Can coral reefs recover from bleaching? The case of Tobago

by Salome Buglass

The bleaching of coral reefs is once again making headlines. Reefs across the tropical Pacific, including the Great Barrier Reef and now reefs in the Indian Ocean, are turning white due to warmer than usual sea temperatures as a result of climate change and the current El Niño. This may be the beginning of a series of mass bleaching events occurring at a global scale, similar to those observed in 1998, 2005, and 2010. Caribbean coral reefs may be the next to experience extensive bleaching, starting at the end of the region’s summer (~August 2016). How coral communities recover from the aftermath of bleaching events is a key question concerning marine scientists and managers as it will determine the survival of coral reefs on an increasingly warming planet.

Bleaching in 2010 at Speyside, Tobago

Bleaching in 2010 at Speyside, Tobago

When sea surface temperatures rise above the normal high for the year, it stresses corals causing them to expel the colorful algae that live inside the corals’ tissue and which provide the corals with their brilliant color and most of their energy needs. Bleached corals are weak and the longer they remain in this state, the more susceptible they become to infectious diseases and vulnerable to partial or complete mortality. Severe bleaching events often lead to significant decline in coral “cover” – the fraction of the reef covered by living corals — and changes in the average colony size. For instance, the average colony size declines as a result of partial mortality or fragmentation. Considering that larger corals tend to have greater reproductive output, a decline in abundance and mean size of coral colonies can greatly slow down the ability of the corals to reproduce, regrow, and thus recover following disturbances such as bleaching.

After witnessing the bleaching among the coral reefs that surround my home island of Tobago back in 2010, I decided to dedicate my Master’s thesis to studying the impact and recovery of these coral communities. With Simon Donner from the University of British Columbia and Jahson Alemu from the Trinidad and Tobago’s Institute of Marine Affairs, I examined changes in coral demographics over time (2010-2013) across three near-shore reef systems with different proximity to urban land. In addition, we tallied the juvenile corals at each reef, as their abundances are indicative of different species’ ability to reproduce sexually and survive. We also assessed sediment deposition and composition at each site using simple PVC pipe traps, as high levels of sedimentation are known to affect the growth stages in a coral’s life cycle. Continue reading

The Vancouver Climate Agreement

This week, the students in my course “Climate Change: Science and Society” completed the “22nd” Conference of the Parties to the UN Framework Convention on Climate Change (UNFCCC). Groups of students representing 24 different countries worked feverishly for a week to complete an agreement. Without further ado, we present to you the Vancouver Climate Agreement:



1. All countries present agree that forests, land use changes, and other sinks shall be included when calculating net greenhouse gas emissions, excluding unintentional and unavoidable natural disasters such as pests and fires. Greenhouse gas emissions including emissions from land use, land use change, and forestry should be reported every five years. Continue reading

Corals suggest El Niño may become more frequent

By Jessica Carilli, Assistant Professor UMass Boston


The author at work

In a warming world, key ocean-atmosphere processes, like the El Niño / Southern Oscillation, are expected to change. An important question is whether the frequency or nature of climate oscillations like El Niño will change in the future.

During El Niño, trade winds that normally blow warm surface waters from east to west across the equatorial Pacific Ocean weaken. The extra-warm surface waters that normally pile up in the west, called the Warm Pool, slosh back towards the east, shutting off upwelling off South America and reducing fishery productivity there. Rainfall patterns also change, moving to the east over the central Pacific and causing droughts in the west. El Niño also has global knock-on effects, like higher rainfall in California and drought in Australia.

In 2010, Simon Donner and I went to the Gilbert Islands, in the Republic of Kiribati, to collect core samples from large coral heads with the intent to learn how climate and the local coral reefs had changed over the past century.

Corals build their calcium carbonate skeletons from seawater, and in the process record changes in their environment – like water temperature and salinity – within the chemistry of their skeletons. Coral skeletons also have rings like trees, so assigning dates to the resulting skeletal environmental records is straightforward.

Gilbert Islands study region map

Butaritari is near the northern edge of the Gilbert group of Kiribati

The Gilbert Islands sit near the eastern edge of the Warm Pool, and are particularly sensitive to a recently discovered variant of El Niño, called El Niño Modoki or central Pacific El Niño. These events seem to be increasing in frequency, which makes this region particularly interesting.

Along with a team of researchers in Australia, we reconstructed water temperature and salinity at Butaritari, in the northern Gilbert Islands, from 1959-2010 and compared trends to other Pacific coral records. The gradient in water temperature from east to west across the Pacific is intrinsically linked to an atmospheric circulation cell called the Walker Circulation, comprised of the trade winds at the surface, rising warm air and rainfall in the west, and sinking, cool dry air in the east.

The records from Butaritari indicate that waters there have not warmed as much as water farther east along the equator. This means the west-east water temperature gradient has weakened over the past half century, and that the Walker Circulation – which breaks down during El Niño events – is weakening.

A weaker Walker Circulation in turn means that El Niño events will be more likely to occur. We could therefore be in for a future of increased El Niño events, which has consequences for fisheries, farming, and freshwater availability – not to mention increased likelihood of natural disasters like flooding and wildfire in some parts of the world.

The original publication can be accessed here or contact Jessica for a PDF.