Deep-living and diverse Antarctic seaweeds as potentially important contributors to global carbon fixation

The study addresses the prevailing assumption from global habitat models that Antarctica offers little suitable habitat for marine macroalgae, implying negligible Antarctic contributions to global macroalgal carbon fixation. Observations in the Antarctic, particularly south of 71°S, are sparse and largely focused on the Antarctic Peninsula. Yet where studied, Antarctic macroalgal standing biomass can be comparable to or exceed some Arctic and temperate systems, providing habitat and driving benthic food webs with potential for carbon sequestration. Climate change is rapidly altering Antarctic coastal environments via glacial retreat, changing sea-ice extent and persistence, and shifts in sediment and phytoplankton dynamics, all of which affect seafloor light and hence macroalgal depth distributions and productivity. Light availability at the seabed is a fundamental constraint on macroalgal distribution. The research aims to document southern high-latitude macroalgal assemblages, including their depth ranges in the Ross Sea, assess whether observed deep-living populations are consistent with physiological light limits, and estimate Antarctic macroalgae’s potential contribution to regional and global carbon fixation.

• Global models predict minimal suitable Antarctic habitat for macroalgae, with large brown macroalgae considered limited in southern distribution and depth range.
• Prior records show Antarctic macroalgae with substantial biomass and ecological roles, mainly from the Antarctic Peninsula; canopy-forming brown algae like Himantothallus grandifolius have verified occurrences to 50–70 m in the Peninsula, and historical southernmost observations near 72°S in the Ross Sea at ~20 m.
• Deep-living algae in polar regions (especially rhodophytes and phaeophytes) are reported below 40–60 m, with some unverified dredge records down to ~100 m in Antarctica; recent ROV studies verified red algae at ~100 m.
• Macroalgal detritus transport to deep water and burial indicates potential for long-term sequestration, possibly enhanced in cold polar environments; Antarctic deep export rates appear high.
• Sea-ice dynamics, glacial retreat, sediment inputs, and phytoplankton phenology strongly modulate light attenuation and thus macroalgal cover and depth distribution, with possible gains from newly ice-free areas but offsets from increased disturbance and turbidity.
• Global summaries of macroalgal NPP show a wide range, with Antarctic rates potentially comparable to temperate/tropical estimates under favorable light conditions.

Study area and surveys: Two NIWA RV Tangaroa voyages (Jan–Feb 2021 and 2023) surveyed coastal Ross Sea, Antarctica (northern Victoria Land, ~71–75°S), targeting five ice-free locations from Robertson Bay (west of Cape Adare) to Terra Nova Bay. A total of 25 stations were surveyed between 40 and 250 m depth.

Seafloor mapping and imaging: Un-navigated shallow coastal areas were mapped with a Kongsberg EM302 multibeam echosounder (MBES). Benthic imagery was collected using the Deep-Towed Imaging System (DTIS) along 200–500 m transects (parallel to shore), at 2.0–3.5 m altitude. DTIS recorded continuous HD video (Sony HDR PJ 760VE, 1080p) and 24 MP stills (Nikon D3200) every 10 s; a low-resolution real-time video aided altitude and habitat targeting. Position and depth were tracked using SIMRAD HiPAP. Parallel lasers enabled image scaling. A Seabird Micocat CTD recorded salinity, temperature, and depth during deployments. Sea-ice presence affected exact deployment depths.

Navigation and data handling: DTIS positions were plotted in real time using OFOP; navigation, camera commands, and annotated observations (substrata, assemblages) were logged and archived via the vessel’s DAS.

Imagery processing: Taxonomists processed DTIS imagery. Clearly attached macroalgal thalli were identified to the highest possible taxonomic level using video, higher-resolution stills, and collected specimens. Functional groups were annotated using modern taxonomy where possible; otherwise, a simplified scheme based on CATAMI was applied. Densities (m−2 or m−3 along transects) were computed as abundance divided by transect area (length × width), with width derived from laser scaling at multiple points. Ortho-mosaics were created from frame-grab stills using Agisoft Metashape and scaled using laser references at six evenly distributed points per transect.

Specimen collections: In 2023, benthic samples were taken at Robertson Bay, Cape Adare, and the Possession Islands using a Van Veen Grab (0.13 m2) and an Agassiz Sled (1.15 m × 0.37 m). These confirmed identifications from imagery. Reference and voucher specimens (including for genetic sequencing) were prepared and deposited at the Museum of New Zealand Te Papa Tongarewa Herbarium (WELT).

Seafloor light modelling: Photosynthetically active radiation at the seabed (EBed) was estimated using the R package coastalLight, integrating global bathymetry, surface PAR, and water clarity to calculate seabed irradiance. Where light attenuation coefficients (Ka) were missing, regional average Ka values were used. EBed was calculated as EBed = I·e^(−Ka·d), where I is surface irradiance and d is depth. For each of the five regions, EBed was extracted along three replicate transects perpendicular to shore spanning shallow water to >100 m. Limited bathymetry constrained retrievals <50 m at some sites.

Productivity modelling: Published photosynthetic parameters for three Antarctic macroalgae/functional groups (Himantothallus grandifolius, Palmaria decipiens, and crustose coralline algae, CCA) were compiled (α, R, Pm). For every grid cell with EBed, net primary productivity (NPP) was calculated using P = Pm·(1 − e^(−α·EBed)) + R. The compensating irradiance (NPP = 0) defined depth thresholds.

Extrapolation of carbon fixation: For each species, Antarctic-wide potential NPP was summed over pixels (1/240°) within observed/assumed latitudinal ranges (H. grandifolius: 60–72.5°S; P. decipiens: 60–75°S; CCA: 60–80°S) and longitudes (Ross Sea: 160–172.5°E; Antarctica: −180° to 180°). Only cells with NPP > 0 were included. Totals were corrected for mean coverage observed across transects shallower than 90 m (P. decipiens 6.6%; H. grandifolius 4.3%; CCA 1.4%), for suitable substratum availability (assumed 30% of area), and for a 5-month growing season. Sums were converted to Tg C yr−1 and compared to global macroalgal carbon fixation estimates. Statistical modelling for depth-density relationships used GAMs; all analyses were conducted in R.

• Deep-living assemblages: Dense, attached canopy-forming brown algae (Himantothallus grandifolius) were observed at 74–95 m depth at multiple northern Victoria Land Coast locations (Robertson Bay, Cape Adare, Possession Islands, Cape Hallet). Crustose coralline algae (CCA) occurred as deep as 125 m; rhodophytes were recorded to 99 m.
• Species richness: Up to 14 potential macroalgal species were recorded at 50–70 m depths, including multiple rhodophytes, chlorophytes, phaeophytes, and unidentified red algae.
• Drift export: Drift algae from several groups were common beyond 100 m, with H. grandifolius observed as drift at >200 m, indicating high export/transport rates to deeper waters.
• Seafloor light: Modelled EBed at surveyed sites ranged from >0.1 mol m−2 day−1 (sufficient for photosynthesis) to frequent values <0.01 mol m−2 day−1 (below typical thresholds). Three mechanisms likely explain deep occurrences: exceptional low-light adaptation, frequent transport of attached substrata or dislodgement, and periods where realised EBed exceeds model estimates.
• Physiological depth thresholds (from literature parameters and EBed): Palmaria decipiens exhibits net carbon gain from 0–70 m (mean compensation depth ~45 m); H. grandifolius to ~75 m (mean ~50 m); CCA to ~125 m (mean ~80 m). Some observed red algal occurrences extended beyond modelled light-based capabilities.
• Carbon fixation estimates (Tg C yr−1): Ross Sea total 1–2.7; Antarctic total 12–37. As a share of global macroalgal carbon fixation, Antarctic macroalgae could contribute 0.9–2.8%. Species-specific Antarctic contributions: H. grandifolius 3–14 Tg (0.3–1.1% global), P. decipiens 1.3–7.9 Tg (0.6–1.2%), CCA 7.5–15 Tg (0.1–0.6%).
• Comparative NPP: Modelled NPP in Antarctic settings is comparable to some temperate and many tropical/subtropical macroalgal estimates under favorable light conditions.
• Paradigm shift: Findings contradict global model outputs that suggest minimal Antarctic macroalgal coverage and show macroalgae are more abundant, deeper-living, and likely underrepresented in global benthic primary productivity and blue carbon assessments.

The findings directly challenge the paradigm that Antarctic coasts provide little suitable habitat for macroalgae and that their contribution to global carbon fixation is negligible. Verified observations of dense, attached macroalgae at 60–125 m, including canopy-forming H. grandifolius, demonstrate that Antarctic taxa occupy deeper photic zones than widely assumed. Coupled with frequent deep-water drift, this supports significant carbon export and potential sequestration pathways. Physiological modelling using species-specific photosynthetic parameters and seafloor light estimates shows that observed depth distributions generally align with low-light adaptation limits for H. grandifolius and CCA, though some red algal occurrences surpass modelled thresholds, implying either episodic higher light availability, local clarity not captured by global products, or transport/deposition effects. The regional extrapolations suggest Antarctic macroalgae may account for 0.9–2.8% of global macroalgal carbon fixation—non-negligible in the context of Southern Ocean carbon sinks where phytoplankton dominate but benthic contributions may be undercounted. Climate-driven changes—reductions in sea-ice cover, glacial retreat, altered meltwater/sediment input, and phytoplankton bloom timing—will modulate seafloor light and disturbance regimes, potentially expanding suitable habitat yet also increasing turbidity and iceberg scouring. Thus, macroalgal blue carbon gains in newly ice-free areas could be offset by enhanced disturbance or reduced light at depth. Improving light models, bathymetric resolution, and in situ validation is essential for refining Antarctic benthic productivity estimates and for managing blue carbon within protected areas such as the Ross Sea MPA.

This study documents some of the deepest verified records of Antarctic macroalgae, including dense, attached H. grandifolius stands to ~95 m and CCA to 125 m, and reveals high abundances of drift algae beyond 100–200 m. Observations combined with light and physiological modelling indicate strong low-light adaptation and suggest Antarctic macroalgae are more widespread and productive than global models predict. Antarctic macroalgae may contribute up to 2.8% of global macroalgal carbon fixation, underscoring their potential significance to blue carbon and long-term sequestration in the Southern Ocean. Future research should prioritize: high-resolution, long-term in situ seabed light measurements; improved coastal bathymetry; broader surveys of biomass and species composition across latitudinal and depth gradients; quantification of export and burial pathways; and evaluation of climate-driven changes (sea ice, glacial melt, sediment/nutrient inputs) on macroalgal habitat and productivity. Integrating these data will enhance carbon budgeting and inform conservation and management within the Ross Sea MPA and other Antarctic coastal regions.

• Seafloor light modelling uncertainty: Limited long-term in situ seabed irradiance measurements in Antarctica impede validation; modelled EBed may overestimate realised light due to unaccounted variability (atmospheric/oceanographic conditions, sea-ice cover and floes).
• Water clarity data gaps: Missing or sparse Ka values required use of regional averages, introducing uncertainty in EBed estimates.
• Bathymetric constraints: Global bathymetry products lack sufficient resolution in some coastal areas (e.g., Wood Bay), limiting light estimates, especially <50 m.
• Attribution of attached vs drift: High prevalence of drift algae at depth complicates determination of true habitable depth ranges and attached population densities.
• Operational constraints: Sea-ice presence and movement affected deployment depths and site access, potentially biasing spatial coverage.
• Extrapolation assumptions: Regional coverage corrections (mean cover percentages), substratum suitability (assumed 30%), and growing season length (5 months) introduce uncertainty into Antarctic-wide NPP estimates and carbon fixation totals.