Despite how the ocean has influenced the natural world throughout history, human-triggered global warming is now changing ocean chemistry through a process called ocean acidification. Described by some researchers as “the other CO2 problem”, this phenomenon refers to the reduction in the pH of seawater over time – a change that comes with negative impacts on life above and below the waves. How does ocean acidification affect marine life and humans and what can we do to mitigate its impact?

With its powerful tides and vast expanses that stretch into the horizon, the ocean captivates us. It is the cradle of early life – over billions of years our oceans have witnessed the evolution of simple cellular organisms to the wonderfully complex communities we see today. Whether we are standing with our feet in the surf or landlocked thousands of miles from the coast, we are surrounded by the ocean’s influence. By distributing heat, sequestering carbon, and storing solar radiation, it drives weather and climate across the globe. 

What is Ocean Acidification?

Ocean acidification refers to a decrease in the pH of seawater due to increased levels of carbon dioxide (CO2) in the atmosphere. Our oceans are carbon sinks – think of them as sponges that soak up excess carbon from the atmosphere.

By natural processes, CO2 absorbed by the ocean reacts with seawater to create carbonic acid, a weak acid that breaks apart into ions of different charges (imagine ions as Lego pieces that make up a larger structure– in this case,  carbonic acid). These include hydrogen ions and bicarbonate ions. The latter disassociates further to produce additional ions of hydrogen and carbonate. Animals like corals, shellfish, oysters, and urchins  –collectively referred to as calcifiers – use carbonate to build their shells and skeletons.

How does this process change when more CO2 is added to the mix? Since the industrial revolution, the amount of CO2 in the atmosphere has risen nearly 50%, jumping up to nearly 420 parts per million. Our seas currently soak up more than a quarter of the CO2 emitted from human activity. More CO2 in the ocean means more carbonic acid is produced, resulting in extra hydrogen and bicarbonate ions in seawater. pH is determined by the number of free hydrogen ions in a solution; the more they are, the lower the pH (and the more acidic the water). Additional CO2 in the water also leads to a decrease in the bioavailability of carbonate, making it harder for calcifiers to build their shells. 

How Does Ocean Acidification Affect Marine Life?

The National Oceanic and Atmospheric Administration determined that the pH of the ocean’s surface water has dropped by 0.1 pH units in the last 200 years. This may sound insignificant. However, it is important to note that pH increases and decreases on a logarithmic scale; a drop of 0.1 represents a 30% increase in acidity. The impacts of this change, combined with other factors like rising temperatures and pollution, are felt by species across trophic levels. Multiple studies show that the net growth (or calcification) of different corals has decreased significantly, leading to a loss of habitat and biodiversity. 

One study found that half of all coral coverage has been lost since the 1950s due to a variety of factors including ocean acidification. At the microscopic level ocean acidification is altering populations of certain species of phytoplankton, a crucial part of ocean food webs. Lower pH has a strong negative impact on mollusks, with shell dissolution observed in oysters, shellfish, urchins, and sea butterflies. Acidic conditions may interfere with regeneration and wound recovery in certain corals used for restoration, hampering ecosystem recovery efforts (Hall et al., 2015). In addition to direct impacts on marine life, acidification ramps up the damaging effects of local stressors like pollution and agricultural runoff, weakening the resilience of important coastal ecosystems.

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Impacts of Ocean Acidification on Humans

One of the ocean’s most amazing attributes is the interconnectedness of its ecosystems and processes. Ocean communities are mosaics of life where a change in one species can be felt through a community. The impacts of acidification act similarly. They ripple out with the potential to alter entire ecosystems, and these ripples do not stop at the surface. With livelihoods and resources under threat, humans are already experiencing the effects of ocean acidification. 

Coral reefs provide important buffer zones to coastal communities, absorbing the blow from storm surges and extreme weather events. With fewer, weaker coral reefs protecting our coasts, we can expect upticks in coastal flooding resulting in loss of property and even loss of life. The shellfish industry, valued at over one billion dollars in the US, faces an uphill battle under acidifying conditions. With economically important species like oysters and lobsters in decline, some coastal cities built on the seafood industry are on the brink of economic collapse

Acidification amplifies already significant declines in marine life due to overfishing and will have serious consequences for millions around the world who get their protein from eating fish. If acidification continues as projected under current conditions, the consequences will become more and more apparent to those whose livelihoods centre around the ocean.

Promising Research

It is easy to slip down a rabbit hole of gloom and doom when learning about the inevitable consequences of ocean acidification. However, there are bright spots we can look to when good news feels scarce. Dr. Emily Hall, a senior researcher and ocean acidification expert at Mote Marine Laboratory & Aquarium in Florida, discussed some of her lab’s positive findings in an interview.

“Looking at different genotypes of corals is a big focus that we’ve had for the past few years,” she said. “It’s been really interesting to see that even within a single species, different genotypes within that species show different responses.” 

Research conducted by Dr. Hall and other collaborators found that specific genotypes of endangered staghorn coral (Acropora cervicornis) have the adaptive capacity to withstand conditions associated with climate change, including lower pH (Muller et al., 2021). While it may be difficult to predict how some organisms fare after combining other factors like warming events, she’s hopeful that adaptive evolution and high genetic diversity will help species survive. 

Dr. Hall also discussed the importance of ecosystem restoration in combating the harmful effects of ocean acidification. Her laboratory is working to understand how seagrass and mangrove ecosystems take up excess carbon dioxide, creating pockets of refugia that buffer against global change. Despite the challenge of studying a changing climate, she remains hopeful: “Our lab and our restoration scientists are looking on the positive side, looking at ways to boost these ecosystems and get them to grow more.” 

Scientists at the University of Washington are also finding positive results surrounding the potential of aquatic plants to suck up extra CO2 in seawater. Scientists teamed up with farmers and biofuel experts to examine how kelp and seaweed can remove excess carbon, protect marine life from the erosive effects of acidification, and create a more sustainable shellfish industry. This work shows promise for future mitigation efforts and will likely play an important role in protecting our oceans from anthropogenic change.  

Solutions to Ocean Acidification? 

Beyond the ocean’s monetary value and ecosystem services, it has intrinsic value that should not be forgotten. As we watch the damaging impacts of climate change unfold, we should ask ourselves: Do humans have the right to cause potentially irreversible change for the purpose of maintaining a lifestyle reliant on fossil fuels and consumption? Do we want to imagine a future where we no longer share a world with sea butterflies or vibrant coral reefs? 

While reducing fossil fuel consumption is an essential part of preventing these losses, the unfortunate reality is that change is more complex than simply riding a bike to work. Many cities and towns are not walkable, low-carbon lifestyles are often not feasible, and legislators are slow to enact meaningful change that will lead to independence from fossil fuels. 

According to Dr. Hall, this should not dissuade us from trying. “Any little bit you can do personally to reduce CO2 really does make a difference.” She explained that seemingly small actions like carpooling, reusing bottles, and empowering others can rub off and inspire more people to make better choices for the environment. Even those living in landlocked states can make choices that are better for the ocean. Learning about our local environments, keeping our backyards clean, and planting local vegetation are simple ways we can protect our waterways. The benefits of healthy ecosystems on land add up and ultimately find their way to the ocean. “We need everybody on board to make sweeping global change, and that starts locally,” she concluded.

Learning about ocean acidification and why it matters is an important part of fighting this global problem. Many are unaware of this issue and would be motivated to care simply by having a better understanding of the impacts. For those wanting to learn more, networks and organisations like NOAA’s Sea Grant and Woods Hole’s Oceanographic Institution provide excellent resources that break down the science of ocean acidification. 

Under business as usual projections, CO2 emissions will intensify and vulnerable species will feel increasing stress from an acidifying ocean. Without a collaborative effort to decrease emissions and swift legislative action on a global scale, reducing these impacts will be an uphill battle. While consequences are already being felt in and out of the water, there is still time for humanity to change course and reduce the impacts still to come. Our oceans, the life they hold, and the people who need them depend on it.   

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Sources
Hall, E. R., DeGroot, B. C., & Fine, M. (2015). Lesion recovery of two scleractinian corals under low pH conditions: Implications for restoration efforts. Mar. Pollut. Bull., 100(1), 321–326. doi: 10.1016/j.marpolbul.2015.08.030
Muller, E. M., Dungan, A. M., Million, W. C., Eaton, K. R., Petrik, C., Bartels, E., Kenkel, C. D. (2021). Heritable variation and lack of tradeoffs suggest adaptive capacity in Acropora cervicornis despite negative synergism under climate change scenarios. Proc. R. Soc. B., 288(1960), 20210923. doi: 10.1098/rspb.2021.0923