The Southern Ocean: The Hub of the World’s Ocean

At the bottom of the world, the Southern Ocean is the “hub” of the global ocean—connecting the Pacific, Atlantic, and Indian Oceans (Figure 1) [1]. It flows clockwise around the Antarctic continent and is dominated by the Antarctic Circumpolar Current, the coldest, biggest, and one of the fastest currents in the ocean. The colder surface waters of the Southern Ocean are separated from the warmer waters of the neighboring south Atlantic, south Pacific, and Indian Oceans by frontal systems that act like underwater boundaries. Frontal systems, which can be up to 100s of km wide, are boundaries between ocean waters that have different temperatures and saltiness. Water currents, some flowing very deep below the surface, can flow across these fronts and into other oceans. These water currents are full of salt, nutrients, and oxygen, and they are very important for a healthy global ocean and Earth System.

The Southern Ocean Ecosystem Is Globally Connected

The Southern Ocean ecosystem includes vast numbers of unique and remarkable organisms—from microscopic species of free-floating algae (called phytoplankton) and tiny animals (like zooplankton), bigger plankton (like jellyfish), to fish and squid, all the way up to seabirds and marine mammals, as well as species that live on the sea floor. Up until recently, the Southern Ocean ecosystem was thought to be largely separated from the rest of the world. However, scientists have made exciting new discoveries about how well the Southern Ocean ecosystem is connected with the global ocean and how important it is in the Earth System [1].

Many species that live in the Southern Ocean affect the nutrient concentrations in the currents that flow from the Southern Ocean to the rest of the world. Similarly, species (and physical processes) in other parts of the global ocean influence the nutrient concentrations of waters flowing back into the Southern Ocean. Smaller organisms (such as phytoplankton and zooplankton) are swept in and out of the Southern Ocean by currents, and larger species (such as squid, fish, and whales) can swim in and out of the Southern Ocean. Many of the species that spend some of their time on land (like seals, penguins, and other seabirds), migrate in and out of the Southern Ocean to find food or complete their life cycles. Together, all these currents, movements, and migrations mean that the Southern Ocean ecosystem is strongly connected with all other ocean ecosystems.

Movement of Small Organisms In and Out of the Southern Ocean

Phytoplankton and zooplankton drift in the currents of the Southern Ocean. Phytoplankton get their energy from sunlight and nutrients from seawater and provide food for zooplankton and other organisms. Zooplankton in turn provide food for many other larger species including squid, fish, marine mammals, and seabirds. One very important zooplankton species is Antarctic krill, Euphausia superba [2]. These large (a few millimeters to several centimeters in length) shrimp-like crustaceans are found throughout much of the Southern Ocean and have a biomass which is estimated to exceed that of all humans on the planet! Krill is a key species in Southern Ocean food webs, and it is the target of a commercial fishery. Phytoplankton and zooplankton are not very strong swimmers and they are transported by ocean currents into or out of the Southern Ocean across the frontal systems. These organisms may also get trapped in very large whirlpools (eddies) that can drift across fronts and deposit them in various parts of the ocean. Antarctic krill, other larger zooplankton species, and small fish can swim and move short distances and up or down the water column, where they encounter currents that may move them in or out of the Southern Ocean. Other larger marine species like fish and squid are stronger swimmers, which allows them to move into or out of the Southern Ocean as they grow and develop.

Movement of Large Organisms In and Out of the Southern Ocean

Large animals, such as various species of seabirds and seals, that live year-round in the Southern Ocean must be able to cope with long winter periods when sea ice covers much of the ocean and there is low light, cold temperatures, and little food. During this period, many animals can reduce their activity or use up energy they stored as fat during the summer months of plentiful food. Every year, vast numbers of whales, seals, and seabirds deal with the long, cold winters by swimming or flying north to warmer waters of the Atlantic, Pacific, or Indian Oceans. Albatrosses are one type of large, migrating seabird. There are 22 species in the albatross family, of which 18 breed and feed in the southern hemisphere oceans. Many albatross populations are declining due to hooking or entanglement in longline fishing gear, or collision with trawler cables, invasive species like mice in their breeding colonies, diseases, or climate change. Petrels are another type of seabird. Some petrel species breed in the Antarctic region while others feed there but breed further north. Although most petrels are small, their population sizes can run into the tens of millions, making them major consumers of prey, including Antarctic krill. Some seabird species are examples of truly amazing migrations into and out of the Southern Ocean. The small Arctic tern breeds in the Arctic during the summer. To escape the Arctic winter, it flies across the equator and all the way down to the Southern Ocean, feeding around Antarctica during the southern summer. Two other bird species, sooty shearwaters and south polar skuas, do the complete opposite: they breed in the Southern Ocean and then fly to the northern hemisphere, experiencing an eternal summer!

Scientists have recently estimated just how many seabirds and whales migrate in and out of the Southern Ocean each year, by fitting animals with tags that can be tracked by satellite or that record light and can be used estimate position by a process called geolocation, combined with surveys done from the ground or the air (Figure 2) [1]. An amazing 68.5 million seabirds leave the Southern Ocean as the Antarctic winter approaches and migrate back in the spring. Around 600,000 whales also migrate in and out of the Southern Ocean each year. These numbers may change in the future with the anticipated recovery of Southern Ocean whale populations. Whale numbers in the Southern Ocean were very low throughout the second half of the 20^th century because they were hunted for many years before this was banned in the 1980s. Since the late 1990s and 2000s there have been truly spectacular increases in the numbers of some whale species, such as the humpback whale [3].

These new and exciting studies of animal movements reveal the enormity of the annual migration of Southern Ocean species. These migrations mean that the seabirds and whales are part of food webs in ecosystems outside of the Southern Ocean, where they may feed, breed, or die. This means that migrating animals transfer vast amounts of energy and nutrients into and out of other ecosystems.

Southern Ocean Ecosystem Change Has Global Consequences

There are major concerns about how future climate change and other human pressures, such as fishing, tourism, and pollution, will impact the Southern Ocean ecosystem [1, 4, 5]. Scientists know that climate-related changes are already affecting many Southern Ocean species—changing their numbers, where they live, and how well they function and reproduce. Changes have already been observed in many species of phytoplankton, zooplankton, fish, large marine mammals, and seabirds, for example. Along with climate-related changes, the increasing whale populations and commercial fishing of fish and Antarctic krill will also affect the balance of this ecosystem.

The new perspective about how connected the Southern Ocean ecosystem is to the rest of the world’s ecosystems means that changes in the Southern Ocean will have global consequences. We now understand that the Southern Ocean ecosystem is linked not just to other ocean ecosystems, but to the Earth’s climate system as well. Southern Ocean ecosystems can help to absorb carbon dioxide from the atmosphere, locking it up for hundreds or thousands of years in the deep ocean and helping limit atmospheric warming. Nutrients exported from the Southern Ocean also help fuel productivity in other ecosystems in the global ocean. The Southern Ocean also supports economies through fishing and tourism (Figure 3). The Southern Ocean ecosystem is, therefore, important to the planet and everyone on it [1].

The good news is that there is a lot that people can do to protect the Southern Ocean ecosystem. Scientists and politicians can and must work together to ensure the future health of the Southern Ocean ecosystem and the continued recovery of whales. One exciting and challenging next step is for scientists to develop realistic predictions or “best estimates” of how climate change and human activities will affect the Southern Ocean ecosystem. These predictions are important for making decisions in time to keep the Southern Ocean ecosystem—and the Earth as a whole—healthy. Educating as many people as possible about the global importance of the Southern Ocean ecosystem will help scientists to make sure our global ocean can continue to thrive and provide people with benefits like food, fascinating species, and climate regulation. This is an exciting new time for everyone who is trying to protect the Southern Ocean.

Glossary

Global Ocean: ↑ The oceans of the world are all connected together, with waters flowing from one ocean region to another. Together they are known as the global ocean.

Frontal System: ↑ A boundary between ocean waters that have different temperatures and saltiness. Frontal systems can be hundreds of km wide.

Earth System: ↑ All the interacting physical, chemical, and biological processes on the planet. It includes all the land, oceans, atmosphere, and poles, as well as humans and other animals.

Phytoplankton: ↑ Microscopic marine algae that float and drift in the ocean. They get their energy from sunlight and nutrients from seawater. They provide food for zooplankton and other organisms.

Zooplankton: ↑ Small animals in the ocean. Some are tiny and microscopic, others are millimeters or even several centimeters in size. Most are not strong swimmers and drift with the ocean currents.

Food Webs: ↑ A food web is the network of who-eats-whom interactions in an area of the ocean. Food webs vary between different areas of the ocean.

Fishery: ↑ A local fishing operation or where fishing is targeted at a particular species of fish or shellfish.

Water Column: ↑ The ocean waters at different depths in a particular place, from the surface to the seabed.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We acknowledge our colleagues who contributed to the Murphy et al. [1] paper. This paper has developed as part of activities of the Marine Ecosystems Assessment for the Southern Ocean (MEASO), a core contribution to the Integrating Climate and Ecosystem Dynamics (ICED) in the Southern Ocean programme. ICED is a regional programme of the global Integrated Marine Biosphere Research (IMBeR, which is sponsored by the Scientific Committee on Oceanic Research, SCOR, and Future Earth) project and a co-sponsored programme of the Scientific Committee on Antarctic Research (SCAR). EM, NJ, RP, and JJ were supported by the Ecosystems ALI-Science project of the British Antarctic Survey. EH was supported by the United States National Science Foundation Grant OPP-1643652. We thank the reviewers for their comments on the manuscript.