w11
Benthic Ecosystems of the Antarctic Seafloor
Endemism and Richness:
of the species found in Antarctic benthic ecosystems are unique to the region and found nowhere else on Earth.
Total species richness in Antarctica is estimated at approximately , which is considered as rich and diverse as the Great Barrier Reef.
Geographic Isolation:
The high level of endemism is primarily attributed to long-term geographic isolation.
The separation between Antarctica and other continents (the Gondwana break apart) became complete approximately ago.
Adaptations to Extreme Conditions:
Food Limitation: Organisms must adapt to highly seasonal food inputs. They build up lipid (fat) reserves during the summer.
Flexible Feeding Strategies:
The sea star Odenesta can switch from scavenging to deposit feeding and can use its tube feet to capture drifting particulate matter.
The sea urchin Sterochinus feeds opportunistically when food becomes available.
Reproductive Timing: Most organisms align reproduction with spring phytoplankton blooms and pulses.
Brooding: There is a high incidence of brooding (carrying offspring, similar to a male seahorse) rather than releasing pelagic larvae into ocean currents.
Microbial Pathways: Feeding on microbial and detrital pathways is critical for survival over winter when fresh material is scarce.
Growth and Longevity:
Organisms are generally cold-adapted with narrow thermal tolerance ranges.
Many species are slow-growing and long-lived. For example, some Glass sponges ( sponge) are estimated to be up to old.
Gigantism: Many species grow significantly larger than their relatives in temperate or tropical waters.
Sea Cucumbers: Filter-feeding sea cucumbers in the Ross Sea reach sizes of and are physically massive.
Sea Spiders: Antarctic species can grow to several centimeters in diameter, significantly larger than temperate species.
Drivers and Biodiversity of Benthic Habitats
Physical Drivers:
Temperature: Some species, like certain ice fish, have extremely narrow temperature tolerance ranges.
Light and Ice: Macroalgal (microalgae/seaweed) communities are restricted to coastal areas or Subantarctic Islands where ice breaks out frequently, allowing light for photosynthesis. Invertebrate-dominated communities prevail where ice is persistent.
Iceberg Scouring:
Icebergs can be several hundred meters deep and act like a bulldozer, mowing down everything on the seafloor.
Intermediate Disturbance Hypothesis: In shallow areas ( deep), occasional scouring creates a patchwork mosaic of successional stages, which can lead to higher diversity than areas with no scouring or extreme recurring scouring.
Distribution Patterns:
Depth Gradient: Abundance and diversity generally decrease with increasing depth.
Currents: Moderate to strong currents favor sessile invertebrates (filter feeders like corals and sponges). Low current areas with high primary productivity favor deposit feeders.
Substrate: Muddy basins host deposit feeders, while hard substrates host sessile communities.
Dropstones: Stones dropped by melting ice provide "hard substrate islands" in otherwise muddy environments.
Mapping and Research:
Vanessa Lucia (IMAS): Developed "Seamap Antarctica" to quantify and map species distributions.
Biodiversity Hotspots: These are generally found in shallower areas with colder waters and moderate current flow.
of identified biodiversity hotspots occur in the coolest water regions of the shelf.
Physiology and Adaptations of Antarctic Benthic Fish
Endemicity: Antarctic benthic and demersal fish show high endemicity but relatively low diversity, dominated by the family Notothenids (including ice fish and toothfish).
Thermal Adaptations:
Antifreeze Proteins: Many fish have proteins in their blood to prevent ice crystal formation, as water temperatures can reach .
Colder Metabolic Rates: Biological processes occur slowly in these temperatures.
Buoyancy and Blood:
Lack of Swim Bladders: Evolutionarily, many species lost swim bladders and achieve neutral buoyancy through low-density bones and high lipid content.
Clear Blood: Some ice fish lack hemoglobin entirely. Because cold water is oxygen-rich, oxygen is transported directly in the plasma. These fish possess large hearts, large blood volumes, and large gill surface areas to compensate.
Nesting: Some species, like ice fish in the Weddell Sea, produce demersal eggs and create massive nesting sites. One colony was found containing thousands of nests guarded by ice fish.
Fisheries: Regulated by Camelar (Commission for the Conservation of Antarctic Marine Living Resources), focusing on Antarctic and Patagonian toothfish and mackerel icefish.
Pelagic Food Webs and Trophic Pathways
Biomass Distribution:
Large biomass is concentrated at the base of the food chain (microbes and phytoplankton).
Biomass decreases as body size increases (top predators like whales and seals).
Primary Producers:
Diatoms: Dominate coastal systems and ice edges.
Pennate Diatoms: Long, skinny structures adapted to live inside brine channels within sea ice.
Haptophytes (Phaeocystis): Form large jelly-like aggregations in open water and marginal ice blooms.
Secondary Producers and Mid-Trophic Levels:
Zooplankton (amphipods, pteropods, copepods, jellies).
Mesopelagic Fish: Live between deep; prey on secondary producers.
Food Web Models:
Classical View: Phytoplankton > Krill > Whales.
Salp Food Chain: Often considered an "evolutionary dead end" because salps are mostly water with low protein/fat. They become important when krill are scarce but provide poor nutrition.
Regional Dominance:
Polar Front: Dominated by the copepod/mesopelagic fish pathway; Antarctic krill are less common.
Marginal Ice Zone: Highly krill-centric due to phytoplankton blooms.
Shelf Zone: Includes silverfish and squid as key components.
Biology and Life Cycle of Antarctic Krill
Keystone Species: Euphausia superba (Antarctic krill) is the central keystone species.
Biological Characteristics:
Diet: Consume primarily diatoms and some copepods.
Behaviour: Form massive, dense swarms (similar to bait fish) and undergo strong dial migration (moving up and down the water column to avoid predators).
Shrinking Adaption: Krill can moult and reabsorb tissue to shrink in body size when food is scarce.
Life Cycle and Ice:
Juveniles are sea ice-associated over winter.
Sea ice provides shelter from predators and a food source (diatoms and detritus on the underside of the ice).
Krill Fishery: Mainly focused on the West Antarctic Peninsula and islands like South Georgia and South Shetland; managed by Camelar.
Marine Megafauna: Whales, Seals, and Birds
Whaling History:
Exploration was driven by whaling and sealing for oil and blubber (used for lighting).
: Norwegian whalers opened a station at South Georgia.
of whales were successively exploited. "Right whales" were named for being the "right" (easiest) whales to harvest.
: A global moratorium on commercial whaling was established, managed by the International Whaling Commission (IWC).
Antarctic Seals:
Leopard Seals: Top territorial predators, long; hunt penguins and fish.
Crabeater Seals: Most abundant seal; of Antarctic seal stocks. They eat krill using specialized teeth that act like baleen filters.
Weddell Seals: Friendly seals that inhabit fast ice; use teeth to keep breathing holes open year-round.
Penguins and Birds:
Approx. in Antarctica.
Emperor Penguins: Largest penguin (). Breed over winter on fast ice. Chicks fledge in spring/summer to align with the food bloom.
Snow Petrols: Use ice for resting and predator avoidance.
Arctic Ecosystem Comparisons: Polar Cod and Polar Bears
Arctic Food Web:
The central pathway is the Antarctic cod (Polar cod), which feeds on crustaceans.
Top predator: Polar Bear. Sea ice is a vital hunting platform for catching seals, walruses, and whales.
Decline: Decreasing Arctic sea ice thickness and early breakup force bears further north, leading to population declines.
Whale Differences:
Baleen whales visit both poles but generally do not forage under ice.
Minke Whales: The only whales that occupy deep ice packs; they have a hard rostrum (nose) to poke through thin ice for breathing.
Environmental Change: Warming, Acidification, and Sea Ice Loss
Drive by Carbon Dioxide:
Warming Pathways: Affects salinity, water masses, iceberg scouring, and causes sea ice/ice shelf loss.
Acidification: Affects the entire water column, particularly calcifying organisms.
The Sea Ice "Heartbeat":
Antarctic sea ice crashed in . This trend of low extent continued through , , and into . This is seen as a potential tipping point.
Regional Trends: The West Antarctic Peninsula is warming faster and losing ice more rapidly than East Antarctica.
Impact of Environmental Change on Ecosystem Components
Benthic Communities:
Loss of cold-adapted species and invasion of king crabs (rare on the shelf currently).
Changes in dominance from corals/calcifiers to sponges due to acidification.
Plankton and Krill:
Phenology Mismatch: A change in the timing of blooms can cause predators to miss their peak food period.
Krill vs. Salps: Loss of sea ice may lead to fewer krill and higher dominance of the less nutritious salp food chain.
Hatching Success: Increased levels have been shown to reduce the hatching rate of Antarctic krill.
Emperor Penguins:
Fast ice melt before chicks fledge causes colony failure (chicks cannot swim without adult waterproof feathers).
One study predicts emperor penguins could be quasi-extinct by . They are currently listed as Endangered on the IU CN Red List.
Questions & Discussion
Question: Is the Arctic benthic system similar to the Antarctic one, and does the lack of large predators (like penguins) in the Arctic influence the benthic community?
Response: Benthic systems in both hemispheres are influenced by cold-water minerals, sponges, currents, and food availability. Arctic systems still have demersal fish that behave as predators, eating brittle stars, amphipods, and worms on the seafloor, so the predatory dynamics at that level are similar even if the specific megafauna differ.
Question: Regarding the shift to a salp-based food chain, is that considered a bad alternative?
Response: Generally, yes. Salps have significantly lower nutritional value compared to Antarctic krill. While salps are part of the ecosystem, they represent a poor quality food source in terms of protein and fat for higher vertebrates.