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Andersson, AJ, Mackenzie FT, Ver LM.  2003.  Solution of shallow-water carbonates: An insignificant buffer against rising atmospheric CO2. Geology. 31:513-516.   10.1130/0091-7613(2003)031<0513:soscai>2.0.co;2   AbstractWebsite

Model predictions suggest that the saturation state of surface ocean waters with respect to carbonate minerals will decline during the twenty-first century owing to increased invasion of atmospheric CO2. As a result, calcareous organisms may have difficulty calcifying, leading to production of weaker skeletons and greater vulnerability to erosion. Alternatively, it has been suggested that there will be no significant impact on coral reef ecosystems because any changes in saturation state and pH will be restored by dissolution of metastable carbonate minerals. To resolve this controversy, we employ a physical-biogeochemical box model representative of the shallow-water ocean environment. Numerical simulations demonstrate that the carbonate saturation state of surface waters could significantly decrease and hamper the biogenic production of CaCO3 during the twenty-first century. Similarly, the average saturation state of marine pore waters could decline significantly, inducing dissolution of metastable carbonate phases within the pore-water-sediment system. Such dissolution could buffer the carbon chemistry of the pore waters, but overlying surface waters of reefs and other shallow-water carbonate environments will not accumulate sufficient alkalinity to buffer pH or carbonate saturation state changes owing to invasion of atmospheric CO2.

Andersson, AJ, Mackenzie FT.  2011.  Technical comment on Kroeker et al. (2010) Meta-analysis reveals negative yet variable effects of ocean acidification on marine organisms. Ecology Letters, 13, 1419–1434. Ecology Letters. 14:E1-E2.: Blackwell Publishing Ltd   10.1111/j.1461-0248.2011.01646.x   AbstractWebsite

Meta-analysis of experimental results has been interpreted to imply that the calcification response of organisms depositing high Mg-calcite is more resilient to ocean acidification than organisms depositing aragonite/calcite. This conclusion might be biased by inadequate recognition and categorisation of high Mg-calcite according to mineral solubility.

Andersson, AJ, Bates NR, Mackenzie FT.  2007.  Dissolution of carbonate sediments under rising pCO(2) and ocean acidification: Observations from Devil's Hole, Bermuda. Aquatic Geochemistry. 13:237-264.   10.1007/s10498-007-9018-8   AbstractWebsite

Rising atmospheric pCO(2) and ocean acidification originating from human activities could result in increased dissolution of metastable carbonate minerals in shallow-water marine sediments. In the present study, in situ dissolution of carbonate sedimentary particles in Devil's Hole, Bermuda, was observed during summer when thermally driven density stratification restricted mixing between the bottom water and the surface mixed layer and microbial decomposition of organic matter in the subthermocline layer produced pCO(2) levels similar to or higher than those levels anticipated by the end of the 21st century. Trends in both seawater chemistry and the composition of sediments in Devil's Hole indicate that Mg-calcite minerals are subject to selective dissolution under conditions of elevated pCO(2). The derived rates of dissolution based on observed changes in excess alkalinity and estimates of vertical eddy diffusion ranged from 0.2 mmol to 0.8 mmol CaCO3 m(-2) h(-1). On a yearly basis, this range corresponds to 175-701 g CaCO3 m(-2) year(-1); the latter rate is close to 50% of the estimate of the current average global coral reef calcification rate of about 1,500 g CaCO3 m(-2) year(-1). Considering a reduction in marine calcification of 40% by the year 2100, or 90% by 2300, as a result of surface ocean acidification, the combination of high rates of carbonate dissolution and reduced rates of calcification implies that coral reefs and other carbonate sediment environments within the 21st and following centuries could be subject to a net loss in carbonate material as a result of increasing pCO(2) arising from burning of fossil fuels.

Andersson, AJ, Mackenzie FT, Lerman A.  2005.  Coastal ocean and carbonate systems in the high CO(2) world of the anthropocene. American Journal of Science. 305:875-918.   10.2475/ajs.305.9.875   AbstractWebsite

The behavior of the ocean carbon cycle has been, and will continue to be, modified by the increase in atmospheric CO(2) due to fossil fuel combustion and land-use emissions of this gas. The consequences of a high-CO(2) world and increasing riverine transport of organic matter and nutrients arising from human activities were investigated by means of two biogeochemical box models. Model numerical simulations ranging from the year 1700 to 2300 show that the global coastal ocean changes from a net source to a net sink of atmospheric CO(2) over time; in the 18th and 19th centuries, the direction of the CO(2) flux was from coastal surface waters to the atmosphere, whereas at present or in the near future the net CO(2) flux is into coastal surface waters. These results agree well with recent syntheses of measurements of air-sea CO(2) exchange fluxes from various coastal ocean environments. The model calculations also show that coastal ocean surface water carbonate saturation state would decrease 46 percent by the year 2100 and 73 percent by 2300. Observational evidence from the Pacific and Atlantic Oceans shows that die carbonate saturation state of surface ocean waters has already declined during recent decades. For atolls and other semi-enclosed carbonate systems, the rate of decline depends strongly on the residence time of the water in the system. Based on the experimentally observed positive relationship between saturation state and calcification rate for many calcifying organisms, biogenic production of CaCO(3) may decrease by 42 percent by the year 2100 and by 85 to 90 percent by 2300 relative to its value of about 24 x 10(12) moles C/yr in the year 2000. If the predicted change in carbonate production were to occur along with rising temperatures, it would make it difficult for coral reef and other carbonate systems, to exist as we know them now into future centuries. Because high-latitude, cold-water carbonates presently occur in waters closer to saturation with respect to carbonate minerals than the more strongly supersaturated waters of the lower latitudes, it might be anticipated that the cool-water carbonate systems might feel the effects of rising atmospheric CO(2) (and temperature) before those at lower latitudes. In addition, modeling results show that the carbonate saturation state of coastal sediment pore water will decrease in the future owing to a decreasing pore water pH and increasing CO(2) concentrations attributable to greater deposition and remineralization of land-derived and in situ produced organic matter in sediments. The lowered carbonate saturation state drives selective dissolution of metastable carbonate minerals while a metastable equilibrium is maintained between the pore water and the most soluble carbonate phase present in the sediments. In the future, the average composition of carbonate sediments and cements may change as the more soluble Mg-calcites and aragonite are preferentially dissolved and phases of lower solubility, such as calcites with lower magnesium content, increase in percentage abundance in the sediments.

Andersson, AJ, Kuffner IB, Mackenzie FT, Jokiel PL, Rodgers KS, Tan A.  2009.  Net Loss of CaCO(3) from a subtropical calcifying community due to seawater acidification: mesocosm-scale experimental evidence. Biogeosciences. 6:1811-1823. AbstractWebsite

Acidification of seawater owing to oceanic uptake of atmospheric CO(2) originating from human activities such as burning of fossil fuels and land-use changes has raised serious concerns regarding its adverse effects on corals and calcifying communities. Here we demonstrate a net loss of calcium carbonate (CaCO(3)) material as a result of decreased calcification and increased carbonate dissolution from replicated subtropical coral reef communities (n=3) incubated in continuous-flow mesocosms subject to future seawater conditions. The calcifying community was dominated by the coral Montipora capitata. Daily average community calcification or Net Ecosystem Calcification (NEC=CaCO(3) production - dissolution) was positive at 3.3 mmol CaCO(3) m(-2) h(-1) under ambient seawater pCO(2) conditions as opposed to negative at -0.04 mmol CaCO(3) m(-2) h(-1) under seawater conditions of double the ambient pCO(2). These experimental results provide support for the conclusion that some net calcifying communities could become subject to net dissolution in response to anthropogenic ocean acidification within this century. Nevertheless, individual corals remained healthy, actively calcified (albeit slower than at present rates), and deposited significant amounts of CaCO(3) under the prevailing experimental seawater conditions of elevated pCO(2).

Andersson, AJ, Mackenzie FT.  2004.  Shallow-water oceans: a source or sink of atmospheric CO2? Frontiers in Ecology and the Environment. 2:348-353.   10.1890/1540-9295(2004)002[0348:soasos]2.0.co;2   AbstractWebsite

The shallow-water ocean environment is of great importance in the context of global change and is heavily impacted by human activity. This study evaluates the effects of human activity on the CO2 exchange between the atmosphere and the surface water of shallow-water oceans. The evaluation is based on changes in net ecosystem metabolism, net ecosystem calcification, and atmospheric CO2 concentrations, as seen in a process-driven biogeochemical box model. Numerical simulations show that this air-sea interface has probably served as a net source of CO2 to the atmosphere for much of the past 300 years, but has recently switched, or will switch soon, to a net sink of CO2, because of rising atmospheric CO2 and increasing inorganic nutrient load.

Andersson, AJ, Krug LA, Bates NR, Doney SC.  2013.  Sea-air CO2 flux in the North Atlantic subtropical gyre: Role and influence of Sub-Tropical Mode Water formation. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 91:57-70.   10.1016/j.dsr2.2013.02.022   AbstractWebsite

The uptake of atmospheric carbon dioxide (CO2) into the mid-latitudes of the North Atlantic Ocean through the production of wintertime Sub-Tropical Mode Water (STMW) also known as Eighteen Degree Water (EDW) is poorly quantified and constrained. Nonetheless, it has been proposed that the EDW could serve as an important short-term sink of anthropogenic CO2. The objective of the present investigation was to determine sea-air CO2 gas exchange rates and seawater CO2 dynamics during wintertime formation of EDW in the North Atlantic Ocean. During 2006 and 2007, several research cruises were undertaken as part of the CLIMODE project across the northwest Atlantic Ocean with the intent to study the pre-conditioning, formation, and the evolution of EDW. Sea-air CO2 exchange rates were calculated based on measurements of atmospheric pCO(2), surface seawater pCO(2) and wind speed with positive values denoting a net flux from the surface ocean to the atmosphere. Average sea-air CO2 flux calculated along cruise tracks in the formation region equaled -18 +/- 6 mmol CO2 m(-2) d(-1) and -14 +/- 9 mmol CO2 m(-2) d(-1) in January of 2006 and March of 2007, respectively. Average sea-air CO2 flux in newly formed outcropping EDW in February and March of 2007 equaled -28 +/- 10 mmol CO2 m(-2) d(-1). These estimates exceeded previous flux estimates in this region by 40-185%. The magnitude of CO2 flux was mainly controlled by the observed variability in wind speed and Delta pCO(2) with smaller changes owing to variability in sea surface temperature. Small but statistically significant difference (4.1 +/- 2.6 mu mol kg(-1)) in dissolved inorganic carbon (DIC) was observed in two occurrences of newly formed EDW in February and March of 2007. This difference was explained either by differences in the relative contribution from different water masses involved in the initial formation process of EDW or temporal changes owing to sea-air CO2 exchange (similar to 25%) and vertical and/or lateral mixing (similar to 75%) with water masses high in DIC from the cold side of the Gulf Stream and/or from below the permanent thermocline. Based on the present estimate of sea-air CO2 flux in newly formed EDW and a formation rate of 9.3 Sv y (Sverdrup year = 10(6) m(3) s(-1) flow sustained for 1 year), CO2 uptake by newly formed EDW may constitute 3-6% of the total North Atlantic CO2 sink. However, advection of surface waters that carry an elevated burden of anthropogenic CO2 that are transported to the formation region and transformed to mode water may contribute additional CO2 to the total net uptake and sequestration of anthropogenic CO2 to the ocean interior. (c) 2013 Elsevier Ltd. All rights reserved.

Andersson, AJ, Mackenzie FT, Bates NR.  2008.  Life on the margin: implications of ocean acidification on Mg-calcite, high latitude and cold-water marine calcifiers. Marine Ecology-Progress Series. 373:265-273.   10.3354/meps07639   AbstractWebsite

Future anthropogenic emissions of CO(2) and the resulting ocean acidification may have severe consequences for marine calcifying organisms and ecosystems. Marine calcifiers depositing calcitic hard parts that contain significant concentrations of magnesium, i.e. Mg-calcite, and calcifying organisms living in high latitude and/or cold-water environments are at immediate risk to ocean acidification and decreasing seawater carbonate saturation because they are currently immersed in seawater that is just slightly supersaturated with respect to the carbonate phases they secrete. Under the present rate Of CO(2) emissions, model calculations show that high latitude ocean waters could reach undersaturation with respect to aragonite in just a few decades. Thus, before this happens these waters will be undersaturated with respect to Mg-calcite minerals of higher solubility than that of aragonite. Similarly, tropical surface seawater could become undersaturated with respect to Mg-calcite minerals containing >= 12 mole percent (mol%) MgCO(3) during this century. As a result of these changes in surface seawater chemistry and further penetration of anthropogenic CO(2) into the ocean interior, we suggest that (1) the magnesium content of calcitic hard parts will decrease in many ocean environments, (2) the relative proportion of calcifiers depositing stable carbonate minerals, such as calcite and low Mg-calcite, will increase and (3) the average magnesium content of carbonate sediments will decrease. Furthermore, the highest latitude and deepest depth at which cold-water corals and other calcifiers currently exist will move towards lower latitudes and shallower depth, respectively. These changes suggest that anthropogenic emissions of CO(2) may be currently pushing the oceans towards an episode characteristic of a 'calcite sea.'

Andersson, AJ, Yeakel KL, Bates NR, de Putron SJ.  2014.  Partial offsets in ocean acidification from changing coral reef biogeochemistry. Nature Climate Change. 4:56-61.   10.1038/nclimate2050   AbstractWebsite

Concerns have been raised about how coral reefs will be affected by ocean acidification(1,2), but projections of future seawater CO2 chemistry have focused solely on changes in the pH and aragonite saturation state (Omega(a)) of open-ocean surface seawater conditions surrounding coral reefs(1-4) rather than the reef systems themselves. The seawater CO2 chemistry within heterogeneous reef systems can be significantly different from that of the open ocean depending on the residence time, community composition and the main biogeochemical processes occurring on the reef, that is, net ecosystem production (NEP = gross primary production autotrophic and heterotrophic respiration) and net ecosystem calcification (NEC = gross calcification gross CaCO3 dissolution), which combined act to modify seawater chemistry(5-7). On the basis of observations from the Bermuda coral reef, we show that a range of projected biogeochemical responses of coral reef communities to ocean acidification by the end of this century could partially offset changes in seawater pH and Omega(a) by an average of 12-24% and 15-31%, respectively.

Andersson, AJ, Gledhill D.  2013.  Ocean Acidification and Coral Reefs: Effects on Breakdown, Dissolution, and Net Ecosystem Calcification. Annual Review of Marine Science. 5:321-348.   doi:10.1146/annurev-marine-121211-172241   AbstractWebsite

The persistence of carbonate structures on coral reefs is essential in providing habitats for a large number of species and maintaining the extraordinary biodiversity associated with these ecosystems. As a consequence of ocean acidification (OA), the ability of marine calcifiers to produce calcium carbonate (CaCO3) and their rate of CaCO3 production could decrease while rates of bioerosion and CaCO3 dissolution could increase, resulting in a transition from a condition of net accretion to one of net erosion. This would have negative consequences for the role and function of coral reefs and the eco-services they provide to dependent human communities. In this article, we review estimates of bioerosion, CaCO3 dissolution, and net ecosystem calcification (NEC) and how these processes will change in response to OA. Furthermore, we critically evaluate the observed relationships between NEC and seawater aragonite saturation state (Ωa). Finally, we propose that standardized NEC rates combined with observed changes in the ratios of dissolved inorganic carbon to total alkalinity owing to net reef metabolism may provide a biogeochemical tool to monitor the effects of OA in coral reef environments.

Andersson, AJ, Mackenzie FT, Lerman A.  2006.  Coastal ocean CO(2)-carbonic acid-carbonate sediment system of the Anthropocene. Global Biogeochemical Cycles. 20   10.1029/2005gb002506   AbstractWebsite

[1] There is little doubt that human activities such as burning of fossil fuels and land use practices have changed and will continue to change the cycling of carbon in the global coastal ocean. In the present study, two biogeochemical box models were used to investigate the consequences of increasing atmospheric CO(2) and subsequent ocean acidification and increasing riverine transport of organic matter and nutrients arising from human activities on land on the global coastal ocean between the years 1700 and 2300. Numerical simulations show that the net flux of CO(2) between coastal ocean surface water and the atmosphere is likely to change during this time from net evasion to net invasion owing to increasing atmospheric CO(2), increasing net ecosystem production arising from increasing nutrient loading to this region, and decreasing net ecosystem calcification due to lower carbonate ion concentration and subsequent lower surface water saturation state with respect to carbonate minerals. Model calculations show that surface water saturation state with respect to calcite will decrease 73% by the year 2300 under a business-as-usual scenario, which in concert with increasing temperature will cause overall biogenic calcification rate to decrease by 90%. Dissolution of carbonate minerals increased by 267% throughout the model simulation. This increase was in part due to increased invasion of atmospheric CO(2), but mainly due to greater deposition and remineralization of land-derived and in situ produced organic matter in the sediments, producing CO(2) that caused pore water pH and carbonate saturation state to decrease. This decrease, in turn, drove selective dissolution of metastable carbonate minerals. As a consequence, the relative carbonate composition of the sediments changed in favor of carbonate phases with lower solubility than that of an average 15 mol% magnesian calcite phase. Model projected changes in surface water carbonate saturation state agree well with observations from the Hawaiian Ocean Time series and the calculated air-sea CO(2) exchanged agrees well with a recent independent estimate of this flux derived from measurements from diverse coastal ecosystems scaled up to the global coastal ocean area.

Andersson, AJ, Bates NR, Jeffries MA, Freeman K, Davidson C, Stringer S, Betzler E, Mackenzie FT.  2013.  Clues from current high CO2 environments on the effects of ocean acidification on CaCO3 preservation. Aquatic Geochemistry.   10.1007/s10498-013-9210-y  
Andersson, AJ, Mackenzie FT, Gattuso J-P.  2011.  Effects of ocean acidification on benthic processes, organisms, and ecosystems. Ocean Acidification. ( Gattuso J, Hansson L, Eds.).:xix,326p.., Oxford England ; New York: Oxford University Press Abstract

The ocean helps moderate climate change thanks to its considerable capacity to store CO2, through the combined actions of ocean physics, chemistry, and biology. This storage capacity limits the amount of human-released CO2 remaining in the atmosphere. As CO2 reacts with seawater, it generates dramatic changes in carbonate chemistry, including decreases in pH and carbonate ions and an increase in bicarbonate ions. The consequences of this overall process, known as "ocean acidification", are raising concerns for the biological, ecological, and biogeochemical health of the world's oceans, as well as for the potential societal implications. This research level text is the first to synthesize the very latest understanding of the consequences of ocean acidification, with the intention of informing both future research agendas and marine management policy. A prestigious list of authors has been assembled, among them the coordinators of major national and international projects on ocean acidification.

Andersson, AJ.  2014.  The oceanic CaCO3 cycle. Treatise on Geochemistry. Vol. 8( Holland HD, Turekian KK, Eds.)., Oxford: Elsevier   10.1016/B978-0-08-095975-7.00619-7  
Andersson, AJ, Mackenzie FT.  2012.  Revisiting four scientific debates in ocean acidification research. Biogeosciences. 9:893-905.   10.5194/bg-9-893-2012   AbstractWebsite

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 CO2 chemistry in shallow water coastal areas, (2) experimental manipulation of marine systems using CO2 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 pCO(2) that is significantly higher than expected from equilibrium with the atmosphere and is strongly linked to biological processes; (2) addition of acid, base or CO2 gas to seawater can all be useful techniques to manipulate seawater chemistry in ocean acidification experiments; (3) estimates of calcification or CaCO3 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.

Andersson, AJ, Kline DI, Edmunds PJ, Archer SD, Bednaršek N, Carpenter RC, Chadsey M, Goldstein P, Grottoli AG, Hurst TP, King AL, Kübler JE, Kuffner IB, Mackey KRM, Paytan A, Menge B, Riebesell U, Schnetzer A, Warner ME, Zimmerman RC.  2015.  Understanding ocean acidification impacts on organismal to ecological scales. Oceanography magazine. 28:10-21.
Andersson, A.  2015.  A fundamental paradigm for coral reef carbonate sediment dissolution. Frontiers in Marine Science. 2   10.3389/fmars.2015.00052   Abstract

The long-term success of coral reefs depends on a positive balance of calcium carbonate production exceeding dissolution, erosion, and material export. As a result of ocean acidification, coral reefs could transition from net accretion to net erosion owing to decreasing rates of calcification and increasing rates of chemical dissolution and bioerosion. Here, I present a fundamental paradigm that aims to explain the main driver of carbonate sediment dissolution on coral reefs based on theory and a new empirical dataset of pore water carbonate chemistry from the Bermuda coral reef platform. The paradigm shows that carbonate sediment dissolution is most strongly controlled by the extent of organic matter decomposition in the sediments, but that the magnitude of dissolution is influenced by how much decomposition is required to reach pore water undersaturation with respect to the most soluble bulk carbonate mineral phase present in the sediments, a condition defined as the Carbonate Critical Threshold (CCT). Decomposition of organic matter beyond the CCT under aerobic conditions results in stoichiometric proportional dissolution of carbonate sediments. As ocean acidification proceeds over the next several decades, the extent of organic matter decomposition required to reach the CCT will decrease, carbonate dissolution will increase, and subsequently the accumulation of carbonate sediments will decrease. Since drastic reductions in anthropogenic CO2 emission are unlikely in the foreseeable future, the paradigm shows that active controls and reduction of organic matter input to coral reefs at the local scale might be an effective mitigation strategy to prevent or delay coral reefs transitioning to a state of net dissolution.

Anthony, KRN, Diaz-Pulido G, Verlinden N, Tilbrook B, Andersson AJ.  2013.  Benthic buffers and boosters of ocean acidification on coral reefs. Biogeosciences. 10:4897-4909.   10.5194/bg-10-4897-2013   AbstractWebsite

Ocean acidification is a threat to marine ecosystems globally. In shallow-water systems, however, ocean acidification can be masked by benthic carbon fluxes, depending on community composition, seawater residence time, and the magnitude and balance of net community production (NCP) and calcification (NCC). Here, we examine how six benthic groups from a coral reef environment on Heron Reef (Great Barrier Reef, Australia) contribute to changes in the seawater aragonite saturation state (Omega(a)). Results of flume studies using intact reef habitats (1.2m by 0.4 m), showed a hierarchy of responses across groups, depending on CO2 level, time of day and water flow. At low CO2 (350-450 mu atm), macroalgae (Chnoospora implexa), turfs and sand elevated Omega(a) of the flume water by around 0.10 to 1.20 h(-1) - normalised to contributions from 1m(2) of benthos to a 1m deep water column. The rate of Omega(a) increase in these groups was doubled under acidification (560-700 mu atm) and high flow (35 compared to 8 cm s(-1)). In contrast, branching corals (Acropora aspera) increased Omega(a) by 0.25 h(-1) at ambient CO2 (350-450 mu atm) during the day, but reduced Omega(a) under acidification and high flow. Nighttime changes in Omega(a) by corals were highly negative (0.6-0.8 h(-1)) and exacerbated by acidification. Calcifying macroalgae (Halimeda spp.) raised Omega(a) by day (by around 0.13 h(-1)), but lowered Omega(a) by a similar or higher amount at night. Analyses of carbon flux contributions from benthic communities with four different compositions to the reef water carbon chemistry across Heron Reef flat and lagoon indicated that the net lowering of Omega(a) by coral-dominated areas can to some extent be countered by long water-residence times in neighbouring areas dominated by turfs, macroalgae and carbonate sand.

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Bates, NR, Amat A, Andersson AJ.  2010.  Feedbacks and responses of coral calcification on the Bermuda reef system to seasonal changes in biological processes and ocean acidification. Biogeosciences. 7:2509-2530.   10.5194/bg-7-2509-2010   AbstractWebsite

Despite the potential impact of ocean acidification on ecosystems such as coral reefs, surprisingly, there is very limited field data on the relationships between calcification and seawater carbonate chemistry. In this study, contemporaneous in situ datasets of seawater carbonate chemistry and calcification rates from the high-latitude coral reef of Bermuda over annual timescales provide a framework for investigating the present and future potential impact of rising carbon dioxide (CO(2)) levels and ocean acidification on coral reef ecosystems in their natural environment. A strong correlation was found between the in situ rates of calcification for the major framework building coral species Diploria labyrinthiformis and the seasonal variability of [CO(3)(2-)] and aragonite saturation state Omega(aragonite), rather than other environmental factors such as light and temperature. These field observations provide sufficient data to hypothesize that there is a seasonal 'Carbonate Chemistry Coral Reef Ecosystem Feedback' (CREF hypothesis) between the primary components of the reef ecosystem (i.e., scleractinian hard corals and macroalgae) and seawater carbonate chemistry. In early summer, strong net autotrophy from benthic components of the reef system enhance [CO(3)(2-)] and Omega(aragonite) conditions, and rates of coral calcification due to the photosynthetic uptake of CO(2). In late summer, rates of coral calcification are suppressed by release of CO(2) from reef metabolism during a period of strong net heterotrophy. It is likely that this seasonal CREF mechanism is present in other tropical reefs although attenuated compared to high-latitude reefs such as Bermuda. Due to lower annual mean surface seawater [CO(3)(2-)] and Omega(aragonite) in Bermuda compared to tropical regions, we anticipate that Bermuda corals will experience seasonal periods of zero net calcification within the next decade at [CO(3)(2-)] and Omega(aragonite) thresholds of similar to 184 mu moles kg(-1) and 2.65. However, net autotrophy of the reef during winter and spring (as part of the CREF hypothesis) may delay the onset of zero NEC or decalcification going forward by enhancing [CO(3)(2-)] and Omega(aragonite). The Bermuda coral reef is one of the first responders to the negative impacts of ocean acidification, and we estimate that calcification rates for D. labyrinthiformis have declined by > 50% compared to pre-industrial times.

Beman, JM, Chow CE, King AL, Feng YY, Fuhrman JA, Andersson A, Bates NR, Popp BN, Hutchins DA.  2011.  Global declines in oceanic nitrification rates as a consequence of ocean acidification. Proceedings of the National Academy of Sciences of the United States of America. 108:208-213.   10.1073/pnas.1011053108   AbstractWebsite

Ocean acidification produced by dissolution of anthropogenic carbon dioxide (CO2) emissions in seawater has profound consequences for marine ecology and biogeochemistry. The oceans have absorbed one-third of CO2 emissions over the past two centuries, altering ocean chemistry, reducing seawater pH, and affecting marine animals and phytoplankton in multiple ways. Microbially mediated ocean biogeochemical processes will be pivotal in determining how the earth system responds to global environmental change; however, how they may be altered by ocean acidification is largely unknown. We show here that microbial nitrification rates decreased in every instance when pH was experimentally reduced (by 0.05-0.14) at multiple locations in the Atlantic and Pacific Oceans. Nitrification is a central process in the nitrogen cycle that produces both the greenhouse gas nitrous oxide and oxidized forms of nitrogen used by phytoplankton and other microorganisms in the sea; at the Bermuda Atlantic Time Series and Hawaii Ocean Time-series sites, experimental acidification decreased ammonia oxidation rates by 38% and 36%. Ammonia oxidation rates were also strongly and inversely correlated with pH along a gradient produced in the oligotrophic Sargasso Sea (r(2) = 0.87, P < 0.05). Across all experiments, rates declined by 8-38% in low pH treatments, and the greatest absolute decrease occurred where rates were highest off the California coast. Collectively our results suggest that ocean acidification could reduce nitrification rates by 3-44% within the next few decades, affecting oceanic nitrous oxide production, reducing supplies of oxidized nitrogen in the upper layers of the ocean, and fundamentally altering nitrogen cycling in the sea.

Bresnahan, PJ, Wirth T, Martz TR, Andersson AJ, Cyronak T, D’Angelo S, Pennise J, Melville KW, Lenain L, Statom N.  2016.  A sensor package for mapping pH and oxygen from mobile platforms. Methods in Oceanography. 17:1-13.   10.1016/j.mio.2016.04.004   Abstract

A novel chemical sensor package named “WavepHOx” was developed in order to facilitate measurement of surface ocean pH, dissolved oxygen, and temperature from mobile platforms. The system comprises a Honeywell Durafet pH sensor, Aanderaa optode oxygen sensor, and chloride ion selective electrode, packaged into a hydrodynamic, lightweight housing. The WavepHOx has been deployed on a stand-up paddleboard and a Liquid Robotics Wave Glider in multiple near-shore settings in the Southern California Bight. Integration of the WavepHOx into these mobile platforms has enabled high spatiotemporal resolution pH and dissolved oxygen data collection. It is a particularly valuable tool for mapping shallow, fragile, or densely vegetated ecosystems which cannot be easily accessed by other platforms. Results from three surveys in San Diego, California, are reported. We show pH and dissolved oxygen variability >0.3 and >50% saturation, respectively, over tens to hundreds of meters to highlight the degree of natural spatial variability in these vegetated ecosystems. When deployed during an extensive discrete sampling program, the WavepHOx pH had a root mean squared error of 0.028 relative to pH calculated from fifty six measurements of total alkalinity and dissolved inorganic carbon, confirming its capacity for accurate, high spatiotemporal resolution data collection.

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Courtney, TA, Andersson AJ, Bates NR, Collins A, Cyronak T, de Putron SJ, Eyre BD, Garley R, Hochberg EJ, Johnson R, Musielewicz S, Noyes TJ, Sabine CL, Sutton AJ, Toncin J, Tribollet A.  2016.  Comparing chemistry and census-based estimates of net ecosystem calcification on a rim reef in Bermuda. Frontiers in Marine Science. 3   10.3389/fmars.2016.00181   Abstract

Coral reef net ecosystem calcification (NEC) has decreased for many Caribbean reefs over recent decades primarily due to a combination of declining coral cover and changing benthic community composition. Chemistry-based approaches to calculate NEC utilize the drawdown of seawater total alkalinity (TA) combined with residence time to calculate an instantaneous measurement of NEC. Census-based approaches combine annual growth rates with benthic cover and reef structural complexity to estimate NEC occurring over annual timescales. Here, NEC was calculated for Hog Reef in Bermuda using both chemistry and census-based NEC techniques to compare the mass-balance generated by the two methods and identify the dominant biocalcifiers at Hog Reef. Our findings indicate close agreement between the annual 2011 census-based NEC 2.35±1.01 kg CaCO3•m-2•y-1 and the chemistry-based NEC 2.23±1.02 kg CaCO3•m-2•y-1 at Hog Reef. An additional record of Hog Reef TA data calculated from an autonomous CO2 mooring measuring pCO2 and modeled pHtotal every 3-hours highlights the dynamic temporal variability in coral reef NEC. This ability for chemistry-based NEC techniques to capture higher frequency variability in coral reef NEC allows the mechanisms driving NEC variability to be explored and tested. Just four coral species, Diploria labyrinthiformis, Pseudodiploria strigosa, Millepora alcicornis, and Orbicella franksi, were identified by the census-based NEC as contributing to 94±19% of the total calcium carbonate production at Hog Reef suggesting these species should be highlighted for conservation to preserve current calcium carbonate production rates at Hog Reef. As coral cover continues to decline globally, the agreement between these NEC estimates suggest that either method, but ideally both methods, may serve as a useful tool for coral reef managers and conservation scientists to monitor the maintenance of coral reef structure and ecosystem services.

Courtney, TA, Lebrato M, Bates NR, Collins A, de Putron SJ, Garley R, Johnson R, Molinero JC, Noyes TJ, Sabine CL, Andersson AJ.  2017.  Environmental controls on modern scleractinian coral and reef-scale calcification. Science Advances. 3   10.1126/sciadv.1701356   AbstractWebsite

Modern reef-building corals sustain a wide range of ecosystem services because of their ability to build calcium carbonate reef systems. The influence of environmental variables on coral calcification rates has been extensively studied, but our understanding of their relative importance is limited by the absence of in situ observations and the ability to decouple the interactions between different properties. We show that temperature is the primary driver of coral colony (Porites astreoides and Diploria labyrinthiformis) and reef-scale calcification rates over a 2-year monitoring period from the Bermuda coral reef. On the basis of multimodel climate simulations (Coupled Model Intercomparison Project Phase 5) and assuming sufficient coral nutrition, our results suggest that P. astreoides and D. labyrinthiformis coral calcification rates in Bermuda could increase throughout the 21st century as a result of gradual warming predicted under a minimum CO2 emissions pathway [ representative concentration pathway (RCP) 2.6] with positive 21st-century calcification rates potentially maintained under a reduced CO2 emissions pathway (RCP 4.5). These results highlight the potential benefits of rapid reductions in global anthropogenic CO2 emissions for 21st-century Bermuda coral reefs and the ecosystem services they provide.

Cyronak, T, Andersson AJ, D'Angelo S, Bresnahan P, Davidson C, Griffin A, Kindeberg T, Pennise J, Takeshita Y, White M.  2018.  Short-term spatial and temporal carbonate chemistry variability in two contrasting seagrass meadows: Implications for pH buffering capacities. Estuaries and Coasts. 41:1282-1296.   10.1007/s12237-017-0356-5   AbstractWebsite

It has been hypothesized that highly productive coastal ecosystems, such as seagrass meadows, could lead to the establishment of ocean acidification (OA) refugia, or areas of elevated pH and aragonite saturation state (Omega(a)) compared to source seawater. However, seagrass ecosystems experience extreme variability in carbonate chemistry across short temporal and small spatial scales, which could impact the pH buffering capacity of these potential refugia. Herein, short-term (hourly to diel) and small-scale (across 0.01-0.14 km(2)) spatiotemporal carbonate chemistry variability was assessed within two seagrass meadows in order to determine their short-term potential to elevate seawater pH relative to source seawater. Two locations at similar latitudes were chosen in order to compare systems dominated by coarse calcium carbonate (Bailey's Bay, Bermuda) and muddy silicate (Mission Bay, CA, USA) sediments. In both systems, spatial variability of pH across the seagrass meadow at any given time was often greater than diel variability (e.g., the average range over 24 h) at any one site, with greater spatial variability occurring at low tide in Mission Bay. Mission Bay (spatial Delta pH = 0.08 +/- 0.08; diel Delta pH = 0.12 +/- 0.01; mean +/- SD) had a greater average range in both temporal and spatial seawater chemistry than Bailey's Bay (spatial Delta pH = 0.02 +/- 0.01; diel Delta pH = 0.03 +/- 0.00; mean +/- SD). These differences were most likely due to a combination of slower currents, a larger tidal range, and more favorable weather conditions for photosynthesis (e.g., sunny with no rain) in Mission Bay. In both systems, there was a substantial amount of time (usually at night) when seawater pH within the seagrass beds was lower relative to the source seawater. Future studies aimed at assessing the potential of seagrass ecosystems to act as OA refugia for marine organisms need to account for the small-scale, high-frequency carbonate chemistry variability in both space and time, as this variability will impact where and when OA will be buffered or intensified.

Cyronak, T, Andersson AJ, Langdon C, Albright R, Bates NR, Caldeira K, Carlton R, Corredor JE, Dunbar RB, Enochs I, Erez J, Eyre BD, Gattuso JP, Gledhill D, Kayanne H, Kline DI, Koweek DA, Lantz C, Lazar B, Manzello D, McMahon A, Melendez M, Page HN, Santos IR, Schulz KG, Shaw E, Silverman J, Suzuki A, Teneva L, Watanabe A, Yamamoto S.  2018.  Taking the metabolic pulse of the world's coral reefs. Plos One. 13   10.1371/journal.pone.0190872   AbstractWebsite

Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.