Coral Cores for Science

I spent last week in St. John, US Virgin Islands, on what my friend Lauren called a “Mama Science Vacation.” That was a great term for it – a vacation from our overwhelming house renovations, and from Southern California traffic, and spending 90% of my brain capacity calculating when Ryder might next need to eat or sleep or use the toilet, and feeling guilty I didn’t have enough enriching activities planned for him. 

This week gave me a chance to reconnect with my adult self, think deeply about science, and see wild coral reefs again in person for the first time in more than a year. It’s amazing how enriching it is to be physically immersed in the environment I’ve been studying mostly remotely for the past decade. Today I saw the most extensive stand of Acropora cervicornis,  the endangered “Staghorn coral,” that I have ever witnessed. These corals used to dominate shallow Caribbean reefs, along with their relatives Acropora palmata, or “Elkhorn coral.” But both species of Acropora all but totally died off decades ago from a disease that swept through the Caribbean. Today, we saw not only thriving large colonies of adult Acroporids but also lots of juveniles – and interestingly these were often growing on dead colonies of the same species. This recalled a study I read (which of course I can't find, now) that found coral larvae of particular species preferentially settle on dead colonies of their own kind – perhaps because of some lingering chemical cues those skeletons exude?

This doesn’t have any particular bearing that I can think of right now towards my own work, but I find it really fascinating.

So, what have we been doing here in St. John? The short of it is that I’m starting a new project in collaboration with colleagues at the University of San Diego, where I’m aiming to tie records of past water quality based on coral skeletal chemistry to data they have been collecting using sediment traps and other instruments. You might remember that corals build their skeletons from chemicals in seawater, and slowly grow larger over time—thus, their skeletons record changes in water quality. Annual changes in skeletal density also provide a lovely chronometer for these chemical records. If we can tie these recent records together, I can extend records of runoff farther into the past (perhaps a century or more) using core samples from large old corals. To see if this idea will work, the first step is to collect short cores from living corals to tie to the sedimentology datasets. Hence, this trip.

Coral cores are collected using an underwater drill by divers using SCUBA equipment. Scientists either use a hydraulic drill driven by a hydraulic engine in a boat at the surface, and long hoses that deliver the hydraulic fluid down to the drill. These rigs tend to be giant, heavy, and awkward. I helped collect cores from fossil corals on land using a hydraulic drilling system at Tabuaeran Atoll way back in the dark ages of 2005, and quickly learned that unless I was going to bring along a bevy of much stronger people as field assistants, this wasn’t something little me could handle. I opted for a pneumatic drill, driven by compressed air – these drills are much smaller though less powerful (so it takes longer to collect an equivalent core length).

The cool thing about pneumatic drills is that they can be driven by an on-ship, gasoline-powered air compressor (ideally), but if this is unfeasible, they can also be driven by SCUBA tanks. Theoretically this shouldn’t be a big problem—if you have gotten to a place set up for SCUBA, there should also be a supply of tanks available to use to drive the drill. Shipping around 200-lb air compressors is, on the other hand, rather difficult and expensive. So for this trip, I opted for the SCUBA tank option.

The trip was thrown together somewhat last-minute, though it had been in the wings of planning for more than a year. Suddenly, various logistical issues came together and made it necessary to jump and get the trip organized. I was able to gather two worthy field assistants – my friend Lauren Freeman who I knew at Scripps, and a grad student at USD named Whitney who had spent at least part of the past 7 years at the Virgin Islands Environmental Resource Station (VIERS), where we were headed.

 

We converged in St. Thomas, where we rented much-too-large of a car and drove across the island, stopped to stock up on groceries and then get WD40 at Home Depot (how civilized), took the car ferry to St. John, and again drove across that island to VIERS. There, we settled into a 2-bedroom ensuite cabin with open, screened sides, a small kitchen, and an open air shower out back. Multiple cabins sat in the jungle encircling a large open area with picnic tables and a fire pit, all connected by elevated boardwalks ready for the rainy season. We slept to the sounds of tree frogs and insects, and the intermittent heart-stopping crash of mangos falling on the corrugated metal roof.

The first day consisted of getting the ladies oriented to the coral drilling gear, getting ourselves oriented to the workings of VIERS, and identifying sites to target via snorkel. We were able to swim around most of Great and also Little Lameshur bays, looking for the right sized colonies of the right coral species at the right depth. After discussions with the folks who oversee permitting for the Virgin Islands National Park, and following preliminary scouting Whitney had completed last year, I had decided to target a coral called Siderastrea siderea, or “Massive Starlet coral,” partly because it is more abundant than the coral I’ve worked with previously, Orbicella (previously Montastraea) faveolata, and partly because it seems a bit hardier than Orbicella. Coral reefs are in bad shape around the world these days, but particularly in the Caribbean. I believe that some collection is Ok for scientific purposes that are justified – but I still would rather minimize my small collection footprint. I aimed to do this by taking samples from corals that have shown evidence they may persist into the future amongst the onslaught of human impacts (sediment and nutrient runoff, overfishing, and climate change are the biggest culprits).

The next day, we got to work. It was—how shall I put this—basically complete underwater chaos. The tanks were lighter underwater than we anticipated and kept trying to escape off into the blue, the 50 foot air hose turned into a bird’s nest and got tangled with the tanks (which we’d daisy-chained together with a rope), nobody had enough dive weights on for this kind of work and couldn’t easily stay put at the target coral, the surge didn’t help things, and we hadn’t yet worked out how to communicate efficiently underwater. Thus we wasted huge chunks of time writing long epics on our dive slates and getting confused by one another. The worst part was that the drill ate up much more air than we anticipated – each tank was only lasting about 6 minutes. Using 6 SCUBA tanks to drive the drill, we collected a measly 2.5cm diameter, 4cm long core over the course of 2 dives, which took in total about 4.5 hours (including loading and unloading the boat at the dock, motoring to the site, mooring, getting geared up, etc.). It was not looking good for my goal of collecting ten 10cm-long cores in the 3 remaining diving days we had that week.

 

That evening my friend Rich—who had done some dive-tank-driven coral coring the previous year with my gear—gave some suggestions over email for improvement. The major idea was to float the tanks at the surface: because the air metered out of the tanks wouldn’t be under additional pressure from the water, the same volume of air would be delivered at a slower pace, and each tank should drive the drill for a longer time. We also decided to make a bridle with dive weights to hang over the drill; we were using a smaller, lighter core barrel than I had previously used, and we thought that the weights might help the coring move along at a more reasonable pace. The skeleton of Siderastrea was much more dense than Orbicella, so I had expected things to go slowly – but not ten times more slowly, as they had our first day.

Floating the tanks at the surface using a lift bag worked—but it was also chaotic. One person had to corral the tanks together (they were daisy-chained with rope but the currents and waves made them each constantly try to escape) and change the regulator from one tank to the next as they emptied. This turned out to be rather exhausting and caused much skull-banging and seasickness for the tank exchanger, but we were ecstatic to see that each tank now lasted about 12 minutes. With the lead-weight bridle and the tank flotilla, we were able to collect 2 whole cores in the time we’d taken the previous day to collect half a core. Things were looking up.

Each dive, we improved. We next commandeered a small kayak to hold all of the drilling tanks, which made things much easier (except when it capsized close to the rocks). Importantly, our underwater communication improved, and Lauren and Whitney quickly figured out what tasks needed to be done and developed their own drilling techniques to combat the surge while avoiding damage to other parts of the reef.

 

We finished collecting all ten cores with a day to spare, so were able to go out with a local retiree who knows practically every inch of Coral Bay – the adjacent developed watershed in which I plan to collect future cores to contrast with the runoff history in the undeveloped watersheds of the National Park. He took us snorkeling to identify future collection sites for another trip, plus to see the secret spot where we were staggered by prolific thickets of Acropora corals.

This fall comes the next fun part: geochemical analysis of these precious and hard-won samples at my new lab at UMass Boston. I can’t wait.