Motivation: Where do CO2 emissions end up?
You’ve probably heard that one way you can help fight climate change is by planting a tree. This works because trees perform photosynthesis, absorbing carbon dioxide (CO2) from the atmosphere and storing it in their wood and leaves. As trees lose their leaves, some of the CO2 is returned to the atmosphere from their decomposition but some is left over and stored in the soil. It might not seem like much but it adds up: globally, forests absorb about 1/3 of human CO2 emissions from fossil fuel burning, helping to slow global warming.
Coral reefs are often called the “rainforests of the sea” because of their amazing biodiversity (Figure 1 ). 25% of all the animal species living in the ocean call reefs home. But do reefs also mimic forests in their ability to absorb CO2? This question has proved difficult to answer because of the complexity of the ecosystem. Zooxanthellae –tiny algae living inside coral skeletons—play the role of trees in the reefs as the photosynthesizers and the ultimate source of energy for the rest of the reef’s inhabitants. Like in a forest, much of the CO2 absorbed in photosynthesis is decomposed and returned to the atmosphere. But there is an additional process unique to reefs: as corals build their calcium carbonate skeletons in a process known as calcification, they release CO2. To determine to overall impact of reefs on climate change, scientists therefore have to determine if the CO2 absorbed by photosynthesis is more or less than the CO2 released by calcification.
Methods: Measuring the fluxes
Most previous studies of the ocean’s CO2 releases have relied on taking water samples at the ocean surface and measuring the CO2 dissolved in the water. From the concentration in the water and an estimate of the wind speed, it’s possible to calculate how quickly CO2 will diffuse into the atmosphere. The problem with this approach is that it relies on individual measurements. This makes it very difficult to see how the CO2 flux changes over time or over different parts of the reef.
A new study from a research group from the University of Queensland, Australia, presents an innovative new way to address this question. Instead of collecting individual samples, the authors measured the CO2 exchange directly by floating an instrument on top of the reef (Figure 2). The instrument, called an eddy covariance tower, continuously monitors the concentration of CO2 in the air. Then, it uses a series of complex calculations based on the wind speed and direction to determine if CO2 is flowing from the water to the atmosphere or vice versa. These towers have been very useful for measuring CO2 exchange on land, where they can be fastened to a hard surface, but this is the first time someone’s tried strapping on pontoons and floating them over a reef!
Results: are reefs a source or a sink of CO2?
The ability to continuously measure CO2 turned out to be critical, as the CO2 flux cycled dramatically over the course of the day and differed among the sections of the reef. In the reef flat (a sandy-bottomed zone between the main reef and a lagoon), there was an overall release of CO2 from the ocean to the atmosphere (Figure 3). However, this changed over a daily cycle, peaking in the afternoon when CO2 release from calcification was probably strongest. Overnight, the ocean absorbed a little CO2 but not enough to make up for the losses during the day. This suggests that unlike forests on land, coral reefs might be releasing rather than storing CO2 because of the calcification process.
The lagoons in the reef, however, appeared to be absorbing CO2 (Figure 4). This is probably because there was more photosynthesis by algae living in the lagoon muds, and less CO2 released from coral building than in the reef flat. The total impact of the reef on climate change therefore depends on the balance between the two types of environment. CO2 was absorbed by the lagoons quicker than it was released the reef flats, but the flats are twice as big as the lagoons in this reef. In the end, they balance each other out: the reef seems to be releasing just as much CO2 as it absorbs. The “rainforests of the sea” are stunningly beautiful and provide a huge range of benefits to fisheries and ocean ecosystems, but absorbing CO2 might not be one of them.