Squeezing in two dives on our last day
October 28, 2013
As expected, the wind rose last night and by 6:00 a.m., the start of our day, it was blowing between 25 and 30 knots – too windy and rough to launch ROV. The forecast was for the winds to drop through the day, so we decided to wait it out and hope for the best. It turned out that it didn’t take long. After only about an hour, we were able to start the dive, and by 8:30 a.m., we were working on the same urchin caging experiment at 1,000 meters that was the focus of our first day on this cruise.
Two years ago, we placed some urchins in mesh cages (six feet by six feet by one foot) on the seafloor at three depths to measure their rates of growth and to test whether they affect the smaller infaunal worms, crustaceans, and mollusks that live in the surface sediments nearby.
We placed sets of cages at depths of 200, 600, and 1,000 meters. At each depth, there were three cages with urchins and three without urchins. At the 600-meter site, we also placed three empty cage frames—cages without any mesh.
When the experiment was initiated, we collected a large number of sediment samples around the cages. This sediment was sieved through a fine mesh to remove the mud, allowing us to identify and count the animals in each sample. These samples allowed us to characterize the benthic macrofaunal community at the three sites.
Now, at the end of the experiment, we are taking sediment samples from within the cages to see if the macrofaunal community in cages with urchins has changed compared to cages without urchins.
We will also study communities in the cages with no mesh to test whether there is an effect of the cage itself on the benthic community. We expect that that cages do have an effect, because the cages slow currents a bit, leading to the accumulation of sediment inside the cages, compared to outside the cages.
Ultimately, we hope the results of this experiment will reveal how strongly urchins affect other seafloor animals on the muddy continental slope—and specifically how they affect the macrofauna.
As described in our October 24 cruise log, we also hope to learn something about how fast urchins grow. We’ve counted rings in the urchin’s calcium carbonate tests (shells), but we don’t know if these rings are created once per year – like tree rings – or at some other frequency.
During today’s first dive, we dove to the 1,000-meter site to collect urchins and animals from the three cages there. After about three hours of work on the bottom, we’d used up all of the sediment cores on the ROV, and had collected urchins and other animals from all of the cages. We then gathered all of the empty cages, tied them together with a long rope, and pulled them back up to the surface under the ROV. The ROV and the cages were on the surface by 1:00 p.m.
The Western Flyer wasn’t scheduled to arrive in Moss Landing until 6:00 p.m., and we wanted to make the most of the precious sea time we had available. We had just enough time to move the ship about eight kilometers toward Moss Landing, and complete a quick ROV dive at the urchin cage experiment at 600 meters depth.
We dove on the site and had about one hour to begin sampling the cages at this site. On the descent we passed through a massive school of juvenile hake – pretty cool. It was also exciting to revisit the cage experiment after not seeing it for two years.
The cages were heavily covered by brown mats of amphipod tubes. These little relatives of ‘sand fleas’ make mucous tubes. We watched them scurry around feeding and interacting with each other – sort of like a large amphipod “apartment complex.” The amphipods are probably an important source of food for a variety of predators that gather round the cages.
We collected video of each of the cages at 600 meters, and made plans for how to continue the experiment when we return to this site during our next cruise in December. Then we brought the ROV back up to the surface and headed for home.
It was about a two hour steam from the last dive site to Moss Landing. It is always nice to head home, with the wind at our backs. As we neared shore, there were about 20 or 30 humpback whales blowing all around, some breaching from time to time. You can never tire of such special treats.
All in all, this was an excellent cruise. Nearly everything went off without a problem. The weather was mostly great, and the crew of the Western Flyer and the ROV Doc Ricketts pilots were cheerful, helpful, and efficient. It is always a pleasure to go to sea on MBARI ships. We completed as much as we could have expected, and look forward to our next chance to experience the deep sea.
— Jim Barry