Synoptic evaluation of carbon cycling in the Beaufort Sea during summer: contrasting river inputs, ecosystem metabolism and air–sea CO2 fluxes

[摘要] The accelerated decline in Arctic sea ice and an ongoing trend toward moreenergetic atmospheric and oceanic forcings are modifying carbon cycling inthe Arctic Ocean. A critical issue is to understand how net communityproduction (NCP; the balance between gross primary production and communityrespiration) responds to changes and modulates air–sea CO₂ fluxes.Using data collected as part of the ArcticNet–Malina 2009 expedition in thesoutheastern Beaufort Sea (Arctic Ocean), we synthesize information on seaice, wind, river, water column properties, metabolism of the planktonic foodweb, 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 anassessment of NCP against air–sea CO₂ fluxes. During the fieldcampaign, 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 exposedto the atmosphere. We detected some areas where the surface mixed layer wasnet autotrophic owing to high rates of primary production (PP), but theecosystem was overall net heterotrophic. The region acted nonetheless as asink for atmospheric CO², with an uptake rate of−2.0 ± 3.3 mmol C m⁻² d⁻¹ (mean ± standarddeviation associated with spatial variability). We attribute this discrepancyto (1) elevated PP rates (> 600 mg C m⁻² d⁻¹) over the shelfprior to our survey, (2) freshwater dilution by river runoff and ice melt,and (3) the presence of cold surface waters offshore. Only the MackenzieRiver delta and localized shelf areas directly affected by upwelling wereidentified 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 represented37.4% 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, andbenthos, contributed to 24, 10, and 12%, respectively. The range ofproduction-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 wastwofold (~ 5.9 g C m⁻²) the biomass of pelagic heterotrophs(~ 2.4 g C m⁻²), in accord with high vertical carbon fluxes onthe shelf (956 ± 129 mg C m⁻² d⁻¹). Threshold PP (PP atwhich NCP becomes positive) in the surface layer oscillated from 20 to152 mg C m⁻² d⁻¹, with a pattern from low-to-high values as thedistance from the Mackenzie River decreased. We conclude that (1) climatechange is exacerbating the already extreme biological gradient across theBeaufort shelf–basin system; (2) the Mackenzie Shelf acts as a weak sink foratmospheric CO₂, suggesting that PP might exceed the respiration ofterrigenous and marine organic matter in the surface layer; and (3) shelfbreak upwelling can transfer CO₂ to the atmosphere, but CO₂outgassing can be attenuated if nutrients brought also by upwelling supportdiatom production. Our study underscores that cross-shelf exchange of waters,nutrients and particles is a key mechanism that needs to be properlymonitored as the Arctic transits to a new state.

[学科分类] 地球化学与岩石