The Southern Ocean food web was long treated as a short, Antarctic-krill-centric energy chain, but analysis of the largest circumpolar dietary database ever compiled (McCormack et al. 2021) confirmed that alternative energy pathways - mesopelagic fish, other krill species, copepods, and squid - dominate predator diets in the Indian and West Pacific sectors while Antarctic krill dominate in the Atlantic and East Pacific. That analysis was confined to austral-summer diet composition. The open problem it left is to convert this qualitative, summer-only map of who-eats-whom into a quantitative, region-specific, and seasonally resolved understanding of the food web: estimating energy and biomass flux rates along trophic pathways (which requires prey biomass densities, consumption rates, and assimilation efficiencies), resolving winter and under-ice structure, filling systematically underrepresented links such as gelatinous prey using complementary methods, determining circumpolar sampling designs, and building and validating alternative region-specific food-web, mass-balance, and end-to-end models (krill-dominated versus alternative configurations) to evaluate ecosystem responses to climate change, fisheries, and whale recovery. As of 2026 the core remains open: winter food-web structure is unquantified, circumpolar energy fluxes have not been estimated from diet data, and only single-region end-to-end or whole-of-ecosystem models (the Cooperation Sea and East Antarctica) and a proposed model-ensemble protocol exist, with no validated circumpolar model set or scenario comparison.

Background

The Southern Ocean (here, waters south of 40 degrees S) supports a characteristic polar food web: the network of who-eats-whom linking phytoplankton through zooplankton and fish to seabirds and marine mammals. For decades the prevailing view was a short, highly efficient krill-centric chain, in which Antarctic krill (Euphausia superba), a swarming shrimp-like crustacean, served as the dominant conduit of energy from primary producers to predators at higher trophic levels (positions in the feeding hierarchy). Over the last fifteen years, diet data have revealed alternative energy pathways in which other mid-trophic organisms - other krill species, copepods (small planktonic crustaceans), mesopelagic fish such as lanternfish (myctophids), and squid - dominate energy transfer in some regions.

To test this regional hypothesis, McCormack et al. (2021) analyzed roughly 26,000 dietary observations from the open-access SCAR (Scientific Committee on Antarctic Research) Southern Ocean Diet and Energetics Database (compiled largely from stomach-content analysis) and compared food-web structure across the four major sectors of the Southern Ocean during austral summer (the Southern Hemisphere summer, roughly November to March). They confirmed significant regional differentiation: Antarctic krill dominate mid-trophic diets in the Atlantic and East Pacific sectors (Scotia Arc and western Antarctic Peninsula), while mesopelagic fish and other krill species dominate in the Indian and West Pacific sectors (East Antarctica and the Ross Sea).

The authors spelled out what this map cannot deliver. First, it describes only summer: winter food-web structure beneath and around sea ice, and the seasonality of trophic link strengths, are essentially unknown because winter sampling is logistically difficult. Second, diet composition is not energy flow: converting observed diet fractions into quantitative rates of energy or biomass transfer requires additional parameters - prey biomass densities, consumption rates, and assimilation efficiencies (the fraction of ingested food a predator actually absorbs) - that are poorly constrained at regional scale. Third, stomach contents systematically underrepresent gelatinous prey (which digest rapidly) and many planktonic groups; complementary methods - stable-isotope ratios and fatty-acid biomarkers (chemical diet tracers integrated over weeks to months), DNA metabarcoding (sequencing prey DNA in gut or scat samples), and acoustic and net-based biomass surveys - are needed to fill these missing links, together with historical records not yet incorporated into the SCAR Southern Ocean Diet and Energetics Database. Fourth, optimal circumpolar sampling and monitoring designs able to resolve latitudinal, seasonal, and interannual variation, and to distinguish Antarctic from subantarctic webs (the subantarctic zone being the warmer waters just north of the Antarctic zone), remain to be determined. Finally, the explicit next step was building and testing alternative, region-specific food-web and mass-balance models - for example Ecopath with Ecosim, which constructs budgets of biomass and energy flows among functional groups subject to mass-balance constraints, and end-to-end models that couple physical, biogeochemical, and food-web components - to evaluate ecosystem responses to climate change and ocean acidification, fisheries, and the recovery of great whale populations after industrial whaling. The paper states that this model-building step depends on expanding the datasets and estimating the energy-flux parameters above.

A companion review by the same group (McCormack et al. 2021, Frontiers in Ecology and Evolution) documented the state of the modelling field: regional food-web models existed for only some Southern Ocean sectors, no whole-of-ecosystem model had been published for any Southern Ocean region, and model validation procedures were undeveloped. A related comparative study (Hill et al. 2021) showed that published regional models can support robust cross-regional comparison if differences in model currency, aggregation scheme, and energetic parameters (so-called model personality) are reported and standardized. Since then, circumpolar biomass estimates for plankton, krill, and salps (barrel-shaped planktonic filter-feeding tunicates related to sea squirts) have become available (Yang et al. 2022), supplying a key parameter class, but the full agenda - flux quantification, winter structure, sampling design, and validated alternative models exercised under climate, fishery, and whale-recovery scenarios - remains uncompleted.

Problem Statement

For the Southern Ocean south of 40 degrees S, move from the qualitative, austral-summer map of regional food-web structure to a quantitative, region-specific, and seasonally resolved representation of the food web and its dynamics. Following the source's own stated limitations and future directions, this comprises the following components:

  1. Quantify energy (or biomass/carbon) flux rates through the trophic pathways of each major Southern Ocean sector by combining observed diet fractions with prey biomass densities, consumption rates, and assimilation efficiencies - including the currently underrepresented gelatinous and planktonic prey.

  2. Constrain seasonal structure - in particular winter and under-ice food-web structure and the seasonality of trophic link strengths, which are poorly known because of limited winter sampling.

  3. Determine circumpolar sampling and monitoring designs capable of characterizing spatial (latitudinal, and Antarctic versus subantarctic) and temporal (seasonal, interannual) variation in food-web structure, and of determining priorities for data collection and research effort.

  4. Build and test alternative, region-specific food-web, mass-balance, and end-to-end models - for example krill-dominated versus copepod-, fish-, or squid-dominated configurations - and use them to evaluate Southern Ocean ecosystem responses to the three main drivers of change: climate change (including ocean acidification), fisheries, and the recovery of great whales.

Together, these components express the source's overarching objective: a validated, region-resolved, scenario-capable quantitative model system of the Southern Ocean food web, whose construction requires the observational and parameter advances in components 1-3 and whose credibility is tested by the scenario analyses of component 4. All four components are required and are not interchangeable alternatives. Here "validated" follows the companion review's convention (McCormack et al. 2021, Frontiers in Ecology and Evolution): model behaviour is shown to be satisfactory for the intended task for the right reasons, assessed against observations from the relevant region and period rather than merely fitted.

References

  1. McCormack SA, Melbourne-Thomas J, Trebilco R, Blanchard JL, Raymond B, Constable A (2021). Decades of dietary data demonstrate regional food web structures in the Southern Ocean. Ecology and Evolution 11(1): 227-241. doi:10.1002/ece3.7017. https://doi.org/10.1002/ece3.7017
  2. McCormack SA, Melbourne-Thomas J, Trebilco R, Griffith G, Hill SL, Hoover C, Johnston NM, Marina TI, Murphy EJ, Pakhomov EA, Pinkerton M, Plagányi É, Saravia LA, Subramaniam RC, Van de Putte AP, Constable AJ (2021). Southern Ocean Food Web Modelling: Progress, Prognoses, and Future Priorities for Research and Policy Makers. Frontiers in Ecology and Evolution 9: 624763. doi:10.3389/fevo.2021.624763. https://doi.org/10.3389/fevo.2021.624763
  3. Hill SL, Pinkerton MH, Ballerini T, Cavan EL, Gurney LJ, Martins I, Xavier JC (2021). Robust model-based indicators of regional differences in food-web structure in the Southern Ocean. Journal of Marine Systems 220: 103556. doi:10.1016/j.jmarsys.2021.103556. https://doi.org/10.1016/j.jmarsys.2021.103556
  4. Yang G, Atkinson A, Pakhomov EA, Hill SL, Racault M-F (2022). Massive circumpolar biomass of Southern Ocean zooplankton: Implications for food web structure, carbon export, and marine spatial planning. Limnology and Oceanography 67(11): 2516-2530. doi:10.1002/lno.12219. https://doi.org/10.1002/lno.12219
  5. Queirós JP, Borras-Chavez R, Friscourt N, Groß J, Lewis CB, Mergard G, O'Brien K (2024). Southern Ocean food-webs and climate change: A short review and future directions. PLOS Climate 3(3): e0000358. doi:10.1371/journal.pclm.0000358. https://doi.org/10.1371/journal.pclm.0000358
  6. Xing L, Tang J, Tian S, Barrier N (2023). Simulating impacts of fishing toothfish on the pelagic community in the Cooperation Sea, Southern Ocean. Regional Studies in Marine Science 68: 103227. doi:10.1016/j.rsma.2023.103227. https://doi.org/10.1016/j.rsma.2023.103227
  7. Murphy K, Fierro-Arcos D, Rohr T, Green D, Novaglio C, Baker K, Ortega-Cisneros K, Eddy TD, Harrison CS, Hill SL, Eskuche-Keith P, Cataldo-Mendez C, Petrik CM, Pinkerton M, Spence P, Stollberg I, Subramaniam RC, Trebilco R, Tulloch V, Palacios-Abrantes J, Bestley S, Bianchi D, Boyd P, Buchanan PJ, Bryndum-Buchholz A, Coll M, Corney S, Datta S, Everett JD, Forestier R, Fulton EA, Guibourd de Luzinais V, Heneghan R, Mason JG, Maury O, McMahon CR, Murphy EJ, Richardson AJ, Tittensor DP, Spillias S, Steenbeek J, Veytia D, Blanchard J (2025). Developing a Southern Ocean Marine Ecosystem Model Ensemble to Assess Climate Risks and Uncertainties. Earth's Future 13(3): e2024EF004849. doi:10.1029/2024EF004849. https://doi.org/10.1029/2024EF004849
  8. Xing L, Grüss A, Barrier N, He J, Tang J (2025). Global sensitivity and uncertainty analyses of an ecosystem model for simulating food web dynamics in the Cooperation Sea, Southern Ocean. Fisheries Research 288: 107345. doi:10.1016/j.fishres.2025.107345. https://doi.org/10.1016/j.fishres.2025.107345
  9. Lovvorn JR, Nissen C, Bahl AA, Behrens E, Brooks CM, Christensen K, Dinniman MS, Dove LA, DuVivier AK, Hofmann EE, Kwasniewski H, Kwon YS, Stammerjohn SE, Xing L (2026). End-to-end modeling for the Ross Sea Region Marine Protected Area: a review of available tools for conservation objectives. Frontiers in Marine Science 13: 1886905. doi:10.3389/fmars.2026.1886905. https://doi.org/10.3389/fmars.2026.1886905
  10. Stollberg I, Bestley S, Constable A, Fulton EA, Porobic J, Green D, O'Sullivan D, Mori M, Corney S, Sumner M, Melbourne-Thomas J (2026). An end-to-end model framework for understanding and predicting changes in Antarctic marine ecosystems. Ecological Modelling 520: 111708. doi:10.1016/j.ecolmodel.2026.111708. https://doi.org/10.1016/j.ecolmodel.2026.111708