|Map of Antarctica|
Most people will probably never travel across the Southern Ocean either by ship or plane to the massive southern continent of Antarctica that anchors the South Pole. The Antarctica continent, drifted with tectonic plates into its current position between 30 to 60 million years ago (Ivany, et al., 2008). Once the continent was in place the cold Antarctic Circumpolar Current (ACC) encircled it, and the land cooled and ice accumulated eventually covering most of the land to a depth of several thousand feet. As the continent cooled, so did the surrounding Southern Ocean, to a temperature several degrees colder than any other. The chill waters of the ACC separate the Southern Ocean from the slightly warmer waters to the north. Having formed at least 30 million years ago, the ACC formed a significant barrier to colonization by species from the north. Biological isolation due to the ACC and colder water temperatures of the seas around Antarctica provided a chance for a highly unusual fauna to develop, particularly in the most southerly waters of the Ross Sea, one of Sylvia Earle’s “Hope Spots.”
In one talk at the IMCC meetings, Richard Aronson, of the Florida Institute of Technology, described some of the work he and colleagues have undertaken to evaluate changes in the waters around Antarctica as a result of human activities and climate change, including the effects of warming and ocean acidification. He reviewed three changes in relation to populations of crabs, sharks, and introduced species.
While quite cold, Southern Ocean waters are rich in nutrients, and the long daylight periods of austral summer result in an explosion of productivity by phytoplankton, the floating plants that serve as food for the shrimp-like krill that form the base of a food chain supporting seabirds, penguins, seals and the great whales that used to thrive in these waters. Although the season of daylight to permit phytoplankton productivity is very short, or perhaps because of this, much of the very high productivity of the water column not consumed by krill sinks to the seafloor to support a high biomass of unique fish and invertebrates that evolved for tens of millions of years in biological isolation due to the cold
|Image of a Sea Spider|
Antarctic Circumpolar Current. The extreme cold preserves much of the sinking organic matter to serve as food for higher organisms such as sponges and gorgonians instead of simply being metabolized by bacteria. Due to the evolutionary isolation, there is a unique assemblage of seafloor species that evolved without being exposed to fish with crushing jaws such as sharks and rays, or similar crushing-clawed crustaceans, the crabs and lobsters, essentially none of which are found on the Antarctic continental shelf. As a result creatures like sea spiders, that are tiny elsewhere in the world, reach dinner plate size in Antarctica. A tendency to gigantism is also shared by other species from the bottom of the sea in the Southern Ocean waters, adding to the uniqueness of the fauna. Likewise, ancient shellfish called brachiopods, absent or uncommon in most of the world’s oceans, are abundant in Southern Ocean shelf sediments.
The isolation of the seafloor on the Antarctic continental shelf may be coming to an end as a result of
climate change however, at least around the Antarctic Peninsula. Aronson has been working with colleague Sven Thatje, who in 2007 noticed a species of king crab, similar to those found off Alaska, in deeper waters off the continental shelf of Antarctica. The water there is warmer than on the shelf by
|Sven Thatje with sub-Antarctic king crab species|
just a bit, and by looking at prior distribution data he was able to show a temperature limit to crab distribution (Hall and Thatje, 2010).
Their studies confirmed that crab larvae don’t survive at temperatures below 0.5 degrees C, and so are restricted to waters at depths from 500-1600m in the Southern Ocean. Thatje notes that since the 1950s the average ocean temperature has increased by 1 degree Centigrade (1.8 degrees Fahrenheit). Aronson’s colleague has pointed out that while other invertebrates in the Southern Ocean share with Antarctic fish an anti-freeze in their blood, “Crabs have a problem in cold water. They cannot flush magnesium out of their blood. So when they are already moving slowly because of the cold, the magnesium acts as a narcotic causing them to pass out and die.” However, if the continental shelf waters warm, “The crabs are on the doorstep. They are sitting in deep water only a couple hundred bathymetric metres away from the slightly cooler shallow waters in the Antarctic shelf environment.” In some areas around Antarctica, the shelf break is not a sharp drop off like most places, and so even a slight warming could mean an invasion of these crabs, and significant changes to the fauna of the continental shelf.
During a joint Swedish-American cruise from the Antarctic Peninsula to the Ross Sea from November 2010 to February 2011, Aronson and his colleagues used an autonomous vehicle to make seafloor video transects to determine whether warming was facilitating movement of crabs onto the continental shelf, where they have so long been absent. The expedition data showed masses of crabs right at the shelf break in the Bellingshausen Sea west of the Antarctic Peninsula, leading Thatje to conclude: “The pace of changes that we are observing here in the Antarctic, which is the remote continent on this planet, is quite frightening.” Aronson has pointed out that “If you look at the warming trends on the [Antarctic] peninsula, you would expect that the crabs would come back in 40 or 50 years. But boom, they’re already here.”
He continued to describe research that has shown that sharks too may soon find a place in warming Southern Ocean waters. Like crabs, the extreme cold has kept both sharks and rays from the Antarctic continental shelf (Aronson, et al., 2007). Cheryl Wilga, who has conducted these studies with Brad Seibel, found that “water only needs to remain above freezing year round for it to become habitable to some sharks.” If this warming takes place, not only will there be probable effects on the seafloor invertebrates, but also on the resident fish. She has pointed out that: “Ice fishes – the only bony fish that now lives in Antarctic waters, because it has antifreeze in its system – will face a new threat as well [from sharks if they move in].”
Humans effects other than warming are also a threat to the Southern Ocean ecosystem. I asked Aronson whether there had been any documentation of invasive species from ship traffic. The answer was yes: an arctic and sub-arctic crab, Hyas araneus, has recently been taken in trawls from Antarctica from waters as shallow as about 850m. It is believed to have come from northern European fishing fleet ships. Like the king crabs, its expansion onto the continental shelf is temperature limited, at least for the time being.
In addition to the specific effects of warming on the Southern Ocean, Aronson notes that he and others are also concerned about ocean acidification effects, which they have discussed in a recent paper (Aronson, et al., 2011). His co-author, James McClintock has pointed out that “The Southern Ocean is the canary in the coal mine with respect to ocean acidification. This vulnerability is caused by a combination of ocean mixing patterns and low temperature enhancing the solubility of carbon dioxide.” Aronson concludes that “Simultaneous action at local, regional and global scales is needed if we are to halt the damage being done to the marine ecosystems of the Southern Ocean.”
While such action is discussed at an international level, the idea of a pelagic protected area for the Ross Sea “Hope Spot” advocated by Sylvia Earle is moving forward. For the past forty years penguin researcher David Ainley has studied changes in Antarctica, and believes that a tipping point of warming is already occurring along the Antarctic Peninsula. He is spearheading an effort to coordinate the nations that jointly manage Antarctica to protect the Ross Sea as a marine reserve, and has established a working group to protect what he has called “The Last Ocean” (http://www.lastocean.co.nz/). Whereas he expects the Ross Sea to stay cold, it can serve as a refugia for Antarctic species of all kinds, from the invertebrates on the seafloor up to and including the penguins and whales. Aronson was clear that “This is the last pristine marine ecosystem on Earth and it could get destroyed.” The fact that people are now realizing how fast changes in the Antarctic Peninsula are taking place may help promote the preservation of the Ross Sea. Stay tuned.
Written by Dr. Philip McGillivary
Aronson, R.B., S. Thatje, A. Clarke, L.S. Peck, D.B. Blake, C.D. Wilga, and B.A. Seibel. 2007. Climate Change and Invasibility of the Antarctic Benthos. Annl. Rev. Ecology, Evolution, and Systematics 38:129-154. DOI: 10.1146/annurev.ecolsys.38.091206.095525. Online at: http://www.annualreviews.org/
Hall, S. and S. Thatje. 2010. Temperature-driven biogeography of the deep-sea family Lithodidae (Crustacea: Decapoda: Anomura) in the Southern Ocean. Polar Biology (published online, October). DOI: 10.1007/s00300-010-0890-0
Ivany, L.C., K.C. Lohmann, F. Hasiuk, D.B. Blake, A. Glass, R.B. Aronson and R.M. Moody. 2008. Eocene climate record of a high southern latitude continental shelf: Seymour Island, Antarctica. Geol. Soc. Am. Bull. 120: 659-678. Online at: http://gsabulletin.gsapubs.