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Climate Change

How will climate change affect the Southern Ocean ecosystems?

Reviewing: Henley, S. F. et al. “Changing biogeochemistry of the Southern Ocean and its ecosystem implications.” Frontiers in Marine Science 7 (2020): 581.

DOI: https://doi.org/10.3389/fmars.2020.00581

 

The Southern Ocean is important to the global ocean

 

Adelie Penguins on an iceberg in the Southern Ocean

If you turn a globe upside down, you will find the Southern Ocean that surrounds Antarctica. While the Southern Ocean only covers about a twentieth of the earth’s surface, many oceanographers deeply care about this part of the ocean.

Although the Southern Ocean is very remote from any land that people live on permanently, it plays a very important role in global ocean circulation (ocean waters moving around globally). If you mark a parcel of water at the Southern Ocean surface or intermediate depth (0-1000m) with a red dye and follow it, you will see it move northward, then pass through oceans around the globe, starting from the Pacific then ending up in the Atlantic before returning back to where it started. And because the Southern Ocean surface waters are rich in nutrients like nitrate, phosphate and iron that are necessary for the growth of marine organisms but are not always available in all parts of the ocean, waters from the Southern Ocean can supply these nutrients to the global ocean and help maintain the marine ecosystem.

Seawater circulating through the global ocean (from Wikimedia Commons). Cold water from the surface and mid-depth Southern Ocean flows northward into the Pacific, flows back out and moves through the Indian Ocean and the Atlantic Ocean before returning to the Southern Ocean.

 

The Southern Ocean is changing!

Recent climate change caused by humans has been affecting the Southern Ocean, as well as other parts of the global ocean. Humans have released lots of carbon dioxide to the atmosphere while burning fossil fuels, and about half of this carbon dioxide gets into the Southern Ocean waters, making the waters more acidic. More carbon dioxide in the atmosphere have also trapped more heat, and the Southern Ocean absorbs the extra heat, so that ice sheets covering the Southern Ocean are melting and the direction and strength of winds over the Southern Ocean have changed. How are all these changes going to affect the Southern Ocean waters, the organisms that live in those waters and our climate? A team of scientists have looked into this question to gain more insight on the potential changes we will see in the Southern Ocean in the near future.

 

The Southern Ocean will take up more carbon dioxide in the future

The bottom of an ice sheet is colored by algae (a type of phytoplankton) living under it. (from Wikimedia Commons)

Carbon dioxide in the atmosphere gets absorbed into the ocean by two major pathways: 1) dissolving in water (just like the fizz in your can of soda) or 2) getting absorbed by phytoplankton. Phytoplankton are basically plants of the sea, and just like land plants, they take up carbon dioxide and use energy from sunlight to produce oxygen. Now, with the climate change in the Southern Ocean, ice sheets will melt and phytoplankton under the ice sheets will be exposed to more sunlight. Melting of ice sheets also release trapped materials from earth that are rich in iron, which is a nutrient that stimulates growth of phytoplankton. These changes will enable phytoplankton to grow faster and take up additional carbon dioxide from the atmosphere.

 

 

Marine organisms living in the Southern Ocean will be affected by climate change

 

Humpback whale feeding on krill (from Wikimedia Commons). Many marine life, including fish, seals, birds, penguins and whales feed on krill.

The Southern Ocean hosts a wide variety of organisms, ranging from tiny microbes like phytoplankton and zooplankton to larger mammals like seabirds and penguins. These organisms are all connected together via the marine food web (as larger organisms feed on smaller organisms). However, climate changes in the Southern Ocean will affect the whole ecosystem. For instance, ocean warming and more sea ice melting may change the types/species of phytoplankton that can thrive best in warmer and fresher seawater. This change can potentially reduce krill populations, and affect larger organisms like whales, seals, fish and penguins that feed on krill. Southern Ocean acidification may also affect the types/species of organisms that can live in more acidic waters, as organisms like diatoms and pteropods have shells that dissolve in acidic water and would be outcompeted by others without shells. These changes in phytoplankton and zooplankton populations will move up through the food web and potentially affect the whole Southern Ocean ecosystem in the coming years.

 

Takeaway: why would we care about these predictions?

The Southern Ocean is vital to supplying nutrients like iron to other parts of the global ocean, and to absorbing large amounts of carbon dioxide from our atmosphere. Learning more about the processes of carbon dioxide getting absorbed into the ocean from the atmosphere or responses of phytoplankton to higher carbon dioxide will help us better understand how climate change is going to affect the Southern Ocean, and eventually the global ocean and its ecosystems.

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