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You might have read stories of coral bleaching events, where coral reefs turn stark white after stress from high ocean temperatures.

Are they due to climate change, or other factors? Can some reefs recover better than others? What else might affect the recovery and survival of these reefs? Since scientists can’t artificially warm up an entire coral reef, they look for natural experiments. One is running this year—an El Niño with record-setting ocean temperatures. 

Sea surface temperature map of the world during 2016's El Niño. Dark areas show where temperatures are higher than normal.
Pacific Ocean temperature differences during this winter’s record-setting El Niño. Dark red areas are much warmer than average.

El Niño is the common name for extraordinarily warm seawaters that sometimes appear around Christmas off the west coast of South America. In some other years, La Niña brings colder waters there. Scientists refer to the irregular patterns of El Niño, La Niña, and neutral years as the El Niño–Southern Oscillation, or ENSO. A USGS research scientist plans to take advantage of this year’s El Niño to investigate coral reefs in an unusual setting—the Pacific coast of Panama. 

Lauren Toth is a physical scientist and Mendenhall Postdoctoral Research Fellow at the St. Petersburg Coastal and Marine Science Center in Florida. Toth’s research combines multiple scientific specialties to determine the limits on coral reef growth across broad spans of time and space. She hopes this information will help guide resource management decisions affecting coral reefs. 

Toth answered questions by telephone from her office in Florida on February 8th, shortly before departing for three weeks of research offshore Panama. This interview was edited for clarity and length. 

Map of Panama, the Gulf of Panama and the Pacific Coast is shown to the south.
Map of Panama, with the Pacific Coast on the southern (lower) side. The Gulf of Panama is also shown to the southern side of Panama.

How did you choose Panama? 
It’s always fascinated me that there even are reefs in Pacific Panama, in an area where you have a 5-meter tidal range and the water is murky all the time. You have, every 10 years or so, a huge El Niño event that knocks back all the corals, and you have upwelling. These are all conditions that should keep reefs from growing. Understanding why and how they grow under those extreme conditions can be really informative. 

How is your research related to El Niño? 
This project in Panama is an extension of the work that I did during my dissertation. We looked at core records of reefs and found a 2,000 [to] 2,500-year gap. Reefs really weren’t growing during that time. Using regional climate records we were able to relate that gap to a period where there was an increase in either the frequency or intensity of the El Niño–Southern Oscillation. 

A scientist drives a 20 foot long aluminum tube into a coral reef to collect a coral core.
Collecting a coral core in the Gulf of Chiriqui, Panama, by forcing a 20-foot aluminum tube into the reef. This core retrieved about 6,000 years of reef history.

That makes sense, because we know reefs in that area now are really, really strongly affected by both El Niño and La Niña. Stronger or more frequent events could’ve shut down the development of those reefs in the past. 

Pacific Panama is a really interesting place to do coral reef research because these reefs, in some cases within 50 kilometers of one another, experience very different conditions. What we’re interested in with this upcoming El Niño is how some of these very localized differences, whether there is upwelling or not, whether a particular reef is a little more sheltered than another, might impact how the reefs respond. 

The overall goal is to use El Niño as kind of a proxy for what’s going to happen everywhere as the climate continues to warm. There are studies that have shown that, as a result of those previous El Niños, there are certain corals that may now be genetically predisposed to tolerate warm temperatures. This will be another test to see whether we’re selecting for corals that’ll handle warmer temperatures. 

In California, everyone hopes this year’s El Niño brings lots of rain. Is it the same in Panama? How will that affect the reefs? 
In Panama, it’s actually drier during El Niño events. You also don’t have a lot of clouds. This actually might make things a little worse for the corals, because there is no cloud cover to keep the light off the reefs. During the first El Niño that was documented in Panama, that ’82–’83 event, the El Niño completely shut down the upwelling, so things got really, really hot, and corals died everywhere. 

The intense sunshine warms up the reefs? 
Under normal conditions, you want to have a lot of light for corals to grow. But when the water is also really warm, too much light can be a bad thing. You get a bleaching response in corals under high temperature stress: they expel the algae symbionts that live in their tissues. Corals rely on the symbionts for most of the food that they produce. 

A coral getting bleached by extreme water temperatures isn’t a big deal if those temperatures don’t last very long, but if they last through a whole season, that could be a death sentence. 

Over thousands of years, they bounce back? 
I think that was the really hopeful message that came out of my doctoral research. In the last 2,000 years or so, the reefs in Pacific Panama have actually been doing pretty well. It’s just whether enough of the corals are going to be able to survive the current conditions to be able to repopulate these areas. 

What are the broader implications of your research? 
I think this research can be applied pretty broadly to our understanding of how reefs will respond to climatic changes in the future. If we can find pockets where these reefs in Panama are more resistant, we might be able to learn something about how to protect reefs elsewhere. 

Is this a multi-year research project? 
The plan is to have all of the monitoring going over a three-year period. This is just the very beginning of that experiment. 

What are you measuring? 
We’re setting up permanent monitoring quadrats on six reefs, half of which are in the upwelling zone, half of which are outside of it. We’re choosing random sites on the reef. Each quadrat will be one meter by one meter. We’ll hammer in pieces of rebar at four corners, so we’ll know how to come back to the exact same spots. We’ll photograph those quadrats every six months to look at how coral color changes, how coral mortality changes, and possibly recruitment of new corals. 

A brain coral spawns white larvae, which are taken away by an underwater current.
Brain coral spawning coral larvae. The larvae will settle on other surfaces and grow into new coral in a process called recruitment.

We’re also installing calcification monitoring stations. Our group has blocks that are set out on the reef and a coral that sits on the top of the cinder block. Every six months, it gets weighed so that we know how much it’s calcifying [growing]. We’ll get an idea of how calcification varies seasonally and how it might vary in response to [El Niño and La Niña events]. 

A cinder block calcification station sits on the seafloor, a live coral rests on top.
A cinder block calcification monitoring station offshore Florida, with a live coral on top.

Then we’re setting up bioerosion monitoring experiments. We have coral discs [sliced from cores] that have been pre-weighed and CAT scanned so we know the exact density and skeletal structure of those discs. We’re putting those out on the reefs for a couple of years. [Then we’ll] pick them up and run them through the CAT scan again. 

What is bioerosion? 
Bioerosion is just the breakdown of coral, usually dead coral, by all kinds of different organisms that like to eat down the rock. In Panama, it’s things like sea urchins, bivalves, and sponges. 

Describe the oceanographic sensors. 
Those will be on the same part of the reef that all of our ecological experiments will be on. They look like a metal cylinder that has a whole bunch of sensors on it that measure water temperature, salinity, light, and chlorophyll A, which gives you an idea of the nutrients in the water. 

Have you had any scary moments out there? 
I did almost hit a humpback whale with a boat once. That was terrifying! We were in a very small dinghy, it was humpback whale migration season, and they just pop up out of nowhere. [On the other hand], it’s very cool to be diving and the whole reef is just reverberating with the whale calls. 

A humpback whale breaches the surface of the ocean near a coast.
A humpback whale breaches the surface of the ocean near a coastline. Humpback whales migrate towards warmer waters during the cold winter months.

Your work in Panama is an offshoot of your main project in Florida? 
Yes. The main part of my Mendenhall project is to look at trends in reef development over the last 10,000 years or so in Florida. 

How is that going? 
Great! We collected a whole bunch of new cores last summer and we now have really good coverage of the whole Florida Keys. Reefs throughout the Keys stopped growing about 3,500 years ago. There were still nice looking reefs after that, with high coral cover, but reef accretion really stopped around that time. It had to do with a combination of overall environmental shifts caused by changing sea level. 

A diver operates an underwater coral core drill, drilling into a coral reef.
USGS scientist Lauren Toth operates an underwater coral core drill offshore Florida.

Why are coral reefs so important? 
Corals and coral reefs are primarily important because of the structure that they build. That structure provides habitat to countless marine organisms, the fish that are so important for all the fisheries. In Florida, we’re concerned a lot about the impacts that hurricanes have. If we’re losing our reefs, we can see an impact on the force of waves that we see during storms. 

Will coral reefs be able to keep up with sea level rise? 
The way in the past that coral reefs have kept up, especially during times of really fast sea level rise, is the corals just recruit to a shallower area. When they reproduce, those juvenile coral settle in a shallower spot, and then reefs start growing there. Corals will be able to keep pace with sea level rise as long as there are not other conditions that are keeping them from living in that area. 

For more information: 
USGS Coral Reef Ecosystem Studies: USGS CREST studies 
ENSO Drove 2500-Year Collapse of Eastern Pacific Coral Reefs, Toth, et al, Science, 
USGS El Niño Information: USGS El Niño