The Secret World of Sea Ice

With the premier of the BBC’s new documentary series Frozen Planet having aired earlier this week, I wanted to hijack some of the buzz around the polar regions of our planet to make a blog post I’ve been planning on writing for a while now.

This all starts with my boyfriend’s subscription to The Economist. In one of the magazines he was sent a few weeks ago, there was an article about melting sea ice in the Arctic that he pointed out to me. If you would like to read it, you can find it online here. The gist of the article is that sea ice is melting a lot faster than models have predicted, and it puts forward a few reasons for this. While that is interesting and important in tackling the overall issue, that’s not what I’m going to talk about.

The photo used in The Economist's article "Beating a Retreat". The polar bear has become the poster child for climate change.

The picture they’ve used, and what has really become the emblem for global climate change, is a polar bear. These animals are quite vulnerable when it comes to a warming planet as they need sea ice to hunt. The thing about the Arctic is that underneath the ice, there isn’t actually any land there. It is just a giant piece of ice floating on top of the planet. So if there is less sea ice, there is less territory for the polar bears to hunt over winter. This probably goes a long way to explaining why more polar bears have been reported near built up areas in the tundras of Canada, they’ve been pushed south because they just can’t go north any more.

So you might think that on the other end of the Earth, in the Antarctic, melting sea ice is less of a problem because there is a land mass underneath the ice. Well, out there sea ice still extends out over the water, and while there are no polar bears there, other animals still live on and under it. However, the melting of it has much more dire consequences to the food webs and ecosystems of the poles than most people realise. Sea ice supports a much more crucial group of organisms, one that would cause the whole system to collapse if it disappeared. I’ll give you one guess as to what that is. There’s a hint in the title of this blog. Yes, of course, it’s phytoplankton!

The bottom of Antarctic sea ice, stained green from the algae living inside it. Photo by Chris Fritsen, retrieved from The Antarctic Sun (antarcticsun.usap.gov)

If you watched the first episode of The Frozen Planet, you may have noticed that in the shots where they are diving under the ice, everything has a green tinge to it. The reason for this is plankton. Polar phytoplankton are unique in that they spend a part of their life cycle living inside the ice. This is made possible through the weird and unique properties of sea ice. The water molecules in seawater freeze and form ice crystals and leave empty pockets, where the salt accumulates and forms what are called brine channels. These form tiny networks of tunnels that allow phytoplankton, zooplankton and bacteria to live inside it during the winter months and form their own microscopic ecosystem within it.

The microbial food web hidden away inside sea ice. Phytoplankton photosynthesise, and are eaten by zooplankton. Bacteria decompose both groups, and provide nutrients algae need to carry on photosynthesising.

This is vital for so many reasons. First of all, it means that when the annual sea ice melt occurs in spring, it causes a huge phytoplankton bloom as they are all released back into the open water. In turn, that causes a subsequent zooplankton bloom, which then goes on to feed animals like fish and squid, which in turn feed whales, seals, birds…. you can see where I’m going with this. The problem is that with warmer temperatures, the sea ice receeds further back every summer, and less ice is formed at winter. Less sea ice means less algae packed away in it, and therefore a much smaller bloom in the springtime.

In the Antarctic, this has sever implications for what is arguably the keystone species of the Southern Ocean, Euphausia superba: Antarctic krill. I’m sure most people have at least some idea as to what krill is, but in case you don’t, they are tiny shrimp-like animals, which are quite famous for being the sole item on the menu for the world’s biggest animal, the blue whale. In Antarctica, they support most of the food web by allowing the transfer of energy between the primary producers locked away in the ice (the photosynthesising phytoplankton) and the the higher animals living out in the open water. The key fact here is that krill and their offspring spend the winter months under the sea ice that projects out from the land, and they do so in huge swarms. There is only one way they can survive the winter, and that is by munching on the sea ice algae. They literally scrape the underside of the ice with specialised appendages and dine on the microscopic plankton. Once the sea ice melts, the krill move back out into open waters and support most of the Southern Ocean foodweb.

A simplified diagram of the Antarctic food web, showing the importance of krill

Some published studies have shown that in years when the extent of the sea ice (i.e. how far it extends from the land out into the sea) has been quite low, krill stocks have suffered.  Not only do they have a smaller refuge for the winter and less food to keep them going, but there is a smaller plankton bloom, as mentioned. Krill get out-competed when food is scarce by another kind of invertebrate, the salps. Salps are pretty much just tubes of jelly that float through the water filtering out any plankton they can get, and are very good at getting lots of food when there is little of it around. Krill by comparison are quite picky, and feed by sort of plucking things out of the water column, so are less effective when food is scarce. Salps are also very good at growing, much faster than pretty much any multi-cellular animal because they can reproduce asexually (basically clone themselves). They can form huge colonies by doing this, ensuring they filter as much food out of the sea as possible. Because the formation of sea ice over the winter months has been dropping in the last 50-100 years, salps have gradually been taking over from krill as the dominant grazers in the Southern Ocean.

Salpa thompsoni, a kind of salp found in the Southern Ocean. Usually, many of these individuals join together to form a colony. Photo by Laurence Madin, retrieved from Woods Hole Oceanographic Institute website (whoi.edu)

So, this is good news for salps, but bad news for pretty much everything else. As far as nutrition goes, salps are kind of nothing. As I said, they are not much more than gelatinous tubes and have little nutritional value to most things. Nothing really eats them, so if they succeed in taking over krill as the main grazer in the Southern Ocean, the food web is likely to collapse. Many of the species we find only in Antarctica might vanish, and other migratory species would desperately need to find new grounds. Southern humpback whales, for example, return to the Antarctic after breeding in the unproductive waters of the equator to start feeding again and pile on their fat reserves. If there is no krill to support the fish they eat, they will have to rapidly find another option. Krill themselves are of commercial importance to humans too, fished in hudreds of thousands of tonnes each year for use as food in aquaculture.

There is, however, a flip side to this. Salps produce very heavy faecal pellets that sink to the depths quite rapidly, and when the animal itself becomes clogged up with plankton, it sinks too. This means salps export a lot of carbon to the deep ocean, a carbon sink, which prevents excess CO2 from being released into the atmosphere. So there is potentially a feedback system there, and salps could actually help to slow down warming a little and restore the formation of sea ice. The only problem is it could well come at the cost of the incredible diversity of Antarctica.

So there you have it, that’s why sea ice is important. Not only do animals need it to live on, but they rely on the growth of the organisms that live in it. I’m very hopeful that The Frozen Planet will introduce people to the beauty of our polar regions and show them just how much it’s worth saving. I feel I must also mention that the polar regions are the areas most vulnerable to ocean acidification at the moment, and will probably start to become “acidified” as soon as the year 2030. This is something that will happen in our lifetimes. We really do need to do something about this now, while the damage is still relatively reversible. If we delay, we could witness the disappearance of a unique and beautiful ecosystem before we’ve really even had the chance to fully understand it.