Antarctic sealife faces iceberg onslaught
26 September 2011, by Tamera Jones
Scientists have provided the first proof that a warming climate around the West Antarctic Peninsula is damaging the region's marine life.
They found that shrinking winter sea ice, caused by warming, has led to a large and rapid increase in the number of icebergs in the area over the last 25 years.
More icebergs means more battering of the shallow sea floor, which has killed off countless marine creatures called bryozoans.
Bryozoans are bottom-dwelling animals. Some are just millimetres long and look like corals, while others reach up to a metre tall and wide and resemble seaweed. They may not be a familiar name, but, 'in Antarctica, they're one of the most common and abundant species that live in the shallows,' says Dr David Barnes from the British Antarctic Survey (BAS), who led the study.
Indeed, bryozoans are responsible for a large proportion of the biodiversity on the continental shelf around the West Antarctic Peninsula, and Barnes says it's highly likely that what's happening them is also happening to other life on the region's seabeds.
'Typically it's very difficult to demonstrate a clear link between climate change and biology. You have to have a long, detailed time-series, which is what we had.'
David Barnes, British Antarctic Survey
'When icebergs smash into the seabed, it's violent and very little survives that action. What we're seeing is that these massive impacts on seabed biology are happening more and more frequently,' he says. 'It's worrying, because these animals are struggling to reach maturity before they are killed off.'
Barnes and his co-author Terri Souster, who's also from BAS, suspected that icebergs grinding against the seabed would harm marine life, but they found proving this far from easy.
They set up grids of 25 concrete markers at three different depths – 5, 10 and 25 metres – on the seabed around Rothera research station. They then went back every year to inspect them to see how the seabed coped with so-called iceberg scouring.
'Surveying just one grid could take a couple of days. You've got low visibility, leopard seals, not to mention freezing water, so it's a lot of work in a challenging environment,' says Barnes.
He and Souster also analysed long-term records of the duration of winter sea ice, which scientists call fast ice. This type of ice holds icebergs together, stopping them crashing around. The researchers also compared the numbers of bryozoan colonies in the different regions of their grids.
'We choose bryozoans, because they're abundant, common and don't move, so are much easier to study than rare or mobile species,' explains Barnes.
They found that since 1986, fast ice has lasted for more than 160 days in good years. But in bad years, it's typically only lasted much less than 100 days. What's more, bad years are becoming more frequent, and the fast ice isn't lasting as long.
This means more icebergs smashing into the seabed, with a devastating effect on bryozoans.
In 1997, around 1 in 1000 colonies reached five years old – beyond the age they reach sexual maturity. But now, less than 1 in 1000 colonies make it their third birthday. 'Now we've got to the point where the population is made up of animals nearly all of which are too young to reproduce,' says Barnes.
'Typically it's very difficult to demonstrate a clear link between climate change and biology. You have to have a long, detailed time-series, which is what we had,' he adds.
He stresses that satellites wouldn't have picked this up, because they have trouble distinguishing between different types of sea ice like icebergs and fast ice. Not just that, but data from satellites is typically on the wrong scale to easily detect biological changes.
'Satellites might have a 25 square kilometre footprint, but you need much more detailed resolution to notice physical changes or changes that might be relevant to seabed biology. It's much easier to do this with ground-based measurements.'
Now he and Souster have uncovered a clear link between climate change and biological change in the West Antarctic Peninsula, Barnes says the next thing is to assess, 'other effects of this massive physical impact on biology. Has this changed the community on the seafloor to one more dominated by pioneers? Is there any change in the amount of carbon this part of ocean life can bury now?'
The findings are published in Nature Climate Change.