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Will less ice be good or bad for Arctic ecosystems? Scientists discuss.

• 18 Feb 2013, 10:15
• Roz Pidcock

A new study suggests thinning Arctic sea ice could be boosting growth of algae underneath it by allowing more sunlight through - which sounds like good news for Arctic ecosystems. But other research suggests it's more complicated. We look at what scientists are saying about the new findings.

The area covered by sea ice in the Arctic is declining by about four per cent per decade. The latest satellite data suggest it's also getting significantly thinner.

A big question is what effect this will have on Arctic fish and mammal populations. At the base of the marine food chain, changes to algae will affect the whole Arctic ecosystem.

Blooming algae

Scientists can't know for sure the ecological consequences of thinning ice due to greenhouse gas warming. But looking at changes in ice cover over shorter timescales can offer some clues.

The new study, just  published in the journal Science, measured biological activity in the Central Arctic during 2012. Sea ice cover decreases in summer as part of a natural cycle. But in 2012, the summer minimum reached a record low.

Using camera-guided and remotely operated sampling devices, the scientists found huge quantities of algae underneath the melting ice. They attributed this to more sunlight being able to reach the algae below. As Professor Kevin Arrigo from Stanford University in California told Carbon Brief, this effect is not unheard of:

"The results are certainly consistent with the massive under-ice bloom we discovered in the Chukchi Sea in 2011 as part of ICESCAPE - which was also the result of thinning sea ice and increasingly widespread melt ponds."

Seafloor bonanza

More surprisingly for the scientists, the seabed directly below the thinning ice was also covered in patches of algae. As microbiologist and lead author Professor Antje Boetius put it:

"The seabed at a depth of more than 400 metres was littered with clumps of ice algae".

It's thought this is because as the ice melts, algae are released and fall to the seabed. There, algae become food for sea floor-dwelling animals such as sea cucumbers and brittle stars.

What's not eaten is metabolised in the seabed by bacteria, a process which uses up oxygen. Data from tiny sensors showed the seabed under thinning ice was depleted of oxygen - an indication that a lot of biological activity had taken place.

Long term benefits?

The fact that over a seasonal cycle, elements of the Arctic ecosystem appeared to benefit from thinning ice led the scientists to suggest the longer term decline in ice cover due to climate change could also be a positive thing.

But other scientists are wary of drawing this conclusion. Partly because there has been uncertainty over how much algae is produced during the bloom at the surface and how much makes it down to the seafloor. Past estimates of both haven't always matched up. Professor Paul Wassman from the University of Tromso told us:

"Indeed, [the new research] comes as a surprise … I think this is a challenge for the entire community of biological oceanographers in the Arctic Ocean."

Wassman suggests a lot more research is needed to support the new study, but he welcomes the findings, saying:

"There is no reason to doubt the results of this paper … I more than willing to revise my ideas and [I'm] actually thankful to this publication as it questions our generic understanding."

Complicating factors

There's also more to consider than just the amount of sunlight  getting through - such as the availability of nutrients. As Dr Ekaterina Popova from the National Oceanography Centre told us:

"[W]e need to be careful and not try to jump to a conclusion that Arctic Ocean is turning into an ecological paradise as ice becomes thinner and retreats. Productivity of this ocean basin is severely limited by nutrients."

The Arctic Ocean exists in layers, which do not easily mix. Warming could strengthen those layers, limiting mixing even further. This could mean once algae use up all the nutrients at the surface, not enough makes it up to the surface to sustain the initial growth burst.

As the oceans absorb carbon dioxide from the atmosphere they become more acidic. Scientists are concerned about even small shifts in ocean acidity, as it can affect how some marine algae grow their shells, which can be crucial to their survival.

Research like this helps shed light on some of the uncertainties over the effects of climate change on Arctic ecosystems - but the debate continues. As with most aspects of climate science, the reality is likely to be far more complicated than any single study can capture.