National Geographic

Icebergs are hotspots for life

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Blogging on Peer-Reviewed ResearchSay the word iceberg, and most people are likely to free-associate it with ‘Titanic’. Thanks to James Cameron (and, well, history too), the iceberg now has a reputation as an cold murderous force of nature, sinking both ships and Leonardo DiCaprio. But a new study shows that icebergs are not harbingers of death but hotspots of life.

Icebergs are hotspots for Antarctic life.In the late 1980s, about 200,000 icebergs roamed across the Southern Ocean. They range in size from puny ‘growlers’, less than a metre long, to massive blocks of ice, larger than some small countries.

They may be inert frozen lumps, but icebergs are secretly in the business of nutrient-trafficking. As the ice around Antarctica melts in the face of global warming, some parts break free from the parent continent and strike out on their own. As they melt, they release stored minerals into the water around them, and these turn them into mobile homes for a variety of life.

Kenneth L. Smith Jr, from the Monterey Bay Aquarium Research Institute, and other scientists from San Diego discovered the true extent of these icy ecosystems by studying two icebergs floating in the Antarctic Weddell Sea. Even the smaller of the two, W-86, has a surface area larger than 17 football pitches. The larger one, A-52 was over a thousand times bigger, with a surface area of 300 km2 and extending 230 metres into the freezing waters.

Smith and crew identified the duo through satellite imaging, and tracked them down by boat. Their ship spiralled around the blocks of ice collecting water samples as it went, from a dangerously close distance of a few hundred feet to a safer five miles away.

Diatoms cover the underside of icebergs and form a ring of life around them.The skies above the two icebergs were patrolled by seabirds, including Cape petrels and Antarctic fulmars. Below the water, Smith explored the icebergs’ undersides with a remote-operated vehicle and found them teeming with life.

Below W-86, he saw a lattice-like surface, and the ridges of these were home to diatoms (right). These single-celled algae are part of the phytoplankton, microscopic creatures that make their energy from the sun and form the basis of the ocean’s food web. In between these diatom-covered ridges, baby icefish and segmented worms swam among the lattice’s nooks and crannies.

A-52 was even more varied, with large caves extending deep into the iceberg’s core. The team found diatoms here too, along with Antarctic krill (below), small shimp-like animals with a taste for diatoms. Among these were various invertebrates – comb jellies, colonial jellyfish-like animals called siphonophores, and predatory torpedo-shaped worms called chaetognaths.

Further out, the area immediately around the iceberg was void. But just beyond that, the ice was encircled by another halo of phytoplankton. These creatures, along with the diatoms on the ice itself, were thriving on the nutrients released by the melting ice, such as iron.

When Smith exposed diatoms to tiny mineral-rich particles filtered from his collected water samples, they grew slowly and steadily, while other diatoms cultured in normal water did not.

Diatoms fed by meltwater nutrients from icebergs, act as food for krill.These drifting islands of ice were dragging entire communities along with them. As they drift and melt, they release small amounts of important nutrients. That triggers the growth of creatures at the bottom of the food chain and provides the foundations for larger animals like krill and seabirds. A-52 alone enriched a massive ring of water about the size of the Isle of Man.

To estimate the effect of other icebergs, Smith used satellites to count the number of bergs in a sample area. Within this space, the satellites spotted almost a thousand individual icebergs that, together, covered less than 0.5% of the ocean’s surface. But even this small amount was enough to enrich over 39% of the Southern Ocean!

By providing support for phytoplankton, the icebergs were also inadvertently helping to mitigate the effects of climate change. Just like land plants, phytoplankton make their own energy through photosynthesis. And just like land plants, they absorb carbon dioxide to do so. By eating the phytoplankton and excreting the remains, krill cause carbon to fall down into the ocean depths in a rain of droppings.

So even as their parent continent melts and releases carbon into the atmosphere, icebergs serve to draw planet-warming carbon away from the air and transfer it to the deepest sea. Smith believes that climate modellers need to take this into account to better predict the effects of melting Antarctic ice.

The disappearing ice can reveal underlying rock which absorbs more heat, hastens melting and releases even more trapped carbon – this is known as ‘positive feedback’. But as the ice melts, icebergs break off and these help to suck in carbon from the atmosphere – this is ‘negative feedback’. The next task is to understand how these two processed balance out.

Reference: Smith Jr, Robison, Helly, Kaufmann, Ruhl, Shaw, Twining and Vernet. 2007. Free-drifting icebergs: hot spots of chemical and biological enrichment in the Weddell Sea. Science doi.10.1126/science.1142834.

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There are 8 Comments. Add Yours.

  1. Lilian Nattel
    May 17, 2009

    That’s so interesting–and also a little hopeful.

  2. Ed Snack
    May 17, 2009

    You are aware, aren’t you, that the ice extent around Antarctica is at historical highs ? I mean, before making a statement like “As the ice around Antarctica melts in the face of global warming…” wouldn’t it be simple to check if that is a factual statemen ? Yes, some shelves around the Antarctic peninsula have broken up, but that represents a tiny minority of Antarctic ice.

  3. Daniel J. Andrews
    May 17, 2009

    Apparently the ice area around Antarctica is not at historical highs although the trend in sea ice coverage is upwards. See official sea ice area graph here:
    http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.area.south.jpg
    For the year 2008 it seems the ice extent was close to the 4th or 5th lowest amount since 1979 (but this may or may not be relevant as I explain below). The years 2004-2007 though did have some historical highs.
    From the authors of a paper in Nature about Antarctic warming…
    …”while sea ice in Antarctica has been increasing on average, there have been significant declines off the West Antarctic coast for the last 25 years, and probably longer”
    If you don’t have a subscription to Nature you can get a summary (by the authors) here:
    http://www.realclimate.org/index.php/archives/2009/01/state-of-antarctica-red-or-blue/
    And an update here:
    http://www.realclimate.org/index.php/archives/2009/02/antarctic-warming-is-robust/
    As you can see from the papers increase in ice extent is not necessarily incompatible with warming and increased melting. Warmer air holds more moisture which results in more snowfall which can increase ice coverage if accumulation is faster than melting. It still may be melting faster than ever but if precipitation is heavier than before the ice sheet will grow. That means you can have more ice melting into the oceans at the same time the ice is increasing.
    So (increased) ice melting around Antarctica is not incompatible with a greater ice extent, which is not incompatible with global warming, and thus the statement is indeed factual.
    Ice extent around Antarctica being at historical highs is currently NOT a factual statement. Two years ago it would have been…but not now. And as pointed out, even if it was currently factual it would not contradict the warming trend measured.
    —-
    Sometimes there is confusion over “historical sea ice extent” when people refer to different measurements. Are they referring to sea ice extent or sea ice thickness? Or perhaps seasonal ice or perennial ice? On what part of the continent? Eastern, western, interior, perimeter?
    For example, in the Arctic there was much ado last year about how great an area was covered by sea ice. That was seasonal ice though and was much thinner. Its extent is based on the temperature and weather from that particular year, and not from any long term trends. A cold year will give you more seasonal ice. A warm year will give you less seasonal ice. Just as a few cold years and greater ice extent is not indicative of global cooling neither is a few warm years and less sea ice indicative of global warming. That is weather, not climate.
    The much more important measurement is the amount of perennial ice (thick ice) because the variations seen in that ice reflect longer term trends. Arctic perennial ice has been declining for decades. That is climate, not weather.
    Naturally the Arctic is ocean surrounded by land and the Antarctic is land surrounded by ocean so the two poles will react differently to changing climate. One may lose its ice, one may gain, or eventually both may lose but the southern pole may lag by decades or centuries. We live in interesting times.

  4. Brian Schmidt
    May 18, 2009

    Ed Snack: No. Extent doesn’t equal mass. Even if it did, more precip could increase mass and still have more melting and bergs. Here for more:
    http://scienceblogs.com/deltoid/2009/05/the_australians_war_on_science_38.php

  5. Ed Snack
    May 18, 2009

    Uh, Deltoid, ho ho ho, you be quoting Tim Lambert as an authority ? True, extent doesn’t (necessarily) equal mass, but you have signally failed to demonstrate the opposite, or that there is more precipitation and hence more ice. The original statement is just “knee jerking”.

  6. Daniel J. Andrews
    May 19, 2009

    Ed S…I posted a reply a couple of days but since it had links in it, it is being held up. I pointed out, among other things, that ice extent around Antarctica is not at historic highs. Google Cryosphere Today. Your statement may have been true in 2004-2007, but not now.
    In addition, the Steig et al paper published in Nature this past January on Antarctic warming also addresses some of your points. If you don’t have subscription access to Nature, check realclimate where they have two posts on this paper…since they’re climatologists I’m sure you’ll find them more reliable than Joe/Jane blogger.
    The link between warmer air holding more moisture and precipitating out as snow when the air hits hits land and increasing snow volume/ice extent is discussed either in those posts or the comments (or perhaps the paper itself…can’t remember now but I’m sure you’ll be able to find it as this is quite a common discussion topic, having been known for well over a hundred years).

  7. Ed Yong
    May 19, 2009

    Daniel – the spam filter didn’t like the multiple links in the original comment. I’ve restored it now. Thanks for chipping in.

  8. Daniel J. Andrews
    May 19, 2009

    Thanks Ed Y. I figured something like that happened. No biggie. It was mainly a drive-by comment as I usually only discuss climate on climate blogs where real experts will comment on things too (learn a lot that way). I’m a biologist not a climatologist so I’m sure a real climatologist would have worded things differently (caveat emptor, in other words).
    I got sucked into the climate thing while doing my graduate work in the Arctic and I spent several years doing a crash course in climatology as it relates to ecology.
    btw, I see you had lunch with Sir David Attenborough. I am supremely overwhelmingly jealous. He also fueled my love for the natural world from way back and I owe him more than I can ever say.
    -dan (ex-pat brit canuck)

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