Ocean Acidification

What is ocean acidification?

Chemical dynamics of atmospheric carbon dioxide combining with ocean water. Image courtesy of Phil Munday of James Cook University.
Chemical dynamics of atmospheric carbon dioxide combining with ocean water. Image courtesy of Phil Munday of James Cook University.

Ocean acidification is the changing of the carbonate chemistry of Earth's oceans due to absorption of carbon dioxide (CO2). About 30% of atmospheric CO2 is absorbed by the world's oceans. This absorption is changing their delicate chemical balance, making it difficult for calcifying organisms, such as corals, molluscs, and shellfishes, to produce their carbonate skeletons.

As increased amounts of carbon dioxide are absorbed into the oceans, their waters are becoming more acidic. The carbon dioxide mixes with water, forming carbonic acid and lowering pH levels. This changing chemistry is slowing the calcification rates of corals and other calcifiers.

Calcification rates in the tropics may decrease by 30% over the next century (Kleypas et al. 1999). This decline will be particularly harmful to coral reefs, fishes, and other organisms that rely on reefs for habitat.

Ecological effects of ocean acidification

Damselfish sheltering in a branching coral colony.
Damselfish sheltering in a branching coral colony.

Ocean acidification could disrupt the marine food web by destroying coral reef habitat, reducing biodiversity, and potentially causing various species to go extinct. Such habitat destruction could affect fishing, tourism, and any human activity that involves the oceans.

It is known that 10% of coral reef fishes rely specifically on corals; however, studies have shown that the abundance of 75% of species of coral reef fishes declined following decreases in coral abundance and that 50% of these fish species showed a decline >50% (Wilson et al. 2006).

Small juvenile fishes live in or near live corals, making coral reefs important fish habitat. Decreases in rates of calcification due to ocean acidification could lead to loss of coral reefs and declines in abundance of fish populations.

Estimated aragonite saturation states of the surface ocean from the year 1765 through 2100 in 5-year increments (Feely et al., 2009), based on 
        the modeling results of Orr et al. (2005) and a Business-As-Usual CO2 emissions scenario. 
        The distributions of deep-sea coral banks are from Guinotte et al. (2006).
Estimated aragonite saturation states of the surface ocean from the year 1765 through 2100 in 5-year increments (Feely et al., 2009), based on the modeling results of Orr et al. (2005) and a Business-As-Usual CO2 emissions scenario. The distributions of deep-sea coral banks are from Guinotte et al. (2006).

What is being done about ocean acidification?

Mosean Hale-Aloha mooring
                 that measures surface-water CO2.  Photo courtesy of the NOAA Pacific Marine Environmental Laboratory.
Mosean Hale-Aloha mooring that measures surface-water CO2. Photo courtesy of the NOAA Pacific Marine Environmental Laboratory.

In 2009, the Federal Ocean Acidification Research and Monitoring Act was passed into law and acknowledged that the Earth's ocean chemistry is changing and the importance of understanding both the ecological and social impacts of these changes.

A national research plan has been established to understand and predict how ecosystems will respond to ocean acidification. Specifically, this plan addresses these three hypotheses:

  1. Rate and magnitude of acidification will vary across time, space, and depth as a consequence of local and regional geochemical, hydrological, and biological mechanisms
  2. Ocean acidification will change ecosystem structure, function, and biodiversity via both direct and indirect effects
  3. Heterogeneity in species-specific responses, local environmental and regional considerations will confer a broad range of vulnerabilities that differ both locally and regionally

Why is it important to monitor both open-ocean and nearshore carbonate chemistry?

Most of what we know about changes in carbonate chemistry that have and will lead to increased acidity (decreased pH) or ocean acidification is based on open-ocean observations. Global climate models that predict future changes in carbonate chemistry are likewise for the open ocean. We don't know if the waters of coastal and nearshore coral reefs will change in the same way as waters of the open ocean might change or if the biogeochemical processes of nearshore environments will result in different rates of change.

Monitoring of ocean acidification

For its Pacific Reef Assessment and Monitoring Program (Pacific RAMP), CRED has study sites at more than 50 islands and atolls in the central and western Pacific across gradients of biogeography, ocean and environmental conditions, and human activities.

Corals and other biota of coral reefs affect their surrounding waters through the normal physiological processes of respiration and photosynthesis. Because of this relationship, it is important to monitor the changing carbonate chemistry that surrounds reefs. CRED long-term monitoring sites focus specifically on coral reefs instead of open-ocean areas.

Carbonate chemistry

Map showing locations where CRED carbonate chemistry instruments have been or will be deployed.
Map showing locations where CRED carbonate chemistry instruments have been or will be deployed.

CRED scientists deploy a broad range of oceanographic instruments and collect water samples at study sites to measure the following water chemistry parameters: dissolved inorganic carbon (DIC), total alkalinity, temperature, salinity, and concentrations of chlorophyll-a and dissolved inorganic nutrients. Each of these parameters is measured in both surface and bottom waters with a sampling interval of 2-3 years.

In addition, daily water sampling is conducted with moored autonomous (MAP-CO2) buoys, such as the Hale-Aloha mooring shown above, to measure changes in water chemistry over a period of 24–48 hours.

These measurements are important to derive the pH and saturation state of waters surrounding coral reefs and will be important predictors of the effects of ocean acidification on calcification rates of corals and other organisms.

Ecological effects

To date, most of the ocean acidification research that targets biological effects has focused on laboratory response experiments rather than examination of what is occurring naturally. In response to this reality, in 2010, CRED initiated a number of ocean acidification projects to better understand and monitor biological responses to ocean acidification in the natural environment. These projects include examination of spatial patterns and temporal trends in (1) coral calcification, extension, and growth rates, (2) net calcium carbonate accretion rates, (3) biodiversity of cryptic organisms, and (4) rates of bioerosion.

Coral calcification, extension, and growth rates

CRED, in collaboration with the Woods Hole Oceanographic Institution, has collected coral cores from massive reef-building corals and samples of branching corals to examine coral calcification, extension, and growth. Cores are analyzed with a computed tomography (CT) scanner to determine the density of their calcium carbonate skeletons over the past few decades (or centuries for long cores). These scans provide a historical record of marine conditions that might help in prediction of the effects of ocean acidification on the ability of corals to calcify.

Diagram illustrating coral-coring process and core analysis. Photo courtesy of the Cohen Lab, Woods Hole Oceanographic Institution.
Diagram illustrating coral-coring process and core analysis. Photo courtesy of the Cohen Lab, Woods Hole Oceanographic Institution.
CT scan of a coral core. Photo courtesy of the Cohen Lab, Woods Hole Oceanographic Institution.
CT scan of a coral core. Photo courtesy of the Cohen Lab, Woods Hole Oceanographic Institution.

Net calcium carbonate accretion rates

A CAU two years after deployment.
A CAU two years after deployment.

Rates of net calcium carbonate accretion are monitored with calcification accretion units (CAUs), which allow for recruitment and colonization of crustose coralline algae and hard corals. By measuring net accretion we can determine how much calcium carbonate is deposited on a coral reef in a given period of time. Total net accretion on coral reefs can be calculated by measuring the change in weight of CAUs deployed on the reef for periods of 2-3 years. We hypothesize that net accretion will vary based on island, region, and habitat and will change over time. By monitoring net accretion on coral reefs, CRED will be able to detect changes in calcification rates.

In association with the Scripps Institution of Oceanography of the University of California San Diego, CRED also will use CAUs to monitor what calcifying organisms are present. Photographs are taken of each CAU and analyzed to determine community composition. Community composition varies across islands and gradients.

As an example of variation in composition and calcification, the photographs below show CAU plates retrieved from Rose Atoll and Taʻu Island in American Samoa. The plates from Rose Atoll are dominated by calcifying red algae and have a higher rate of calcification than do the plates from Taʻu. Through analysis of water chemistry and benthic communities, CRED scientists are trying to determine why these differences occur. It is important to document these differences now so that changes in calcification rate or community structure that are caused by ocean acidification can be identified in the future.

Plates from CAUs retrieved from Rose Atoll. Plates from CAUs retrieved from Taʻu Island in American Samoa.
Plates from CAUs retrieved from Rose Atoll and Taʻu Island in American Samoa.

Biodiversity of cryptic organisms

An ARMS three years after deployment at Rose Atoll.
An ARMS three years after deployment at Rose Atoll.

CRED has deployed autonomous reef monitoring structures (ARMS) to examine the biodiversity and community structure of the cryptobiota community. The cryptobiota community is targeted for biodiversity and community composition measurements because it is the most numerically abundant and diverse community on a reef system (Ginsburg 1983), and the organisms within this community are extremely important in reef trophic dynamics (Hutchings 1985; Enoch 2010), and are vulnerable to direct and indirect effects of acidification as their calcification processes, life histories, and habitats respond to changes in carbonate chemistry (Byrne 2011).

Recent examinations into ecosystem responses to ocean acidification in and around shallow-water (0.5 - 3 m) CO2 vents have attributed a decrease in diversity, biomass, and trophic complexity of the cryptobiota community to the modified carbonate chemistry along venting sites (Kroeker et al. 2011). ARMS research will help lessen the critical gap in understanding how ecosystem dynamics may change under ocean acidification in the natural environment.

Rates of bioerosion

A bioerosion block deployed with a CAU. Photo courtesy of Megan Donahue of HIMB.
A bioerosion block deployed with a CAU. Photo courtesy of Megan Donahue of HIMB.

Bioerosion is an important process to consider in determination of the effects of ocean acidification. If a reef is eroding faster than it is accreting, its coral skeleton will weaken and, eventually, massive reef structures will convert to rubble, sand, and silt, thereby altering the entire ecosystem.

In collaboration with the Donahue Lab at the Hawaiʻi Institute of Marine Biology (HIMB) of the University of Hawaiʻi at Mānoa, CRED has deployed bioerosion blocks at study sites in the main and Northwestern Hawaiian Islands to monitor rates of bioerosion. In collaboration with NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML), CRED will be expanding the number of monitoring sites where bioerosion blocks are deployed in the Pacific.

References

Brainard R, Bainbridge S, Brinkman R, Eakin CM, Field M, Gattuso JP, Gledhill D, Green A, Gramer L, Hendee J, Hoeke R, Holbrook S, Hoegh-Guldberg O, Lammers M, Manzello D, McManus M, Moffitt R, Monaco M, Morgan J, Obura D, Planes S, Schmitt R, Steinberg C, Sweatman H, Vetter O, Wilkinson C, Wong K
2010. An international network of coral reef ecosystem observing systems (I-CREOS). In: Hall J, Harrison DE, Stammer D (eds.) Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society (Vol. 2), Venice, Italy, 21-25 September 2009. ESA Publication WPP-306. DOI: 10.5270/OceanObs09.cwp.09
Byrne M
2011. Impact of ocean warming and ocean acidification on marine invertebrate life history stages: vulnerabilities and potential for persistence in a changing ocean. Oceanography and Marine Biology: An Annual Review. 49: 1-42
Enoch IC
2010. Motile cryptofauna of an eastern Pacific coral reef: biodiversity and trophic contribution. Ph.D. diss., 219 p. +Appendices. Open Access Dissertations. Paper 497.University of Miami, Coral Gables, FL.
Feely RA, Doney SC, Cooley SR
2009. Ocean Acidification: Present conditions and future changes in a high- CO2 world. Oceanography. 22(4): 36-47
Ginsburg, RN
1983. Geological and biological roles of cavities in coral reefs. In: Barnes DJ (ed.) Perspectives on coral reefs, p.148-153. D. J. Barnes. Australian Institute of Marine Science, Townsville, Australia, 277 p.
Hutchings PA
1985. Cryptofaunal communities of coral reefs. Acta Oceanologica Sinica 5: 603-613.
Kleypas JA, Buddemeier RW, Archer D, Gattuso JP, Langdon C, Opdyke BN
1999. Geochemical consequences of increased atmospheric carbon dioxide on coral reefs. Science 284: 118-120.
Kleypas JA, Feely RA, Fabry VJ, Langdon C, Sabine CL, Robbins LL
2006. Impacts of ocean acidification on coral reefs and other marine calcifiers: a guide for future research, report of a workshop held 18–20 April 2005, St. Petersburg, FL, sponsored by NSF, NOAA, and the U.S. Geological Survey, 88 p.
Kroeker KL, Micheli F, Gambi MC, Martz TR
2011. Divergent ecosystem responses within a benthic marine community to ocean acidification. Proceedings of the National Academy of Sciences, USA 108(35): 14515-14520.
Orr JC, Fabry VJ, Aumont O, Bopp L, Doney SC, Feely RA, Gnanadesikan A, Gruber N, Ishida A, Joos F, Key RM, Lindsay K, Maier-Reimer E, Matear R, Monfray P, Mouchett A, Najjar RG, Plattner G-K, Rodgers KB, Sabine CL, Sarmiento JL, Schlitzer R, Slater RD, Totterdell IJ, Weirig M-F, Yamanaka Y, Yool A.
2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437: 681-686.
Wilson SK, Graham NAJ, Pratchett MS, Jones GP, Polunin NVC
2006. Multiple disturbances and the global degradation of coral reefs: are reef fishes at risk or resilient? Global Change Biology 12(11): 2220-2234.