Interannual variability (2000–2013) of mesopelagic and bathypelagic particle fluxes in relation to variable sea ice cover in the eastern Fram Strait

Abstract

The Fram Strait connects the Atlantic and Arctic Oceans and is a key conduit for sea ice advected southward by the Transpolar Drift and northward inflow of warm Atlantic Waters. Continued sea ice decline and “Atlantification” are expected to influence pelagic–benthic coupling in the Fram Strait and Arctic as a whole. However, interannual variability and the impact of changing ice conditions on deepwater particle fluxes in the Arctic remain poorly characterized. Here, we present long-term sediment trap records (2000–2013) from mesopelagic (200 m) and bathypelagic (2,300 m) depths at two locations (HGIV and HGN) in the Fram Strait subjected to variable ice conditions. Sediment trap catchment areas were estimated and combined with remote sensing data and a high-resolution model to determine the ice cover, chlorophyll concentration, and prevailing stratification regimes. Surface chlorophyll increased between 2000 and 2013, but there was no corresponding increase in POC flux, suggesting a shift in the efficiency of the biological carbon pump. A decrease in particulate biogenic Si flux, %opal, Si:POC, and Si:PIC at mesopelagic depths indicates a shift away from diatom-dominated export as a feasible explanation. Biogenic components accounted for 72% ± 16% of mass flux at 200 m, but were reduced to 34% ± 11% at 2,300 m, substituted by a residual (lithogenic) material. Total mass fluxes of biogenic components, including POC, were higher in the bathypelagic. Biomarkers and ∂13C values suggest both lateral advection and ice-rafted material contribute to benthic carbon input, although constraining their precise contribution remains challenging. The decadal time series was used to describe two end-members of catchment area conditions representing the maximum temperatures of Atlantic inflow water in 2005 at HGIV and high ice coverage and a meltwater stratification regime at HGN in 2007. Despite similar chlorophyll concentrations, bathypelagic POC flux, Si flux, Si:POC, and Si:PIC were higher and POC:PIC was lower in the high-ice/meltwater regime. Our findings suggest that ice concentration and associated meltwater regimes cause higher diatom flux. It is possible this will increase in the future Arctic as meltwater regimes increase, but it is likely to be a transient feature that will disappear when no ice remains.

Release of ballast material during sea-ice melt enhances carbon export in the Arctic Ocean

“…Recent observations from the same sediment trap time-series show an increase in surface chlorophyll concentrations between 2001 and 2013 ( 46 ), while POC export in the vicinity of the ice-edge did not increase. This may indicate a decrease in the export efficiency of surface production in the Fram Strait, which could be connected to a reduction of particle ballasting vie terrigenous material in the sea-ice vicinity.…”

“…This may indicate a decrease in the export efficiency of surface production in the Fram Strait, which could be connected to a reduction of particle ballasting vie terrigenous material in the sea-ice vicinity. It should be noted though, that it is not clear to what extent potential shifts in phytoplankton community composition may drive a reduction of the export efficiency in the Fram Strait ( 42 , 46 ). In conclusion, though gypsum ballasting is projected to increase in future, it is unclear if gypsum release will compensate for ballasting via terrigenous material and lead to an increase in the quantity of Arctic carbon export, at least in the Fram Strait.…”

“…The moored long-term SMT were deployed annually from 2001 to 2013 (not every year and month were fully covered, see Table S1 ; see Refs. 29 and 46 for further information) between 180 and 340 m depth (except from 2009 July 20 to 2010 July 15 where the deployment depth was 80 m; see Table S1 ). In the years after 2013, the protocol was changed and these trap results still need quality control on their comparison to this time-series.…”

The Arctic summer microbiome across Fram Strait: Depth, longitude, and substrate concentrations structure microbial diversity in the euphotic zone

“…Decadal records of sinking particles in the Arctic Ocean revealed long‐lasting effects of a warm‐water anomaly; stimulating small phytoplankton while larger diatoms decreased in abundance, coincident with shifting bacterial composition (Cardozo‐Mino et al, 2023). These dynamics have major consequences for carbon export and benthopelagic coupling (Jacquemot et al, 2022; Kohlbach et al, 2023; Salter et al, 2023). Future ocean scenarios predict substantial ecosystem shifts in the Arctic, supported by a changing microbiome structure at higher temperatures (Ahme et al, 2023).…”