Polar Biology Lecture Notes

Academic Profile and Positionality

• Speaker Information: Dr. Michelle LaRue

  • Current Role: Professor at Gateway Antarctica, School of Earth and Environment (SEE), University of Canterbury.

  • Academic Background: PhD (2014) in Conservation Biology from the University of Minnesota.

  • Specializations: Spatial ecology, population dynamics of penguins and seals, remote sensing, Geographic Information Systems (GIS), and statistics.

  • Field Experience: 12 field seasons in Antarctica.

• Whakataukī (Māori Proverb): "He waka eke noa" — translated as "A canoe which we are all in with no exception."

Current Status of Polar Sea Ice: NSIDC 2025 Update

• Antarctic Sea Ice Minimum (March 6, 2025 Reporting):

  • On March 1, 2025, Antarctic sea ice reached its annual minimum extent of $1.98 \times 10^6\,km^2$ ($764,000$ square miles).

  • This ties with 2022 and 2024 for the second lowest extent in the $47$-year satellite record.

  • This marks the fourth consecutive year that Antarctic sea ice has fallen below the $2.0 \times 10^6\,km^2$ ($772,000$ square miles) threshold.

• Comparative Statistics:

  • The minimum is $860,000\,km^2$ ($332,000$ square miles) below the 1981-2010 average minimum of $2.84 \times 10^6\,km^2$.

  • It is $190,000\,km^2$ ($73,000$ square miles) above the record low set on February 21, 2023.

  • The 2024 maximum was $200,000\,km^2$ ($77,000$ square miles) above the 2023 record-low maximum, but $1.55 \times 10^6\,km^2$ ($598,000$ square miles) below the 1981-2010 average maximum.

• Temporal Trends:

  • 1979–2012: Period of relative stability.

  • 2013–2015: Period of mostly above-average ice extents.

  • 2016–2022: Period of mostly below-average extents.

  • 2023–2025: Significant decline, with 2023 being the record low. Scientists speculate the Southern Ocean sea ice may have entered a "wholly new state" outside of observed variability.

• Geographic Observation of Ice:

  • High-concentration ice currently remains mostly in the Weddell Sea.

  • Low-concentration ice is present in the Amundsen and Ross Seas.

  • Isolated patches persist along the coasts of Enderby Land and Wilkes Land.

  • The minimum date (March 1) was five days later than the median date (February 24).

Radiative Balances: Albedo and the Greenhouse Effect

• Definition of Albedo: A measure of how well a surface reflects solar energy, expressed on a scale from $0$ to $1$.

  • $0$: A perfect absorber of light.

  • $1$: A perfect reflector of light.

• Comparative Albedo Values:

  • Ocean: $0.06$ (absorbs the vast majority of solar energy).

  • Sea Ice: $0.70$.

  • Snow: $0.90$ (highly reflective).

• The Greenhouse Effect:

  • Natural Greenhouse Effect: Solar radiation enters the atmosphere; some heat is re-radiated into space while some is trapped by natural levels of Greenhouse Gases ($GHGs$), maintaining habitability.

  • Human Enhanced Greenhouse Effect (Global Warming): Increased concentrations of $CO_2$, $CH_4$, and $N_2O$ trap more re-emitted heat, resulting in less heat escaping into space and a warmer planet.

Polar Geography and Physical Boundaries

• Arctic Geography: The Arctic Ocean is essentially bounded by land (continents of North America, Europe, and Asia).

• Antarctic Geography: Antarctica is a continent surrounded by the Southern Ocean. Its physical boundary is effectively defined by the Antarctic Circumpolar Current (ACC) rather than landmasses.

• Ice as a Unique Feature:

  • Functions as a physical barrier to sunlight.

  • Serves as a biological platform for animals to stand, breed, and grow upon.

  • Changes seasonally in extent and thickness.

• Pagophilic Species: Derived from the Greek for "ice-loving," referring to species that depend on sea ice for their life cycle.

Sea Ice Classification and Characteristics

• Types of Sea Ice:

  • Pack Ice: Mobile ice located in the middle of the ocean, not attached to land.

  • Fast Ice: Ice that is "fastened" or connected to the coastline or land.

• Thickness: Ranges from a few centimeters to several meters. This thickness determines the ice's stability and suitability as a habitat.

Polynyas: Ecological Oases

• Definition: Areas of open water surrounded by sea ice.

• Formation Mechanisms:

  • Latent Heat Polynyas: Formed by physical forces such as winds and currents.

  • Process: Katabatic winds (strong, cold winds flowing down from the Antarctic Ice Sheet, which has a maximum height of $4776\,m$) push sea ice away from the shelf ice or coast. As new ice forms in the open water, it is immediately pushed away, creating a persistent opening.

• Biological Importance:

  • Provide access to the water for air-breathing marine animals (penguins, seals, whales) to forage.

  • Often areas of high primary productivity due to increased sunlight penetration compared to ice-covered regions.

  • Case Study: Emperor penguin colony distribution is heavily associated with the location of coastal polynyas (e.g., the $50$ coastal polynyas identified around the Antarctic continental shelf).

Antarctic Marine Biology

• Key Species and Food Web:

  • Southern Ocean Food Web: Heavily dependent on Krill and Silverfish.

  • Foraging Strategy: Colonial animals (like penguins) are generally central place foragers, meaning they must return to a specific location (the colony) to feed chicks or pups, making the health of the ocean immediately surrounding the colony critical for survival.

• Environmental Adaptation: Animals must adapt to extreme cold and seasonal changes in light and ice availability.

Arctic Marine Biology

• Primary Productivity:

  • Productivity follows the changing ice edge rather than being strictly tied to coastal polynyas.

  • Spring: Dominated by diatom blooms.

  • Summer: Dominated by coccolithophore blooms.

  • Productivity is currently increasing as sea ice decreases.

• Polar Bears:

  • The only bear species considered a marine mammal.

  • An indicator species for the health of the Arctic ecosystem.

  • Unlike Antarctic penguins, they are not central place foragers.

  • Diet: Forage for ringed seals ($Pusa\,hispida$), bearded seals ($Erignathus\,barbatus$), walrus, and whale carcasses.

  • Energy Requirements: An adult female ($175\,kg$) requires high-energy food sources. Field metabolic rates are significantly higher than terrestrial herbivores like giant pandas.

• Pinnipeds and Seabirds:

  • Walrus: Use ice floes and land to raise young; they are benthic foragers (feeding on the seafloor) and need ice to rest between dives.

  • Arctic Seals: Short weaning periods, sometimes lasting only one to two weeks.

  • Arctic Seabirds: High diversity in nesting (geese on flat lands; puffins and cormorants on cliffs). Primarily surface foragers that dive only a few meters.

Comparison of the Poles

Global Connectivity and Implications

• The "One Ocean" Concept: Represented by the Spilhaus Projection, emphasizing that all oceans are connected.

• Climate Modulation: Both poles act as climate modulators and air conditioners for the planet.

• Human Impact: Human decisions regarding fossil fuels, CFCs, and harvests (fishing) have cross-spatial and cross-temporal effects.

  • Complexity: The same factor (fossil fuel burning) can cause different manifestations, such as dramatic sea ice loss in the Arctic while historically causing slight increases in the Antarctic due to changing wind patterns, though this trend is now shifting toward record lows in the south as well.

• Major Summary Points:

  • Ice (pack and fast) defines polar biology.

  • Polynyas and ice edges drive productivity.

  • Large predators serve as vital indicators for environmental change.

  • Global impacts: "What happens in the Antarctic does not stay in the Antarctic."