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Ocean acidification refers to a reduction in the pH of the ocean over an extended period time, caused primarily by uptake of carbon dioxide (CO2) from the.
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In addition, when marine organisms die and fall to the ocean floor, CO 2 is released through the process of decomposition. Under the worst-case scenarios outlined above, with seawater pH dropping to between 7. Under such conditions, marine calcifiers would have substantially less material to maintain their shells and skeletons. Laboratory experiments in which the pH of seawater has been lowered to approximately 7. As a result, their small size places them at higher risk of being eaten by predators.

Furthermore, the shells of some organisms—for instance, pteropods , which serve as food for krill and whales —dissolve substantially after only six weeks in such high-acid environments.

Changes in seawater chemistry

Larger animals such as squid and fishes may also feel the effects of increasing acidity as carbonic acid concentrations rise in their body fluids. In addition, many marine scientists suspect the substantial decline in oyster beds along the West Coast of the United States since to be caused by the increased stress ocean acidification places on oyster larvae.

It may make them more vulnerable to disease. Physiological changes brought on by increasing acidity have the potential to alter predator-prey relationships. Some experiments have shown that the carbonate skeletons of sea urchin larvae are smaller under conditions of increased acidity; such a decline in overall size could make them more palatable to predators who would avoid them under normal conditions. In turn, decreases in the abundance of pteropods, foraminiferans , and coccoliths would force those animals that consume them to switch to other prey.

What is Ocean Acidification?

The process of switching to new food sources would cause several predator populations to decline while also placing predation pressure on organisms unaccustomed to such attention. Many scientists worry that many marine species, some critical to the proper functioning of marine food chains, will become extinct if the pace of ocean acidification continues, because they will not have sufficient time to adapt to the changes in seawater chemistry.

The deeper waters of the ocean are naturally more acidic than the upper layers, since CO 2 that dissolves at the surface descends with dense, cold water as part of the thermohaline circulation. In midlatitude waters and in waters closer to the poles, many so-called cold-water coral communities are found at depths that range from 40 to 1, metres about to 3, feet —as opposed to their warm-water counterparts, the tropical coral reefs, which are rarely found below metres feet.

Since about the year , studies have shown, increased acidity has raised the saturation horizon about 50 to metres about to feet in midlatitude and polar waters. This change is enough to threaten cold-water coral communities, and some scientists fear that additional communities will be placed at risk if the boundary approaches the surface of the ocean. A decline in cold-water marine calcifiers would result in a decline in reef building, and other marine organisms that depend on corals for their habitat and food would decline as well.

Scientists also predict that, if ocean acidification were to increase worldwide, warm-water coral communities, which often supply food and tourism revenue to people who live near them, would suffer similar fates. In addition, scientists predict that the reduction of marine phytoplankton populations due to rising pH levels in the oceans will produce a positive feedback that intensifies global warming.

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Journal of Experimental Marine Biology and Ecology. Archived from the original on 3 September Retrieved 4 June Uncovering the Mechanism" PDF. Archived from the original PDF on 6 November Spicer; Stephen Widdicombe A noisier ocean at lower pH" PDF. Archived from the original PDF on 30 October All Things Considered, 12 August Archived from the original on 11 December Retrieved 28 September Meta-analysis reveals complex marine biological responses to the interactive effects of ocean acidification and warming, Ecol Evol.

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What is Ocean Acidification?

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Carbon capture and storage Efficient energy use Low-carbon economy Nuclear power Renewable energy. Individual action on climate change Simple living. These organisms make their energy from combining sunlight and carbon dioxide—so more carbon dioxide in the water doesn't hurt them, but helps. Seagrasses form shallow-water ecosystems along coasts that serve as nurseries for many larger fish, and can be home to thousands of different organisms.

Under more acidic lab conditions, they were able to reproduce better, grow taller, and grow deeper roots—all good things. However, they are in decline for a number of other reasons—especially pollution flowing into coastal seawater—and it's unlikely that this boost from acidification will compensate entirely for losses caused by these other stresses.

Some species of algae grow better under more acidic conditions with the boost in carbon dioxide. But coralline algae , which build calcium carbonate skeletons and help cement coral reefs, do not fare so well.

How pollution is changing the ocean's chemistry

Most coralline algae species build shells from the high-magnesium calcite form of calcium carbonate, which is more soluble than the aragonite or regular calcite forms. One study found that, in acidifying conditions, coralline algae covered 92 percent less area, making space for other types of non-calcifying algae, which can smother and damage coral reefs. This is doubly bad because many coral larvae prefer to settle onto coralline algae when they are ready to leave the plankton stage and start life on a coral reef. One major group of phytoplankton single celled algae that float and grow in surface waters , the coccolithophores , grows shells.

Early studies found that, like other shelled animals, their shells weakened, making them susceptible to damage. But a longer-term study let a common coccolithophore Emiliania huxleyi reproduce for generations, taking about 12 full months, in the warmer and more acidic conditions expected to become reality in years. The population was able to adapt, growing strong shells.

It could be that they just needed more time to adapt, or that adaptation varies species by species or even population by population. While fish don't have shells, they will still feel the effects of acidification. Because the surrounding water has a lower pH, a fish's cells often come into balance with the seawater by taking in carbonic acid. This changes the pH of the fish's blood, a condition called acidosis. Although the fish is then in harmony with its environment, many of the chemical reactions that take place in its body can be altered.

Just a small change in pH can make a huge difference in survival. In humans, for instance, a drop in blood pH of 0. Likewise, a fish is also sensitive to pH and has to put its body into overdrive to bring its chemistry back to normal. To do so, it will burn extra energy to excrete the excess acid out of its blood through its gills, kidneys and intestines.

It might not seem like this would use a lot of energy, but even a slight increase reduces the energy a fish has to take care of other tasks, such as digesting food, swimming rapidly to escape predators or catch food, and reproducing. It can also slow fishes growth. Even slightly more acidic water may also affects fishes' minds. While clownfish can normally hear and avoid noisy predators, in more acidic water, they do not flee threatening noise. Clownfish also stray farther from home and have trouble "smelling" their way back.

This may happen because acidification, which changes the pH of a fish's body and brain, could alter how the brain processes information. Additionally, cobia a kind of popular game fish grow larger otoliths —small ear bones that affect hearing and balance—in more acidic water, which could affect their ability to navigate and avoid prey. While there is still a lot to learn, these findings suggest that we may see unpredictable changes in animal behavior under acidification.

The ability to adapt to higher acidity will vary from fish species to fish species, and what qualities will help or hurt a given fish species is unknown. A shift in dominant fish species could have major impacts on the food web and on human fisheries. But to predict the future—what the Earth might look like at the end of the century—geologists have to look back another 20 million years.

The main difference is that, today, CO 2 levels are rising at an unprecedented rate— even faster than during the Paleocene-Eocene Thermal Maximum. Researchers will often place organisms in tanks of water with different pH levels to see how they fare and whether they adapt to the conditions. They also look at different life stages of the same species because sometimes an adult will easily adapt, but young larvae will not—or vice versa. Studying the effects of acidification with other stressors such as warming and pollution, is also important, since acidification is not the only way that humans are changing the oceans.

In the wild, however, those algae, plants, and animals are not living in isolation: So some researchers have looked at the effects of acidification on the interactions between species in the lab, often between prey and predator. Results can be complex. In more acidic seawater, a snail called the common periwinkle Littorina littorea builds a weaker shell and avoids crab predators—but in the process, may also spend less time looking for food.

Boring sponges drill into coral skeletons and scallop shells more quickly. And the late-stage larvae of black-finned clownfish lose their ability to smell the difference between predators and non-predators, even becoming attracted to predators. For example, the deepwater coral Lophelia pertusa shows a significant decline in its ability to maintain its calcium-carbonate skeleton during the first week of exposure to decreased pH. But after six months in acidified seawater, the coral had adjusted to the new conditions and returned to a normal growth rate.

There are places scattered throughout the ocean where cool CO 2 -rich water bubbles from volcanic vents, lowering the pH in surrounding waters. Scientists study these unusual communities for clues to what an acidified ocean will look like.


Researchers working off the Italian coast compared the ability of 79 species of bottom-dwelling invertebrates to settle in areas at different distances from CO 2 vents. For most species, including worms, mollusks, and crustaceans, the closer to the vent and the more acidic the water , the fewer the number of individuals that were able to colonize or survive. Algae and animals that need abundant calcium-carbonate, like reef-building corals, snails, barnacles, sea urchins, and coralline algae, were absent or much less abundant in acidified water, which were dominated by dense stands of sea grass and brown algae.

Only one species, the polychaete worm Syllis prolifers , was more abundant in lower pH water. The effects of carbon dioxide seeps on a coral reef in Papua New Guinea were also dramatic, with large boulder corals replacing complex branching forms and, in some places, with sand, rubble and algae beds replacing corals entirely. One challenge of studying acidification in the lab is that you can only really look at a couple species at a time. To study whole ecosystems—including the many other environmental effects beyond acidification, including warming, pollution, and overfishing—scientists need to do it in the field.

Scientists from five European countries built ten mesocosms—essentially giant test tubes feet deep that hold almost 15, gallons of water—and placed them in the Swedish Gullmar Fjord. After letting plankton and other tiny organisms drift or swim in, the researchers sealed the test tubes and decreased the pH to 7.

Now they are waiting to see how the organisms will react , and whether they're able to adapt. If this experiment, one of the first of its kind, is successful, it can be repeated in different ocean areas around the world. If the amount of carbon dioxide in the atmosphere stabilizes, eventually buffering or neutralizing will occur and pH will return to normal. This is why there are periods in the past with much higher levels of carbon dioxide but no evidence of ocean acidification: But this time, pH is dropping too quickly.