Are coral reefs not going to grow in the near future?

Written by Lynn Bonomo

Background

Climate change has become a huge area of study in coral reefs in response to the dangers of coral bleaching, ocean acidification, and reef destruction. The extent of these threats have recently been highlighted by Dr. Rebecca Albright from the California Academy of Sciences. In a recent paper in Nature, Dr. Albright has shown that net calcification (same amount of calcium carbonate is produced and dissolved) of coral reefs is more diminished in ocean habitats than previously thought. Previous studies have taken place in laboratory settings with limited species. This is the first study to explore the effects in the ocean and with a community of coral reef organisms.

The Study

The study took place over 30 days, September to October, in 2016 at One Tree Island in Australia (Figure 1). This island is part of the Great Barrier Reef and is a unique location; the island consists of several lagoons that set up a special environment, in which there is a current in only one direction during low tide events. This allowed Dr. Albright and her team to modify the water chemistry and produce a potential 50 years in the future scenario of low pH in the ocean, due to increasing levels of CO2 in the ocean as a consequence of  climate change.

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Figure 1: Location of One Tree Island in the Great Barrier Reef. Points indicate the location of the island in relation to Australia (left hand picture);  picture of One Tree Island and surrounding reef (right hand image). Image from Google Maps.

In order to test the high concentrations of CO2 in the oceans the team carried out several field tests. The team filled an entire 15,000 liter tank with sea water and added CO2 into the tank on experimental days. The CO2 was added until the water on the reef after dilution was lowered to a pH of around 7.9, compared to the regular pH of 8.1 that occurs naturally. The team added Rhodamine WT dye to the tank in order to be able to track the spread of the water on both the control and experimental days through heat maps. The team then pumped the dyed water over the reef and observed the spread and took water samples from two lines across the reef (Figure 2). The samples were measured by collecting data on pH, alkalinity, salinity, nutrients, and dye concentrations at each water sample site. The experimental conditions (lowered pH) were tested for 20 days and the control days (just dye) were tested for 10 days. The experiments took into account the natural systems daily and hourly fluctuations by comparing the data to monitors that were placed in the lagoons and at nearby locations.

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Figure 2: Heat map showing the spread of the reduced pH water from the tank during experimental days. The open circle dots are where water samples were taken from each day. Image taken from (Albright et al. 2018)

Results and why it matters…

Previous lab studies underestimated the effect of corals ability to grow through calcification, with the study of individual species of coral showing a range between 15% to 28% lower calcification than normal. Dr. Rebecca Albright and her co-authors determined that mixed coral reef communities in nature reduced calcification by 40% at the lower pH. This indicates a huge problem to future coral reefs. If the coral reefs cannot undergo calcification, they will be unable to grow and could even start reducing in size. This is a severe problem that can affect both coral reef communities, coastal habitats, and human lives that depend on coral reefs for food, monetary income, or protection from weather. The research presented by Dr. Alright contributes to growing research efforts  showing the importance of reducing carbon dioxide and other greenhouse gas emissions immediately.

Reference:

Albright, Rebecca et al. 2018. “Carbon Dioxide Addition to Coral Reef Waters Suppresses Net Community Calcification.” Nature (555): 516–19. http://www.nature.com/doifinder/10.1038/nature25968 (March 16, 2018).

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