Indicator Highlight: Ocean Acidifcation

A favorite pastime of many is traveling to an orchard to pick fruits in the summer or autumn months, and heading home to bake some delicious fruity treats. If you go at the peak of the season, the fruits are abundant, dangling off every tree, vine, and bush. With every step, you can add fruit to your bag until it's overflowing, too heavy to carry and ready to be taken home to make fruit pies, tarts, cobblers, and more. If you go to the orchards outside of peak season, the fruit selection is limited. You have to travel further to find ripe fruits, spending energy with every travail up and down the rows. Instead of making all of the baked goods, you’re limited on the amount of fruits that you can bring home. When fruit resources are limited, the ability to make delicious baked goods is also limited. While the availability of fruit is seasonal and limits your ability to make delicious baked goods, the availability of some minerals in our oceans is primarily a global, human-caused issue that limits the ability for many of our favorite seafoods to grow and survive.

Dissolved within our ocean’s waters are minerals and gasses. Certain types of these minerals are absorbed by many marine species, like oysters, crabs, corals, shell-building plankton and abalone, as “ingredients” that are essential to create and grow their shells. As more and more carbon dioxide gas from human activity dissolves into our oceans, the availability of these essential mineral building blocks becomes limited.

Slowed growth and survival of shelled species is just one impact that ocean acidification has on our nation’s ecosystems. Continue reading for more information on the science behind ocean acidification, more ways it impacts our ecosystems, and how ocean acidification impacts you.

What is Ocean Acidification?

The ocean acts like a sponge for carbon dioxide in the atmosphere. As levels of carbon dioxide increase in the atmosphere from human activity such as burning fossil fuels and changing land use, the ocean absorbs more carbon dioxide as well. When seawater absorbs carbon dioxide, a series of chemical reactions occur with two main consequences. First, the changes in chemistry result in more hydrogen ions (H⁺) in the seawater. While the ocean isn’t acidic, this increases the acidity and is why we call this process ocean acidification. Small increases in acidity can have big consequences to marine life. These chemical reactions that take place as atmospheric carbon dioxide is absorbed into our oceans are just a few of the steps of the global carbon cycle. Second, the released hydrogen ions act like free agents able to react with other chemicals in the ocean and compete with important molecules needed to form calcium carbonate, an essential building block mineral for building shells and skeletons for some marine life. More information about the science behind measuring our ocean acidification indicator metrics can be found at the bottom of this article under “Digging Deeper” and on the indicator page.

Digging Deeper: How do we measure ocean acidification?

We can characterize the carbonate system in the ocean using four parameters. When tracked over time, these parameters characterize the degree and dynamics of ocean acidification, and allow the calculation of biologically important indices. This dashboard provides two of these parameters, pH and pCO2, and the biologically relevant index Ω, the saturation state of important biomineral calcium carbonate. Learn more about Big 4 for measuring ocean acidification. 

pH - a scale of small changes, big consequences
The concentration of hydrogen ions determines a water’s acidity. Acidity is measured with pH, a logarithmic scale from 0 to 14, where 0 is the most acidic (the most hydrogen ions), 7 is neutral, and 14 is the most basic (the least hydrogen ions). Since it is a logarithmic scale, small changes in pH means big changes in acidity. For example, since the Industrial revolution, the global average pH has changed from 8.2 to 8.1. It seems small, but that is about a 26% increase in acidity.
 

Ω - saturation state of an important mineral building block
The concentration of hydrogen ions in the water affects the availability of other minerals, like calcium carbonate (CaCO₃), the main mineral found in shells of many marine animals. There are two forms of this mineral: aragonite and calcite, each with different saturation states. Researchers measure the saturation (shown as Ω) of these minerals as a measure of its availability for biological processes such as shell building. Typically, as acidity increases, calcium carbonate becomes harder to form, as it takes more energy to do so. Decreased calcium carbonate availability can result in the inability for young individuals to grow shells, decreased growth, increased susceptibility to disease, and potentially death. Researchers observed these effects in pteropods, important planktonic snails that form the base of food webs, oysters, and other shellfish.
 

pCO2 - how much carbon dioxide is in the ocean?
The partial pressure of CO2, or pCO2, tells us how much carbon dioxide is in seawater. This information helps us understand ocean carbonate chemistry and biological productivity. Regional processes have strong influences on pCO2, as it is affected by several factors including temperature, salinity, ocean currents and upwelling, biological activity and salinity. pCO2 increases when the ocean absorbs more CO2 from the atmosphere. 

pCO2 can be particularly variable near coastal zones as seasonal and coastal dynamics occur. Biological productivity can greatly impact this measure. As phytoplankton bloom (grow), they take in carbon dioxide and reduce pCO2. Conversely, when phytoplankton die, decomposition releases carbon dioxide into the seawater. This measure is strongly influenced by temperature. Warmer water holds less carbon dioxide gas; therefore, colder regions tend to have higher amounts of carbon dioxide. 

Figure demonstrating the relationship between pH, carbon dioxide (CO2), and mineral availability for shell growth. As carbon dioxide increases, water becomes more acidic, and mineral availability decreases. 

Connections and Consequences

Changes in ocean acidification have been monitored and tracked systematically and synoptically since the 1980s using stationary time series sites, mapped CO2 partial pressure data, decadal hydrographic cruises, and ocean and Earth models. While increasing anthropogenic atmospheric carbon has caused a steady rise in ocean carbon dioxide, ocean systems also experience natural fluctuations in pH as well. Ocean waters vary in pH, pCO2, and aragonite saturation state (Ω) temporally by year, by season, and even throughout a 24-hour day. Similarly, these measures vary based on location, with latitudinal differences (North – South), and distance from shore. These natural variations are driven by changes in physical oceanography (ex. tides), biological processes (ex., photosynthesis from primary producers), and geochemical reactions (ex., erosion). Some of the largest natural variability in water measurements can be found in estuarine and coastal ecosystems. Ocean acidification due to human activities may further increase the amount of variability and extremes observed in these ecosystems.

The pteropod, or “sea butterfly”, is a tiny sea creature about the size of a small pea. Pteropods are eaten by organisms ranging in size from tiny krill to whales. The photos above show that a pteropod’s shell dissolves over 45 days when placed in sea water with pH and carbonate levels projected for the year 2100. Photo credit: Bednarsek et al.,  David Liittschwager/National Geographic Stock. 

The status and trend of pCO2, pH, and Aragonite Saturation State in the Southeast United States Large Marine Ecosystem. pCO2 values are above the 90th percentile of historic values, while pH and Saturation State are below the 10th percentile. No trend is apparent for pCO2 and pH, but the Aragonite Saturation state is trending downward. NOAA EIWG, 2023

How Does Ocean Acidification Affect Species and Ecosystems?

• Dungeness crab are an important commercial and recreational fishing species along the West Coast of the U.S. Young Dungeness crabs, known as megalope, have been found to be susceptible to acidifying ocean conditions. Megalope caught in acidic environments were smaller and had thin, damaged shells (also known as “carapaces”), likely a result of their inability to easily absorb essential shell-growing minerals from the seawater. Scientists also observed damage to tiny hair-like structures called “mechanoreceptors”, which act like whiskers on a cat to sense the environment and help crab megalope detect tiny wave vibrations from prey and predators.
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• Northern sand lance are an important forage fish in the Gulf of Maine and the Stellwagen Bank National Marine Sanctuary.They are also a key prey species for sea birds and migrating humpback whales. Increased ocean acidity has been shown to decrease survival in early life stages of sand lance. Fish embryos release an enzyme into the egg when they are ready to hatch. This enzyme breaks down the “shell”, or outer membrane of the fish egg called the “chorion”, and allows the fish to exit the egg. The ability of this enzyme to break down the egg chorion decreases under acidic conditions. The impacts of ocean acidification on sand lance survival, and likely the number of sand lance available as prey in the ecosystem, may affect the well-being of larger marine species like sea birds and whales throughout the entire ecosystem food chain.
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• Corals - A coral is made up of individual animals called polyps. Stony coral species create their strong, hard skeletal structure by extracting the mineral, aragonite, from seawater. These hard structures in the shapes of tables, pillars, brains, etc. are what we envision when we think of coral reefs. Ocean acidification decreases the amount of available aragonite in seawater, resulting in less dense, weaker coral structures. Less dense corals are more vulnerable to damage from storms waves and algae-grazing fish species. Weaker corals may limit the availability of coral reefs as an important coastal habitat and ecosystem. 

It is important to note that ocean acidification poses a different threat to corals than warming sea surface temperatures and heat wave events. During times of thermal stress, corals experience bleaching, when their colorful symbiotic algae, zooxanthellae, exit the coral polyps, resulting in the white skeleton showing. Thermal stress affects hard and soft corals as well as other benthic organisms like sponges that live on reefs. This is different from the effects of ocean acidification which directly impacts the corals by weakening their skeletons. The impacts of climate change, including marine heatwaves and ocean acidification greatly impact corals. Multiple stressors and these cumulative impacts on corals, like coral bleaching and fragile skeletons, lead to degraded health.

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How Could Ocean Acidification Affect Me?

Effects on Seafood Production

Ocean acidification impacts commercial, recreational, subsistence, and traditional fisheries participants, by impacting the seafood species that end up on our plates. These include tasty items such as clams, mussels, scallops, and crabs, all of which make shells from calcium carbonate. Many of these are valuable fisheries that support our coastal communities and traditional cultures.

Ocean acidification has a broad impact on the shellfish industry. On the east coast, Atlantic surfclams respond to ocean acidification by feeding less and growing more slowly that can delay the time it takes to reach the minimum catch size allowed for fishermen. On the west coast oyster aquaculture farms sometimes are unable to purchase young, larval oysters to seed their oyster beds and grow adult oysters for harvest for that supports jobs in the local community. 

Acidified ocean conditions stymie efforts to restore and recover red abalone in California, once a thriving fishery with historical cultural significance to local tribes, by reducing growth and reproduction especially in those adults that experience more acidic waters when young. Dungeness crab, a highly valuable fishery, suffer a dual effect from ocean acidification that not only thins the shells of the bottom dwelling clams and muscle the adult crabs feed on but also causes damage to the carapace (crab shells) and hair-like structures that smaller developing larval crabs use to sense their environment. 

Ocean acidification may reduce shellfish populations in the future that could lead to shifts in communities reliant on shellfish fisheries. Still, there is hope. As research and experience grow, some impacts of ocean acidification on calcifying organisms can be mitigated. Aquaculture rearing facilities can now buffer intake seawater to support shellfish past the more vulnerable young stages. Some research suggests that farming seaweed, which uses CO2 for growth, near oyster farms could improve local conditions for shellfish aquaculture in open waters.

Effects on Ecosystem Services and Tourism

Healthy clam, mussel and oyster beds in nearshore waters help protect the shoreline by reducing wave and tidal action that leads to erosion. They also help reduce coastal inundation that is amplified by sea level rise. Decreases in shellfish populations, and the hard structures formed by these animals, as a result of ocean acidification increases the risk of erosion and flooding in many coastal communities. These important ecosystem services provided by shellfish are referred to as living shorelines and are often more effective protection than hardscaping such as concrete or rock seawalls. 

The shellfish in living shorelines also filter algae and other particles from the seawater and can improve water quality by trapping sediments and reducing nutrient loading from adjacent lands. This can help reduce the potential for harmful algal blooms that can create toxins that affect aquatic species and can reduce air quality for people near the water. These ecosystem services can also be performed by shellfish aquaculture. Decreases in shellfish populations due to ocean acidification may reduce water quality and increase the changes of harmful algal blooms along our coasts.

Coastal communities may have some dependence on tourism related to healthy coastlines and recreational fisheries. Healthy coral reefs not only protect shorelines but are a major draw for tourists interested in snorkeling or diving to see the abundant life supported by healthy reefs.  And the known, negative effects of ocean acidification on these coral reefs could impact tourism by decreasing healthy coral ecosystems that people pay to go see.

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