Ocean Acidification is a Possible Contri-butor to Ancient Reef Crises

Ocean acidification<!–[if supportFields]> XE “acidification” <![endif]–><!–[if supportFields]><![endif]–> in recent years has been linked to decreases in calcification rates in reef-building corals. Little work has been done to analyze past coral<!–[if supportFields]> XE “coral” <![endif]–><!–[if supportFields]><![endif]–> reef crises and ocean acidification<!–[if supportFields]> XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–> events. Coral reefs have one of the most complete fossil records due to their calcium carbonate<!–[if supportFields]> XE “calcium carbonate (CaCO3)” <![endif]–><!–[if supportFields]><![endif]–> skeletons. Thus, the coral reef fossil record has been widely studied and documented. Ocean acidification in prehistoric times has been connected with several possible mechanisms resulting in large releases of CO2 into the marine environment. Like the fossil record, geologic evidence of these events has been collected in a database. Kiessling and Simpson (2011) investigated a possible relationship between past extinction<!–[if supportFields]> XE “extinction” <![endif]–><!–[if supportFields]><![endif]–> events, reef crises, and ocean acidification trends by examining data from the paleological databases. The authors found that four out of five past reef crises could be partially attributed to ocean acidification and global warming. Out of these five major events, only two had geological proof of a concurrent ocean acidification event. —Emily Putnam
Kiessling, W., Simpson, C., 2011. On the potential for ocean acidification<!–[if supportFields]>XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–> to be a general cause of ancient reef crises. Global Change Biology 17, 56–67.

Kiessling and Simpson from the Leibniz Institute for Research on Evolution and Biodiversity first determined major reef crises by calculating changes in reef volume per time interval and defining crises as significant outliers below the normal. Extinction intensities were found by calculating the rate of extinction<!–[if supportFields]>XE “extinction” <![endif]–><!–[if supportFields]><![endif]–> from the PaleoBiology Database. The selectivity of these extinctions was determined in two ways––by comparing calcified organisms with all others, and by comparing physiologically pH sensitive organisms with all others. The authors also took into consideration the possibility that ocean acidification<!–[if supportFields]>XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–> could have impacted the preservation of the fossil record by measuring the completeness of the record for groups at each time interval.
Kiessling and Simpson found that reef volume changes vary widely over time and only the most severe decreases in volume can be identified as crises. A total of five reef crises were found from the existing records––Late Devonian, Early Triassic, earliest Triassic, Early Jurassic and early Eocene. Four out of five of these crises followed elevated extinction<!–[if supportFields]> XE “extinction” <![endif]–><!–[if supportFields]><![endif]–> rates of calcifying organisms, with three of these showing a preferential extinction of these organisms. Five marine biodiversity<!–[if supportFields]>XE “biodiversity” <![endif]–><!–[if supportFields]><![endif]–> crises were found––the end-Ordivician, Late Devonian, end-Permian, end-Triassic, and end-Cretaceous. Degradation of the fossil record agreed with fossil preservation hypotheses for acidified conditions.
Analysis of the geologic evidence for possible ocean acidification<!–[if supportFields]> XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–> events revealed that four out of five reef crises could have been at least partially due to ocean acidification<!–[if supportFields]> XE “acidification” <![endif]–><!–[if supportFields]><![endif]–>. Ocean acidification was not connected to mass extinctions. Thus, ocean acidification in today’s world may play a role in reef stress, but may not lead to mass extinctions. Despite the attempt to connect the past with the present, the authors could not offer more information relating the magnitude of the crises with CO2 levels to determine the potential impact of the current ocean acidification event on coral<!–[if supportFields]>XE “coral” <![endif]–><!–[if supportFields]><![endif]–> reefs. Global warming was considered to be an equally likely cause of reef degradation and crisis as ocean acidification. Kiessling and Simpson therefore concluded that the greatest danger to coral reefs comes from the double threat of global warming and ocean acidification.

Ocean Acidification Favors the Domin-ance of Turf Algae over Kelp Forests

Research into habitat effects of ocean acidification<!–[if supportFields]> XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–> are primarily concerned with calcifying coral<!–[if supportFields]> XE “coral” <![endif]–><!–[if supportFields]><![endif]–> reefs. While reefs are critical ecosystem builders in tropical areas, these are not the only habitats that could potentially be harmed by acidification<!–[if supportFields]> XE “acidification” <![endif]–><!–[if supportFields]><![endif]–>. A recent study has examined the effects of increased temperature and ocean acidification associated with global warming on temperate habitats dominated by kelp forests (Connell and Russell 2010). The early stage of temperate algal communities is dominated by algae that form a turf over large rocky surfaces. Following a period of relative stability, kelp (also a type of alga) overtakes the turf algae and creates a forest<!–[if supportFields]> XE “forest” <![endif]–><!–[if supportFields]><![endif]–> habitat. Connell and Russell hypothesized that removal of turf algae would result in a greater recruitment of kelp and that turf algae would increase in abundance in high temperature and pH conditions. They found that these hypotheses were true, further solidifying the perception that ocean acidification will have far-reaching effects even in communities not dominated by calcifying organisms. —Emily Putnam
Connell, S.D., Russell, B.D., 2010. The direct effects of increasing CO2 and temperature on non-calcifying organisms: increasing the potential for phase shifts in kelp forests. Proceedings of the Royal Society B: Biological Sciences 277, 1409–1415.

Connell and Russell from the Southern Seas Ecology Laboratories at the University of Adelaide first tested the ability of turf algae to inhibit kelp recruitment. The authors removed turf algae from square meter plots that were found less than 5 meters from existing kelp canopies. These sites were chosen for their location along a metropolitan coastline. After one year, the number of kelp recruits was recorded and compared to control plots where no turf algae were removed. Mesocosm experiments were set up in large 40-liter tanks where turf algae specimens were exposed to one of four combinations of current summer maximal ocean temperature and pH and predicted temperature and pH for the year 2050. After 14 weeks, visual estimations of algal growth on previously uninhabited substrate were made. The algae from this substrate was then scraped from the substrate, dried for two days and weighed. Chlorophyll fluorescence was measured to determine the photosynthetic, or quantum, yield of the algae. Quantum yield is calculated comparing the relative fluorescence following exposure of the algae to a weak beam of red light and then to a strong beam of light.
Kelp recruitment was greater for all sites where turf algae had been removed. Increased temperature led to an increase in turf algae cover, but increased CO2 had no effect. Increased temperature and CO2 combined led to an even greater increase in turf coverage than increased temperature alone. Quantum yield increased slightly with CO2 increases but decreased with temperature increases. The synergistic effects of increased temperature and acidity on turf coverage suggests that the dual threat of acidification<!–[if supportFields]>XE “acidification” <![endif]–><!–[if supportFields]><![endif]–> and global warming would further favor the dominance of turf algae at sites that were once home to large kelp forests. While action can be taken to reduce nutrient levels as a means of reducing turf algae on a local scale, any mitigation will eventually be overcome by these global stresses. The gradual dominance of turf algae will also likely result in a further weakening of existing kelp forests due to reduced resilience. While the results from this study apply directly to coastlines with local human populations, even remote habitats will probably be affected by changes in seawater chemistry and temperature. As habitats shift and change, local herbivorous species distributions will adjust accordingly. Thus, temperate marine communities face as much risk as tropical communities in the face of global warming and ocean acidification<!–[if supportFields]> XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–>.

Early Growth and Development of the Tropical Fish Acanthochromis polyacanthus unaffected by Ocean Acidification

Effects on the calcification and growth due to ocean acidification have been established for several marine invertebrate species. However, little research has been done to examine the effects of ocean acidification on fish. Of the available research, most papers have focused on the acid-base regulatory mechanisms of adult fish, but very few have considered the effects of acidification on juveniles. Munday et al. (2011) studied whether ocean acidification affected the early life cycle of the tropical reef fish, Acanthochromis polyacanthus. The authors believed that an increase in acidity should affect skeletal growth by reducing calcification. They found that increases in dissolved CO2 did not significantly affect the growth and development of these reef fishes. The results suggest that marine fish species may be more tolerant to changes in seawater pH than invertebrates in the same locations. —Emily Putnam
 Munday, P.L., Gagliano, M., Donelson, J.M., Dixson, D.L., Thorrold, S.R., 2011. Ocean acidification does not affect the early life history development of a tropical marine fish. Marine Ecology Progress Series 423, 211–221.

Munday and his colleagues at James Cook University studied the effects of acidification on the growth of A. polyacanthus individuals for the first three weeks of their lives. Freshly hatched A. polyacanthus juveniles were reared in four acidification conditions––the current pH, the projected pH for the year 2100, and two intermediate acidities. At the end of three weeks, the fish were killed and preserved for study. The standard length and weight of each fish was measured to ascertain overall growth. Otoliths, or small ear bones, were removed and examined by photographic analysis to determine the level of asymmetry. Each fish was then stained and measurements were taken for 29 skeletal reference points. Statistical analyses compared the effects due to parentage versus that of acidified conditions.
Munday et al. found that standard length and weight was not significantly different among the treatments. The otoliths were expected to be more strongly impacted by acidified environments because they are composed of aragonite––a compound that is harder to form as pH decreases. However, the authors found that there was no effect of acidification on any aspect of the otoliths. Skeletal measurements agreed with otolith and standard growth measurements––26 of 29 skeletal references were not different amongst the treatments. Statistical analysis showed that these three reference points were not enough to conclude that acidified conditions affected the development of the juvenile. All growth measurements were more strongly connected to genetic variation than to environmental constraints.
Even though early life stages are considered to be more susceptible to environmental conditions, Munday et al. found no negative effects on the early life of A. polyacanthus. They concluded that marine fishes may be more tolerant to ocean acidification than their invertebrate counterparts. One possible explanation for the observed results is that A. polyacanthus juveniles spend all of their early lives on the reef, where large fluctuations in dissolved CO2 levels are common. Thus, this species may be especially tolerant to changes in ocean pH. The authors also posit that 3 weeks may be sufficient for the juveniles to develop to a life stage where they can actively regulate internal acid-base chemistry to overcome the effects of acidification. Taken together, the authors conclude that more expansive studies of species and juvenile-based growth be undertaken to fully consider the effects of ocean acidification of early life development.

Velvet Swimming Crab Necora pu-ber able to Compensate for Medium-Term Exposure to Ocean Acidification

Tolerance to ocean acidification has not been very well studied, and even fewer studies have examined the physiological basis for this tolerance. The velvet swimming crab Necora puber has been shown to be tolerant to acidification on a short-term basis. A recent study examined the physiological implications of a medium-term exposure to acidified conditions (Small et al. 2010). The authors examined a variety of parameters to attempt to get a complete look at how this species tolerates decreases in pH. They found that most physiological processes, including thermal tolerance and immune response, were unaffected by lowered pH levels and that the crabs were able to compensate for these harsh conditions. A decrease in oxygen uptake suggests that decreased energy consumption is associated with internal pH regulation, even though more of the remaining energy consumption is presumably diverted to active excretion of hydrogen ions from the gills. This will likely make the crabs less active predators. —Emily Putnam
Small, D., Calosi, P., White, D., Spicer, J.I., Widdicombe, S., 2010. Impact of medium-term exposure to CO2 enriched seawater on the physiological functions of the velvet swimming crab Necora puber. Aquatic Biology 10, 11–21.

Small and his colleagues at the University of Plymouth studied the effects of acidification on a variety of physiological characteristics of a medium–term exposure length of 30 days. Male N. puber individuals were collected and held in three acidification conditions––the current pH, the projected pH for the year 2100, and a lower value to mimic conditions of extra CO2 inputs in addition to acidification. After 30 days, crabs were placed in a chamber for 50 minutes and the amount of oxygen taken up by the crabs was measured as an indicator of metabolic rate. The upper thermal tolerance was determined by finding the temperature at which the crabs began to spasm and the temperature at which they were unable to right themselves from an upturned position. Haemolymph, or circulatory fluid, was extracted by needle and tested for total CO2 levels and pH. From these values, pCO2 and carbonate ion concentrations were determined by calculation. The immune response to acidification was determined by examining lipid peroxide concentrations in haemolymph samples. Calcium and magnesium haemolymph concentrations were found by atomic absorption spectrometry. Levels of shell mineralization were found by dissolving a portion of the shell, or chelae, in acid and measuring calcium and magnesium ion concentrations.
The oxygen uptake decreased under acidified conditions, and total CO2 levels in haemolymph decreased as a function of decreasing pH. Haemolymph calcium and magnesium concentrations also decreased in acidified conditions, but magnesium concentration in the shell increased slightly at lower pH. Thermal tolerance, lipid peroxidation, and calcium ion concentration in the shell were unaffected by acidification.
Small et al. concluded that N. puber is able to compensate for a medium-term exposure to ocean acidification. This compensation comes without affecting thermal tolerance, or immune response. Decreases in metabolic activity corresponded to increases in calcium in the haemolymph and magnesium in the chelae. Since no impact upon mineralization was observed, the authors concluded that shell dissolution is not the most important process in buffering the body against acidified conditions in the medium term. The ability to buffer pH is likely energetically expensive, which could incur extra costs on physiological processes not examined here. The authors conclude that the decrease in oxygen consumption observed corroborates other papers as a proposed mechanism for conservation of energy and regulation of internal pH in harsh external conditions. As a key predator in its ecosystem, tolerance for ocean acidification will likely give this species an ecological advantage over other predators. However, since oxygen uptake limitations were observed, there could be additional restrictions on the ability of these predators to maintain existing population dynamics in the face of ocean acidification.

Temperature Rise and Ocean Acidifica-tion Impacts Physiological Processes of the Brittlestar Ophiocten sericeum

The Arctic<!–[if supportFields]> XE “Arctic” <![endif]–><!–[if supportFields]><![endif]–> is considered one of the most threatened areas on the planet. Climate change has already greatly impacted the oceanic landscape of this region. Further increases in temperature and acidity are believed to affect the Arctic more strongly than any other region on Earth. Ocean acidification<!–[if supportFields]> XE “acidification” <![endif]–><!–[if supportFields]><![endif]–> has been shown to negatively affect the calcification and growth of many marine organisms. Temperature increases have placed organisms under great thermal stress and narrowed the habitat range of species, especially in the Arctic. Wood et al. (2011) examined the physiological effects of temperature and acidity increases on the Arctic brittlestar Ophiocten sericeum<!–[if supportFields]> XE “Ophiocten sericeum” <![endif]–><!–[if supportFields]><![endif]–>. The authors found that high temperature had no effect on metabolic rate, but resulted in a decrease in muscle density. The results also suggest that a lower pH increased metabolism. Taken together, the authors found that increased temperature and acidity due to climate change could result in decreased survival over the long term.—Emily Putnam

Wood, H., Spicer, J., Kendall, M., Lowe, D., Widdicombe, S., 2011. Ocean warming and acidification<!–[if supportFields]> XE “acidification” <![endif]–><!–[if supportFields]><![endif]–>; implications for the Arctic<!–[if supportFields]> XE “Arctic” <![endif]–><!–[if supportFields]><![endif]–> brittlestar Ophiocten sericeum<!–[if supportFields]> XE “Ophiocten sericeum” <![endif]–><!–[if supportFields]><![endif]–>. Polar Biology 34, 1–12.

Wood and colleagues at the Plymouth Marine Laboratory tested the effects of increased temperature and increased acidity on the physiology of O. sericeum. Six experimental setups were used––three with ambient temperature seawater and three at a higher temperature. The setups at each temperature were further broken down by pH––current ocean pH, the mid-level pH predicted for the year 2100, and the extreme pH level predicted for the year 2100. Some of the brittlestars were chosen for amputation and either 25% or 70% of an arm was removed. All brittlestars were then randomly placed into one of the experimental setups, where they remained for twenty days. After this time, brittlestars that had been left complete were placed into a metabolic chamber for 2 hours. The amount of oxygen respired was measured. These brittlestars then had one complete arm amputated for examination under microscope of the size and density of muscle cells, changes in calcium content, and the thickness of the outer layer of tissue. Previously amputated brittlestars were measured for the amount of arm regenerated following exposure to acidified and/or heated water. Regeneration was measured by comparing the length of the arm after twenty days to the length just after amputation.
Metabolic oxygen uptake increased under conditions of increased acidity, but not increased temperature. Higher temperatures resulted in a decrease in muscle density and lower calcium contents. Regeneration was affected by both increased acidity and temperature, though pH had a greater effect. Functional regeneration was greater at the more basic pH and greater in all of the high temperature setups.
The increase in metabolic rate at lower pH indicates a highly stressful environment for this species. The brittlestars compensate for the low pH with an increased energy demand. Temperature was expected to have an effect on metabolic rate, but no increase was detected. The authors posit that the temperature used in this study was not sufficiently high to find a significant effect on metabolism. The decrease in muscle at high temperatures suggest that pre-existing muscle may have provided an additional energy source for the brittlestars. Wood et al. concluded that increased acidification<!–[if supportFields]> XE “acidification” <![endif]–><!–[if supportFields]><![endif]–> resulted in an increase in energy demand. This energy demand created additional stresses for the brittlestars, forcing them to use reserve energy. This idea was further supported by the faster regeneration in acidified and hotter waters. The brittlestars use energy faster and require more energy than can be adequately taken in from their surroundings. The authors conclude that O. sericeum may be able to survive short-term increases in temperature and acidity, but long-term survival may not be possible under the current predictions.

Temperature Rise and Ocean Acidification Impacts Physiological Processes of the Brittlestar Ophiocten sericeum

The Arctic is considered one of the most threatened areas on the planet. Climate change has already greatly impacted the oceanic landscape of this region. Further increases in temperature and acidity are believed to affect the Arctic more strongly than any other region on Earth. Ocean acidification has been shown to negatively affect the calcification and growth of many marine organisms. Temperature increases have placed organisms under great thermal stress and narrowed the habitat range of species, especially in the Arctic. Wood et al. (2011) examined the physiological effects of temperature and acidity increases on the Arctic brittlestar Ophiocten sericeum. The authors found that high temperature had no effect on metabolic rate, but resulted in a decrease in muscle density. The results also suggest that a lower pH increased metabolism. Taken together, the authors found that increased temperature and acidity due to climate change could result in decreased survival over the long term.––Emily Putnam
Wood, H., Spicer, J., Kendall, M., Lowe, D., Widdicombe, S., 2011. Ocean warming and acidification; implications for the Arctic brittlestar Ophiocten sericeum. Polar Biology 34, 1–12.

Wood and colleagues at the Plymouth Marine Laboratory tested the effects of increased temperature and increased acidity on the physiology of O. sericeum. Six experimental setups were used––three with ambient temperature seawater and three at a higher temperature. The setups at each temperature were further broken down by pH––current ocean pH, the mid-level pH predicted for the year 2100, and the extreme pH level predicted for the year 2100. Some of the brittlestars were chosen for amputation and 25% or 70% of an arm was removed. All brittlestars were then randomly placed into one of the experimental setups, where they remained for twenty days. After this time, brittlestars that had been left complete were placed into a metabolic chamber for 2 hours. The amount of oxygen respired was measured. These brittlestars then had one complete arm amputated for examination under microscope of the size and density of muscle cells, changes in calcium content, and the thickness of the outer layer of tissue. Previously amputated brittlestars were measured for the amount of arm regenerated following exposure to acidified and/or heated water. Regeneration was measured by comparing the length of the arm after twenty days to the length just after amputation.
Metabolic oxygen uptake increased under conditions of increased acidity, but not increased temperature. Higher temperatures resulted in a decrease in muscle density and lower calcium contents. Regeneration was affected by both increased acidity and temperature, though pH had a greater effect. Functional regeneration was greater at the more basic pH and greater in all of the high temperature setups.
The increase in metabolic rate at lower pH indicates a highly stressful environment for this species. The brittlestars compensate for the low pH with an increased energy demand. Temperature was expected to have an effect on metabolic rate, but no increase was detected. The authors posit that the temperature used in this study was not sufficiently high to find a significant effect on metabolism. The decrease in muscle at high temperatures suggest that pre-existing muscle may have provided an additional energy source for the brittlestars. Wood et al. concluded that increased acidification resulted in an increase in energy demand. This energy demand created additional stresses for the brittlestars, forcing them to use reserve energy. This idea was further supported by the faster regeneration in acidified and hotter waters. The brittlestars use energy faster and require more energy than can be adequately taken in from their surroundings. The authors conclude that O. sericeum may be able to survive short-term increases in temperature and acidity, but long-term survival may not be possible under the current predictions.

Ocean Acidification Negatively Impacts the Productivity of Photosynthetic Coral Symbionts

Many corals have a symbiotic relationship with symbiotic dinoflagellates. These photosynthetic symbionts are vital to the survival and health of their coral hosts. Ocean acidification has been shown to inhibit the calcification coral structures, but few studies have focused on the impact on the symbiotic dinoflagellates. Crawley et al. (2010) tested the impact of ocean acidification on several key aspects of dinoflagellates of the genus Symbiodinium associated with the coral Acropora formosa. The authors found that a decrease in ocean pH resulted in an increase of the pigment chlorophyll a per cell, an increase in xanthophyll de-epoxidation, and a decrease in photosynthetic capacity per chlorophyll. Phosphoglycolate phosphatase (PGPase), an enzyme that enables carbon fixation by the symbionts, was also impacted by an increase in ocean acidity. These findings suggest symbiont productivity might increase under conservative increases in ocean acidity, but will likely decrease according to the current trajectory.––Emily Putnam
Crawley, A., Kline, D.I., Dunn, S., Anthony, K., Dove, S., 2010. The effect of ocean acidification on symbiont photorespiration and productivity in Acropora formosa. Global Change Biology 16, 851–863.

Crawley and colleagues at the University of Queensland performed a variety of tests to assess both physiological changes due to increased CO2 and the genetic expression of the enzyme PGPase. The physical tests executed were the respirometry assays, cell counting, and pigment analysis. Respirometry assays are designed to examine photosynthesis as a product of controlled amounts of light. This test determines respiration that is produced without light, as well as the photosynthetic efficiency and capacity of the symbionts. The symbiont cells were counted for each trial to see whether acidification impacted the number of dinoflagellates found on a coral specimen. The pigments produced by the symbionts were separated by chromatographic techniques. The authors specifically looked for the pigment xanthophyll and evidence of xanthophyll de-epoxidation. Xanthophyll de-epoxidation is an important indicator for this study because the process of de-epoxidation quenches extra energy from light and protects the symbiont from damage.
Genetics testing centered on expression of PGPase. The sequence for PGPase was obtained from a database and then compared with similar sequences from other photosynthetic organisms­­, including diatoms and terrestrial plants. RNA was extracted from A. formosa specimens. Quantitative real time reverse transcription polymerase chain reaction was performed to measure the expression of PGPase in the symbionts. A test was done using coral RNA with no symbionts to ensure that the primers used to isolate the sequence coding for PGPase did not amplify the coral’s RNA as well.
The chlorophyll a concentrations per cell increased along with increases in acidity. The density of cells was not affected by acidification. Xanthophyll de-epoxidation also increased as a result of acidification. PGPase expression was reduced by 50% in the severely acidified conditions. Crawley et al. claim that a coincidental decrease in photosynthetic productivity suggests a link between PGPase expression and photosynthetic output, but will require further testing to confirm this hypothesis. Productivity increases under conservative increases in acidity, but did not for the current trajectory of ocean acidification. The authors posit that sharp increases in acidity may overwhelm the symbionts capacity for dissipating energy and that the loss of these systems may be involved in the loss of productivity.

Ocean Acidification Reduces Recruitment of the Reef-Building Coral Acropora palmata

Coral reefs are one of the most diverse ecosystems on the planet. As CO2 emissions grow, the oceans absorb a greater amount of CO2 in a process known as ocean acidification. Recent studies have focused on the impact of ocean acidification on corals’ ability to grow a calcified skeleton. However, very little experimentation has been done to examine the impact of acidification on the early life stages of coral. A recent study has shown that an increase in dissolved CO2 has reduced the recruitment abilities of Acropora palmata juveniles (Albright et al. 2010). Albright et al. found that increased acidity resulted in a decrease in fertilization, settlement, and early growth. Reductions in recruitment can severely impact the size of coral reefs and limit the ability of reefs to recover from large-scale disasters. Thus, ocean acidification places further stress on an already threatened ecosystem.––Emily Putnam
Albright, R., Mason, B., Miller, M., Langdon, C., 2010. Ocean acidification compromises recruitment success of the threatened Caribbean coral Acropora palmata. Proceedings of the National Academy of Sciences 107, 20400–20404.

Albright and colleagues at University of Miami and the National Oceanic and Atmospheric Administration performed assays to determine the effect of increased ocean acidity on the three components of recruitment––the successful development of larvae, the ability of larvae to settle and the growth of larvae after settling. A fertilization assay, a settlement assay, and a growth assay were executed. Each test was performed at dissolved CO2 levels either consistent with today’s values, at the lowest value estimated for the year 2100, or at the highest estimate for the year 2100. The fertilization assay determined the number of successful fertilizations for a range of optimal sperm concentrations as a function of dissolved CO2. The settlement assay determined the number of larvae settled onto tiles. The growth assay measured the upward growth of the larvae once they were settled. Taken together, these assays measure the recruitment success of Acropora palmata juveniles.
Albright et al. found a significant decrease in the recruitment success of Acropora palmata juveniles. Ocean acidification reduced fertilization success by an average of 12–13%. The number of settled larvae was reduced by 45% at the low estimate and by 69% at the high estimate for dissolved CO2 levels in the year 2100. Growth was limited by 39% at the low end of the range and by 50% at the high end. The authors conclude that there was a net reduction of 52­­–73% in larval recruitment. Further studies are necessary to determine whether the mechanisms involved in reducing recruitment success are direct or indirect.
Reduced growth in larval settlement stages further supports the observation of reduced coral skeleton growth in adults under acidified conditions. The authors have found that ocean acidification will negatively impact larval coral stages in addition to the widely recognized impacts on adult stages. The magnitude of this problem is further revealed when you consider that approximately 75% of coral species reproduce in the same way as Acropora palmata. Thus, coral communities throughout the globe may be at risk for reductions in recruitment.
Catastrophic events further degrade and damage coral reefs. Global climate change has contributed to an increase in frequency of catastrophic storms and to an increase in global seawater temperatures. These events have severely weakened the resiliency and health of coral reef systems throughout the world. The success of recruitment is vital to recovery following destructive events. An increase in acidity over the next century will further limit recovery and reduce coral population sizes. 

Fish Prefer the Smell of Predators in Acidified Conditions

Ocean acidification as a result of increased CO2 emissions by human activity has been shown to have a variety of secondary effects on marine organisms. Dixson et al. (2010) studied the ability of orange clownfish, Amphiprion percula, to detect the presence of predators by smell under acidic conditions. Newly hatched and settlement stage larvae prefer waters that contain no trace of other fish—either predatory or non-predatory. The authors simulated the more acidic conditions predicted for the year 2100 to determine whether fish raised in more acidic conditions would be able to detect predators. Although newly hatched larvae in acidic conditions navigated away from predator signals, settlement stage larvae preferred water containing the chemical signature of predators over waters without any chemical signal and waters containing the chemical signature of non-predators. The results suggest that clownfish may exhibit risky settlement choices as oceans continue to acidify.—Emily Putnam

Dixson, D.L., Munday, P.L., Jones, G.P., 2010. Ocean acidification disrupts the innate ability of fish to detect predator olfactory cues. Ecology Letters 13, 68–75.­

          Dixson and colleagues at James Cook University compared the ability of settlement-stage larvae and newly hatched Amphiprion percula to detect predators. Two species of predators were chosen­, Cephalopholis cyanostigma and Pseudochromis fuscus. Two species of non-predators, Acanthurus pyroferus and Siganus corallinus, were chosen as further controls on the ability of the clownfish to detect predator signals. The clownfish were raised in water either at pH levels consistent with today’s oceans or at pH levels predicted for the year 2100. The fish were then placed individually into chambers with two streams of water. These tests focused on six combinations: normal water vs. normal water, predator 1 vs. normal water, predator 2 versus normal water, non-predator 1 vs. normal water, non-predator 2 vs. normal water, predator 1 vs. non-predator 1, predator 2 vs. non-predator 2. The clownfish were monitored for stream choice every five seconds for two minutes. To account for side preferences unrelated to chemical signals, the fish were allowed to rest for one minute while the streams were switched, then monitoring was repeated.
          Although newly hatched and settlement-ready clownfish raised in today’s waters preferred normal water and water with non-predatory chemical cues over those of predators, Amphiprion percula larvae raised in acidified conditions preferred water containing the chemical signatures of predatory fish and could not make a distinction between the chemical signatures of predatory vs. non-predatory fish. Newly hatched clownfish from acidified conditions followed the preferences of fish from today’s waters. The difference between newly hatched and settlement-age fish reflects the vulnerability of these two ages. Newly hatched fish by today’s standards would avoid the chemical signals of all fish and swim to the open ocean, whereas settlement-ready fish need to settle on reefs where they are more likely to be near other fish species. This could account for the discrepancy in sensing abilities between newly hatched and settlement-age orange clownfish individuals.
The larvae’s ability to detect predators can be directly related to the chance of survival. Fish that are unable to detect chemical cues will be more likely to settle near predators and will increase their chance of becoming food. This risky behavior could contribute to a large decrease in the population sizes and could even lead to extinction. One important thing to note is that the fish in the acidification samples were treated using acidity levels predicted for 2100. It is possible that a gradual increase of ocean acidity could instigate the adaptation of senses to be able to distinguish predators even under acidified conditions. However, the ability to adapt and the exact pH levels at which these fish are affected has yet to be determined. Dixson et al. have shown that increases in ocean acidity can affect the ability of fish to determine the presence of a predator and to distinguish between predators and non-predators.