 g C mâ2), in accord with high vertical carbon fluxes on the shelf (956 ± 129 mg C mâ2 dâ1). Threshold PP (PP at which NCP becomes positive) in the surface layer oscillated from 20 to 152 mg C mâ2 dâ1 , 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 shelfâbasin 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