Tide pool in the University of California Bodega Marine Reserve.
Tide pool in the University of California Bodega Marine Reserve. Credit: Dr. Dwayne Meadows, NOAA/NMFS/OPR

Simply by breathing, tide pool critters acidify their environment, affecting their ability to build their shells and skeletons. A team of researchers noticed this intriguing effect while testing equipment along the rocky California coast.

In tide pools at the Bodega Marine Reserve, research scientist Lester Kwiatkowski of the Carnegie Institution for Science in Stanford, Calif., and colleagues observed a dramatic change in tide pool pH between day and night while they were evaluating some lab instruments. The researchers looked closer, sampling the water during low tide at periodic times during the day and night to study its chemistry, and they discovered a seesaw pattern in which the pools’ pH dropped each night and then reverted to more alkaline conditions during the day.

In a paper published earlier this month in Scientific Reports, the researchers describe the pattern as an influence of two different types of “breathing” that alternately dominate tide pool chemistry: When the Sun shines during the day, photosynthesizing organisms such as algae take up carbon dioxide (CO2) and release oxygen, which raises the pH of the tide pool. After sunset, that process stops, and other organisms continue to respire by taking up oxygen and releasing CO2—similar to the way humans breathe—dropping the pH and acidifying the water, Kwiatkowski said.

The researchers realized that this microcosm of life offered a highly accelerated analogue of the global phenomenon of ocean acidification.

The kinds of “breathing” taking place and their effects on water chemistry weren’t entirely new information. But the researchers realized that this microcosm of life offered a highly accelerated analogue of the global phenomenon of ocean acidification.

“As the communities respire, they produce CO2, which lowers the pH in a very similar way to what we would expect to happen on decadal and centennial time scales due to climate change,” said Kwiatkowski, the lead author on the new paper.

Hard Day’s Night

The oceans are acidifying worldwide from people pumping vast quantities of CO2 into the air. When the ocean absorbs CO2, the molecule reacts with water to form carbonic acid, which builds up and threatens marine organisms that have shells or skeletons made from the compound calcium carbonate.

To create crystals of calcium carbonate, many sea creatures collect calcium ions and carbonate ions from seawater. Corals, for instance, use calcium carbonate to make skeletons, whereas other creatures, including snails, clams, and mussels, create shells. A more acidic ocean can interfere with this shell- or skeleton-building process.

The tide pools studied by Kwiatkowski and his colleagues host skeleton builders such as coralline algae and shell builders such as mussels and limpets as well as noncalicifying organisms such as sea grass and red, brown, and green algae.

From the chemistry of the water, the researchers gleaned information about the growth and dissolution of the calcium-rich frameworks of the inhabitants. The team specifically looked at how the aragonite saturation state—a number that tells scientists how much aragonite (a form of calcium carbonate) is available for creatures to use—affected tide pool creatures at different times of the day. A high saturation state value means there is enough aragonite available for the organisms to use. When this number is low, calcium carbonate already incorporated into sea creatures’ bodies dissolves back into the water.

The researchers found that during the day when water pH was less acidic, the calcifying organisms grew more, even when the availability of aragonite was relatively low. On the flip side, at night, when acidity rose again, these organisms were much more sensitive to the availability of aragonite, and their shells and skeletons began to dissolve.

Many of the photosynthesizing organisms also calcify, Kwiatkowski noted, so he speculates that perhaps the extra energy provided by photosynthesis makes them less sensitive to the availability of aragonite and therefore more resilient against any daytime dissolution.

Real Effects of Ocean Acidification

As humans pump more and more carbon dioxide and other greenhouse gases into the atmosphere, the ocean’s pH is expected to drop even more.

Beyond identifying a handy natural laboratory for studying ocean acidification and its effects, the new findings also forebode worsening nighttime stress to come for tide pool inhabitants. In the past century, greenhouse gas emissions have already increased the acidity of the ocean by 30%. And as humans pump more and more CO2 and other greenhouse gases into the atmosphere, the ocean’s pH is expected to drop even more. As a result, tide pools will start each night more acidic than they already are, leading to deeper dissolution of shells and skeletons in the nocturnal hours. “You’re going to have an ocean acidification effect on top of the respiration effect and that is likely to increase the rates of nighttime dissolution,” Kwiatkowski said.

“Tide pool communities are considered more resilient to change because they are adapted to large swings in carbonate chemistry on a daily basis,” said Joanie Kleypas, a marine ecologist and biologist at the National Center for Atmospheric Research in Boulder, Colo., who was not involved in the research. “The paper emphasizes the need to take into account both the biological and geochemical aspects of calcification.”

Because this research covered only four specific tide pools, it remains to be seen how widely the researchers’ findings apply, Kwiatkowski said. Given that creatures living in the studied tide pools—mussels, crabs, algae, sea grass, and more—are ubiquitous in coastal temperate zones, he suspects that his team’s observations have broad significance.

—JoAnna Wendel, Staff Writer

Correction, 21 April, 2016: An earlier version of this article incorrectly stated that the pH of the ocean dropped by 30%. This article has been updated to state that the acidity of the ocean has increased by 30%.

Citation: Wendel, J. (2016), Tide pools mimic climate change in everyday cycle, Eos, 97, doi:10.1029/2016EO049425. Published on 1 April 2016.

Text © 2016. The authors. CC BY-NC-ND 3.0
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