Some Like it Acidic — We Hope

Corals that form structures such as these need strong skeletons to withstand the ceaseless erosion by waves and currents. Photograph by Stuart Sandin.

by Nichole Price, member of the Benthic Team

It is widely realized that the health and sustainability of coral reefs are under constant assault from a variety of human activities including overfishing and pollution, as well as from the rising sea surface temperatures due to human-driven climate change. But there is another threat, one recognized more recently and thus less widely known, that concerns scientists and policy makers alike: ocean acidification. This oceanographic process is driven by the release of excess carbon dioxide (CO2) into the atmosphere, a byproduct of our burning fossil fuels. As the carbon dioxide mixes with the ocean surface waters, the seawater chemistry changes in such a way that the carbonate needed by seaweeds and animals to form shells or external skeletons is less available. As a result, these organisms may find it harder to grow and their structures may be weaker. Essentially, the drop in pH (a unit of measure for acidity) is giving the reefs osteoporosis; the coral and algal skeletons that form the reef structure are expected to become more brittle and porous, and may even dissolve as conditions worsen. And worsen they will, as the excess carbon dioxide we have already sent into the atmosphere continues to move into the oceans.

On this brief cruise we cannot document the increasing acidity, as this process is happening over time periods measured in decades. Instead  our goal is to record the natural daily and seasonal variation in pH on a reef to help us establish the acid tolerances of the reef organisms. The pH rises during the day as the photosynthesizing algae take up CO2 from the water, falls again at night as all the organisms on the reef continue to respire, releasing CO2. What are the highs and lows that corals and seaweeds experience every day? What groups of species do we find clustered in the areas of the reef that experience naturally low pH? These will likely be best able to withstand the increasing acidity that lies ahead.

Both crustose coralline algae and green calcifying algae deposit a form of calcium carbonate that adds strength to the reef structure. Photograph by Jen Smith.

In order to answer these kinds of questions, we are combining physical measurements by novel, custom-made pH sensors from the Scripps Institution of Oceanography with carefully crafted techniques for sampling reef organisms by SCUBA divers. This way we will be able to correlate the natural seawater chemistry fluctuations with the local rates of growth. For us, one of the biggest thrills is to strategically locate the sensors on the reef to record the data, then bring them back to the lab on the Hanse Explorer where we crack them open—fingers crossed that the sensor had indeed logged the data. These same sensors were previously deployed on a cruise last April in collaboration with the Coral Reef Ecosystem Division of NOAA (National Oceanographic and Atmospheric Administration). However, never before this cruise has this vital data been collected on such remote, intact coral reefs.