Ozone depletion may leave a hole in phytoplankton growth

New research suggests that the growth of phytoplankton is reduced by 56% when stratospheric ozone drops below 17% or less than 300 Dobson Units (DU).

The research, conducted by scientists from the Australian Government Antarctic Division, the Antarctic Climate and Ecosystems Cooperative Research Centre, and the University of Tasmania, sounds a warning that ozone depletion over Antarctica could have a bigger impact on marine life in the Southern Ocean than previously thought.

Antarctic Division biologist, Dr Andrew Davidson, Honours student Nina Cadman, and their colleagues (Kelvin Michael, Manuel Nunez, Simon Wotherspoon and Ben Raymond), used satellite data to determine concentrations of ozone and phytoplankton chlorophyll over 10 million square kilometres of the marginal ice zone around the East Antarctic coast during November and December, between 1997 and 2000.

Ozone depletion affects living organisms by allowing more damaging solar ultraviolet-B radiation (UVBR) to reach Earth. Since the mid- 1970s, man-made chlorofluorocarbons and halons have caused a profound decline in stratospheric ozone over Antarctica. As a result, ozone concentrations during spring (September-November) commonly fall below 50% (around 180 DU) and may decline below 30% of pre-ozone depletion levels — more than doubling the amount of UVBR reaching the Earth’s surface. Some ozone depletion even persists until February, leading to a 50–100% increase in UVBR around the height of summer.

‘Thus, UVBR is enhanced throughout the period of greatest biological production in Antarctic waters,’ Dr Davidson said.

Comparing satellite measurements of chlorophyll and ozone, the research team found that the rate of chlorophyll accumulation at ozone concentrations less than 300 DU was around half that at higher ozone concentrations. This resulted in 56% less chlorophyll being accumulated in the marginal ice zone over November and December at low ozone concentrations.

‘Interestingly, the 56% inhibition we obtained equated to around a six percent inhibition of each phytoplankton generation over the two months of our study,’ Dr Davidson said.

‘Previous studies suggest Antarctic phytoplankton are inhibited around six percent by ozone depletion in the marginal ice zone but, crucially, have neglected to consider any cumulative effect of UV-induced inhibition of successive generations.’

Phytoplankton is the base of the marine food web, acting either directly or indirectly as a food source for all other organisms. As a result, Dr Davidson said such marked inhibition would profoundly reduce the amount of food available for other organisms in Antarctic waters. It could also affect global climate, as phytoplankton production facilitates the transfer of carbon from the atmosphere to the deep ocean for geological time periods, thereby reducing the accumulation rate of greenhouse gas and global warming.

However, Dr Davidson cautions that there are limitations associated with this preliminary research.

‘Ozone rarely falls below 300 DU around the height of summer and we require more information at this critical time. In addition, changes in ozone concentration coincide with changes in a range of environmental variables. So we cannot exclude the possibility that the ozone acts, partly or wholly, as a proxy for another unmeasured environmental factor that influences chlorophyll accumulation,’ he said.

The team is addressing this, and other questions, through continued research.