THE ozone hole above Antarctica may not be damaging life in the ocean below
after all. If Californian researchers are right, then increased ultraviolet
radiation is having scarcely any effect on the growth of marine plankton, the
base of the ocean鈥檚 food chain.
The team, led by Kevin Arrigo of Stanford University in Palo Alto, has
created computer models of phytoplankton growth over a year in the southern
hemisphere before and after the ozone hole appeared in the 1980s. They included
such factors as the position of the ozone hole, cloud cover, and UV-B strength,
the type of ultraviolet radiation that increases as atmospheric ozone
declines.
To find out what increased UV-B did to phytoplankton, the researchers
compared two models: one based on data from 1992, a year with a yawning ozone
hole and the other with the same parameters except for the ozone levels, which
were taken from 1978, a year of 鈥渘ormal鈥� conditions before the hole
appeared.
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Over the southern hemisphere ecosystem as a whole, they found that primary
phytoplankton production decreased by only about 1 per cent in 1992, which is
significantly lower than other estimates.
Arrigo鈥檚 work does not discount the results of a number of studies showing
that increased UV-B can stunt phytoplankton growth by 10 per cent or more in
localised areas or in the laboratory
(快猫短视频, 8 August 1998, p 24).
The difference is that his study looked at the big picture of UV-B for the
whole ocean.
In previous studies, researchers scaled up measurements of plankton growth
beneath the hole and elsewhere to calculate an overall effect for the whole
Southern Ocean. But although they knew that factors such as cloud cover were
important, they are difficult to include in such calculations.
鈥淥n a cloudy day under a deep hole, there鈥檚 still not nearly as much UV flux
as on a clear day with no hole,鈥� says Arrigo. Another important factor is that,
at any given time, around 80 per cent of the southern hemisphere鈥檚 ozone hole is
over ice. So only a small fraction of phytoplankton in Southern Ocean waters
feels the full brunt of ozone depletion, he says. Both these factors were
incorporated into the new models.
Raymond Smith, at the University of California, Santa Barbara, who did
landmark research on the effect of increased UV-B on phytoplankton, says that
marine plankton have adapted to cope with the hole. 鈥淚t鈥檚 obvious that [the
impact] isn鈥檛 going to be catastrophic,鈥� he says. 鈥淲e鈥檝e had the ozone hole for
a decade and a half and the system is still there.鈥�
Although Arrigo鈥檚 results are good news, he says we shouldn鈥檛 stop worrying
about ozone depletion, because phytoplankton is only one component of the
ecosystem. 鈥淭here may be species shifts going on that no one is aware of,鈥� he
says.
And we shouldn鈥檛 forget that Southern Ocean phytoplankton could be stretched
to their limit in absorbing ultraviolet radiation. 鈥淩ight now, they鈥檙e keeping
up,鈥� he says, 鈥渂ut if the problem gets worse who knows?鈥�