Benthic fauna declined on a whitening Antarctic continental shelf

The study investigates how increasing sea-ice cover (whitening) in much of Antarctica over recent decades has affected benthic communities on the continental shelf, a response that has been poorly documented compared to areas experiencing ice retreat. Antarctic benthos is highly diverse, slow-growing, and adapted to cold, food-limited environments. While glacier and ice-shelf retreat in West Antarctica and the Antarctic Peninsula have been linked to increased pelagic productivity and benthic blue carbon gains, large regions such as the Weddell Sea have seen increasing sea-ice extent through 2014. The authors pose the question of how benthic abundance, biomass, community composition, and megafaunal structural complexity have changed on a whitening shelf and how these changes relate to sea-ice cover, productivity, iceberg area, and the Southern Annular Mode (SAM).

Previous work highlights Antarctic benthic biodiversity, endemism, and high biomass, shaped by cold isolation and life-history traits such as slow growth and late maturity. Disturbance by iceberg scouring drives small-scale variability. Detecting temporal change is challenging due to patchiness, slow growth, and limited standardized time series. Climate-related benthic changes have been documented mainly in response to glacier/sea-ice retreat, iceberg scouring, and ice-shelf collapse, including rapid sponge expansion and blue carbon increases in regions of ice loss. In contrast to the Arctic, two-thirds of Antarctica, including the Weddell Sea, saw increasing sea-ice extent from 1979 to a 2014 maximum. Limited long-term Antarctic shelf studies exist, e.g., McMurdo Sound showing decadal benthic changes related to grounded icebergs and a positive relationship between bryozoan growth and open-water extent in the eastern Weddell Sea. A key gap is the lack of standardized, repeated, quantitative community-scale time series from whitening Antarctic shelves, which this study addresses for the northeastern Weddell Sea.

Study area: Kapp Norvegia/Auståsen (KNA) on the northeastern Weddell Sea shelf, delineated via GIS as ~9,180 km². Depth range for analyses was 100–700 m. Sampling and data: Eight R/V Polarstern expeditions (ANT-VI/3, VII/4, XIII/3, XV/3, XVII/3, XXI/2, XXVII, PS82) provided data from 1988, 1989, 1996, 1998, 2000, 2004, 2011, and 2014. After excluding BENDEX experimental stations, 59 multibox corer stations (total 307 cores; each core 20×12×45 cm) were analyzed. Sediments were sieved at 0.5 mm; retained macrofauna were fixed in 5% borax-buffered formalin and later enumerated and weighed for biomass. Abundance and biomass were compiled for 35 taxa and aggregated into four feeding guilds (deposit feeders, suspension feeders, scavengers, predators) following a predefined classification; colonial taxa had presence/absence abundance. Megafaunal 3D structural complexity: Imagery (video/photo) transects were analyzed for most expeditions (no images available for 1989); categories 0–5 captured successional stages from freshly scoured (0) to apex communities (5) dominated by large megafauna (e.g., glass sponges, gorgonians). Frequencies were converted to percentages using fuzzy coding to estimate annual proportions of each 3D category. Environmental drivers: Summer (Nov–Mar) SAM index (Marshall), sea-ice cover (NSIDC Sea Ice Index), daylight fraction (NOAA Solar Geometry), iceberg area (ALTIBERG v2.1 small icebergs), and a summer productivity index computed as the product of ice-free days and daylight fraction per pixel within KNA. The productivity index was validated against satellite Chl a and particulate organic carbon (SeaWiFS/MODIS) with significant correlations (p<0.005; r>0.7). For each factor, summer means and anomalies were calculated for 1986–2014 where possible. Statistical analysis: Expedition-level means of abundance and biomass (total and by feeding guild) and imagery-based complexity were used to compute anomalies and temporal patterns. Spearman correlations with 9,999 permutations tested associations of productivity and iceberg area with total and guild-specific abundance and biomass, and with 3D complexity; positive coefficients indicate increases with the driver, negative indicate reductions. Temporal lags (same year t0, 1-year t1, 2-year t2) were considered. Blue carbon extrapolation: Based on summed biomass anomalies and an estimated whitening shelf area of 1.85×10^6 km² (from IBCSO and ArcGIS measure), the authors extrapolated potential Antarctic-scale changes, contingent on geochemical corroboration.

• Macrobenthos declines: Relative to the late 1980s, total macrobenthic abundance halved and biomass declined by roughly two thirds over 1988–2014. The most pronounced drop occurred around 2000, coincident with a SAM maximum. Representative station means (±SE) illustrate the trend: abundance 6,388±1,710 ind m−2 (1988) to 697±359 (2000), partial rebounds thereafter; biomass 9.90±2.95 g C m−2 (1988) to 1.40±0.83 (2000), with very low 0.02±0.02 in 2014. - Community composition shift: While the proportional contribution of feeding guilds to abundance stayed relatively constant, biomass composition shifted from suspension-feeder dominance in the 1980s–1990s to deposit-feeder dominance in the 2000s, consistent with reduced vertical flux of fresh organic matter. - Megafaunal structure: Imagery documented a decline in fully 3D apex communities and an increase in early successional stages, indicating reduced megafaunal complexity (notably large glass sponges). - Environmental context: Over the study, SAM transitioned to a more positive phase, summer sea-ice cover increased strongly, iceberg area showed no clear trend, and the productivity proxy decreased. - Driver-response correlations: Positive correlations between productivity and both abundance and biomass were found for the total community and all major feeding guilds (p<0.05). Suspension-feeder biomass responded immediately (year t0) to productivity, while deposit feeders, scavengers, and predators showed delayed biomass responses (t1–t2); abundances exhibited ~2-year phase shifts, consistent with slow reproduction and recruitment. Abundance (total and by guild) was inversely correlated with iceberg area (p<0.05), whereas biomass appeared largely independent of iceberg area. - Blue carbon implications: If patterns observed at KNA extend across whitening Antarctic shelves, Antarctic zoobenthic blue carbon storage could have decreased by ~18.9×10^6 t over the period of increasing sea-ice extent up to 2014, in contrast to increases in West Antarctica and the Antarctic Peninsula.

The findings demonstrate, for the first time across the full continental shelf depth range, a multi-decadal decline in benthic abundance, biomass, and megafaunal structural complexity in a region experiencing increased sea-ice cover. The mechanistic link is consistent with reduced pelagic primary production and diminished particle flux to the seafloor under higher sea-ice cover, to which suspension feeders are particularly sensitive. Delayed responses of deposit feeders and higher trophic levels reflect indirect or time-lagged energy pathways. Although iceberg area was negatively related to abundance, biomass and megafaunal 3D complexity did not show clear dependence on iceberg changes at the depths studied, contrasting with shallow coastal systems where scouring dominates. Extrapolated to the wider whitening Antarctic shelves, these changes imply reduced benthic blue carbon storage, altering biogeochemical cycling and diminishing a potential negative feedback to climate change. The study’s end coincides with a 2014 peak in Antarctic sea-ice extent, followed by record declines by 2017; a possible shift towards reduced sea ice in the Weddell Sea could reverse productivity limitations and enhance benthic carbon drawdown. However, warming intrusions and projected temperature increases may challenge stenothermal Antarctic benthos, complicating future trajectories. Sustained, integrated observations are needed to disentangle interacting drivers (sea ice, productivity, temperature) and forecast ecosystem feedbacks to climate.

This work provides a 26-year, standardized quantitative record linking increased sea-ice cover to decreases in benthic abundance, biomass, a shift from suspension to deposit feeder biomass dominance, and reduced megafaunal structural complexity on the northeastern Weddell Sea shelf. The strong coupling of benthic biomass—especially suspension feeders—to productivity underscores food limitation as a key control in Antarctic shelf ecosystems. If representative, these trends suggest a net reduction in Antarctic zoobenthic blue carbon during the period of whitening, with implications for climate feedbacks. Looking ahead, potential declines in sea-ice extent could enhance productivity and benthic carbon storage, yet concurrent ocean warming may impose physiological limits on many benthic taxa. Future research should combine continued long-term, standardized benthic monitoring with geochemical measurements of carbon sequestration, improved mechanistic models that integrate sea-ice dynamics, productivity, and temperature stress, and expanded spatial coverage to assess representativeness across Antarctic shelves.

• Spatial scope: Results are from the Kapp Norvegia/Auståsen region and may not capture heterogeneity across Antarctic shelves; extrapolations to the continent assume representativeness and require geochemical corroboration. - Temporal sampling: Only eight expeditions over 1988–2014 provide discrete temporal snapshots rather than continuous annual coverage; imagery was unavailable for 1989 and limited in later years. - Gear constraints: Multibox corer sampling (0.5 mm mesh) under-represents large megafauna (e.g., glass sponges), necessitating separate, semi-quantitative imagery-based assessments of 3D complexity. - Productivity proxy: The productivity index (ice-free days × daylight) is a proxy validated against satellite Chl a and POC but is not a direct measurement; sub-ice blooms and other processes may not be fully captured. - Iceberg metric: Iceberg area used pertains to small icebergs from ALTIBERG; potential effects of giant icebergs or scouring frequency/intensity at fine spatial scales are not fully resolved. - Potential confounders: Concurrent environmental changes (e.g., warming intrusions, circulation variability) could influence benthos alongside sea-ice and productivity changes.