The ozone hole over Antarctica could be having a bigger impact on life in the region than anyone realised. In clear summer skies, lower ozone levels allow significantly more UVB light to reach the ocean and damage DNA.
Now an analysis of east Antarctic waters suggests that higher levels of UVB light can significantly reduce phytoplankton blooms. “If you have a substantial reduction in the amount of plant material, that’s going to have all sorts of knock-on effects for the rest of the food web,” says Andrew Davidson of the Australian Antarctic Division in Kingston, Tasmania.
Davidson’s team studied the marginal ice zone. These regions are estimated to produce between a quarter and two-thirds of all phytoplankton in the Southern Ocean. The team used satellite data to study levels of chlorophyll, an indicator of overall phytoplankton levels, and ozone concentration in five regions during November and December, from 1997 to 2000. They only considered data for periods when there were at least six cloudless days out of 10.
As you would expect during a summer phytoplankton bloom total chlorophyll increased. However, when ozone levels fell below 300 Dobson units, chlorophyll accumulation was just 45.7 per cent of that at ozone levels above 300 Dobson units. Ozone dipped below that threshold on 27 per cent of the days in the team’s study period.
However, Kevin Arrigo at Stanford University, California, says that on average, chlorophyll concentrations in Antarctic waters under the ozone hole have not changed since the late 1970s, when stratospheric ozone was much higher than today. “This suggests to me that the ozone hole is having very little impact on overall chlorophyll concentrations in the Antarctic.”
“If you have a substantial reduction in plant material, that’s going to have a knock-on effect on the food web”
It has been difficult to pinpoint the impact of UVB on phytoplankton, Davidson says, because the amount of plant material in Antarctic waters varies by up to 25 per cent from year to year. “It’s a noisy signal – and detecting the effect of UVB against that noisy background is extremely difficult,” he says.
Nevertheless, Davidson says this is the first time anyone has tracked changes in actual chlorophyll levels in the ocean and linked them to ozone levels.
Phytoplankton are a food source for krill, which are eaten by seals, whales, fish and more than 50 species of birds. Antarctic krill populations have fallen by 80 per cent since the 1970s – a drop attributed to warmer temperatures. Higher UV levels may also be playing a role in changing Antarctic communities, says Davidson, who presented his findings at a meeting of the Scientific Committee for Antarctic Research in Hobart, Tasmania last week. Annual depletions of ozone in the stratosphere are predicted to last for at least another 50 years.
, 22 July 2006, issue 2561