What just one degree of warming does to the floor of the Antarctic ocean

1° Celsius doesn’t sound like a lot, but warming the seabed just slightly has a huge impact on the species living there.

In the next 50 years, ocean temperatures are expected to increase by 1°C. To get a sense of how rising temperatures will impact marine communities, researchers lowered heated plates into the Antarctic ocean. These plates warmed some patches of seabed by 1°C and others by 2°C, which is the projected temperature increase over the next 100 years. After nine months of slightly warmer temperatures, the balance of species living in the areas was significantly disrupted. A type of marine worm grew on average 70 percent larger than normal under the warmer conditions. One species of bryozoan, a tiny colonial organism, flourished as well, ultimately crowding out other species. Gail Ashton, the researcher behind the study, tells us more about it and what it can tell us about the impact of warming oceans more broadly.

The researchers deploy concrete slabs to support the heated settlement panels. Credit: Gail Ashton

ResearchGate: What motivated this study?

Gail Ashton: We want to better understand the implications of projected ocean warming on marine communities. We were excited to be able to warm the sea floor in situ and observe a whole community response.

RG: Why study only one degree of warming?

Ashton: One degree is projected as a likely scenario for ocean warming in 50 years, and therefore keeps the experiment within the realms of reality and ecological relevance. It also represents a small but significant jump for the animals. You can imagine that if we warmed by four degrees, just the shock of that immediate change may have affected them negatively, and we would not have known whether it was the shock or the absolute temperature that was driving responses.

RG: How did you warm the area of seabed you studied?

Ashton: Essentially, we placed heating elements within a plastic block on the seafloor. Imagine taking your electric stove to the bottom of the ocean and setting the temperature to +1. Then imagine the logistics and infrastructure that might be associated with that whilst working in Antarctica! Both the surface of the panels and a layer of water about 2mm thick from the panels is reliably warmed to the set temperature.

Credit: Sabrina Heiser

RG: How long did you warm these areas?

Ashton: Nine months. We wanted to run the experiments for longer, but icebergs threatened to destroy the panels and associated results, so we decided to cut the experiment there.

RG: What happened?

Ashton: We observed increased growth coverage by organisms in the 1°C and 2°C treatments. Results were most clear on the 1°C panels, and more variable in the 2°C ones.

RG: Was this what you were expecting?

Ashton: Me, no. I was expecting to have to interrogate the results statistically to tease out any significant changes. I was not expecting such a visible difference. My co-authors who have spent longer working at polar latitudes did expect these differences, but again, not to the same degree.

RG: Based on the changes in this experimental setting, what impact do you think a one-degree temperature increase on a wider scale might have?

Ashton: It's difficult to extrapolate with confidence beyond the experimental setting. But, I would expect increased growth by organisms living on the seafloor. I would expect the composition of that community to change, with some winners and some losers. I expect these impacts to ripple up the food chain with changes in grazers, scavengers, and predators.

More broadly, the results add to increasing evidence of ecosystem level change with a one-degree temperature increase. This research points to more extensive change at the poles than we would have predicted.

RG: What timeline can we expect for this increase?

Ashton: The temperature increase is consistently projected for a 50-year timescale. So we may see these rates of increase on that same 50-year horizon. The reality is that we are already seeing increases in growth associated with ocean warming. Who knows what other natural or anthropogenic drivers may also come into play in that timescale.

RG: How might this affect humans?

Ashton: If you step back even further, seafloor communities are a vital part of ocean ecosystems, holding the bulk of biodiversity in the oceans and providing important functions like nutrient cycling, carbon capture, and habitat provision. If you value the functions that the ocean provides – whether that's your food, recreational opportunities, biodiversity, resilience to change, marine products – then it all affects you.

Featured image courtesy of Gail Ashton.