For the first time, researchers from the SponGES project collected year-round video footage and hydrodynamic data from the mysterious world of a deep-sea sponge in the Arctic. Deep-sea sponge beds are often compared to the rich ecosystems of coral reefs and form true oases. In a world with no light and no obvious food sources, they provide a habitat for other invertebrates and a haven for fish in an otherwise barren landscape. It is still a mystery how these biodiversity hotspots survive in this extreme environment up to 1500 meters below the water surface. With over 700 hours of footage and data on food supply, temperature, oxygen concentration and currents, NIOZ scientists Ulrike Hanz and Furu Mienis found clues that could help find answers.
Colorful, thriving communities
“The deep sea, in most places, is barren and flat,” says marine geologist Furu Mienis of the Royal Netherlands Institute for Ocean Research NIOZ. “And then, suddenly, we have these sponge grounds that form colourful, thriving communities. It is intriguing how this system sustains itself in such a place.” To better understand this unexpected success, the research team deployed a ‘ground lander’ equipped with sensors and an underwater camera designed specifically for the extreme environment by NIOZ engineers and technicians. The location: a huge underwater mountain in the Norwegian Sea, part of the mid-Atlantic ridge known as Schulz Bank. A year later they picked it up again for evaluation. What they saw and measured was a world where sponges survived in sub-zero temperatures and withstood potential food deprivation, high current speeds and 200-metre underwater waves.
According to Furu Mienis, “We still don’t get why they grow where they grow, but we are off to a good start of a better understanding. Apparently, this seamount and the hydrodynamic conditions create a beneficial system for the sponges.” An important finding was the sponge bottom at the interface between two water masses, where strong internal tidal waves can propagate widely. Data from the sensors showed that water flow at the top of the mountain interacts with the mountain itself to create turbulent conditions with transient high flow velocities of up to 0.7 meters per second, which can be considered “stormy” conditions in the deep sea.
At the same time, water movements around the mountains supply the sponge bottom with food and nutrients from the water layers above and below. The team measured the amount of nutrients sinking from the surface to the sponge bottom and found that fresh nutrients were released only once in this vertical direction during a major event in summer when phytoplankton were blooming. Ulrike Hanz from NIOZ, the lead author of the study, explains: “This isn’t enough to sustain the sponge grounds, which is why we expect that additionally, bacteria and dissolved matter keep the sponges and associated fauna from starving.”
The long-term record shows that Schulz Bank sponges thrive at temperatures around zero degrees Celsius. This is at least 4°C lower than for stony cold-water corals, which also occur in the deep sea. Ulrike Hanz comments: “It is striking that they are alive with temperatures at zero degrees or even less. This is quite extreme, even for the deep sea.” In this environment without food, the cold might actually play a role in the sponge’s survival by lowering its metabolism. And the cold is not the only extremity they face. Video footage of the highest current velocity events in winter show that these “storms” continue to push the sponges to their limits. Hanz goes on to say, “The speeds that we witnessed, might be close to the maximum that they can endure. At the highest speed, we saw some sponges as well as anemones being ripped from the seafloor.”
And the most remarkable thing after hundreds of hours of footage Ulrike Hanz explains: “The image that I started with was almost the same as the image in the end. One year had gone by and everything looked almost the same. It’s just so cold out there that no crazy things are going on. The reef is still very pristinee.” However, Hanz and Mienis stress that this is a very endangered ecosystem: “Sponges can be up to two hundred years old, once damaged it takes ages for them to recover..” And there are potential threats. “Fishery seems to be the biggest one, but there is also the future possibility of deep-sea mining and changes in temperature and turbulence caused by climate change”, says Ulrike Hanz. And Furu Mienis adds, “It is a fragile equilibrium that consists of many tiny components. Take one of those away and the whole system can collapse.” Their research contributes to an initial baseline that could be essential for future protection. Furu Mienis concludes, “We now identified the first natural ranges, and gathered a bit of the information on how these rich sponge grounds can thrive.”
Heiner Kubny, PolarJournal