Geosciences Column: How El Niño triggered Indonesia corals die-off

Geosciences Column: How El Niño triggered Indonesia corals die-off

In the glistening waters of Indonesia, shallow corals – the rain forests of the sea – teem with life.  Or at least they did once. Towards the end of 2015 the corals started to die, leaving a bleak landscape behind. An international team of researchers investigated the causes of the die-off. Their findings, published recently in the EGU’s open access journal, Biogeosciences, are rather surprising.

Globally, corals face tough times. Increasing ocean-water temperatures (driven by a warming climate) are disrupting the symbiotic relationship between corals and the algae that live on (and in) them.

The algae, known as zooxanthellae, provide a food source for corals and give them their colour. Changing water temperatures and/or levels, the presence of contaminants or overexposure to sunlight, put corals under stress, forcing the algae to leave. If that happens, the corals turn white – they become bleached – and are highly susceptible to disease and death.

Triggered by the 2015-2016 El Niño, water temperatures in many coral reef regions across the globe have risen, causing the National Oceanic and Atmospheric Administration (NOAA) to declare the longest and most widespread coral bleaching event in recorded history. Now into its third year, the mass bleaching event is anticipated to cause major coral die-off in Australia’s Great Barrier Reef for the second consecutive year.

The team of researchers studying the Indonesian corals found that, unlike most corals globally, it’s not rising water temperatures which caused the recent die-off, but rather decreasing sea level.

While conducting a census of coral biodiversity in the Bunaken National Park, located in the northwest tip of Sulawesi (Indonesia), in late February 2016, the researchers noticed widespread occurrences of dead massive corals. Similar surveys, carried out in the springs of 2014 and 2015 revealed the corals to be alive and thriving.

In 2016, all the dying corals were found to have a sharp horizontal limit above which dead tissue was present and below which the coral was, seemingly, healthy. Up to 30% of the reef was affected by some degree of die-off.

Bunaken reef flats. (a)Close-up of one Heliopora coerula colony with clear tissue mortality on the upper part of the colonies; (b)same for a Porites lutea colony; (c) reef flat Porites colonies observed at low spring tide in May 2014. Even partially above water a few hours per month in similar conditions, the entire colonies were alive. (d) A living Heliopora coerula (blue coral) community in 2015 in a keep-up position relative to mean low sea level, with almost all the space occupied by corals. In that case, a 15 cm sea level fall will impact most of the reef flat. (e–h) Before–after comparison of coral status for colonies visible in (c). In (e), healthy Poritea lutea (yellow and pink massive corals) reef flat colonies in May 2014, observed at low spring tide. The upper part of colonies is above water, yet healthy; (f) same colonies in February 2016. The white lines visualize tissue mortality limit. Large Porites colonies (P1, P2) at low tide levels in 2014 are affected, while lower colonies (P3) are not. (g) P1 colony in 2014. (h) Viewed from another angle, the P1 colony in February 2016. (i) Reef flat community with scattered Heliopora colonies in February 2016, with tissue mortality and algal turf overgrowth. Taken from E. E. Ampou et al. 2016.

The confinement of the dead tissue to the tops and flanks of the corals, lead the scientists to think that the deaths must be linked to variations in sea level rather than temperature, which would affect the organisms ubiquitously. To confirm the theory the researchers had to establish that there had indeed been fluctuations in sea level across the region between the springs of 2015 and 2016.

To do so they consulted data from regional tide-gauges. Though not located exactly on Bunaken, they provided a good first-order measure of sea levels over the period of time in question. To bolster their results, the team also used sea level height data acquired by satellites, known as altimetry data, which had sampling points just off Bunaken Island. When compared, the sea level data acquired by the tidal gauges and satellites correlated well.

Sea-level data from the Bitung (east North Sulawesi) tide-gauge, referenced against Bako GPS station. On top, sea level anomalies measured by the Bitung tide-gauge station (low-quality data), and overlaid on altimetry ADT anomaly data for the 1993– 2016 period. Note the gaps in the tide-gauge time series. Middle: Bitung tide-gauge sea level variations (high-quality data, shown here from 1986 till early 2015) with daily mean and daily lowest values. Bottom, a close-up for the 2008–2015 period. Taken from E. E. Ampou et al. 2016.

The data showed that prior to the 2015-2016 El Niño, fluctuations in sea levels could be attributed to the normal ebb and flow of the tides. Crucially, between August and September 2015, they also showed a sharp decrease in sea level: in the region of 15cm (compared to the 1993-2016 mean). Though short-lived (probably a few weeks only), the period was long enough that the corals sustain tissue damage due to exposure to excessive UV light and air.

NOAA provides real-time Sea Surface Temperatures which identify areas at risk for coral bleaching. The Bunaken region was only put on alert in June 2016, long after the coral die-off started, therefore supporting the crucial role sea level fall played in coral mortality in Indonesia.

The link between falling sea level and El Niño events is not limited to Indonesia and the 2015-2016 event. When the researchers studied Absolute Dynamic Topography (ADT) data, which provides a measure of how sea level has change from 1992 to 2016, they found sea level falls matched with El Niño years.

The results of the study highlight that while all eyes are focused on the consequences of rising ocean temperatures and levels triggered by El Niño events, falling sea levels (also triggered by El Niño) could be having a, largely unquantified, harmful effect on corals globally.

By Laura Roberts Artal, EGU Communications Officer

References and resources

Ampou, E. E., Johan, O., Menkes, C. E., Niño, F., Birol, F., Ouillon, S., and Andréfouët, S.: Coral mortality induced by the 2015–2016 El-Niño in Indonesia: the effect of rapid sea level fall, Biogeosciences, 14, 817-826, doi:10.5194/bg-14-817-2017, 2017

Varotsos, C. A., Tzanis, C. G., and Sarlis, N. V.: On the progress of the 2015–2016 El Niño event, Atmos. Chem. Phys., 16, 2007-2011, doi:10.5194/acp-16-2007-2016, 2016.

What are El Niño and La Niña? – a video explainer by NOAA

Coral Reef Watch Satellite Monitoring by NOAA

Global sea level time series – global estimates of sea level rise based on measurements from satellite radar altimeters (NOAA/NESDIS/STAR, Laboratory for Satellite Altimetry)

El Niño prolongs longest global coral bleaching event – a NOAA News item

NOAA declares third ever global coral bleaching event – a NOAA active weather alert (Oct. 2015)

The 3rd Global Coral Bleaching Event – 2014/2017 – free resources for media and educators

What is coral bleaching? – an infographic by NOAA
The ENSO (El Niño–Southern Oscillation) Blog by (a NOAA resource)

Imaggeo on Mondays: what corals can tell us about past climate change

Imaggeo on Mondays: what corals can tell us about past climate change

Reconstructing past climates is a tricky task at the best of times. It requires an ample data set and a good understanding of proxies. Add into the mix some underwater fieldwork and the challenge got a whole lot harder! In today’s Imaggeo on Monday’s post, Isaac Kerlow explains how information locked in corals can tell the story of past climates and how important it is, not only to carry out the research, but to communicate the results to the public! If you stick with this post until the end you’ll be rewarded with a super informative video too!

One of the main science communication initiatives at the Earth Observatory of Singapore (EOS) is about producing short films that showcase the scientific research of the principal investigators. The collection of films created by the EOS Art+Media group is called the “Knowledge Capsules” and they are free to view and download on the internet. On this occasion the filmmaking team travelled to Checheng, Southern Taiwan, to document and explain the field methods of the Marine Geochemistry team.

Creating a successful science film for a mainstream audience requires an understanding of the scientists’ methods, theories and goals. During principal photography that takes place during expeditions the filmmaking team needs to stay a step ahead of the game in order to capture the critical moments such as this image where Dr. Nathalie Goodkin passes a sample of Porites coral (a type of stony, finger-like, coral) to a scientist aboard the research vessel.

The Marine Geochemistry team at EOS investigates Earth’s climate history through the study of corals. This region is where the Kuroshio Current intrudes into the South China Sea. The team extracted samples of the Porites coral species that are approximately 300 to 500 years old, as well as seawater in which these corals grow. Because the chemical composition of corals depends on the seawater in which they grow, analysing the coral samples can give an indication of the temperature and salinity of the surrounding seawater. With these results, the team is able to reconstruct global climate systems throughout several centuries.

By Isaac Kerlow, Earth Observatory of Singapore

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at

GeoCinema Online: Oceans

This week on GeoCinema Online, we’re taking a look at all things ocean, bringing what few people see straight to your desktop – or, for that matter, any other shiny viewing device you may posses! Take a dive and find out what plankton get up to in the microscopic world beyond our vision, what corals and communities lie in the cold deep and how oceanographers are working to better understand the sea’s mysteries…

The Secret Life of Plankton

New videography techniques have opened up the oceans’ microscopic ecosystem, revealing it to be both mesmerisingly beautiful and astoundingly complex. Using footage from the Plankton Chronicles project, this film ignites wonder and curiosity about this hidden world that underpins our own food chain.

HERMIONE: Hot Spots In The Cold Deep

The HERMIONE (Hotspot Ecosystem Research and Man’s Impact on European Seas) project is focused on investigating marine ecosystems around the world, from hydrothermal vents and submarine canyons to seamounts, cold seeps and deep basins. They support a huge diversity of life that is vulnerable to the impacts of climate change and human activities. Here, the HERMIONE Scientists are researching their natural dynamics, gathering information can be used to create effective management plans that will help to protect our oceans for the future.

5th Int. Symposium on Deep-Sea Corals

Deep-sea corals contain unique records of ambient ocean water conditions and bear rich benthic habitats that are more biodiverse than their tropical equivalents. This film presents some of the latest developments in our understanding of cold-water corals together with the needs for improving the management of these amazing habitats.

Fathoming the Sea

A film focusing on how oceans work, global climate history, the dynamics of the coastal Waddenzee and the significance of Dutch maritime research.

IODP 342 Newfoundland Documentary (Episode 1)

The mission of IODP (Integrated Ocean Drilling Program) Expedition 342: recover sediments that tell the story of climate change, ocean currents and glaciations over millions of years. The main drilling target: an interval in the geologic past when the Earth was a lot warmer than today. The sediments of the Newfoundland ridges contain enough detail to teach us precisely what happened, when and why. This  is the first of many episodes (also available on YouTube), so join the scientists and their crew to learn what they were looking for and what they’ve found.

In Search Of Tricho (Part 1)

Scientists from the Woods Hole Oceanographic Institution on the research vessel R/V Oceanus undertake a cruise to study health and function of plankton in the western North Atlantic. This is the first in a 3 part series documenting the cruise, the research, and the scientists.

Need a change of scene? Why not take a look at some of the other posts in this series: how telescopes are transforming our understanding of Earth and space or how scientists are working towards better methods of earthquake and eruption forecasting?


The Secret Life of Plankton: Tierny Thys/The Plankton Chronicles Project (source)

HERMIONE: Hot Spots In The Cold Deep: Marum TV/HERMIONE (source)

5th Int. Symposium on Deep-Sea Corals: ScienceMediaNL (source)

Fathoming the Sea: NIOZ, the Royal Dutch Maritime Research Centre/NWO, Utrecht University/ European Science Foundation/NSF/IODP (USA)/Zcene Moving Moving Media Company (source)

IODP 342 Newfoundland Documentary: IODP/Ocean Leadership (source)

In Search of TRICO: Woods Hole Oceanographic Institute (source)