Bleaching continues

Corals on Dongsha are still feeling the heat. Two weeks into the bleaching event, more corals have joined the ranks of the bleached.

A coral colony tagged on May 26 is not bleached.

The good news is that almost all of the corals are still hanging on. They are going it alone though, their symbiotic algae mostly gone. When we look closely as the surface of the bleached corals, the coral polyps are stretching their tentacles out into the water, grasping for food.

By June 1st, the same colony as above has lost most of its

color, appearing bleached. The green on top of the colony

is a layer of mucus. 

Without their photosynthesizing symbionts, the coral animal is left to feed itself. Even though most of the coral's energy is provided by the symbionts, corals are equipped to catch food themselves. Seawater is constantly flowing across the reef, and this seawater carries plankton. Corals cannot move, they live inside the large skeleton that they build, and that skeleton is cemented to the reef. Instead, corals reach their tentacles out into the water and snag food passing by. That most of the corals on Dongsha are still alive despite two weeks without their symbionts suggests these corals are catching some food.

Another possibility is that the corals are not getting enough food without their symbionts and are burning through their fat reserves. If this is the case, the clock is ticking for these corals until those fat reserves run dry. The question is whether these corals can continue to feed themselves, or live off their fat reserves, long enough to acquire new symbionts.

A mucus layer shedding off a bleached coral. Notice the

white color of the coral where the mucus has fallen away.

The thousands of tiny white bumps are the living coral

polyps.

Here are some of the changes that we have observed over the past two weeks:

The coral colonies that are bleaching on Dongsha are spewing out mucus, which coats the colony. Producing lots of mucus is a general coral stress response, the mucus may be a way for the coral to shed its symbionts or harmful bacteria.

When we first observed bleaching on May 26, it was only massive, dome-shaped corals of the genus Porites. Now we are beginning to see the start of bleaching in different species of branching coral.

Bleaching is starting in several different species.

At least a few corals appear to have died from the bleaching event. The colonies are bone-white and no living coral polyps are visible. A thin layer of algae is beginning to cover the skeleton - available hard substrate is a hot commodity on reefs and algae waste little time in taking over. Hopefully the few cases of mortality that we have seen are not a sign of things to come.

Recent coral mortality. A thin layer of algae is covering the

former colony.

The cause of the bleaching event is now more clear. Data from temperature loggers deployed on Dongsha show very rapid warming, over 4°C in just two weeks. While the seasonal range of temperature on Dongsha is around 8°C, the temperature changes slowly and steadily throughout the year. It's most likely the rapid burst of warming that the corals could not handle.

We will continue to keep an eye on the bleaching happening on Dongsha, keep following our blog for more updates in the weeks to come.

- Tom DeCarlo
Joint Program in Oceanography
Woods Hole Oceanographic Institution

Temperature recorded by a logger on the Dongsha

reef flat. The sharp up and down spikes are daily

temperature changes. The 4-5°C warming trend 

beginning around May 9th likely caused the

bleaching event on Dongsha.

Partial bleaching of branching

coral.

Ready for the BIG ones!

Google Earth Image of Dongsha Atoll with approximate

location of DTS cable.

New and full phases of the moon bring "spring" tides - the largest amplitude tides.  Energetic tidal currents flowing over bumps in the ocean floor drive waves within the ocean, aka, "internal waves" (see Science section for a nice explanation).  Spring tides in the South China Sea produce some of the biggest internal waves that have been measured in the ocean… and they'll be heading toward the Dongsha atoll on June 12th (full moon)!

Fiberoptic cable deployed behind a small boat

on the reef flat.  The small dot on the horizon is

the scaffolding platform where the DTS

base station is located.

But, we're ready for them!




Yesterday, we completed the setup of the distributed temperature sensor (DTS), which includes 4 kilometers of fiberoptic cable laid from the lagoon, over the reef flat, and
down the fore reef slope to 50 m depth offshore - acting as an internal wave antenna (see blog post on May 27th for more detail on the DTS).  The DTS measures temperature and along with other instruments we have deployed on the reef, will allow us to better understand how much of the deep offshore water the internal waves bring onto the shallow reef and how it affects the corals that live there.

Aryan, Austin, and Kristen are very excited to have an

empty reel!  Note the yellow "lazy susan" that saved the day!

Deploying 4-km of cable on a reef is no small task.  It requires a lot of teamwork, good timing, and a little luck.  Our luck came in the form of good weather (small waves on the reef crest) and a useful reel mount that operates much like a "lazy susan" on the dining table that we borrowed from a nearby construction site!  We had planned some other methods for spooling out the cable, but the "lazy susan" turned out to be the best option.  Sometimes in field work, improvisation is key!

DTS cable over spur and groove formations

on the fore reef slope.

We still have a lot of work to do surveying the cable and ground-truthing our measurements with more traditional single-point ocean temperature sensors, but we're looking forward to capturing some internal waves in our DTS data!



- Kristen Davis
Coastal Dynamics Lab
University of California, Irvine

 

Kristen places a temperature sensor on the cable to

calibrate the DTS signal.

Robotic reef

Our team hard at work assembling instruments on the 

scaffolding tower.

Coral reefs are growing day in, day out, constantly constructing the calcium carbonate skeletons that form the reef. The reef does not have working hours. Unfortunately, we do.  We can only take small boats onto the reef during daylight hours. That gives us only half of the story of what’s happening on the reef. Photosynthesis dominates in daylight, respiration at night – like the reef inhales all day and exhales in the night. To understand how the reef is growing and “breathing”, we need to sample changes in the seawater chemistry at all times of the day.

Katie inspects the RAS - excited that the RAS is in the

water and starting to collect samples.

One solution is to program “robots” to sample around the clock. On Dongsha, we have instruments deployed on the reef that are continuously measuring the temperature, salinity, pH, and current speed of seawater flowing onto the reef. To measure net ecosystem calcification (NEC), which tells us how quickly the reef is growing, we need to collect bottle samples of seawater after it has flowed across the reef. The calcifying organisms of the reef, including corals and coralline algae, use ions in the seawater to build their shells and skeletons. Using seawater samples, we can measure how quickly these ions are removed from seawater, which gives us a measurement of the reef-scale calcification rate – and how this calcification rate changes as “internal” waves crash onto the reef.

The largest, and most complicated, “robot” that we have deployed on Dongsha collects seawater samples for us at all hours of the day, called the “Remote Access Sampler” or “RAS”. But the RAS is 5 feet tall and weighs over 300 pounds, and we need to deploy it from a small boat at shallow depth (~2 meters at high tide) on the reef 25 kilometers from Dongsha Island. In other reef systems, we assemble the RAS on land and bring it on a boat to where we want to sample. On Dongsha, instead we built a scaffolding tower out on the reef, brought the RAS in pieces to the reef, assembled the RAS on the tower, and lowered it into the water.

Assembling the scaffolding tower and the RAS took several days, but that was time well spent. The Dongsha reef is being sampled around the clock, while we are sleeping or eating dinner on the island. This gives us time to collect additional samples, such as cores of skeleton from individual coral colonies (more on this soon).

 The time-lapse videos below show us lifting and lowering the RAS in the scaffolding tower.

- Tom DeCarlo

Joint Program in Oceanography

Woods Hole Oceanographic Institution

Needle in a haystack

Finding something smaller than a soda can in the ocean is a bit like finding a needle in a haystack. But in this case, finding the needle means recovering very valuable information.

We found a temperature logger deployed on the forereef!

The logger was attached to a buoy to make it easier to find.

In this picture, Tom has just recovered the old loggers and

is about to attach new loggers to the buoy. Photo by Yalan,

National Sun Yat-Sen University.

During our expedition to Dongsha last year, we deployed about two dozen temperature loggers around the atoll. Every fifteen minutes over the past year, these loggers have recorded the temperature of seawater. These temperature records are incredibly important for understanding how internal waves affect corals living on the atoll, as well as identifying warm events that may have caused coral bleaching (see post below on bleaching at Dongsha).

The tricky part is finding our loggers. Dongsha Atoll is about 500 square kilometers, and our loggers are not much bigger than a pen. To make matters worse, after a year in the ocean the loggers will be completely covered with marine life, which acts as camouflage. The key is taking accurate GPS coordinates when we deploy the loggers, and a little bit of luck.

A temperature logger deployed in June 2013 that we just 

recovered. After a year in the ocean, the logger is

completely covered with fleshy and coralline algae, and

even a coral!

So far we have found all the loggers that we left on the reef flat, which is the relatively calm and shallow part of the reef. We haven't had quite the same luck (yet) with loggers on the deeper, rougher forereef - we found 2 out of 8 on our first attempt. Actually, finding any forereef loggers is a huge success because the only way to find them is by SCUBA, and at 70 feet depth we only have 10-15 minutes to search before we need to return to the surface. And of course there is always the possibility that our moorings broke free during storms - one of the largest typhoons of the year passed nearby Dongsha. Though very pleased with the valuable information we have already recovered from the loggers, we are holding out hope that we will still find the rest!

- Tom DeCarlo
Joint Program in Oceanography
Woods Hole Oceanographic Institution.

New technology

Austin and Pat load the DTS cable onto Atoll-1

Today we deployed the first 1 km section of fiber optic cable on the Dongsha reef between the lagoon and our scaffolding tower. Though it presented some challenges, we all made it through in one piece (especially the cable). The next step is to deploy another 3 km of cable to stretch from the lagoon all the way towards the open ocean to the forereef. During the next weeks the 4 km of cable that lies on the reef will collect accurate temperatures within 0.01°C every meter at 30 second intervals. These measurements will allow us to track the cold water brought to the reef by internal waves (see the "Science" section). All of this is accomplished using a technology known as DTS.

Tom swims behind Atoll-1, carefully laying cable across 

the reef. The scaffolding tower that will hold the solar panel

and electronics is in the background.

What is DTS? Distributed temperature sensing was developed for industrial use in the oil industry, but is more commonly being applied to hydrological and environmental applications. Instead of relying on data collected from individual sensors, the DTS records a continuous profile of temperatures along the fiber optic cable. It achieves this by measuring the light reflected back on sensor through a phenomenon known as Raman scattering. The signal consists of two wavelengths, Stokes and antiStokes. Because the antiStokes particle is strongly dependent on temperature, the ratio of the two can be translated into temperature. What does this all mean? Temperature can be cheaply and effectively measured over vast distances in time, using the same type of cable that plugs into your phone or television.

Ultimately, the scaffolding will hold the DTS, a small meteorological station, and solar panels to power the whole system. Stay tuned for progress on that in the coming days!

- Austin Hall
Water Resource Engineering
Oregon State University

Ghost reef

Diving into the water, it is as if we are surrounded by ghosts. Massive coral colonies surround us, but they are all bone-white. These are not actually ghost corals, they are "bleached" corals.

Tom inspects a recently bleached coral on Dongsha.

Coral reefs are typically found in the deserts of the ocean - regions where there are few sources of nutrients to the surface waters where the corals live. The reefs that corals build are like oases in the desert, but corals can only do this with the help of symbionts. Even though corals themselves are animals, within their cells they host photosynthetic algae. Everyone wins - the symbiotic algae provide energy to the coral from the products of their photosynthesis, and the algae receive shelter within the coral and benefit from coral waste products.

However, the symbiosis can go awry. Under certain conditions, typically when exposed to unusually warm temperatures, the symbiotic algae begin to produce chemicals that are toxic to the coral host. In response, the coral kicks out the symbionts. This is "coral bleaching" - the vibrant colors of coral colonies are actually caused by the symbiotic algae; once the algae are gone, the colony appears white because we are actually seeing the calcium carbonate skeleton through the translucent coral tissue. This is bad news for corals: without their symbionts, corals often starve and the colony dies. But bleaching is not always a death sentence - corals can acquire more symbionts from the seawater around them, and sometimes the coral colony fully recovers.

Sadly, we saw on Dongsha yesterday that most of the massive coral colonies are bleaching right now. They are still hanging on - with a close look at the colony surface, we could see that the coral animal is still alive. The question is whether they can hang on long enough to acquire new symbionts. With more than four weeks of our expedition remaining, we will continue to monitor these corals, and try to figure out why they bleached in the first place. Stay tuned for more - and cross your fingers that the corals here on Dongsha can recover!

- Tom DeCarlo
Joint Program in Oceanography
Woods Hole Oceanographic Institution

2 fine days on the atoll

Pat and Austin happy to be on Dongsha!

Spirits were high as we arrived on Dongsha Atoll three days ago. Waiting for us as we got off the plane were three bicycles. The island is small enough to bike across in 3 minutes, but it sure beats walking!

Our first two days on Dongsha Atoll have gone remarkably well. Our first objective was to deploy a series of instruments across the reef to characterize water chemistry and the physics of water flowing over the reef (more on this later). We would have been pleased to accomplish this in the first week, but by the end of our first day all the instruments were in the water.

Some of our instruments ready to go in the ocean

The next step was to build a tower out on the reef. The reef itself all just barely below the surface of the ocean, but our distributed temperature sensor (DTS) needs a platform for an electronics box and a solar panel. Our solution? Build a scaffolding tower out on the reef. Assembling scaffolding underwater on SCUBA is not easy. Luckily, some researchers from the National Sun Yat-sen University and staff at the National Marine Park of Dongsha were interested in what we are doing and offered to help. A few hours later, we have a dry platform on the reef!

So far Tom, Pat, and Austin are on Dongsha. Kristen, Katie, and Aryan arrive on Thursday. We are accomplishing a lot now, but we will need everyone once we begin deploying our larger instruments: the DTS and our remote access sampler (RAS) - an automatic sampler that collects seawater samples for 4 days.

Instruments deployed on the reef measuring temperature
and salinity (left), pressure (right), and we are deploying an
instrument to measure pH (upper right).

Heading out shortly for another day of difficult (but very fun) research!

- Tom DeCarlo
Joint Program in Oceanography
Woods Hole Oceanographic Institution

Our scaffolding tower rising out of the water. Our research
vessel Atoll-2 is in the background.

Staff of the Marine National Park help us load scaffolding on Atoll-2