Living at depths approaching an entire mile, deep-sea Arctic sponges are a scientific curiosity, as they’ve somehow managed to make this extreme environment their home. New evidence gathered over the course of an entire year is taking biologists a step closer to understanding these ecologically important sponges and how they’re able to survive such harsh conditions.
Sponge grounds are an important contributor to deep-sea ecosystems, and they’re often compared to tropical reefs in this regard. Yet very little is known about their environment and the conditions responsible for making life possible, both for deep-sea sponges and the community of animals that live among them. Living more than 4,900 feet (1,500 meters) below the surface, and in waters below the freezing point, these organisms have no access to sunlight, and the food that does become available is exceptionally scarce. Despite this, sponge grounds are a veritable oasis for other invertebrates and fish.
“The deep sea, in most places, is barren and flat,” Furu Mienis, a marine geologist with the Royal Netherlands Institute for Sea Research, said in a statement. “And then, suddenly, we have these sponge grounds that form colourful, thriving communities.” To which she added: “It is intriguing how this system sustains itself in such a place.”
Mienis is the co-author of a new JGR Oceans study detailing a year-long investigation of a sponge ground located at the Schulz Bank of the Mid Atlantic Ridge.
“Apparently, this seamount and the hydrodynamic conditions create a beneficial system for the sponges,” she said.
Mienis’s team deployed a bottom lander — a large rig fitted with an array of sensors — at the summit of an Arctic seamount located in the Norwegian sea. In operation from July 21, 2016, to July 27, 2017, the bottom lander gathered data such as temperature and water oxygen levels, the flow of currents, and even the amount of food that trickled down from the surface. A special camera fitted to the rig managed to capture over 700 hours of footage, offering a long-term perspective of physical changes at the deep-sea sponge ground.
Data gathered at the summit of the Schulz Bank showed that water flowing around the sponge ground is interacting with the seamount itself, “which produces turbulent mixing with temporarily high current speeds,” as the authors write in the study. What’s more, water moving around the seamount is delivering “food and nutrients from water layers above and below toward the sponge ground,” according to the new paper. The team recorded “high” currents clocking in at 0.70 m per second, which for the seafloor is a downright superstorm (watch the video below to the the “high speed” currents).
Only one episode during the course of the year provided food from above; namely, the summer phytoplankton bloom period. The authors said this couldn’t possibly provide enough food for the critters below, and they suspect the presence of other food sources in the form of bacteria and dissolved matter.
Deep-sea sponges along the Schultz Bank were observed to thrive at temperatures below the freezing point, which the authors described as “extreme.” The scientists say these frigid temperatures could be keeping these creatures alive, lowering their metabolic rate such that they can survive the harsh, resource-depleted environment. Interestingly, the high-speed currents appear to be testing the limits of sponges.
“The speeds that we witnessed might be close to the maximum that they can endure,” explained Ulrike Hanz, the first author of the new study, in the NIOZ statement. “At the highest speed, we saw some sponges as well as anemones being ripped from the seafloor.”
As for physical changes to the site, the scientists saw virtually nothing new during the entire year. A year had elapsed and “everything looked almost the same,” said Hanz, as it’s “just so cold out there that no crazy things are going on.”
Despite this serenity, the authors warn that deep-sea sponge grounds need to be protected from human activities, citing fishery and possible deep-sea mining as potential threats. Sad to think these animals can survive such harsh conditions at the Arctic seafloor, but not the meddling hands of humanity.