Is This The Worst Global Coral Bleaching Event Ever?

Great Barrier Reef off the east coast of Australia where currents swirl in the water around corals. Image acquired by Landsat-8 on 23 August 2013. Image Courtesy of USGS/ESA.

Great Barrier Reef off the east coast of Australia where currents swirl in the water around corals. Image acquired by Landsat-8 on 23 August 2013. Image Courtesy of USGS/ESA.

It was announced last week that 93% of the Great Barrier Reef has been hit by coral bleaching due to rising sea temperatures from El Niño and climate change. We first wrote about the third worldwide coral bleaching in October 2015, noting this year’s event could be bad. Those fears would appear to be coming true with the results of Australia’s National Coral Bleaching Task Force aerial survey of 911 coral reefs which found 93% had suffered from bleaching; of which 55% had suffered severe bleaching.

Coral bleaching occurs when water stresses cause coral to expel the photosynthetic algae, which give coral their colours, exposing the skeleton and turning them white. The stress is mostly due to higher seawater temperatures; although cold water stresses, run-off, pollution and high solar irradiance can also cause bleaching.

Bleaching does not kill coral immediately, but puts them at a greater risk of mortality. Recovery is also possible if the water stress reduces and normal conditions return, which is what is hoped for in the Northern Sector of the reef above Port Douglas, where around 81% of corals had suffered severe bleaching – the water quality in this area is good, which should also aid recovery. The reefs fared better further south. Within the Central Sector, between Port Douglas and Mackay, 75 of the 226 reefs suffered from severe bleaching. Whilst in the Southern Sector below MacKay only 2 reefs suffered severe bleaching and 25% had no bleaching.

The news is not all bad. A survey of the coral reefs of the Andaman and Nicobar Islands, a territory of India that marks the dividing line between the Bay of Bengal & Andaman Sea, also published this week shows no evidence of coral bleaching. This survey is interesting for remote sensors as it was undertaken by a remotely operated vehicle, PROVe, developed by India’s National Institute of Ocean Technology. As well as mapping the coral reefs, PROVe has a radiometer attached and is measuring the spectral signatures of the coral in the area, which could be used to support the monitoring of corals from satellites.

Monitoring coral bleaching from space has been done before. For example, Envisat’s MERIS sensor was determined to be able to detect coral bleaching down to a depth of ten metres, or the Coral Bleaching Index (Ziskin et al, 2011) which uses the red, green and blue bands to measure increases in spectral reflectance in bleached corals. Given the size, geographical area and oceanic nature of corals, satellite remote sensing should be able to offer valuable support to the monitoring of their health.

Following the second global bleaching event, in 1997/98, research confirmed that 16 percent of the world’s coral died. Who knows what the outcome of the current event will be?

El Niño causing Coral Bleaching

Variations in Pacific Ocean sea surface height compared to a long term average for the 2015 and 1997/98 El Niño events. Courtesy NASA/JPL-Caltech

Variations in Pacific Ocean sea surface height compared to a long term average for the 2015 and 1997/98 El Niño events. Courtesy NASA/JPL-Caltech

Coral reefs are currently undergoing their third worldwide bleaching event linked to the El Niño effect in the last 20 years, and scientists fear this one could be the worst.

El Niño is the warm phase of the El Niño Southern Oscillation and is associated with a band of warm water that develops in the central and east-central equatorial Pacific. This means waters in the Pacific Ocean are nutrient-poor, and are accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. This causes disruption to weather patterns worldwide including: droughts in Indonesia and Australia, and altering the path of the atmospheric jet stream over America.

Coral reefs cover less than 1% of the earth’s surface, but are some of the most valuable, diverse and vulnerable ecosystems on the planet. Reef building corals thrive in water temperatures between 73° and 84° Fahrenheit, but struggle outside of this range. Climate change is providing a challenge, and when you add on a warming effect like El Niño, the danger for coral reef ecosystems is clear. Worldwide coral bleaching events have occurred in 1997/98 and 2009/10, both of which were El Nino years; and the fear is that this year’s event, which will extend into 2016, could be significant.

Warmer water stresses the coral causing them to expel the photosynthetic algae, called zooxanthellae, living in their tissues. This turns them completely white, hence the term bleaching. Although this does not kill the coral immediately, it does put them at greater risk of dying. For example, half the coral reefs in the Caribbean were lost following a local bleaching event in 2005 – a weak El Niño year.

Satellite data has provided a valuable source of data to monitor the changes in coral reefs. For example, the French Centre National d’etudes Spatiales (CNES) and the USA’s National Aeronautics and Space Administration (NASA), began a mission in 1987 to monitor global ocean changes including measuring sea height by radar altimetry. It began with the TOPEX/Poseidon mission launched in 1992, which provided major data on the way the El Niño effect operated. This was followed Jason-1, launched in 2001, and Jason-2 in 2008. The value of this type of data to monitoring effects like El Niño can be seen at the top of the blog that shows side by side the variations in the Pacific Ocean sea surface height compared to a long term average for the 2015 El Niño and the strong event of 1997/98, with data collected by TOPEX/Poseidon for 1997 and the OSTM/Jason-2 for 2015. Further images and animations from NASA/JPL-Caltech can be found here.

Coral reef ecosystems are a source of food, protection against coastal erosion and provide spawning and nursing grounds for fish. They also provide jobs through fishing and tourism, and are estimated to contribute $29.8 billion to the global economy every year. However, scientists estimate that between 40 and 50% of corals worldwide have been destroyed or lost in the last 50 years. They expect this decline to continue, which could have significant consequences to the human and marine populations that are dependent on them.

Ocean Colour Partnership Blooms

Landsat 8 Natural Colour image of Algal Blooms in Lake Erie acquired on 01 August 2014. Image Courtesy of NASA/USGS.

Landsat 8 Natural Colour image of Algal Blooms in Lake Erie acquired on 01 August 2014. Image Courtesy of NASA/USGS.

Last week NASA, NOAA, USGS and the US Environmental Protection Agency announced a $3.6 million partnership to use satellite data as an early warning system for harmful freshwater algae blooms.

An algae bloom refers to a high concentration of micro algae, known as phytoplankton, in a body of water. Blooms can grow quickly in nutrient rich waters and potentially have toxic effects. Shellfish filter large quantities of water and can concentrate the algae in their tissues, allowing it to enter the marine food chain and potentially causing a risk to human consumption. Blooms can also contaminate drinking water. For example, last August over 40,000 people were banned from drinking water in Toledo, Ohio, after an algal bloom in Lake Erie.

The partnership will use the satellite remote sensing technique of ocean colour as the basis for the early warning system.  Ocean colour isn’t a new technique, it has been recorded as early as the 1600s when Henry Hudson noted in his ship’s log that a sea pestered with ice had a black-blue colour.

Phytoplankton within algae blooms are microscopic, some only 1,000th of a millimetre in size, and so it’s not possible to see individual organisms from space. Phytoplankton contain a photosynthetic pigment visible with the human eye, and in sufficient quantities this material can be measured from space. As the phytoplankton concentration increases the reflectance in the blue waveband decreases, whilst the reflectance in the green waveband increases slightly. Therefore, a ratio of blue to green reflectance can be used to derive quantitative estimates of the concentration of phytoplankton.

The US agency partnership is the first step in a five-year project to create a reliable and standard method for identifying blooms in US freshwater lakes and reservoirs for the specific phytoplankton species, cyanobacteria. To detect blooms it will be necessary to study local environments to understand the factors that influence the initiation and evolution of a bloom.

It won’t be easy to create this methodology as inland waters, unlike open oceans, have a variety of other organic and inorganic materials suspended in the water through land surface run-off, which will also have a reflectance signal. Hence, it will be necessary to ensure that other types of suspended particulate matter are excluded from the prediction methodology.

It’s an exciting development in our specialist area of ocean colour. We wish them luck and we’ll be looking forward to their research findings in the coming years.