Glass sponges take advantage of retreating Antarctic ice shelves

Bremerhaven, 11th July 2013. The breakup and collapse of the Larsen A ice shelf in the western Weddell Sea in 1995 has resulted in fundamental changes to life on the sea bed in less than two decades.  As reported by biologists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in the cover story of the current issue of the scientific journal Current Biology, Antarctic glass sponges have been the prime beneficiaries of the disappearance of the ice shelf. To the surprise of the scientists, the density of these archaic filter-feeders has increased threefold between 2007 and 2011 despite only low plankton food supply and water temperatures of minus 2 degrees Celsius.  The sponges had also grown remarkably quickly and had completely supplanted competitors for food.  The results show that communities at the bottom of the western Weddell Sea react considerably more quickly to climate-related changes than previously thought. 

Glass sponges (Hexactinellida), an archaic group of animals at the basis of the animal kingdom, dominate the shallow seafloor in the Antarctic.  Many biologists believed that glass sponges grew so slowly that two-metre behemoths would have to be around 10,000 years or older.  These assumptions have now been challenged in a new study led by scientists from the Alfred Wegener Institute (AWI) and published in the current issue of the scientific journal Current Biology. 

During a Polarstern expedition to the poorly accessible region of the former Larsen A ice shelf, AWI biologists Laura Fillinger and Claudio Richter, together with colleagues from the University of Gothenburg and the Senkenberg Research Institute and Natural History Museum; succeeded in demonstrating that glass sponges can grow rapidly within a short period of time.  “We were surprised by what we saw on our video screens in 2011 when we lowered our remotely operated vehicle onto the sea bed at a depth of around 140 metres.  In an area, which had revealed large numbers of ascidians and only occasional glass sponges during an earlier expedition with the ice-breaking research vessel ‘Polarstern’ in 2007, four years later we found no ascidians at all.  These pioneer species had completely disappeared, replaced by three times the number of glass sponges, including several juvenile individuals”, reports Laura Fillinger, lead author of the study. 

Until this time scientists had assumed that communities on the Antarctic sea bed only change very slowly because of the very low temperature (minus two degrees Celsius) and patchy supply of food in pack-ice covered waters.  “We now know that glass sponges may undergo boom-and-bust cycles, allowing them to quickly colonize new habitats in a short period of time”, says project leader Prof. Dr. Claudio Richter.

“To the organisms living on the sea bed, the disappearance of the hundred-metre-thick Larsen A ice shelf must have been like the heavens opening up above them”, he adds.  Where cold, darkness and food shortages had previously reigned, sunlight now allows plankton growth in surface waters and, hence, a rain of food comes down to the sea bed.  

 Glass sponges feed on the smallest plankton, which they filter from the water.  The animals grow to a size of up to two metres, and their vase-like bodies provide perfect hiding, spawning and shelter opportunities for fish, invertebrates and many other sea dwellers.  “Like corals, sponges create their own habitats.  To an extent they are like cities on the sea bed. There is something going on wherever they grow, and this attracts other sea dwellers to them”, says Claudio Richter. 

New spaces are being created for such underwater worlds wherever the ice shelves on the Antarctic Peninsula are retreating or breaking up.  However, scientists cannot yet definitively say whether this means that glass sponges will be one of the beneficiaries of climate change.  Laura Fillinger: “There are still too many unknowns to make predictions.  One example is the question of the influence of competitors: currently we are witnessing a fierce competition for space on the sea bed.  Another concerns predators: in our dive in 2011 we hardly saw any of the snails and starfish, which feed on glass sponges.  However, it is possible that these voracious predators will follow suit and wreak havoc.” 

Huge iceberg breaks away from the Pine Island glacier in the Antarctic

Bremerhaven, 9 July 2013. Yesterday (8 July 2013) a huge area of the ice shelf broke away from the Pine Island glacier, the longest and fastest flowing glacier in the Antarctic, and is now floating in the Amundsen Sea in the form of a very large iceberg. Scientists of the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association have been following this natural spectacle via the earth observation satellites TerraSAR-X from the German Space Agency (DLR) and have documented it in many individual images. The data is intended to help solve the physical puzzle of this “calving“.

Scientists from the American space agency NASA discovered the first crack in the glacier tongue on 14 October 2011 when flying over the area. At that time it was some 24 kilometres long and 50 metres wide. ”As a result of these cracks, one giant iceberg broke away from the glacier tongue. It measures 720 square kilometres and is therefore almost as large as the city of Hamburg“, reports Prof. Angelika Humbert, ice researcher at the Alfred Wegener Institute. 

The glaciologist and her team used the high resolution radar images of the DLR earth observation satellite TerraSAR-X to observe the progress of the two cracks and to better understand the physical processes behind the glacier movements. The researchers were thus able to measure the widths of the gaps and calculate the flow speed of the ice. ”Above the large crack, the glacier last flowed at a speed of twelve metres per day“, reports Humbert’s colleague Dr. Dana Floricioiu from DLR. And Nina Wilkens, PhD graduate in Prof. Humbert’s team, adds: “Using the images we have been able to follow how the larger crack on the Pine Island glacier extended initially to a length of 28 kilometres. Shortly before the “birth” of the iceberg, the gap then widened bit by bit so that it measured around 540 metres at its widest point.“

The scientists incorporate these and other TerraSAR-X satellite data in computer simulations using which they are able to model the break and flow mechanisms of the ice masses. “Glaciers are constantly in motion. They have their very own flow dynamics. Their ice is exposed to permanent tensions and the calving of icebergs is still largely unresearched “, explains ice modeller Angelika Humbert. 

The scientist and her team then compare their simulation results with current satellite data such as from TerraSAR-X. If the model agrees with reality, the scientists can conclude, for example, the gliding property of the ground beneath the glacier ice and how the ice flow could behave in the event of further global warming. 

Are ice breaks caused by climate change? Angelika Humbert does not so far see any direct connection: “The creation of cracks in the shelf ice and the development of new icebergs are natural processes“, says the glaciologist. However, the Pine Island glacier, which flows from the Hudson mountains to the Amundsen Sea, was the fastest flowing glacier in the Western Antarctic with a flow speed of around 4 kilometres per year. This speed is less caused by the rising air temperatures, however, and is more attributable to the fact that the wind directions in the Amundsen Sea have altered. ”The wind now brings warm sea water beneath the shelf ice. Over time, this process means that the shelf ice melts from below, primarily at the so-called grounding line, the critical transition to the land ice“, says the scientist. 

For the Western Antarctic ice shelf, an even faster flow of the Pine Island glacier would presumably have serious consequences. “The Western Antarctic land ice is on land which is deeper than sea level. Its “bed” tends towards the land. The danger therefore exists that these large ice masses will become unstable and will start to slide“, says Angelika Humbert. If the entire West Antarctic ice shield were to flow into the Ocean, this would lead to a global rise in sea level of around 3.3 metres. 

Info box: Shelf ice 

The shelf ice, which is 200 to 1200 metres, thick is created by glaciers sliding into the sea. It is therefore an extension of the Antarctic land ice which thins at the edges and floats on the sea. The ice shelf itself rests on the Antarctic continent, reaching a thickness of up to four kilometres and is largely frozen to the rock bottom. A special feature of the Western Antarctic is that large areas of land are below sea level. (AWI press release, july 9,2013)