Can marine plants save us from ocean acidification – what do models say?

Climate change is a big deal in California, and scientists keep searching for solutions. Common imprints of climate change on the daily life of Californians include increased droughts, more frequent and severe wildfires, stronger heatwaves, and less snowpack. Fossil fuel emissions change our climate, and these greenhouse gases present yet another sinister threat: they increase ocean acidity along our treasured California coast. Our more acidic ocean poses big problems for key marine organisms, so it’s important to understand the role of marine plants. Specifically, can marine plants save us from ocean acidification?

Yummy oysters, mussels, abalone, sea urchins and starfish all build their skeletons out of calcium carbonate, the same mineral as limestone. The active ingredient in antacid tablets, calcium carbonate dissolves under more acidic conditions. So, the more acidic the California coast becomes, the harder it is for key players to grow their skeletons. This situation could bring big changes to coastal Californian ecosystems. Just like human communities, ocean ecosystems work best when all members maintain health. When factors limit the ability of key members to grow to full potential, the community changes. Some win, but more lose.

The chemistry of ocean acidification is actually pretty simple. Carbon dioxide, the primary greenhouse gas and contributor to climate change, dominates fossil fuel emissions. As the atmosphere fills with carbon dioxide, the oceans absorb about 30 percent of the emitted gas. In this way, the oceans help fight climate change. They take up carbon dioxide that would otherwise end up in the atmosphere and contribute to further warming. But there’s no such thing as a free lunch. When the oceans absorb carbon dioxide, seawater becomes more acidic. This increase in acidity happens when carbon dioxide reacts with water and forms carbonic acid. This same reaction renders our sparkling water and Coca-Cola acidic.

Can anything be done to limit the effects of ocean acidification along California’s coast?

Well, yes.

We scientists, and marine resource managers, can increase monitoring networks to understand how acidic conditions vary along the California coast. We can take steps to make sure that naturally acidic spots don’t worsen from nutrient pollution and wastewater runoff. We can also evaluate whether nature offers a ready-made solution, seagrass.

Seagrass lines the California coastline. And, like terrestrial grass, seagrass makes its own food. In a process called photosynthesis, these plants combine carbon dioxide and water—in the presence of sunlight—to make sugar. This process removes acid-increasing carbon dioxide from ocean water.

In the same way that humans exhale carbon dioxide produced from metabolizing our food, seagrass produces carbon dioxide when it metabolizes its food. This process, called respiration, returns acid-increasing carbon dioxide to ocean water.

So, if photosynthesis outweighs respiration for an entire seagrass meadow, then seagrasses make the water less acidic—that is, more alkaline. Recently, scientists have wondered whether seagrasses may provide a natural refuge of alkaline waters, where marine organisms may grow healthy calcium carbonate skeletons. Could seagrass protection and restoration become a powerful tool to combat ocean acidification?

For the last two years, I led a group of scientists to study this question in depth. We worked together to develop a computer model of how seagrass changes, and is changed by, the seawater chemistry above it. We used mathematical equations to specify the relationships between seawater chemistry and seagrass growth. In the course of dozens of conference calls and hundreds of emails, we refined this model. Once we had built a working model, we unleashed it to help answer our questions.

Scientists and the public ask me about the value of computer models. We all agree that computer models are not the same as the real world. Rather, they provide simplified representations of the real world. But when built correctly, computer models function as non-invasive tools that help us understand nature’s complex behaviors and interactions. For instance, we used our model to help us understand how the amount of seagrass in a meadow may affect the meadow’s ability to counteract, or buffer against, ocean acidification. In the real world, we do not have an easy way to control the amount of seagrass to study its buffering effect. If we wanted to control the amount of seagrass, we would need to rip out healthy seagrass meadows. This is not something I recommend!

In addition to providing a non-invasive tool for knowledge, computer models cut research time. In our study, we wanted to understand how buffering changes in summer, compared to winter. Setting up the model with the appropriate conditions for summer and winter took five minutes. It took another minute to simulate a month of data on a computer. In the real world, we’d have to wait six months between summer and winter. And we would need to wait anxiously for winter weather that was calm enough to allow us to deploy the equipment needed to measure every factor we had waited all that time to observe. In some winters, these “weather windows” never happen.

So, what did our model say about seagrass?

Well, seagrass meadows might provide some buffering, but they are not a silver bullet to stop ocean acidification along all of coastal California. However, our model results suggest that seagrass meadows might provide localized effects within the meadow. These effects may be equivalent to turning back the clock a few decades on a 150-year-old problem that keeps worsening faster and faster. Hundreds of model simulations revealed the basic reason behind this limited effect: the more seagrass grows, the more food the seagrass has to metabolize to support its current biomass. In this way, seagrass resembles people. As we grow to adulthood, we eat more to support the energy needs of our larger bodies. Eventually, as seagrass reaches its greatest density, photosynthesis and respiration balance each other out. And that balance minimizes the buffering of ocean acidification.

As I mentioned earlier, models imperfectly represent the real world. At the same time, in many countries, scientists perform complementary lab experiments and take field measurements to determine whether seagrass can effectively buffer ocean acidification. Together, these multiple studies and overlapping approaches may provide a more complete answer to the question than our model has by itself. Meanwhile, I’ll keep at work to figure out what else we can do to fight ocean acidification and protect California’s productive coasts.

David Koweek, 31 July 2018. Blog.


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