Ocean Acidification


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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.


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  • Physiological and ecological effects.

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.

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.

Models predict that DMS production will decrease by about 18 percent by from preindustrial levels, which will result in additional radiative forcing corresponding to an atmospheric temperature increase of 0. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind.

Your contribution may be further edited by our staff, and its publication is subject to our final approval. Calcification involves the precipitation of dissolved ions into solid CaCO 3 structures, such as coccoliths. Of the extra carbon dioxide added into the oceans, some remains as dissolved carbon dioxide, while the rest contributes towards making additional bicarbonate and additional carbonic acid.

To maintain chemical equilibrium, some of the carbonate ions already in the ocean combine with some of the hydrogen ions to make further bicarbonate. The increase in concentrations of dissolved carbon dioxide and bicarbonate, and reduction in carbonate, are shown in a Bjerrum plot. Most calcifying organisms live in such waters. The carbonate compensation depth occurs at the depth in the ocean where production is exceeded by dissolution.

Calcium carbonate occurs in two common polymorphs crystalline forms: Aragonite is much more soluble than calcite, so the aragonite saturation horizon is always nearer to the surface than the calcite saturation horizon. Increasing acidity has possibly harmful consequences, such as depressing metabolic rates in jumbo squid , [62] depressing the immune responses of blue mussels, [63] and coral bleaching.

However it may benefit some species, for example increasing the growth rate of the sea star, Pisaster ochraceus , [64] while shelled plankton species may flourish in altered oceans. The report "Ocean Acidification Summary for Policymakers " describes research findings and possible impacts. Although the natural absorption of CO 2 by the world's oceans helps mitigate the climatic effects of anthropogenic emissions of CO 2 , it is believed that the resulting decrease in pH will have negative consequences, primarily for oceanic calcifying organisms.

These span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores , corals , foraminifera , echinoderms , crustaceans and molluscs. However, as ocean pH falls, the concentration of carbonate ions required for saturation to occur increases, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution.

Therefore, even if there is no change in the rate of calcification, the rate of dissolution of calcareous material increases. Corals, [69] [70] [71] coccolithophore algae, [72] [73] [74] [75] coralline algae, [76] foraminifera, [77] shellfish [78] and pteropods [13] [79] experience reduced calcification or enhanced dissolution when exposed to elevated CO 2. The Royal Society published a comprehensive overview of ocean acidification, and its potential consequences, in June When exposed in experiments to pH reduced by 0.


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  4. The fluid in the internal compartments where corals grow their exoskeleton is also extremely important for calcification growth. When the saturation rate of aragonite in the external seawater is at ambient levels, the corals will grow their aragonite crystals rapidly in their internal compartments, hence their exoskeleton grows rapidly. If the level of aragonite in the external seawater is lower than the ambient level, the corals have to work harder to maintain the right balance in the internal compartment.

    When that happens, the process of growing the crystals slows down, and this slows down the rate of how much their exoskeleton is growing. Depending on how much aragonite is in the surrounding water, the corals may even stop growing because the levels of aragonite are too low to pump into the internal compartment. They could even dissolve faster than they can make the crystals to their skeleton, depending on the aragonite levels in the surrounding water.

    What is Ocean Acidification?

    A study conducted by the Woods Hole Oceanographic Institution in January showed that the skeletal growth of corals under acidified conditions is primarily affected by a reduced capacity to build dense exoskeletons, rather than affecting the linear extension of the exoskeleton. Ocean acidification may force some organisms to reallocate resources away from productive endpoints such as growth in order to maintain calcification. In some places carbon dioxide bubbles out from the sea floor, locally changing the pH and other aspects of the chemistry of the seawater.

    Studies of these carbon dioxide seeps have documented a variety of responses by different organisms. In Papua New Guinea , declining pH caused by carbon dioxide seeps is associated with declines in coral species diversity. For example, the elevated oceanic levels of CO 2 may produce CO 2 -induced acidification of body fluids, known as hypercapnia. Also, increasing ocean acidity is believed to have a range of direct consequences. For example, increasing acidity has been observed to: The lower PH was simulated with times the normal amount of CO 2.

    Another possible effect would be an increase in red tide events, which could contribute to the accumulation of toxins domoic acid , brevetoxin , saxitoxin in small organisms such as anchovies and shellfish , in turn increasing occurrences of amnesic shellfish poisoning , neurotoxic shellfish poisoning and paralytic shellfish poisoning. While the full implications of elevated CO 2 on marine ecosystems are still being documented, there is a substantial body of research showing that a combination of ocean acidification and elevated ocean temperature, driven mainly by CO 2 and other greenhouse gas emissions, have a compounded effect on marine life and the ocean environment.

    This effect far exceeds the individual harmful impact of either. Meta analyses have quantified the direction and magnitude of the harmful effects of ocean acidification, warming and deoxygenation on the ocean. Leaving aside direct biological effects, it is expected that ocean acidification in the future will lead to a significant decrease in the burial of carbonate sediments for several centuries, and even the dissolution of existing carbonate sediments.

    The threat of acidification includes a decline in commercial fisheries and in the Arctic tourism industry and economy. Commercial fisheries are threatened because acidification harms calcifying organisms which form the base of the Arctic food webs. Pteropods and brittle stars both form the base of the Arctic food webs and are both seriously damaged from acidification. Pteropods shells dissolve with increasing acidification and the brittle stars lose muscle mass when re-growing appendages. Pteropods are severely affected because increasing acidification levels have steadily decreased the amount of water supersaturated with carbonate which is needed for aragonite creation.

    Arctic food webs are considered simple, meaning there are few steps in the food chain from small organisms to larger predators. For example, pteropods are "a key prey item of a number of higher predators — larger plankton, fish, seabirds, whales". The effects on the calcifying organisms at the base of the food webs could potentially destroy fisheries. For example, decrease in the growth of marine calcifiers such as the American lobster , ocean quahog , and scallops means there is less shellfish meat available for sale and consumption.

    Acidification could damage the Arctic tourism economy and affect the way of life of indigenous peoples. A major pillar of Arctic tourism is the sport fishing and hunting industry. The sport fishing industry is threatened by collapsing food webs which provide food for the prized fish.

    A decline in tourism lowers revenue input in the area, and threatens the economies that are increasingly dependent on tourism. Stabilizing atmospheric CO 2 concentrations at ppm would require near-term emissions reductions, with steeper reductions over time. In order to prevent disruption of the calcification of marine organisms and the resultant risk of fundamentally altering marine food webs, the following guard rail should be obeyed: One policy target related to ocean acidity is the magnitude of future global warming. This would represent a substantial decline in surface ocean pH.

    In the denial, EPA said that risks from ocean acidification were being "more efficiently and effectively addressed" under domestic actions, e. Climate engineering mitigating temperature or pH effects of emissions has been proposed as a possible response to ocean acidification. The IAP [16] statement cautioned against climate engineering as a policy response:. Mitigation approaches such as adding chemicals to counter the effects of acidification are likely to be expensive, only partly effective and only at a very local scale, and may pose additional unanticipated risks to the marine environment.

    There has been very little research on the feasibility and impacts of these approaches. Substantial research is needed before these techniques could be applied. Iron fertilization of the ocean could stimulate photosynthesis in phytoplankton see Iron hypothesis. The phytoplankton would convert the ocean's dissolved carbon dioxide into carbohydrate and oxygen gas, some of which would sink into the deeper ocean before oxidizing. Seagrasses form shallow-water ecosystems along coasts that serve as nurseries for many larger fish, and can be home to thousands of different organisms.

    What is Ocean Acidification?

    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. 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.

    Rising Ocean Temperatures are "Cooking" Coral Reefs

    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.

    Breadcrumb

    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.

    Changes in seawater chemistry

    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. Buffering will take thousands of years, which is way too long a period of time for the ocean organisms affected now and in the near future.

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