Icebergs – What you gonna do when they come for you?

The science bit

Antarctica – the white continent, the most inhospitable place on the planet. Yet there is life here and I do not mean the bunch of scientists rambling around research stations who could not survive without the outside world. I mean colourful self-sustaining life. Where do you ask, since the most colourful life you can see is a penguin and that is black and white? Under the sea is my answer! Antarctic seas are teeming with life, they are wonderfully rich, filled with obscure little critters, some of which might seem familiar from a day’s rock pooling some of which seem to have come straight out of an alien movie but most of them supremely unique and only found in Antarctica.

Worms (Parabolasia corrugatus) that can grow over a meter long and expand or decrease their surface depending on oxygen concentration and sea urchins (Sterechinus neumayeri). These are some of the first animals to return after a disturbance to the seafloor Photo credit: C. Stronach
Worms (Parabolasia corrugatus) that can grow over a meter long and expand or decrease their surface depending on oxygen concentration and sea urchins (Sterechinus neumayeri). These are some of the first animals to return after a disturbance to the seafloor Photo credit: C. Stronach

Yet, like so many other of earths ecosystems they are under threat. In Antarctica some of this threat might come from an unexpected source – Icebergs! Icebergs can be a magnificent sight, providing a magical labyrinth of sculptures on the sea surface and range from being as large as a city to being a small sorry looking heap of ice in the water (perfect for a G&T). However, the saying the tip of the iceberg is a saying for a reason. What we can see of an iceberg on the sea surface is about 10% of the iceberg. A 10 m iceberg on the surface thus, is approximately 90 m deep. This means icebergs can be quite far reaching. Usually, icebergs are frozen into the sea-ice for large parts of the year.

Several icebergs covered in snow and frozen into the sea-ice
Icebergs frozen into sea-ice during winter at Rothera Research Station

Warming temperatures and changes in climate, however, reduce the duration of time icebergs are immobilised and mean icebergs drift around the sea surface a lot more freely moved by currents and wind. How, you might wonder, is this a threat to cheery little critters in the water? In general, there are two types of animals living in the sea, those that live in the water column (pelagic – like fish and jellyfish) and those that live on the seafloor (benthos – like starfish and anemones). Animals living on the seafloor move a lot slower, or might even be attached to the seafloor, than those living in the water column. When icebergs are moving around the sea, their submerged parts are very likely to bump into the seafloor and move along it, thereby scouring it. To the animals living on the seafloor this might be similar to a bulldozer in a forest for an Orangutan. Anything living in the wake of an iceberg will be removed and killed. This might sound harsh, but at a normal rate this is natures way of guaranteeing that there is a space to live for all animals, even for the weaker ones and it makes Antarctica’s seafloors even richer. Yet during 2007-2009 there was very little sea-ice along the West Antarctic Peninsula which meant that icebergs moved a lot more and a lot more freely than they used to. This in turn meant there was a lot more iceberg scouring on the seafloor and when we looked at it, we found that a lot of the animals that live attached to the seafloor had disappeared, the ones that we could find did not grow as old anymore and there was a lot more free space without animals on it.

(A) An iceberg grounded on the seafloor while frozen into the sea-ice and what the seafloor looks like immediately after being scoured by icebergs (B), after 10 years without disturbance (B) and when it has been protected from disturbance (C) – Source Zwerschke et al. (2021).

The seafloor was in crisis! Amount of different animals that could move around, however, stayed the same. We think this is because a lot of them are scavengers (animals that live of anything they can find such as vultures) and they are able to move into freshly scoured areas fairly quickly and munch on anything that is left over – after true vultures of the sea fashion. The outlook for anything else on the other hand, was fairly bleak. But don’t worry this is a good news story. After 2009, we started to see increases in sea-ice duration again. This meant icebergs were being kept in check more and more and the animals on the seafloor could start to recover. We kept checking up on them to see how many of the species had returned and how long they were living and we found that 10 years after the last big pulse of icebergs impact, the animals on the seafloor had recovered and the seafloor was again densely populated with vibrant, beautiful, bustling life.

Monitoring sea-ice cover and the retreat of Sheldon Glacier at a remote outcrop on Adelaide Island.

While 10 years might seem like a lifetime to some, it is actually incredibly fast if you consider how sensitive and delicate the ecosystem in Antarctica is (temperatures in the sea range from -2° to +2°C for comparison temperatures in the shallow Irish Sea are between 6.5° and 15.5°C. When the surface is covered in sea-ice the seafloor is shrouded in perpetual darkness with very little light reaching the bottom, and food in form of teeny tiny algae (phytoplankton) growing in the water column only turns up once a year). Even if you compare it to the average recovery time of a forest which is 40 years it again seems incredibly fast. Forest – 0, Antarctic seafloor – 1, I would say. Yet, before we get our party hats and streamers out, we should pause for a moment and have another good look at the data. What we see is that, actually, sea-ice duration is still not as high as it used to be and, actually, we still see a lot of iceberg impacts. At any rate more than we had seen before 2007.

We counted days of sea-ice cover for every year – dark blue (A) and on average how often the seafloor was hit by icebergs each year – turquoise (B) and how many different types of animals there are for those attached to the seafloor in dark brown and those that are more mobile in light brown (C). The bit marked with the blue bar is the time when there was very little sea-ice. Amended from Zwerschke et al. (2021)

The recovery of the seafloor under these conditions is reassuring and shows that we have some tough little bastards down there which can cope with a lot of adversity. On the other hand, what if this is just a short rebreather, the calm before the storm? What if more years without sea-ice and increased iceberg scouring are under way? How much can these little sea aliens take before they have to give up and will perish forever? There is no doubt, that a decrease in sea-ice is linked to a warming planet and while the fate of a purple starfish might not rock your boat, the prospect of 5 m sea level rise, once all the glaciers have melted should at least give cause to a raised eyebrow and a slightly uncomfortable feeling in your stomach region. Far be it from me to preach, we are all creatures of comfort but there are still little things that every one of us can do and you have heard them all before (cycle, use public transport, up-cycle, eat less meat, don’t fly so often) yet even if the problem seems humongous (sorting climate change is a biggie) and it seems impossible to tackle for the individual –  every little bit helps. You cycling to work or taking the bus instead of the car matters! Swapping to green energy has an impact! Having cauliflower instead of a steak changes everything – in so many aspects! It’s a slow burner, you will not be rewarded the next day but your children or grandchildren might and the anemones will thank you!

Sources

Barnes, D. K. A., & Conlan, K. E. (2007). Disturbance, colonization and development of Antarctic benthic communities. Philosophical Transactions of the Royal Society B: Biological Sciences, 362(1477), 11– 38. https://doi.org/10.1098/rstb.2006.1951

Barnes, D. K. A., & Souster, T. (2011). Reduced survival of Antarctic benthos linked to climate‐induced iceberg scouring. Nature Climate Change, 1(10), 365– 368. https://doi.org/10.1038/nclimate1232

Connell, J. H., & Keough, M. J. (1985). Disturbance and patch dynamics of subtidal marine animals on hard substrata. In S. T. A. Pickett & P. S. White (Eds.), Natural disturbances and patch dynamics (pp. 125– 151). Academic Press.

Gutt, J., & Piepenburg, D. (2003). Scale‐dependent impact on diversity of Antarctic benthos caused by grounding of icebergs. Marine Ecology Progress Series, 253, 77– 83. https://doi.org/10.3354/meps253077

Jones, H. P., & Schmitz, O. J. (2009). Rapid recovery of damaged ecosystems. PLoS One, 4(5), e5653. https://doi.org/10.1371/journal.pone.0005653

Oppenheimer, M., B.C. Glavovic , J. Hinkel, R. van de Wal, A.K. Magnan, A. Abd-Elgawad, R. Cai, M. Cifuentes-Jara, R.M. DeConto, T. Ghosh, J. Hay, F. Isla, B. Marzeion, B. Meyssignac, and Z. Sebesvari, 2019: Sea Level Rise and Implications for Low-Lying Islands, Coasts and Communities. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. https://www.ipcc.ch/srocc/

Zwerschke N., Morley S. A., Peck L. S. , Barnes D. K. A. (2019). Can Antarctica’s shallow zoobenthos ‘bounce back’ from iceberg scouring impacts driven by climate change? Global Change Biology, https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.15617