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

Abstract. 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.