Sunday, June 24, 2018

The voyage of the Dawn Treader: part 3

"No one except Lucy knew that as it circled the mast, it had whispered to her, 'Courage, dear heart,' and the voice, she felt sure, was Aslan's, and with the voice a delicious smell breathed in her face."
- C.S. Lewis in The Voyage of the Dawn Treader

My study sites and the Dawn Treader route.
White outline shows SBNMS. Base map
from Google Earth.
Friends, the day has finally come! The project that I have been preparing for so long has finally begun! All of the preparations, all of the dive training, and all of the practice deploying my samplers are finally paying off. I had a successful day out on the Dawn Treader and visited two out of my three study sites to set the project in motion.

My study sites for this project are in Stellwagen Bank National Marine Sanctuary, a marine protected area just offshore of Cape Cod. The area is best-known as a productive fishing ground and prime whale-watching site (we actually saw a juvenile humpback), but the benthic habitats are diverse and interesting as well. There are numerous shipwrecks in the Sanctuary, and there's also an area known as the Sponge Forest, where the seafloor is covered by large boulders and populated by a diverse community of benthic animals. The scientific question that I'm investigating for my project is actually whether shipwrecks could serve as stepping-stones to facilitate the spread of an invasive species, Didemnum vexillum, to the Sponge Forest. Didemnum vexillum is a nasty species - it's colonial, and it grows in thick mats on the seafloor and smothers other organisms. It's been found on Georges Bank and could be arriving on Stellwagen Bank soon.

My day started at 5 am. I drove to the marina where the Dawn Treader resides and found the boat's captain had already brought her to the dock. All of my gear had been loaded the night before, so I joined the captain, mate, and my dive buddy on board, and we were off. We steamed out of the marina, through the Cape Cod Canal, across Cape Cod Bay, and out to the Sponge Forest. It took about 4 hours altogether. By the time we arrived, I was anxious to get in the water.

Going through the Cape Cod Canal at 6 am
My dive buddy and I suited up, jumped in, and descended through the water column together. When we were about 70' below the surface, I could see the seafloor, and my heart sank. I was descending onto a flat plain of sand. My first thought was that I had gotten the location wrong, that I had given the captain the wrong coordinates. We were supposed to be in an area with boulders, but there were only small rocks to be found. By the time I landed on the sand, I was thoroughly frustrated - there was not a boulder as far as my eyes could see.

Evan, my dive buddy, started deploying the samplers he was carrying as I had instructed him, while I looked all around us for any sign of rock. He pulled out a set of fouling panels, like a flag attached to a rebar pole, and started pushing the rebar into the sand. It wouldn't go. He hammered on top of the rebar. No dice. So he moved the rebar over a foot or two and tried hammering again, and it finally went in. At least that worked, I told myself with a shrug.

With the samplers secured in the sediment, we swam up-current to continue the boulder search. There was mostly sand, but I did see a few small rocks. One of them had a few strands of hydroid on it, and another had a patch of crustose coralline algae and a sea star. That's strange, I thought to myself, crustose coralline algae never lives on isolated rocks, only on big boulder reefs. Swimming a bit further, I noticed soft white papillae emerging from the sand. Those look like papillae from a sponge, I thought, but a sponge wouldn't live buried in the sand. Weird.

I turned to Evan and signaled "Let's go back." The boulders we sought were just not there. As we made our way back to the samplers, though, my observations started to coalesce in a dawning realization. What if we hadn't missed the study site? What if the boulders were there, just buried under the sand? Slowly, everything started to make sense - the sponge papillae, the crustose coralline algae, the difficulty hammering rebar into the sand. The boulders were buried! They had to be!

Back on the boat, Evan and I talked it through. Massachusetts had a very stormy spring this year. There were 3 large nor'easters in a row in March, and the turbulent weather lasted well into June. Strong northeast winds must have redistributed sand on Stellwagen Bank, bringing sediment from offshore to cover the Sponge Forest boulders. It was the only plausible explanation.

I was sad to see such a beautiful habitat disappear and a bit uncertain of the fate of my project. After all, trying to stop invasive species from reaching the Sponge Forest seems a bit futile if the Sponge Forest is buried in sand. Right after the dive, I got seasick, so my mood declined even further. I settled back into a chair on deck and tried to stabilize myself as we steamed to the next site.

Stay tuned for the end of the story in the next post.

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