Warming and acidification threaten glass sponge pumping and reef formation

Several kilometres off the coast of British Columbia, a unique and beautiful sight can be found on the seafloor. Delicate reefs made up of the glass sponge Aphrocallistes vastus provide important and unique habitats for fish and other invertebrates. These glass sponges have a tremendous filtration capacity, critical for the transfer of carbon between all trophic levels of their ecosystems. However, their ability to withstand continued climate change is unknown. We interviewed Dr. Angela Stevenson, lead author of a new study that examines how glass sponges, specifically their ability to pump water and form reefs, are affected by ocean acidification and warming.

An underwater image of a large glass sponge reef with a scuba diver for scale (diver is smaller than one mound, and at least ten mounds are visible in the picture).
Mosaic of individual glass sponges near Texada Island, Strait of Georgia, British Columbia. © Ocean Wise Research Institute

Dr. Stevenson is currently a Postdoctoral Researcher of seagrass Blue Carbon, in the Marine Evolutionary Ecology group in GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany. She conducted this research on glass sponge reefs in BC as a MEOPAR and UBC Ocean Leaders Postdoctoral Fellow in the Department of Zoology and Institute for the Oceans and Fisheries, University of British Columbia in Canada.

What is your background?

I am a benthic ecologist who specializes on crinoids and echinoids in mesophotic to deep-sea habitats. The central theme of my work exploits the strong link between benthos and their predators to learn about deep-water ecology, how we shape it, and how we can benefit from it.

Underwater image of a blonde woman on a scientific dive, holding dive gear and scientific equipment.
Angela on a deep technical scientific dive. © Tadhg O Corcora

For those of us not familiar with your area of research, could you give us a little bit of background on your research project?

In British Columbia, glass sponges uniquely form giant reefs that grow to 19 m in height and span several kilometers of the seafloor. They also host a multitude of fish and invertebrates that live inside and around the barrels of the sponges. Collectively these sponges process considerable volumes of water daily: they filter 100 billion litres of water every day, equivalent to one per cent of the total water volume in the Strait of Georgia and Howe Sound combined. So you can imagine, they have a strong impact on nutrient cycling, bentho-pelagic coupling (connecting the water column with the seafloor), and carbon sequestration (e.g. capturing and storing our emissions) in the NE Pacific Ocean. Because of the importance of this habitat, I wanted to see how it would fare under future climate scenarios, specifically ocean warming and acidification.

Underwater image of a rockfish inside barrel of glass sponge in a reef.
Rockfish inside barrel of glass sponge in a reef near Gambier Island, Howe Sound, British Columbia. © Adam Taylor, Marine Life Sanctuaries Society

“Because of the importance of this habitat, I wanted to see how it would fare under future climate scenarios, specifically ocean warming and acidification.”

What was the motivation or inspiration for this research?

Much of what we know about our oceans comes from studies on coastal and shallow areas, so basically, the intertidal to 30m depth, leaving a huge chunk of our oceans, 30 to 11,000 m, mostly understudied. My work focuses on this part of the ocean, from the mesophotic to deep-sea floor. Glass sponges live in this depth range, and as shallow as 22 m in British Columbia. I wondered how these glass structures would fare under future climate scenarios of warming and acidification. In history, we see many instances of glass sponges performing well under acidification. They survived prehistorical mass extinctions caused by acidified oceans, and also, are found near CO2 seeps in Papua New Guinea. We even find some surviving under rapid warming events in Antarctica. But until this point in time, there had been no experimental work done on glass sponges (the entire class Hexactinellida) to test their sensitivities to climate change, or any other environment change for that matter. The big obstacle here was keeping them alive in captivity, longer than a few weeks. I developed a protocol to keep these sensitive animals in captivity for five months, which allowed us to test the long-term sensitivities to the conditions we exposed them to.

Summary slide of topic, featuring images of siliceous sponges, depictions of mass extinctions with dying icthyosaurs and CO2 being released by a volcanic vent, a melting iceberg, and an image of underwater CO2 vents
Effect of ocean warming, acidification, and their interaction on glass sponges. Image compiled by Angela Stevenson from (c) NOAA (left), Victor Leshyk (middle), Great Barrier Reef Foundation (bottom right).

“…until this point in time, there had been no experimental work done on glass sponges (the entire class Hexactinellida) to test their sensitivities to climate change, or any other environment change for that matter. The big obstacle here was keeping them alive in captivity, longer than a few weeks.”

What was the main question of this research?

How sensitive is reef-building glass sponge Aphrocallistes vastus to ocean warming and acidification?

How did you conduct this research or how did you go about answering your question?

Thirty two juvenile A. vastus sponges were kept in recirculating 250 L aquaria, called mesocosms, for five months. After I had acclimated the sponges to their new home and feeding regime for two weeks, I exposed some to +2 C warming, others to slightly more acidic conditions (pH 7.6) than that experienced in their natural environment, and others to a combination of both warming and acidification. I compared these to ‘control’ sponges, which were exposed to ambient conditions (same temperature and pH found in their natural habitat). To monitored their health throughout the experiment, I measured their filtration capacity with a bright yellow fluorescent dye, noted when they stopped feeding and documented the onset of dead tissue. At the end of the experiment, using a computer-interface tensometer to mimic the dactyl (or pointy leg) of a crab walking on the sponge. I looked at how the strength and stiffness of their skeleton had changed after 4.5 months exposure to warming and acidification and compared these between treatments.

Underwater image of a diver holding several bags with glass sponges
Juvenile glass sponge Aphrocallistes vastus collections during field sampling. Each are contained in their own bags and in situ water for transportation from the seafloor to the lab. Diver: Donna Gibbs, Ocean Wise. © Angela Stevenson
Juvenile glass sponge Aphrocallistes vastus in mesocosm tanks
Juvenile glass sponge Aphrocallistes vastus in their mesocosms, in the lab in UBC. © Angela Stevenson

What were the main findings of your work?

It’s clear from this work that acidification, warming, and their combination have substantial negative effects on the filtration capacity of these glass sponges:

  • filtration slowed 2-3 times and weakened 2-6 times, compared to the control sponges;
  • for those sponges that were exposed to warming (regardless of acidification), 50-60% stopped feeding after just 1 month and onset of this was quick, it happened as quickly as two weeks after the experiment had started;
  • there was earlier onset (by 1 month) of irreversible tissue damage (dead tissue) in sponges exposed to warming;
  • and there was 2-3 times more dead tissue in all sponges compared to the control ones;
  • after 4.5 months in their treatments, the sponges had reduced skeletal strength and stiffness – they broke twice as easily and were twice as flexible as the control sponges, which isn’t beneficial in a highly dynamic system, where strong currents might cause them to bend and lower their feeding efficiency. Because of this and the fact that the reef itself builds up on generations past, these weaker and stiffer skeletons would be expected to slow or completely stop reef formation under future climate scenarios.
An image demonstrating filtration in juvenile glass sponge Aphrocallistes vastus in the lab with a fluorescent dye. The dye is injected next to the sponge on the left side of the image and a large plume of dye can be seen exiting the sponge from the top and towards the right of the image.
Monitoring filtration in juvenile glass sponge Aphrocallistes vastus in the lab with a fluorescent dye. © Abi Hayward
Summary image of results, showing that acidification and warming negatively impact filtration capacity (filtration time slowed 2 - 3.5 times, and filtration strength was 2 - 6 times weaker).
Summary slide of ocean warming and acidification effects on glass sponge filtration. Modified from Stevenson et al., 2020.

“It’s clear from this work that acidification, warming, and their combination have substantial negative effects on the filtration capacity of these glass sponges.”

Did you find anything unexpected?

Periods of prolonged warming have already been observed in the field, at the depth and collection site of the present study and in other parts of the Howe Sound. Our results suggest that irreversible tissue withdrawal could take place in A. vastus after 30 days of exposure to warming (>10.4 C), which could have occurred if it were not for several brief (one week) periods of cooling observed in the summer of 2016. Warming trends pose an immediate stress to glass sponge reefs, as the addition of 0.5 °C to the 2016 pattern would result in 140 consecutive days of warming, a period longer in length and warmer than the sponges were exposed to in the lab in the present study.

Also, in another study (not yet published) I conducted in parallel, I saw an opposite response to warming by feather stars that live in the same parts of the ocean as A. vastus. They grew faster under warming! These kinds of divergent responses make it very difficult to predict how biodiversity will respond to future climate scenarios.

Juvenile glass sponge - Translucent segment in lower half of the sponge shows irreversible tissue damage. Top (beige) half of sponge is comprised of living, healthy tissue.
Juvenile glass sponge Aphrocallistes vastus in aquaria where it underwent lab experimentation at the University of British Columbia. Translucent segment in lower half of the sponge shows irreversible tissue damage. Top (beige) half of sponge is comprised of living, healthy tissue. © Angela Stevenson. Modified from Fig. 4 Stevenson et al., 2020.
Summary image of results, showing that acidification and warming negatively impact sponge feeding (50% stop feeding after 1 month with quick onset of reduced feeding (2 weeks) and early onset of dead tissue by one month. Under all treatments, there was 2 - 3 times more dead tissue than controls).
Summary slide of ocean warming and acidification effects on glass sponge feeding and tissue. Modified from Stevenson et al., 2020.

What is the one take-home of this work that you want everyone to know or remember?

We rarely hear stories of climate change impacts on habitats off the coast of Canada. Our study not only provided a local example, here in BC, but also showed that deeper habitats, like BC’s glass sponge reefs, are extremely sensitive to small fluctuations in temperature and acidity.

under water images combining the first and third images of this post to show how large the reefs are. Right image has on small mound circled, which is the size of the right image with the rockfish hiding in the sponge.
Combine image to provide some perspective of just how big the reefs really are. Left image: © Adam Taylor, Marine Life Sanctuaries Society. Right image: © Ocean Wise Research Institute.

Anything else you’d like to say?

I wanted to end on a positive note and provide actions for citizens: our voice as a community and as consumers really matters. We must put pressure on our provincial and federal governments to implement the laws and policies they are formulating because we only have a couple decades before the warming threshold is reached and causes irreversible damage to nature and thus society. And industry/business owners must be more responsible about the materials/products they put into this world. Ultimately, consumers drive both these markets. So being mindful of what you choose to buy and support will help us all move into the ‘action’ phase of climate adaptation and mitigation.

Also, bottom-up community-led efforts (Marine Life Sanctuaries Society is an excellent example of this) have been the most successful call to action for protecting these habitats and others worldwide. Keep standing up for the environment, let them know you care about it and the climate crisis. Our voices matter and are more important now than ever if we want to curb our emissions.

Underwater photo of a diver shining a light on a glass sponge reef.
Glen Dennison SCUBA diving above Halkett glass sponge reef in Howe Sound, British Columbia. © Adam Taylor, Marine Life Sanctuaries Society

Read Stevenson et al., 2020 here (open access):

Warming and acidifcation threaten glass sponge Aphrocallistes vastus pumping and reef formation

Kristina Barclay, Ocean Acidification Community of Practice, 21 April 2021. Article.

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