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Oceanography

Underwater warehouse

Fauna from the bottom of the sea in Antarctica have resources to stay alive even in winter

ARTHUR GÜTH/USPUnder the ice: life continues for the sea cucumbersARTHUR GÜTH/USP

In the Antarctic winter the sun only peeps over the horizon for three hours a day, the temperature remains at around – 30º Celsius and the sea freezes and becomes part of the continent that is home to the planet’s South Pole. Organisms that live under the water have no way of reaching the surface and the ocean depths become even darker than usual. For the last ten years oceanographer Paulo Sumida, from the University of São Paulo (USP), has been investigating how the fauna at the bottom of the sea in the West Antarctic Peninsula shelf, the point on the continent that is closest to South America, survive until summer returns.

Academics the world over who study this region had already investigated the ecology of organisms that live on the continent and in the waters that surround it. “The plankton that live in the column of water have many strategies for dealing with the lack of food”, says Sumida. One of them is to assume the form of sleeping cysts waiting for more convenient times for finding food and reproducing. Other organisms in suspension in the water, like krill (crustaceans that look like shrimp), feed on the algae they find on the undersurface of the ice. But what about the organisms from the bottom, known as benthos, which depend on food falling from the surface?

To see what happens at depths of between 500 and 640 meters the group explored the sea-bottom at Bellingshausen, on five expeditions between 1999 and 2001. They did this with the help of video cameras pulled by a boat and others that remain on the sea-bottom for months on end and take photographs every 12 hours. The results of the part analyzed by Sumida and Angelo Bernardino, at the time his student in the Oceanographic Institute at USP, and published in November 2008 in the Deep-Sea Research II journal, show that purple sea urchins, delicate sea lilies, relatives of star fish that swim by waving their fine tentacles, orange sea cucumbers and polychaetes, animals similar to prickly worms that are as much as 25 centimeters long, have a store of food at their disposal during the winter.

The discovery is the result of the Food for Benthos on the Antarctic Continental Shelf (Foodbancs) project, coordinated by Craig Smith, from the University of Hawaii, and David DeMaster, from the State University of North Carolina; the oceanographer from USP forms part of this group. The team analyzed 900 video images taken during the expeditions that together cover an area of 1834 sq m. and that allowed the researchers to identify the species that live in different zones, measure how abundant and how big the animals are, see how they produce feces and record the variability of the fauna from one year to another and between different areas and at different periods of the year. The images show fauna that vary a lot according to the year and place: the animals detected by the video images quadrupled between November 1999 and November 2000 and doubled between February 2000 and March 2001. So far this increase in population seems to be the result of the migration of the organisms, rather than an increase in the flow of carbon particles to the bottom. “Many young animals enter the population, which we call recruitment”, explains the researcher.

Paulo Sumida/USPWinter food store
After the surprise of discovering that these animals are active the whole year, the international group is trying to measure how much food is available in the different marine zones in each season of the year, in addition to detailing the life cycle of the organisms that live in them. Sumida explains that in summer – the period of greatest productivity – the marine organisms reproduce so much that there is too much food. So the animals eat more and faster, letting partially digested detritus and even live algae fall to the bottom. These are the algae and the organic material that form the winter food store of the creatures on the bottom. “When the winter cold arrives there is little microorganism activity and decomposition is very slow”, adds Sumida. The food is conserved for months on end in a natural refrigerator.

Understanding this dynamic goes beyond curiosity about the bottom of the sea, a world that is not so distant, but is largely unknown. It also has an important effect when it comes to absorbing and releasing carbon dioxide, one of the main players in global warming: when the algae sink they take with them the carbon dioxide fixed by photosynthesis, generating a shortage on the water’s surface, which therefore absorbs more carbon from the atmosphere. As the water cools down the Antarctic ocean carries a greater quantity of the gas, which ends up trapped when the surface is once more covered by ice. But it does not always stay there: carbon dioxide dissolved in water can be found in deep currents. “These currents roam the oceans and can take as long as 500 years to release the gas”, says the oceanographer.

In the second phase of the project, which is currently on-going, Sumida intends to understand and forecast the effects of the changes he has seen in the landscape and the climate of the frozen continent. “The Antarctic peninsula is the place that is warming fastest in the world”, says the oceanographer, who believes that warming may be responsible for reducing marine productivity in the region.

The last expedition in which one of Sumida’s students is taking part happens this month. When they analyze the new data the researchers hope to understand more about what is happening with the carbon cycle and the ecological dynamic of the region. “The layer of ice is lasting less and less and extends less far”, says Sumida, for whom losing this surface is the equivalent of chopping down a forest: there is less habitat for animals like krill, and this creates important problems in the food chain, which includes  bigger animals, like seals and whales.

The project
The benthic megafauna response to the seasonal depositing of phytodetritus on the western shelf of the Antarctic peninsula (nº 03/02292-7); Modality Regular Research Awards; Coordinator Paulo Yukio Gomes Sumida – IO-USP; Investment R$ 19,500.25

Scientific article
SUMIDA, P.Y.G. Temporal changes in benthic megafaunal abundance and composition across the West Antarctic Peninsula shelf: results from video surveys. Deep-Sea Research II. v. 55, n. 22-23, p. 2.465-2.477. Nov. 2008.

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