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Coral Reefs and Ocean Acidification © 2009 by the Oceanography Society.Oceanography a 2009 by the Ll Rights Reserved or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] ofor Th e The to: [email protected] Oceanography approval Oceanography correspondence all portionthe Send Society. ofwith any articlepermitted only photocopy by Society, is of machine, reposting, this means or collective or other redistirbution This article has This been published in SPECIAL IssUE FEATURE Oceanography BY J OA N A . K L E Y PA S AND KIMBERLY K. YATES journal of The 22, NumberOceanography 4, a quarterly Society. , Volume Coral Reefs and Ocean Acidification © 2009 by The 2009 by Oceanography Society. A ll rights reserved. P ermission is granted to copy this article for use in teaching and research. article for use research. and this copy in teaching to granted is ermission Figure 1. Important calcifying organisms on coral reefs: (A) various reef-building corals on a Samoan reef, (B) coralline algae encrusting rock surfaces on a Samoan reef, (C) Halimeda alga from Puerto Rico, (D) Penicillus algae, (E) reef sand composed of benthic foraminifera from Warraber Island, Torres Strait (average diameter of foraminifera grains is about 1 mm), and (F) echinoderm from the US Virgin Islands. Photo credits: A,B,C, Nathan Smiley; D, Matt Miller; E, Deirdre E. Hart; F, Chris DuFore P O Box 1931, R epublication, systemmatic reproduction, reproduction, systemmatic epublication, R ockville, MD 20849-1931, U S A . 108 Oceanography Vol.22, No.4 AbsTRACT. Coral reefs were one of the first ecosystems to be recognized as future. It explores how ocean acidifica- vulnerable to ocean acidification. To date, most scientific investigations into the tion might affect coral reef systems at effects of ocean acidification on coral reefs have been related to the reefs’ unique multiple scales, from organism, to coral ability to produce voluminous amounts of calcium carbonate. It has been estimated communities, to reef structure. Feely that the main reef-building organisms, corals and calcifying macroalgae, will calcify et al. (2009) provide a review of ocean 10–50% less relative to pre-industrial rates by the middle of this century. This acidification chemistry. decreased calcification is likely to affect their ability to function within the ecosystem and will almost certainly affect the workings of the ecosystem itself. However, ocean EffECTS of OCEAN acidification affects not only the organisms, but also the reefs they build. The decline ACIDIFICATIon on REEF in calcium carbonate production, coupled with an increase in calcium carbonate ORGANISMS dissolution, will diminish reef building and the benefits that reefs provide, such How do I affect thee? – as high structural complexity that supports biodiversity on reefs, and breakwater Let me count the ways… effects that protect shorelines and create quiet habitats for other ecosystems, such as mangroves and seagrass beds. The focus on calcification in reefs is warranted, but the Research on the effects of ocean acidifi- responses of many other organisms, such as fish, noncalcifying algae, and seagrasses, cation on marine ecosystems continues to name a few, deserve a close look as well. to focus on calcifying organisms, and for good reason. Skeletal formation in InTRODUCTION ecosystem functioning does affect reef many organisms that secrete one of the In the 1970s and 1980s, research on coral building. Although reef building may various minerals of calcium carbonate reef ecosystems covered two, often sepa- not be essential to coral communities, (aragonite, calcite, and high-magnesium rate, tracks: biological and geological. the reef structure provides services calcite) changes when exposed to In fact, the Proceedings of the Third to the reef ecosystem by supporting elevated-CO2 conditions. On coral International Coral Reef Symposium1, the biodiversity and creating more surface reefs, the two main calcifying groups— outcome of a meeting held in Miami, area to support the reef community. corals and calcifying macroalgae Florida, in 1977, were cleanly separated These services are important, as is the (Figure 1A, B)—seem particularly sensi- into two volumes: “1. Biology” and reef function as a breakwater protecting tive to ocean acidification. However, “2. Geology.” Over time, arguments shorelines and creating conditions that noncalcifying organisms on coral reefs about the definition of a coral reef promote other productive systems like will also be affected. It is difficult to (e.g., is reef building a necessary litmus mangroves and seagrass beds. predict the overall impact on coral reefs test for defining a coral reef ecosystem?) Coral reefs are the most widely recog- of some species being “winners” and were reconciled in recognition that coral nized ecosystem threatened by ocean others “losers” in a progressively lower- reef ecosystems exist along a continuum, acidification. Changes in seawater chem- pH ocean, but the loss of reef builders from nonreef-building coral ecosystems istry resulting from increased carbon threatens both the biological and geolog- to those that build massive, rapidly dioxide uptake by the ocean impede ical identities of this ecosystem. accumulating structures. Somewhere the basic function of calcium carbonate along that continuum lies a threshold production that is characteristic of many Reef Builders (Corals and between coral communities and coral reef organisms, and that provides the Calcifying Macroalgae) reefs, and across that threshold the coral foundation of coral reef structure. Ocean History communities may or may not func- acidification, therefore, impacts both the The pioneers in reef calcification studies tion similarly. It is not altogether clear biological and geological components have known for some time that calcifica- that the coral community depends on of coral reefs. This article summarizes tion by coral reef communities affects its own reef building. those impacts and provides a glimpse seawater chemistry (Smith and Pesret, What is clear, however, is that into what coral reefs may look like in the 1974; Smith and Key, 1975). Some of 1 The International Coral Reef Symposia have been held every four years since 1969. Oceanography December 2009 109 the early studies of “reef metabolism” Unfortunately, this inference is differs (1) between species, (2) between used the alkalinity anomaly technique— too simple because corals and other the life stages of species (note that a measure of the drawdown of alka- organisms exert energy to control their the mineralogy of the initial calcium linity over time due to precipitation of calcification, and because in most reef carbonate precipitated by larvae is some- calcium and carbonate ions as calcium organisms it is internal and isolated from times a rare, highly soluble form), and carbonate—to quantify calcification seawater. In corals, there is no evidence (3) in some species, between different rates of coral reef communities (Gattuso that carbonate ions are transported stages of calcification. et al., 1993; Kinsey, 1978, 1985; Smith from seawater to the site of calcifica- and Kinsey, 1978). The converse, that tion (McConnaughey et al., 2000), so Calcification seawater chemistry controls coral reef the carbonate ion concentration must Despite our incomplete understanding of calcification rates, was slower to be affect calcification indirectly, or is simply the exact mechanisms that control calcifi- recognized (Smith and Buddemeier, a covariant of some other aspect of cation, decreased skeletal growth in reef- 1992). Some of the first studies to test the carbonate chemistry that controls calcifi- building corals and coralline algae is one impact of ocean acidification on marine cation rate. Several studies have manipu- of the best-known consequences of ocean organisms were conducted on corals lated seawater chemistry by altering acidification (Figure 2). A wide range 2+ 2– – and coralline algae (Gattuso et al., 1998; concentrations of Ca , CO 3 , HCO 3, or of responses has been observed, but on Langdon et al., 2000). The Biosphere 2 pH while holding the others constant in average, a doubling of pre-industrial studies, in particular, suggested that order to tease out which component of atmospheric CO2 concentration results calcification rates of the coral/algal the carbonate system elicits a response in about a 10–50% decrease in the calci- community correlated best with arago- from corals. Those that manipulated fication rate of reef-building corals and nite saturation state (Ωar) (Langdon the calcium ion concentration (Gattuso coralline algae (Kleypas and Langdon, et al., 2000, 2003). This result agrees well et al., 1998; Marshall and Clode, 2002) 2006). The good news is the response is with geochemical studies that show that did cause a calcification response (note reversible (calcification will increase if precipitation rates of inorganic aragonite that marine aquarists are certainly acidification is reversed). The bad news is from seawater can be expressed by the aware that calcium supplements speed it is highly unlikely that ocean acidifica- n empirical equation R = k (Ωar – 1) , coral growth), and thus demonstrated tion can be reversed, and we don’t know where R is the rate of aragonite precipita- the impact of saturation state changes. yet whether corals and coralline algae tion, k is the rate constant, and n is the Ocean acidification, however, does not can adapt to these changes. order of the
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