Biogeosciences Revisiting four scientific debates in ocean acidification research

In recent years, ocean acidification has gained continuously increasing attention from scientists and a number of stakeholders and has raised serious concerns about its effects on marine organisms and ecosystems. With the increase in interest, funding resources, and the number of scientific investigations focusing on this environmental problem, increasing amounts of data and results have been produced, and a progressively growing and more rigorous understanding of this problem has begun to develop. Nevertheless, there are still a number of scientific debates, and in some cases misconceptions, that keep reoccurring at a number of forums in various contexts. In this article, we revisit four of these topics that we think require further thoughtful consideration including: (1) surface seawater CO 2 chemistry in shallow water coastal areas, (2) experimental manipulation of marine systems using CO 2 gas or by acid addition, (3) net versus gross calcification and dissolution, and (4) CaCO3 mineral dissolution and seawater buffering. As a summation of these topics, we emphasize that: (1) many coastal environments experience seawater pCO2 that is significantly higher than expected from equilibrium with the atmosphere and is strongly linked to biological processes; (2) addition of acid, base or CO 2 gas to seawater can all be useful techniques to manipulate seawater chemistry in ocean acidification experiments; (3) estimates of calcification or CaCO 3 dissolution based on present techniques are measuring the net of gross calcification and dissolution; and (4) dissolution of metastable carbonate mineral phases will not produce sufficient alkalinity to buffer the pH and carbonate saturation state of shallow water environments on timescales of decades to hundreds of years to the extent that any potential negative effects on marine calcifiers will be avoided.


Introduction
Ocean acidification (OA), or the other CO 2 problem as it is sometimes referred to, has gained continuously increasing attention and interest from scientists, the media, the public and policy makers in the last several years.Concurrent with this increasing interest and concern for OA, research funding dedicated to the issue has significantly increased, the number of articles both in the scientific literature and popular media has exploded (cf., Gattuso and Hansson, 2011), and even several documentary movies have been produced dealing with this issue (e.g., "Acid Test", "A Sea of Change", "Tipping Point", as well as others).A recent search on Google for "ocean acidification" produced 981 000 results (22 October 2011).This is a small number compared to a search on "global warming", which produced more than 170 million results (22 October 2011), but nevertheless, it still clearly demonstrates increasing public interest and concern for OA.Ten years ago this search would have produced a handful of results.
As early as in the first decades of the 1900s, a number of pioneers investigated links between seawater pH and various biological processes including survival, calcification, metabolism, and spawning in organisms such as barnacles, algae, sea urchins, and oysters (see review in Gattuso and Hansson, 2011and references therein, e.g., McClendon, 1917, 1918;Gail, 1919;Bouxain, 1926a, b;Pryterch, 1929).Other pioneers between the 1950s and 1990s (e.g., Revelle and Suess, 1957;Broecker and Takahashi, 1966;Broecker et al., 1971;Bacastow and Keeling, 1973;Garrels and Mackenzie, 1980;Smith and Buddemeier, 1992; as well as others) began to recognize the potential problem related to increasing anthropogenic CO 2 emissions and partial absorption of the gas in the surface waters of the ocean leading to OA with effects on individual marine organisms and ecosystems.However, it was not until 1999 when Kleypas et al. (1999) published an article in "Science" titled "Geochemical consequences of increased atmospheric CO 2 on coral reefs", and then four years later when Caldeira and Wickett (2003) published an article in "Nature" titled "Anthropogenic carbon and ocean pH" that the issue gained considerable attention.At the present time, new OA research is being published in the scientific literature almost on a daily basis and reports in the popular media appear with a similar frequency.With the increasing number of investigations and interest concerning this environmental problem, it has become increasingly important to recognize and learn from previous experiences, carefully and objectively assess data and results, and not perpetuate overly simplistic interpretations of what can be a nuanced problem.Many recent workshops and publications on OA as well as the "Guide to best practices for ocean acidification research and data reporting" (Riebesell et al., 2010) have made attempts to stress the importance of implementing more rigorous and sophisticated approaches in OA research.Nevertheless, there are still several outstanding scientific debates and in some cases misconceptions that reoccur in the scientific literature, at scientific meetings and workshops, in manuscript and proposal reviews, and in discussions with other scientists and stakeholders that we think are necessary to revisit and synthesize here.The objective of this article is to raise an increased awareness about four of these topics including: (1) surface seawater CO 2 chemistry in shallow water coastal areas, (2) experimental manipulation of marine systems using CO 2 gas or by acid addition, (3) net versus gross calcification and dissolution, and (4) CaCO 3 mineral dissolution and seawater buffering.

Surface seawater CO -carbonic acid system chemistry in shallow water coastal areas
From a forecasting perspective, OA and the associated changes in open ocean surface seawater carbonate chemistry pose little uncertainty and we can fairly accurately predict these conditions using standard CO 2 -carbonic acid system chemical equations for any given scenario of anthropogenic CO 2 emissions and ocean uptake of this gas expected within the next several decades (e.g., Orr et al., 2005;Orr, 2011;Joos et al., 2011).However, it is repeatedly overlooked that in many of the shallow water coastal environments where a significant proportion of benthic calcifying organisms reside and may be at risk from OA (e.g., mussels, oysters, corals, coralline algae) that surface seawater pCO 2 is already at levels significantly higher and pH lower than expected from equilibrium with current atmospheric levels (Fig. 1; e.g., Fagan and Mackenzie, 2007;Bates et al., 2010;Thomsen et al., 2010;Shamberger et al., 2011;Yu et al., 2011;Hofmann et al., 2011).Also many large coastal ecosystems, including reefs, are found adjacent to land masses with major fluvial inputs, and thus, experience large variability in environmental parameters such as temperature, salinity, nutrient and light conditions (e.g., Drupp et al., 2011).showing pCO 2 levels that on average are significantly higher than expected from equilibrium with the atmosphere.On occasion pCO 2 levels at the Hawaii site have been observed to exceed 1000 µatm, which is much higher than the levels predicted to occur in the atmosphere in the year 2100 under a business-as-usual CO 2 emission scenario.Data courtesy: Eric De-Carlo, University of Hawaii, Dwight Gledhill, NOAA AOML, and Chris Sabine, NOAA PMEL.http://www.pmel.noaa.gov/co2/story/Coral+Reef+Moorings In many near-shore environments, the high seawater pCO 2 and low pH conditions arise as a result of net heterotrophic conditions owing to remineralization of organic material exceeding its production (Smith and Hollibaugh, 1993;Frankignoulle et al., 1998;Mackenzie et al., 2004), and/or from the process of calcification in coral reef and carbonatedominated environments, which decreases seawater total alkalinity and releases CO 2 (Ware et al., 1991;Frankignoulle et al., 1994).In regions influenced by upwelling, deep seawater enriched in CO 2 from decomposition of organic material is brought up to shallow depths resulting in lower pH and saturation state with respect to carbonate minerals (Feely et al., 2008).Furthermore, many near-shore environments undergo large diurnal fluctuations in seawater chemistry associated with daytime and nighttime changes in the intensity of photosynthesis/respiration and calcification/dissolution, as well as water advection owing to tidal cycles and changes in winds (Fig. 2; Drupp et al., 2011;Hofmann et al., 2011;Shamberger et al., 2011).Consequently, the ambient CO 2 conditions for these environments and for the marine organisms living within them are significantly higher (or sometimes lower) than for a system currently in equilibrium with the atmospheric CO 2 concentration.These conditions are rarely taken into account or even recognized in the majority of manipulation experiments with marine organisms published to date with a few exceptions (e.g., Andersson et al., 2009;Thomsen et al., 2010;Yu et al., 2011).Because upwelling and/or onwelling are important flows in coastal ocean environments, as atmospheric concentrations of CO 2 continue to rise, shallow water environments experiencing supersaturated CO 2 conditions with respect to the atmosphere could potentially still increase as the CO 2 of the open ocean source water increases.This statement assumes that net ecosystem production (NEP) and net ecosystem calcification (NEC) do not change (Mackenzie et al., 2004).Thus, in such a scenario, future surface seawater CO 2 stabilization values for many shallow coastal environments will significantly exceed the atmospheric partial pressure of CO 2 .Nevertheless, increasing fluvial inputs of nutrients and organic matter to the coastal ocean (Seitzinger et al., 2010;Lerman et al., 2011) could very well alter the trophic status of this environment with subsequent changes to its seawater carbonic-acid system.This change could either exacerbate or alleviate the anthropogenic OA effect in this environment depending on the direction of the change in trophic status (Mackenzie et al., 2004).An increase in net autotrophy might alleviate the effect of anthropogenic OA by fixing more CO 2 into organic material while increasing net heterotrophy would have the opposite effect.Similarly, a reduction in net ecosystem calcification would act as a negative feedback to OA owing to reduced depletion of total alkalinity and generation of CO 2 .
Biological feedbacks to the seawater carbonic-acid system on diurnal to seasonal timescales have been demonstrated for several coastal environments such as coral reefs and seagrass beds (Suzuki et al., 1995;Semesi et al., 2009;Bates et al., 2010), as well as in experimental setups (Andersson et al., 2009;Anthony et al., 2011), but it is obviously much more difficult to predict the biological feedback to carbonate chemistry on inter-annual to decadal timescales.Nevertheless, failure to recognize that these feedbacks exist (both positive and negative) and the fact that many coastal environments already experience CO 2 conditions significantly higher than expected from equilibrium with the atmosphere may result in erroneous conclusions regarding the effect of OA on shallow water marine organisms and ecosystems.Similarly, CO 2 treatment values recommended at several workshops and also in the "Guide to best practices for ocean acidification research and data reporting" (Barry et al., 2010) are highly beneficial for comparisons between experiments and to investigate functional relationships, but it is important to recognize that these levels may not reflect present in situ conditions or the timing of future in situ coastal seawater conditions.In our opinion these target levels should be viewed as recommendations, but not something that is set in stone and mandates all OA research.What is truly important is that researchers determine accurately the complete dissolved CO 2 -carbonic acid system and characterize seawater pCO 2 , pH, HCO − 3 , CO 2− 3 , etc., and their variability within their experimental systems.This includes diurnal or other temporal variability that may occur owing to processes altering the ideal physiochemical conditions within a system.Furthermore, in translating experimental results to natural systems, it is equally important to recognize that seawater CO 2 concentrations in near-shore environments are generally not constant and can be significantly higher or lower than expected from equilibrium with current atmospheric CO 2 concentration.Ultimately, to make robust predictions for the future, one needs to understand the current range of seawater CO 2 conditions experienced by an organism or community in the natural environment and how these conditions may change.
3 Manipulation of marine CO 2 -carbonic acid system using CO 2 gas or by acid addition In order to make predictions for the future on how marine organisms will respond to elevated seawater CO 2 conditions, one of the most logical approaches is to expose them to the anticipated future seawater conditions and record organism responses.Nonetheless, significant care and caution need to be exercised in interpreting the results of these experiments before drawing conclusions and extrapolating them to natural environments and future conditions.After all, a single species short-term CO 2 "shock" experiment in a tank with a duration of days to weeks may involve caveats in terms of the direct translation of results to longer-term gradual changes in CO 2 in natural environments.
There are a number of different methods that can be used to manipulate the seawater CO 2 -carbonic acid system, which have been reviewed in detail elsewhere (Gattuso and Lavigne, 2009;Gattuso et al., 2010).Fundamentally, all methods are based on two different approaches that involve addition of CO 2 gas or addition of acid and/or base (Schulz et al., 2009;Gattuso and Lavigne, 2009;Gattuso et al., 2010).The majority of experiments conducted to date have been based on bubbling with CO 2 gas or addition of a strong acid.If a seawater system is bubbled with CO 2 gas, the system will strive to reach equilibrium with the pCO 2 of this gas phase resulting in an increase in total dissolved inorganic carbon (DIC) while total alkalinity (TA) remains essentially unchanged with subsequent increases in [CO 2aq ], [HCO − 3 ] and [H + ], and a decrease in [CO 2−  3 ].This methodology accurately mimics the chemical changes that will occur as a result of oceanic uptake of anthropogenic CO 2 in a homogeneous gas-seawater system.In contrast, acid addition results in a decrease in the TA of the system with a subsequent change in the equilibrium concentrations of the individual components that differ slightly from the CO 2 bubbling method (see Table 1 in Schulz et al., 2009 andTable 2 in Gattuso andLavigne, 2009, respectively).Recent recommendations suggest that acid manipulation experiments are accompanied, for example, by addition of NaHCO 3 and/or Na 2 CO 3 in order to compensate for the reduction in TA (Gattuso and Lavigne, 2009;Gattuso et al., 2010), but obviously there are many experiments conducted during the past decade in which this was not done, and the validity of these experiments has repeatedly come into question.
Despite the relatively small differences in carbon speciation between acid and CO 2 manipulation experiments, opponents of the former have argued that acid addition experiments and results are not especially useful in the context of OA based on the argument that they do not represent accurately changes that will occur as a result of anthropogenic OA.Technically, these opponents are correct since TA is manipulated and not the DIC with a range of resulting conditions in carbon speciation depending on whether the system is closed, open, static, or continuous-flow (Gattuso and Lavigne, 2009;Gattuso et al., 2010).In some cases these conditions will differ more than in others.For example, a static tank experiment open to the atmosphere could result in significantly different conditions as the system is allowed to equilibrate with the atmosphere and thereby loose CO 2 with a resulting decrease in DIC.This effect is insignificant in a closed or a continuous-flow system (depending on flow-rate) with very small differences in carbon speciation compared to a system bubbled with CO 2 gas.
Equilibrating large volumes of seawater on the order of 10 000 to 100 000 of liters per day with CO 2 gas at a given pCO 2 level for extended periods of time can be challenging and requires significant resources not available to a sufficient number of investigators.Hence, the acid addition method (with or without NaHCO 3 or Na 2 CO 3 addition) may be justified in such circumstances and will continue to be a beneficial and a reasonable approach that could be used in future OA studies.In experiments where researchers are trying to mimic the natural flow regime, for example on a coral reef, with high flow rates and turnover of large volumes of water with associated replenishment of food and nutrients (e.g., Langdon and Atkinson, 2005;Kuffner et al., 2008;Jokiel et al., 2008), acid addition may logistically be a more feasible method than CO 2 bubbling for most researchers.Similarly, acid (or base) addition can be beneficial in experiments evaluating physiological processes and dependence on different carbon species (e.g., Schneider and Erez, 2006;Jury et al., 2010).Nonetheless, depending on the experimental circumstances, investigators should obviously try to mimic natural conditions as much as possible, but the fact that an experiment is conducted based on the acid addition method should not automatically reject it from being a valuable experiment.
The fundamental question at the center of the discussion is: what dissolved inorganic carbon specie(s) is/are important to marine organisms?DIC and TA are terms describing the sum of different dissolved components, which are not fixed at any given DIC or TA value, but a function of several factors including temperature, salinity, and pressure.Functionality of marine organisms probably has little to do with these former variables, but is mainly controlled by the concentration and distribution of the various carbon species in the ambient seawater, the concentration gradients between the external seawater environment and the calcifying fluid and its carbon species distribution, and/or the availability of H + , CO 2aq , HCO − 3 or CO 2− 3 , which are important for a marine organism's metabolic processes such as photosynthesis and calcification (Raven, 1993;Gattuso et al., 1999;Cohen and McConnaughey, 2003;Allemand et al., 2011;Jokiel, 2011).Some researchers have argued that the small differences on the order of a few percent in [HCO −  3 ] between acid addition and CO 2 bubbling may explain contradictory responses in experimental results (e.g., Iglesias-Rodriguez et al., 2008).This is perhaps the case for some organisms, but to make robust and valid conclusions, direct side-by-side experimental comparisons are required.Few direct comparisons have been made to date, but an increasing number of researchers report similar organism responses resulting from acid addition and CO 2 bubbling experiments (Kurihara, 2008;Schulz et al., 2009;de Putron et al., 2011;A. Andersson, unpublished data).
It is important that caution and care are exercised in interpreting experimental results based on the acid addition method, but it is similarly important to be cautious in interpreting any experimental manipulations as they are only a controlled approximation of what is happening in nature.Regardless of the method used, it is critical that researchers accurately characterize and report the complete CO 2 -carbonic acid system parameters as well as other important parameters in their experiments, like nutrient concentrations.

Net versus gross calcification and dissolution
A large number of OA experiments to date have been conducted on corals (Gattuso and Hansson, 2011).So far the results have been relatively consistent demonstrating that the rate of calcification decreases as a result of decreasing seawater carbonate saturation state (cf., reviews in Langdon and Atkinson, 2005;Erez et al., 2011;Andersson et al., 2011).Similarly, experiments or observational studies of subtropical and tropical calcifying communities have shown similar results with decreasing net community calcification as a function of decreasing seawater carbonate saturation state (e.g., Langdon et al., 2000Langdon et al., , 2003;;Yates andHalley, 2003, 2006;Silverman et al., 2007;Andersson et al., 2009;Shamberger et al., 2011).In many cases these results are based on the alkalinity anomaly technique (Smith and Key, 1975;Smith and Kinsey, 1978;Chisholm and Gattuso, 1991) and the assumption that for every mole of CaCO 3 precipitated or dissolved, seawater total alkalinity changes by two moles.Although processes other than calcification and dissolution may influence total alkalinity, such as nutrient uptake and release and ammonia and sulfate reduction, these processes have a relatively minor effect in most coral reef environments relative to the influence of calcification and CaCO 3 dissolution processes.Nevertheless, there could be exceptions to this assumption that need to be carefully considered especially in natural or experimental settings where nutrient and/or organic acid concentrations are high (Dickson, 2010).
A bigger problem arises from the fact that researchers repeatedly overlook that the alkalinity anomaly technique, as well as other techniques used to measure calcification rates of calcifying organisms or communities (e.g., dry or buoyant weight technique, calcium isotope technique, and skeletal density banding; see review in Langdon et al., 2010) are only measuring the net effect of calcification and dissolution, and do not distinguish between these two processes (Langdon et al., 2010;Findlay et al., 2011).Consequently, an observed decrease in net calcification for a calcifying organism or community could result from a decrease in gross calcification or an increase in gross dissolution, or a combination of both of these effects.At this time we know little about the relative sensitivity of these processes and how they will change in response to OA at both the individual organism and larger community/ecosystem scale.The effect of OA on CaCO 3 dissolution has been given very little attention despite the fact that almost all the reported results represent the net effect of calcification and dissolution.Attempts have been made to evaluate the problem based on observations of nighttime dissolution and from environments where calcification is believed to be non-existent or minimal (e.g., Silverman et al., 2007;Andersson et al., 2007), but these estimates are nevertheless still net estimates.Findlay et al. (2011) attempted to estimate the relative contribution from calcification and dissolution for a range of marine benthic calcifiers by exposing both live and dead shells to different seawater pH conditions.Their results showed that dissolution of the dead shells increased significantly as a function of decreasing pH, but net calcification in live organisms was little affected or even increased under the imposed experimental conditions.This finding suggests that the calcifying organisms were able to compensate for the increase in dissolution by up-regulating calcification.However, most live organisms usually have tissues or organic coatings that may act as a protective layer from dissolution (e.g., Morse and Mackenzie, 1990), which is less the case for dead shells or skeletons.Nonetheless, Findlay et al. (2011) observed negative effects on other physiological processes, such as metabolism, health, and behavior, which may indicate that any potential up-regulation of calcification occurred at the expense of these processes.
Although surface seawater in most ocean regions is supersaturated with respect to the major carbonate phases present in sediments and reef frameworks, CaCO 3 dissolution is an ongoing process and occurs in all these regions (Andersson et al., 2011).The dissolution process is either driven by microbial decomposition of organic material (i.e., metabolic dissolution) that creates undersaturated conditions in sediment pore waters or microenvironments, or by bioeroding organisms that actively penetrate carbonate substrates by various acidification mechanisms.Some of these bioeroders are photosynthesizing (e.g., endoliths), and hence, may be more efficient at penetrating carbonate material and contributing to CaCO 3 dissolution during daytime (Tribollet et al., 2006;Tribollet, 2009).The problem at hand is similar to the problem of quantifying gross primary production and respiration and their relative contribution to net primary production.However, an important difference is that photosynthesis primarily occurs when sufficient light is available (i.e., PAR, Photosynthetitically Available Radiation) while both calcification and dissolution can take place at all times.Perhaps some cleverly designed experiments based on isotope addition or some other traceable parameter (e.g., strontium enrichment) could be used to address this problem.Understanding the relative contribution from gross calcification and dissolution to net ecosystem calcification and the processes controlling their magnitude would significantly improve our ability to predict future consequences of OA, for example, to CaCO 3 accretion rates on coral reefs.Until this can be accomplished, it is important to recognize that the alkalinity anomaly technique and other current techniques used to measure calcification are measuring the net effect of calcification and dissolution.Nevertheless, the effects of OA on calcification and dissolution rates are but one concern associated with this major change in seawater chemistry, and there are many other aspects important to an organism's and/or ecosystem's success that also need to be considered (e.g., fecundity, metabolism, recruitment, early development, predator-prey interactions, community structure, etc.).
Following the article by Kleypas et al. (1999) on the "Geochemical consequences of increased atmospheric CO 2 on coral reefs", a couple of abstracts appeared in the "Proceedings from the 9th International Coral Reef Symposium" in Bali proposing that rising CO 2 would not greatly impact reef systems since any reduction in pH would be rapidly compensated for by dissolution of high magnesian calcite minerals (Barnes and Cuff, 2000;Halley and Yates, 2000).This was followed by a lively discussion online on the coral-list (http://coral.aoml.noaa.gov/archive/coral-list-2001.txt) between several individuals arguing whether or not this process, referred to as the magnesian salvation theory (MST), was important.The idea behind the hypothesis arose from the fact that shallow water carbonate sediments contain a significant proportion of abiotic cement and mainly skeletal Mg-calcite minerals (∼25 %; Land, 1967;Morse and Mackenzie, 1990).Biogenic Mgcalcite mineral phases are mainly produced by marine calcifiers such as certain algae, benthic forams, echinoderms, and bryozoans.Mg-calcite is a variety of calcite in which a significant proportion of calcium ions (up to 30 mol %) has been randomly replaced in the mineral structure by smaller magnesium ions causing distortions to the ideal CaCO 3 calcite mineral structure.As a result of this distortion and also because of inclusion of other impurities, such as HCO − 3 , OH − , H 2 O, and SO 2− 4 , Mg-calcite minerals with significant magnesium content are more soluble than both calcite and aragonite, the latter being the mineral phase deposited by corals.As seawater CO 2 continuously increases and the saturation state with respect to carbonate minerals decreases, Mg-calcite phases will be the first mineral phases subject to undersaturated conditions and most likely subject to dissolution, "the canary in a coal mine" of OA (Mackenzie et al., 1983;Morse et al., 2006;Andersson et al., 2008; see also Walter and Morse, 1985, on the complexities tied to this statement in terms of dissolution rates of biogenic phases).Dissolution of carbonate minerals consumes CO 2 and produces total alkalinity: As a result, the pH and saturation state with respect to carbonate minerals increases.Hence, the dissolution process potentially can act as a buffer counteracting the modifications in seawater carbon chemistry imposed by uptake of anthropogenic CO 2 .If carbonate dissolution were able to keep up with the uptake of anthropogenic CO 2 , the seawater would be significantly buffered and its pH would remain unchanged (Fig. 3).In summation, the MST suggests that undersaturated conditions and dissolution of highly soluble Mg-calcite phases as a result of OA will buffer the CO 2 -carbonic acid   3. Hypothetical illustration of a system that receives no buffering (top panel) from dissolution of CaCO 3 minerals, i.e., seawater total alkalinity remains unchanged, and a system that is buffered by CaCO 3 dissolution (bottom panel) in order to maintain a constant pH, i.e., total alkalinity increases and accumulates within the system.
system of seawater and prevent any negative effects on marine organisms.
During the ongoing discussion on the coral-list about the MST, Bob Buddemeier succinctly outlined why dissolution of high Mg-calcite would not significantly buffer seawater from OA (http: //coral.aoml.noaa.gov/archive/coral-list-2001.txt).This fact had also been pointed out previously in a US Department of Energy report edited by Garrels and Mackenzie (1980).Later, Andersson and colleagues (Andersson et al., 2003(Andersson et al., , 2005(Andersson et al., , 2006(Andersson et al., , 2007;;Morse et al., 2006) showed in a number of papers based on a numerical modeling approach that the global coastal ocean, as well as most shallow water environments in general, will not be significantly buffered by dissolution of carbonate minerals on time-scales of decades to hundreds of years.However, these authors have failed to convince skeptics as the issue is continuously being raised or mentioned in a number of forums and settings based on, for example, incorrect thermodynamic calculations (e.g., Loáiciga, 2006; see technical comment by Caldeira et al., 2007) or without clear definition or due consideration of what is required to achieve a significant buffer effect (e.g., Tribollet et al., 2009;Hauck et al., 2011;Tynan and Opdyke, 2011).
The first condition that requires recognition is that evidence of dissolution of CaCO 3 does not automatically mean that a significant buffer effect can be achieved.Some of the confusion associated with this problem may simply be related to how investigators define a significant buffer effect.In the original discussion on the MST, a significant buffer effect referred to a restoration in seawater pH that would prevent or alleviate any negative effects on marine organisms' calcification rates arising from OA.This is the definition adopted by Andersson and colleagues (Andersson et al., 2003(Andersson et al., , 2005(Andersson et al., , 2006(Andersson et al., , 2007;;Morse et al., 2006).In contrast, Garrels and Mackenzie (1980) evaluated how much anthropogenic CO 2 could be taken up by the ocean considering whether a homogenous or heterogenous equilibrium existed between the atmosphere, seawater and the solid CaCO 3 phase (see also Cao and Dai, 2011).In this latter definition, any dissolution and production of total alkalinity could be considered a buffer effect.In this context, whether this process is significant or not requires evaluation relative to the capacity of seawater to take up CO 2 under a homogeneous scenario (i.e., no reaction with CaCO 3 ).Clearly, the thermodynamic definition of a significant buffer effect can be substantially different from a definition that considers the buffer effect in the context of the physiological and metabolic effects on marine organisms and their calcification rates.Hence, the definition and the means of evaluation of a significant buffer effect become important considerations in any discussions of this topic.
On time-scales of several thousands of years, dissolution of carbonate sediments in both shallow coastal waters and most importantly at depth in the ocean attendant by the consequent shoaling of the saturation horizon will act as the ultimate sink of anthropogenic CO 2 , the antacid or buffer of the world ocean (Archer et al., 1998;Broecker, 2003), a process mainly dictated by the overturn rate of the ocean, which is in the order of roughly 1000 yr.So why is it the case that dissolution of shallow water carbonate sediments will not significantly buffer the CO 2 -carbonic system of ocean surface waters on the time-scale of decades to centuries?
The first question we might ask is whether there is enough shallow water reactive high Mg-calcite material available to buffer the surface ocean CO 2 -carbonic acid system?Assuming an average porosity of 50 % and that the top meter of sediments is available for this reaction, there is roughly 10.4 × 10 15 mols, or 125 Pg, of carbon as Mg-calcite available for this reaction (Garrels and Mackenzie, 1980;Andersson et al., 2003Andersson et al., , 2005)).From a stoichiometric standpoint, assuming that the oceanic uptake of 1 mol of CO 2 will be balanced by dissolution of 1 mol of Mg-calcite, this process going to completion could proceed for about 60 yr at an absorption rate of surface seawater of ∼2 Pg C yr −1 (Le Quéré et al., 2009;Takahashi et al., 2009) before the entire reactive Mg-calcite reservoir was fully exhausted.However, to maintain a constant seawater pH, less than 1 mol of CaCO 3 is required for every mol of CO 2 absorbed.Thus, at first glance there appears to be a sufficient mass of reactive Mg-calcite to produce an important buffer effect at this uptake rate for the next several decades.If aragonite dissolving in shallow pore waters were also taken into consideration, even more reactive CaCO 3 would be available (see E. Sundquist for a different opinion in Garrels and Mackenzie, 1980).However, one must keep in mind that most of the anthropogenic CO 2 absorbed by the oceans is taken up in the open ocean and is not available for dissolution of shallow-water carbonate phases unless the acidified water is upwelled and/or onwelled into coastal waters.
Second, we might inquire whether or not anthropogenic emissions of CO 2 will change surface seawater carbon chemistry sufficiently so that seawater will become undersaturated with respect to the average Mg-calcite composition and consequently subject to dissolution?Under current CO 2 emission scenarios, it is highly likely that surface seawater in high latitudes will become undersaturated with respect to aragonite within the present or early part of the next century (e.g., Orr et al., 2005;Orr, 2011).Consequently, for Mg-calcite phases more soluble than aragonite (i.e., phases with more than 8 to 12 mol % MgCO 3 ), seawater will be undersaturated with respect to these phases and potentially subject to dissolution (as discussed below, this is also a kinetic problem) prior to this condition.At intermediate and low latitudes, this question is more difficult to answer because there are currently two different solubility curves for biogenic Mg-calcite minerals and we do not know which one best represents what is going on in the natural environment (e.g., Plummer and Mackenzie, 1974;Bischoff et al., 1993;Morse et al., 2006;Andersson et al., 2008).If we accept the more conservative solubility curve of Bischoff et al. (1993), surface seawater will remain supersaturated in tropical and temperate latitudes even with respect to a biogenic calcite mineral phase containing 18 mol % MgCO 3 by the end of this century under a Business-As-Usual CO 2 emission scenario (IS92a; Andersson et al., 2008).In contrast, based on the higher solubility estimate of Plummer and Mackenzie (1974), surface seawater in tropical and subtropical latitudes is presently close to a metastable equilibrium with respect to a Mg-calcite composition containing 15 mol % MgCO 3 .Shallow water carbonate sediments have an average Mg-calcite composition of approximately 13 to 15 mol % MgCO 3 (Morse and Mackenzie, 1990).Thus, as the ocean becomes more acidic, its surface waters could become undersaturated with respect to the majority of shallow water Mg-calcite phases.These phases then would be subject to dissolution if the higher solubility estimate of Plummer and Mackenzie (1974) most accurately describes the behavior of Mg-calcite phases in the natural environment.However, even if a mineral phase is subject to dissolution, the rate of dissolution (i.e., the kinetics of the reaction) needs to be sufficiently fast relative to the rate of ocean uptake of CO 2 (or CO 2 production from organic matter decomposition), as well as the rate of physical exchange of seawater between the open ocean and shallow water environments, in order to produce a significant buffer effect.The rate of dissolution is not strictly controlled by solubility.For instance, biogenic Mg-calcite minerals of less solubility may dissolve faster than those of greater solubility because of grain microstructural complexity.In other words, this complexity may override thermodynamic constraints resulting in the dissolution of a thermodynamically more stable biogenic mineral phase (Walter and Morse, 1985).CaCO 3 dissolution rates from the natural environment and experiments have been reported in the range from less than 0.1 to as high as 7 mmol CaCO 3 m −2 h −1 (see Table 3 in Andersson et al., 2007) based mainly on nighttime observations.These rates are high and generate substantial quantities of total alkalinity.However, in the context of a potential buffering effect, these rates need to be evaluated relative to the overall rate of calcification of marine organisms during daytime and the rate of flushing with open ocean waters.
The buffering question primarily boils down to a simple mass balance problem where the rate of net dissolution needs to be fast enough in order for total alkalinity to accumulate and thus buffer the CO 2 -carbonic acid system.Fig. 3 illustrates two hypothetical scenarios of (a) no buffering from CaCO 3 dissolution, i.e., no change in total alkalinity, and (b) sufficient buffering and accumulation of total alkalinity in order to maintain a constant seawater pH.The figure demonstrates that by evaluating the total alkalinity balance of a system, it can be determined whether the system is buffered or not from CaCO 3 dissolution.However, it is important to keep in mind that evidence of CaCO 3 dissolution does not mean that the system is sufficiently buffered to prevent negative effects on marine organisms.Previous exclamations of a significant buffer effect on relevant human timescales repeatedly overlook the importance of physical mixing (Barnes and Cuff, 2000;Halley and Yates, 2000).The area of the open ocean involved in the uptake of anthropogenic CO 2 is very large compared to the area of shallow water CaCO 3 sediments that potentially could neutralize this CO 2 .In general, the flushing of these shallow CaCO 3 rich environments with open ocean water is too fast for any significant accumulation of alkalinity within these environments.
One example of a system that experiences accumulation of total alkalinity owing to carbonate mineral dissolution is Devil's Hole, Harrington Sound, Bermuda (Andersson et al., 2007).In summer, a seasonal thermocline develops and separates the mixed layer from the subthermocline layer at this site (Fig. 4).Organic matter remineralized in the subthermocline layer produces CO 2 that causes a reduction in seawater pH and saturation state with respect to carbonate minerals, which begin to dissolve.Metastable Mg-calcite minerals make up a significant proportion of the carbonate sediments in Devil's Hole and Harrington Sound (Neumann, 1965;Andersson et al., 2007), but are virtually absent in the finer grain size fractions of Devil's Hole sediments.Evidence of dissolution can be seen from an increase in total alkalinity and calcium concentrations, which accumulate within the subthermocline layer (Fig. 5).This is also evidence that the system is experiencing a buffer effect.Nevertheless, despite evidence of dissolution of high Mg-calcite minerals and accumulation of alkalinity in Devil's Hole in response to elevated CO 2 , the seawater pH and aragonite saturation state levels are sufficiently low to negatively affect most shallow water marine calcifiers.Notably, based on the average sediment composition and seawater chemistry observed in Devil's Hole (Andersson et al., 2007), these data best agree with what is expected based on the Mg-calcite solubility curve of Plummer and Mackenzie (1974).
In conclusion, at present time and for most shallow water environments, the rate of CO 2 uptake and the rate of physical mixing are too fast relative to the rates of net dissolution of Mg-calcite phases in order for sufficient alkalinity to accumulate and buffer the CO 2 -carbonic acid system of coastal ocean ecosystems, including regional coral reef systems.

Concluding remarks
In this article we have addressed four topics relevant to OA research that in our opinion have been associated with misunderstandings and/or a lack in clarity, which we have encountered in various forums including the scientific literature, conference proceedings, manuscript and proposal reviews, as well as in discussions with fellow scientists.Our intention with this article was to raise more fully the awareness of the community to these topics and stimulate further necessary discussion.In the following, we summarize some of the key points and propose some potential future research avenues and practices to consider including: • For any ocean acidification experiment involving organisms from near-shore environments, what is the natural range and variability of CO 2 conditions experienced by these organisms?In many cases, ambient CO 2 conditions are significantly higher than anticipated for equilibrium with the atmosphere.Hence, it is important to quantify the variability (i.e., minimum, maximum and mean) in seawater carbon chemistry properties in both the natural environment and in experimental setups.It is absolutely critical that the complete CO 2 -carbonic acid system (i.e., H + , CO 2 , HCO − 3 , CO 2− 3 , DIC, TA) be fully characterized.Obviously, it is similarly important to characterize other relevant environmental parameters such as temperature, salinity, nutrient, and light conditions.
• It is usually not justified to discard acid (or base) manipulation experiments simply because these experiments do not exactly mimic the process of anthropogenic OA.
There may be other advantages provided by these experiments that carefully need to be considered such as the ability to evaluate physiological processes and conduct large-volume experiments and replication of natural flow environments.Nevertheless, additional direct side-by-side comparison of acid and CO 2 addition experiments are required to put this discussion to rest.This could also settle some enduring disagreements regarding contradictory results that have been attributed to acid versus CO 2 experimental treatments.
• It is important to recognize that the alkalinity anomaly technique and other approaches used to measure calcification are measuring the net effect of gross calcification and dissolution.To predict accurately the effect of OA, for example, on coral reef communities, it would be beneficial to be able to quantify these processes independently.What are the major controls of gross calcification and dissolution, and how do they change in response to rising seawater CO 2 and decreasing ?
• It is well established that CaCO 3 will dissolve in response to high CO 2 and low pH once an equilibrium threshold has been surpassed.For aragonite and calcite these thresholds are relatively well known in seawater, but for biogenic Mg-calcite minerals they are still uncertain in the natural environment.What are these thresholds, what are the controls, and most importantly what are the controlling factors in the dissolution kinetics of the reactions involving these mineral phases?
• Any attempts to address whether dissolution of CaCO 3 will significantly buffer seawater of a particular carbonate ecosystem (small or large) need to consider the following questions: (1) How much CaCO 3 is available and reactive to changes in seawater CO 2 chemistry?
(2) What are the kinetics of dissolution, i.e., the mechanisms and rates of the reactions?
(3) What is the physical mixing/hydrographic regime?Accumulation of total alkalinity in static experimental vessels has little relevance to most of the natural environment, which is well flushed.
(4) What evidence is there of a buffer effect and how is the significance of this effect defined? Observations of dissolution alone do not automatically mean that a significant buffer effect can be produced.
Based on the current global socio-economic and political situation, it is highly unlikely that there will be a significant reduction or slow-down in CO 2 emissions during the next several decades.Thus, we can conclude with a very high degree of certainty that the atmospheric concentration of CO 2 will continue to increase and as a result the acid-base balance of surface seawater will also continue to change.There is no doubt that this major change in seawater carbon chemistry is of great concern in terms of how it will affect individual marine organisms, communities, and ecosystems, as well as the services and resources these systems provide to humans.To maximize our understanding of this problem, it is becoming increasingly important to recognize and learn from previous experiences, carefully and objectively assess data and results, as well as implement more rigorous approaches in OA research.

Fig. 1 .
Fig. 1.Air and surface seawater pCO 2 (µatm) from two near-shore environments in Hawaii and Puerto Rico during summer of 2008 and 2009, respectively,showing pCO 2 levels that on average are significantly higher than expected from equilibrium with the atmosphere.On occasion pCO 2 levels at the Hawaii site have been observed to exceed 1000 µatm, which is much higher than the levels predicted to occur in the atmosphere in the year 2100 under a business-as-usual CO 2 emission scenario.Data courtesy: Eric De-Carlo, University of Hawaii, Dwight Gledhill, NOAA AOML, and Chris Sabine, NOAA PMEL.http://www.pmel.noaa.gov/co2/story/Coral+Reef+Moorings Figure 1.

Fig. 2 .
Fig. 2. Example of a near-shore mangrove environment in Bermuda, which undergoes large diurnal fluctuations in seawater carbonate chemistry over diurnal cycles owing to tidal flushing (and pumping) and groundwater intrusion as well as changes in ecosystem metabolism (e.g., photosynthesis, respiration, calcification, and CaCO 3 dissolution) between night and day.Surface seawater pCO 2 (black line) and relative tide (red line) over a 24-h period are shown.

Fig.
Fig.3.Hypothetical illustration of a system that receives no buffering (top panel) from dissolution of CaCO 3 minerals, i.e., seawater total alkalinity remains unchanged, and a system that is buffered by CaCO 3 dissolution (bottom panel) in order to maintain a constant pH, i.e., total alkalinity increases and accumulates within the system.

Fig. 4 .
Fig. 4. Summertime vertical profiles from Devil's Hole, Bermuda, showing temperature and pCO 2 (left panel), and aragonite saturation state ( ) and pH-total (right panel).Due to the thermal stratification of the water column, CO 2 produced from microbial decomposition of organic material accumulates within the subthermocline layer producing low -aragonite and pH (see Andersson et al., 2007 for detailed discussion).

Fig. 5 .
Fig. 5. Vertical profiles of [Ca 2+ ] and TA in Devil's Hole, Bermuda.The increase of both parameters in the subthermocline layer (gray shaded area) indicates CaCO 3 dissolution from the sediments.Despite significant dissolution and buffering, -aragonite and pH remain at low levels (see Andersson et al., 2007 for detailed discussion).