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Ocean Acidification

Acid loving algae? How ocean acidification may contribute to harmful algal blooms

The paper: Riebesall et al. 2018. Toxic algal bloom induced by ocean acidification disrupts the pelagic food web. Nature Climate Change Letters, https://doi.org/10.1038/s41558-018-0344-1

What exactly is ocean acidification?

As the most recent National Climate Assessment suggests, humanity stands to suffer immense losses from climate change. Immediate and swift action is required to prevent some of the worst effects projected in coming decades; increased coastal flooding, more intense hurricanes, longer and more intense fire seasons, changing precipitation patterns, and an acidifying ocean.

Of these awful current and anticipated impacts, ocean acidification may be the most difficult to picture. After all, the ocean is not a Marvel-villian-worthy bubbling vat of acid—so what gives?

The ocean isn’t yet a bubbling boil of acid, unlike this acidic reaction. Image credit: Rick González, Flickr

Ocean acidification, or the lowering of ocean pH, happens as the oceans absorb the greenhouse gas carbon dioxide, or CO2. As the carbon dioxide reacts with water, it releases hydrogen ions, lowering the pH and making the system more acidic. This is a natural response of a planet in balance, where the atmosphere and the ocean buffer each other to prevent excesses in either system. Since the advent of widespread agriculture and, especially, the more recent Industrial Revolution, excessive amounts of greenhouse gases have skewed this planetary balance. Trying to keep up, ocean pH has decreased by 0.1 pH units.

While this may look like a tiny change, the pH scale is logarithmic; this means small changes in the unit scale equate to large physical changes. This 0.1 pH unit changes equates to a 25 – 30% increase in acidity. That’s pretty big!

Living organisms have really narrow pH ranges at which they can survive and function normally; most of the enzymes and proteins we need for daily life can only function within a specific pH range, so living outside of an accustomed pH range can really mess up basic cellular processes. Ocean animals that rely on shell-building are particularly sensitive to changes in pH, as a more acidic environment makes it difficult for some organisms to produce shells. Scientists are still figuring out the impacts of changing pH on other ocean organisms and marine communities.

Some shell-producing organisms have trouble forming their shells in more acidic ocean conditions. Image: Wikimedia Commons, Luis Miguel Bugallo Sánchez

The experiment

A recent study by Riebesall et al. set out to fill in some gaps about the impacts of ocean acidification by exploring how changing carbon dioxide concentrations and pH levels could impact phytoplankton communities. Phytoplankton are tiny marine algae that form the base of the ocean food chain, and provide about half of the world’s oxygen.

The research team set up a mesocosm—this is a type of field-based experimental setup that tries to replicate a given natural environment under controlled conditions. Mesocosms can be huge tanks or, as in the case of this study, large seawater bags placed in the actual environment, sealed off from exchange with the surrounding waters during the experimental manipulation.

The team set up their mesocosm near the Canary Islands in the subtropical North Atlantic. Situating the mesocosm bags in the ambient environment allowed the team to subject the phytoplankton communities within their experiment to ambient light, temperature, and physical turbulence, making them more realistic representations of the system at-large. The team treated the mesocosms with different levels of carbon dioxide, to simulate changing ocean acidification levels. The team also gave the mesocosms a dose of nutrients, by adding nutrient-rich deep water from the surrounding ocean and then incubated them for 62 days, while they took measurements on phytoplankton abundance, biomass, particulate flux, and carbonate chemistry of each mesocosm.

The team set up their experiment near the Canary Islands in the Atlantic Ocean. Image credit: World Atlas

This experiment yielded some unexpected results. Vicicitus globosus, a toxic microalgae, appeared in mesocosms treated with elevated carbon dioxide, about a quarter of the way into the experiment.

Toxic algal species like Vicictus globosus can form harmful algal blooms, or HABs; HABs occur when nuisance or toxic phytoplankton species grow in overabundance, altering the normal balanced composition of the phytoplankton community. Many HAB species are less nutritious than other phytoplankton and can produce toxins that harm other critters. V. globosus persisted across the duration of the experiment in mesocosms treated with high carbon dioxide levels, altering the abundance of dinoflagellates in the plankton community. Also, fewer plankton grazers like calanoid copepods were found in the high carbon dioxide mesocosms, presumably because of the toxins produced by blooms of V. globosus.

These community changes also had an impact on nutrient and carbon transport. Typically, surface water systems transfer dead and decaying particulate matter to deeper waters as surface flora and fauna grow, eat, poop, and die.This flux of nutrient-rich material to the depths is hugely important, as it feeds other systems in the deeper ocean while helping to bury carbon, out of reach of atmospheric cycling.  The community changes seen in the experimental mesocosms translated to a change in how each system stored dead or decaying particles; mesocosms that experienced a V. globosusbloom exported about half of the carbon and nutrients compared to mesocosms with low CO2 treatment that did not experience a V. globosus bloom, likely because of altered zooplankton dynamics.

The big picture

Warming ocean temperatures and increased acidification can be directly stressful for many marine organisms. Yet this study demonstrates that ocean acidification also has the potential to cause serious indirect effects by impacting the base of the marine food chain. The stimulated growth and persistence of harmful or nuisance algal species has the capacity to throw marine food webs out of whack by changing the export of organic material to deeper waters and decreasing the amount of available zooplankton that many fish and higher level marine predators rely on for nutrition. At least 4.3 billion people, or more than half of the Earth’s population relies on seafood to help fulfill their protein requirements—shifting baselines in marine food webs could impact how the ocean is able to support these human diet needs.

There are plenty of ways to start addressing climate change in your daily life—hope is not lost! Check out some practical tips here and here and here, and spread the word about how small changes can add up.


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