Amplified Arctic iceberg traffic reshapes benthic biodiversity

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The deep seafloor of the Arctic Ocean remains one of Earth’s least-charted biomes, its physical and biological dynamics concealed beneath a canopy of pack ice. Recent studies, nevertheless, demonstrate that anthropogenic warming and intensified human activity are already imprinting measurable signals at abyssal depths 2 , 3 . The retreat of sea ice 1 , 4 promotes more frequent and extensive ice-edge and under-ice algal blooms 5 , strengthening the biological carbon pump. Diatom-rich aggregates subsequently descend to the seabed, enhancing benthic respiration and stimulating bioturbation 6 , 7 . As sea ice diminishes, increased human accessibility facilitates industrial exploration, tourism and fishing 8 , with bottom trawling, in particular, inflicting long-lived mechanical disturbance on deep-sea habitats 9 , while mobilizing large pools of sedimentary carbon 10 . Moreover, anthropogenic activities have led to a rise in plastic pollution on the deep Arctic seafloor 11 .

By contrast, a slower-acting natural driver of seafloor change is the delivery of glacial ice-rafted debris, particularly its coarse fraction (dropstones), transported by calved icebergs. Once deposited, these erratics become colonization hotspots for hard-bottom fauna 12 , 13 . Figure 1 provides a conceptual overview of this glacier-to-seafloor pathway. In Fram Strait, the prime gateway between Greenland and Svalbard, a long-term deep-sea monitoring site offers a rare window into these changing abyssal environments. Operated by the Alfred Wegener Institute since 1999, the LTER (Long-Term Ecological Research) observatory HAUSGARTEN 14 combines autonomous platforms with annual RV Polarstern expeditions 15 to monitor oceanographic, biogeochemical and ecological parameters from the surface to depths of up to 5,500 m.

Fig. 1: Icebergs transport glacial debris to the deep seafloor. The alternative text for this image may have been generated using AI.

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a , Debris transported by calved icebergs is released during melt and settles to the seafloor, where the coarser fraction (dropstones) provides a hard substrate for colonizing sessile benthic fauna. b , c , Iceberg (height 18 m; b ) and growler with stones and sediments observed in the central Arctic ( c ). d , Iceberg embedded in sea ice with surface stones, potentially reflecting supraglacial debris or exposure of debris-rich basal ice, near HAUSGARTEN. The plan area of the exposed stones was about 1,200 m 2 . Credit: b , d , photographs by M.H.; c , photograph by J. Harding, NHK.

Recent seafloor imagery from HAUSGARTEN has shown increasing patchiness and higher local densities of dropstones in parts of the Fram Strait abyss, far from any calving front (location is indicated in Extended Data Fig. 1 ). Whether this reflects a brief episodic pulse or a sustained rise in iceberg traffic driven by accelerating glacial melt remains unclear, as smaller icebergs are difficult to detect in satellite observations and leave no long-term record of their abundance.

In this study, we took a tiered approach by (1) documenting a short-term increase in dropstone density and the associated impact on benthic communities at one HAUSGARTEN station; (2) boarding and sampling a drifting iceberg to characterize its lithogenic cargo; and (3) mining 40 years of RV Polarstern visual logs to test for changes in glacial ice frequency across Fram Strait. Finally, for sightings embedded in compact pack ice, we reconstructed trajectories using satellite-derived sea ice-motion data to pinpoint likely source glaciers and assess whether their calving fluxes have accelerated. We then ran high-resolution drift simulations that delineate changing pathways and dispersal corridors for debris-laden glacial ice, demonstrating a lithogenic imprint on deep-sea ecosystems hundreds of kilometres downstream.

Dropstones shape benthic biodiversity

The presence and distribution of glacially delivered dropstones at HAUSGARTEN station EG-IV on the East Greenland continental margin (about 2,500 m water depth) were documented using a towed deep-sea camera system deployed from RV Polarstern . Repeated camera transects were conducted during expeditions in 2015 and 2017 along a 2.6 km track, allowing temporal changes within a defined segment of the seafloor to be assessed over a short timescale ( Methods and Extended Data Fig. 1 ).

Image analysis revealed clustered deposition of newly delivered, predominantly small dropstones consistent with melt-out from passing icebergs. Dropstone density in 2017 (1.93 ± 0.01 stones per m 2 , mean ± s.e.) was significantly higher than in 2015 (1.59 ± 0.01) ( t 1  = 6.21, P  = 0.01) (Fig. 2b ), whereas the size distribution of stones differed, and mean stone area decreased from 7.92 ± 1.70 cm 2 in 2015 to 4.25 ± 1.96 cm 2 in 2017 ( χ 2 test, P  = 0.003). The distribution of dropstones on the seafloor did not deviate from randomness in 2015 ( χ 2 test, P  = 0.99), but in 2017, stones were signif…