Sea-ice derived meltwater stratification slows the biological carbon pump: results from continuous observations

Abstract

The ocean moderates the world’s climate through absorption of heat and carbon, but how much carbon the ocean will continue to absorb remains unknown. The North Atlantic Ocean west (Baffin Bay/Labrador Sea) and east (Fram Strait/Greenland Sea) of Greenland features the most intense absorption of anthropogenic carbon globally; the biological carbon pump (BCP) contributes substantially. As Arctic sea-ice melts, the BCP changes, impacting global climate and other critical ocean attributes (e.g. biodiversity). Full understanding requires year-round observations across a range of ice conditions. Here we present such observations: autonomously collected Eulerian continuous 24-month time-series in Fram Strait. We show that, compared to ice-unaffected conditions, sea-ice derived meltwater stratification slows the BCP by 4 months, a shift from an export to a retention system, with measurable impacts on benthic communities. This has implications for ecosystem dynamics in the future warmer Arctic where the seasonal ice zone is expected to expand.

Unprecedented decline of Arctic sea ice outflow in 2018

“…A recent study reported that anomalous reduction of sea ice concentration and extreme deep mixed layers occurred northeast of Fram Strait in the 2017/2018 winter 48 . Another study pointed out that the anomalous ocean stratification in 2018 affected local biogeochemical cycles north of the Fram Strait 49 . The anomalous ice melt reported here can have contributed to the reduction of sea ice cover in this area, while the southerly winds can have advected the meltwater eastward by Ekman transport and contributed to the observed fresher surface layer and shallower warm AW layer observed in the western Eurasian basin in winter 2017 21 .…”

Phytoplankton Surveys in the Arctic Fram Strait Demonstrate the Tiny Eukaryotic Alga Micromonas and Other Picoprasinophytes Contribute to Deep Sea Export

“…The western part of the North Atlantic Ocean near Baffin Bay and the Labrador Sea (Canadian Arctic) and the eastern part, at the Fram Strait and the Greenland Sea, exhibit the “most intense absorption of anthropogenic carbon globally” [ 20 ], and the biological carbon pump plays a key role. In the Canadian Arctic, the abundance of picophytoplankton in the photic zone, specifically Micromonas , has been shown to be increasing due to the influence of climate change at the same time as larger phytoplankton decline [ 10 ].…”

“…This patterning connects to physical dynamics in that coexistence and even enhancement are seen in frontal regions (where the intermingling of water masses occurs) and dominances seemingly arise in water masses with more coherent origins and characteristics. Additionally, the eddying recirculation observed in this area of the Fram Strait could transport species adapted to cold waters towards warmer waters, and vice versa [ 20 ].…”

“…The flux of the Micromonas , based on the quantitative analysis of the trap data, exhibited temporal variation, with peaks in the middle of the summer (second part of July; Figure 3 c). Strong seasonality in eukaryotic microbial community composition and in the export related to meltwater-derived stratification, is reported for the Fram Strait [ 20 , 59 ], and other means of export exist through mesoscale features [ 15 ] in the eastern North Atlantic. Likewise, the influence of seasonality on both biological composition and carbon export appears to be very strong at more temperate and subtropical sites, where sediment traps have been monitored using molecular approaches in the North Pacific gyre (HOT [ 10 ]) or by comparing two seasons in the North Atlantic gyre (BATS; [ 60 ]).…”

“…To determine the taxa that contribute to export, we used amplicon sequencing to examine phytoplankton communities in deep sediments traps (200, 1250 and ~2500 m), with the deepest depth being near/at the seafloor, as well as samples from the above sunlit zone. Our analyses focused on the summer season, which has been shown to be the period with the highest documented carbon flux to the sediment in the Fram Strait [ 20 ]. The observation that Micromonas was omni-present in the transect allowed us to compare its abundance between the seasonally ice-covered Western Fram Strait and the mainly ice-free Eastern Fram Strait.…”