Fake dropstones

Friends, it’s the end of another day at sea, and we have more samples to show for it. The ROV dive today was to sample a long-term experiment at the central Hausgarten station, begun by my colleagues last year. This particular experiment concerns dropstones, which are random rocks deposited on the seafloor by melting icebergs. They’re very common in polar regions, especially near the ice edge. They’re colonized by all sorts of beautiful sponges, anemones, and soft corals, and they create sheltered habitat for amphipods and shrimps. Some of you might remember I wrote a paper aboutdropstone communities as part of my PhD.

Well, dropstones don’t just create habitat for sessile invertebrates; they also add heterogeneity to the seafloor. They alter the bottom current, creating turbulence and velocity gradients. Water flows around them in eddies, eroding or depositing sediment in the stone’s immediate vicinity. All of this affects the animals that live in the sediment, but until now, we didn’t know exactly how. How does the arrival of a dropstone on the seafloor alter the sediment infaunal community around the stone? My colleagues designed an experiment to find out.

This time last year, a series of fake dropstones were made out of cement and outplanted at the central Hausgarten station, in an area where dropstones naturally occur. The cement stones sat on the seafloor for a year and were revisited today with an ROV. Using push-cores, my colleagues, Pitty and Thomas, were able to collect sediment samples adjacent to the stones and far away. The infauna from the samples can now be counted and identified to see how the communities next to a stone differ from the background fauna.

I spent most of the day in the winch control room, watching the live feed from the ROV on a big screen. I wanted to see if anything had settled on the fake stones within the past year, and just as I expected, there were no visible inhabitants. One of the fake stones was actually surrounded by natural stones covered in sponges, so if the sponges had reproduced within the last year, their larvae would definitely have been able to reach the fake stone. The absence of any visible colonists lead me to two hypotheses: (1) Arctic deep-sea sponges do not reproduce on an annual basis, or (2) they grow so slowly that potentially year-old recruits cannot be seen with a camera. We know extremely little about the population dynamics of sponges in the Hausgarten area, so there are plenty of good questions for future research!

When the push-cores came to the surface with the ROV, I helped Pitty and Thomas process the samples. We divided the cores into preserved and frozen portions, then packed the samples away for transport back to Germany. I’ll be very interested to see what the results yield!