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Changes in CaCO₃ dissolution due to ocean acidification are potentially more important than changes in calcification to the future accretion and survival of coral reef ecosystems. As most CaCO₃ in coral reefs is stored in old permeable sediments, increasing sediment dissolution due to ocean acidification will result in reef loss even if calcification remains unchanged. Previous studies indicate that CaCO₃ dissolution could be more sensitive to ocean acidification than calcification by reef organisms. Observed changes in net ecosystem calcification owing to ocean acidification could therefore be due mainly to increased dissolution rather than decreased calcification. In addition, biologically mediated calcification could potentially adapt, at least partially, to future ocean acidification, while dissolution, which is mostly a geochemical response to changes in seawater chemistry, will not adapt. Here, we review the current knowledge of shallow-water CaCO₃ dissolution and demonstrate that dissolution in the context of ocean acidification has been largely overlooked compared with calcification.

Coral reefs have high biological diversity and provide a myriad of ecosystem services to humans, such as fisheries and tourism^{1, 2}. The CaCO₃ coral reef structure provides habitat for a large number of species. Reef structures are formed through the growth and build up of coral aragonite skeletons, red and green calcareous macroalgae, and other calcareous organisms, such as bryozoans, echinoderms and foraminifera. However, for a whole coral reef to be in a state of net accretion, CaCO₃ production (and any external sediment supply) must exceed its loss through physical, chemical and biological erosion and transport and dissolution as follows:

Ocean acidification (OA) generally refers to the lowering of the ocean's pH due to the uptake of anthropogenic CO₂ from the atmosphere. When CO₂ dissolves in sea water it forms H₂CO₃ (carbonic acid), which rapidly dissociates into HCO₃⁻ (bicarbonate ion) and H⁺ (hydrogen ion)³. Some of the excess H⁺ combines with CO₃²⁻ (carbonate ion) to form HCO₃⁻ and the remaining H⁺ lowers the seawater pH (pH = −log [H⁺]) (Fig. 1). The effect of OA on the decreased production of coral reef CaCO₃ (calcification) is well documented; see, for example, refs 4,5,6. However, CaCO₃ production is only part of the equation determining coral reef accretion, and much less is known about the effects of OA on physical, chemical and biological erosion and transport (loss) and dissolution. In particular, the dissolution of coral reef CaCO₃ sediments has largely been neglected by the research community. For example, dissolution is often excluded from coral reef carbonate budgets developed by geologists (see, for example, refs 7,8), while biologists mainly focus on calcification (see, for example, ref. 9). In addition, unlike biologically mediated calcification that could potentially adapt to OA^{10, 11} (but see ref. 12), increasing CaCO₃ dissolution is mostly a geochemical response to changes in seawater chemistry and will increase according to thermodynamic and kinetic constraints^{13, 14}, which cannot adapt to changing carbonate chemistries.

The numbers refer to the processes that break down permeable coral reef sediments: (1) environmental dissolution, or dissolution caused by the properties of sea water; (2) metabolic dissolution, which is induced through respiration and other metabolic processes occurring in sediment pore waters; (3) bioerosion, which can enhance dissolution through the physical and chemical actions of marine organisms; and (4) physical processes in coral reef sediments that interact with chemical and biological dissolution. Photo: © Ray Berkelman, AIMS.

Coral reefs are self-perpetuating ecosystems, with CaCO₃ formation and accretion providing the structural complexity necessary to support their high functional and biological diversity. To predict the accretion of coral reefs under future-ocean carbonate chemistries, it is necessary to determine the balance between CaCO₃ production and destruction processes and the impact of seawater carbonate chemistries on these processes. CaCO₃ stock in coral reefs can be roughly divided into two main pools; coral framework and CaCO₃ sediments. Coral framework refers to what is generally thought of as the reef structure and is where the highest rates of calcification (that is, CaCO₃ production) take place. Living corals can be thought of as a thin veneer of living tissue growing vertically and horizontally through the water column on top of their deposited CaCO₃ skeleton framework. After deposition, these skeletons can undergo diagenetic processes that modify the CaCO₃ minerals over geological timescales, eventually forming sedimentary limestones¹⁵. The coral framework is also subjected to physical and mechanical, chemical and biological breakdown processes that give rise to the formation of permeable CaCO₃ sediments (Fig. 1). Permeable sediments make up the majority of CaCO₃ stored in coral reef ecosystems, accounting for up to 95% of areal benthic coverage¹⁶. Sediments represent the storage of reef-derived CaCO₃ over thousands of years¹⁷ and are important in the development of other geological reef structures by acting as infill¹⁸.

Lateral growth of coral reefs is due to high rates of calcification on the fore reef that generate a constant supply of CaCO₃ material, which is eventually broken down and transported reefward in the direction of the prevailing wind, waves and currents^{19, 20} (Fig. 2). This transported CaCO₃ is then subjected to further breakdown processes, leading to the formation of shallow reef habitats (for example, reef flats and lagoons) as sediment accumulates during times of stationary or low sea-level rise (SLR)^{21, 22}. Besides production via the breakdown and transport of the coral reef framework, sediment can also be formed directly through infaunal CaCO₃ production by organisms such as foraminifera, coralline algae and molluscs^{23, 24}. Once produced, this sediment can be stored within the coral reef ecosystem, lost through dissolution or transported offshore, where it either accumulates as sand banks or beach faces, or is deposited in the deep sea^{7, 25}. Within reef flats and lagoons, sediment can be focused by currents and wave patterns, leading to the formation of low-lying land structures called sand or coral cays²⁶. Therefore, it is important to address how OA will affect all processes (for example, CaCO₃ production, dissolution (including bioerosion) and transport) that lead to the formation of reef habitat, both below and above the high tide mark.

Global production and accumulation values assume a global reef area of 0.6 × 10⁶ km² and all values are in ×10⁹ t CaCO₃ yr⁻¹. A previous study³¹ estimated that loss due to dissolution and offshore transport combined was ~20% of production (0.2 × 10⁹ t CaCO₃ yr⁻¹); however, measured values were not used to constrain this estimate. When the range of conservative net daily CaCO₃ dissolution rates for sediments (0.09–0.50 kg m⁻² yr⁻¹; Table 1) are extrapolated to the global reef area (0.05 × 10⁹–3.0 × 10⁹ t CaCO₃ yr⁻¹) they could account for up to 167% of the total loss term estimated by this previous study³¹. Values for offshore transport were estimated based on the difference between the loss term estimated in ref. 31 and the conservative dissolution rates. Photo by Christian Wild; adapted with permission from ref. 65, © 2008 by the Association for the Sciences of Limnology and Oceanography, Inc.

Studies investigating the calcification responses of whole coral reef communities to OA have generally shown a much stronger response and sensitivity to changes in the aragonite saturation state (Ω_Ar) than studies of individual organisms (see, for example, refs 6,37,38). On average, community calcification is predicted to decrease by 60% per unit decrease in Ω_Ar, but estimates range from 10 to >100%, that is, net dissolution^{39, 40}. The greater sensitivity is most likely the result of the combined effect of decreasing organism calcification rates and increasing CaCO₃ dissolution rates in sediments and microenvironments^{10, 38}. Evidence of net CaCO₃ dissolution is commonly observed at night in the water column in many coral reef environments^{40, 41, 42} and at any time in carbonate sediment pore waters owing to the metabolic processes driving dissolution^{43, 44}.

Mesocosm or enclosure experiments, the Biosphere II artificial reef community and measurements from natural coral reef environments have all shown that CaCO₃ dissolution will increase in response to rising seawater CO₂ and decreasing pH and Ω_Ar (for example, refs 6,38,39,40,45). Some of these results suggest that the process of CaCO₃ dissolution could be an order of magnitude more sensitive to OA than calcification in individual organisms³⁸, but despite this observation, few studies have attempted to quantify the relative contributions of these processes. This is partly due to the fact that our present measurement techniques only measure the net effect of these processes, and thus, it is challenging to quantify gross calcification and dissolution. Nevertheless, separation in time (for example, night net dissolution) and space (most dissolution probably occurs in the sediments) provide initial constraints and a conservative estimate of gross CaCO₃ dissolution rates^{40, 44}.

Based on over 50 years of research (Fig. 3), the dissolution of permeable reef sediments can be grouped under three processes: (1) environmental dissolution, or dissolution caused by the properties of the bulk sea water^{29, 46}; (2) metabolic dissolution, which is induced through respiration and other metabolic processes occurring in sediment pore waters^{44, 47}; and (3) bioerosion, which can enhance dissolution through the physical and chemical actions of marine organisms^{48, 49} (Fig. 1). Environmental and metabolic dissolution can largely be grouped under the broader term of chemical dissolution, whereas biological dissolution or bioerosion can occur via physical and chemical processes. In addition, a number of physical processes (for example, wave action, currents, advective porewater flow) in coral reef sediments can interact with chemical and biological dissolution. As such, it is difficult to separate out the long-term physical, chemical and biological drivers of coral reef CaCO₃ sediment dissolution because they act synergistically (Fig. 1).

This is by no means a comprehensive list, but rather an overview of the work that has been done over the past several decades. There are many additional important studies that deserve recognition. References are provided in the Supplementary Information.

There are at present many marine environments that already experience seawater CO₂ and pH levels similar or even more extreme than those levels anticipated as a result of oceanic uptake of anthropogenic CO₂ over this and the next century. The distribution and composition of different CaCO₃ mineral phases in these environments provide important clues in terms of the potential response to future OA, although extensive uncertainty still exists with respect to mineral solubility and kinetics. Consequently, the timing of critical thresholds (for example, when a coral reef ecosystem will be net dissolving) and rates of dissolution on relevant timescales of ongoing anthropogenic OA are major gaps in our knowledge. However, in general we have a good understanding of how CaCO₃ minerals respond to rising CO₂ and lower pH and Ω. For example, the CaCO₃ content of sediments becomes increasingly scarce as a function of depth and changes successively according to mineral stability. Similarly, we know that the rate of dissolution increases as a function of increasing seawater undersaturation^{73, 74}. Selective dissolution according to mineral stability has been reported for a seasonally stratified environment in Bermuda exposed to high CO₂ and low pH conditions during summer⁷⁵. Observations from the eastern tropical Pacific, which naturally experiences high CO₂, low pH and low Ω owing to upwelling, show high rates of bioerosion, low reef cementation and poorly developed reefs under these conditions (see, for example, ref. 76). Furthermore, observations from the volcanic CO₂ vent sites in Italy and Papua New Guinea have shown evidence of dissolution of individual calcifiers, both live and dead^{77, 78}. In Papua New Guinea, no positive reef accretion occurred below an average seawater pH of 7.7 (ref. 77).

Coral reefs are subject to many anthropogenic stressors, such as nutrient over-enrichment (eutrophication), climate change (for example, bleaching due to rising temperatures and increases in the intensity of tropical cyclones), overfishing, sedimentation, disease and pollution that may act synergistically with OA to enhance CaCO₃ sediment dissolution. Excess terrestrially derived nitrogen and phosphorus and organic matter are delivered to coral reefs via river runoff, groundwater and atmospheric deposition⁷⁹. Increased nitrogen and phosphorus loads could enhance reef primary production and net autotrophy, which could lead to enhanced bioerosion^URL: