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2018
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.

Paulsen, ML, Andersson AJ, Aluwihare L, Cyronak T, D'Angelo S, Davidson C, Elwany H, Giddings SN, Page HN, Porrachia M, Schroeter S.  2018.  Temporal changes in seawater carbonate chemistry and carbon export from a Southern California estuary. Estuaries and Coasts. 41:1050-1068.   10.1007/s12237-017-0345-8   AbstractWebsite

Estuaries are important subcomponents of the coastal ocean, but knowledge about the temporal and spatial variability of their carbonate chemistry, as well as their contribution to coastal and global carbon fluxes, are limited. In the present study, we measured the temporal and spatial variability of biogeochemical parameters in a saltmarsh estuary in Southern California, the San Dieguito Lagoon (SDL). We also estimated the flux of dissolved inorganic carbon (DIC) and total organic carbon (TOC) to the adjacent coastal ocean over diel and seasonal timescales. The combined net flux of DIC and TOC (FDIC + TOC) to the ocean during outgoing tides ranged from - 1.8 +/- 0.5 x 10(3) to 9.5 +/- 0.7 x 10(3) mol C h(-1) during baseline conditions. Based on these fluxes, a rough estimate of the net annual export of DIC and TOC totaled 10 +/- 4 x 10(6) mol C year(-1). Following a major rain event (36 mm rain in 3 days), FDIC + TOC increased and reached values as high as 29.0 +/- 0.7 x 10(3) mol C h(-1). Assuming a hypothetical scenario of three similar storm events in a year, our annual net flux estimate more than doubled to 25 +/- 4 x 10(6) mol C year(-1). These findings highlight the importance of assessing coastal carbon fluxes on different timescales and incorporating event scale variations in these assessments. Furthermore, for most of the observations elevated levels of total alkalinity (TA) and pH were observed at the estuary mouth relative to the coastal ocean. This suggests that SDL partly buffers against acidification of adjacent coastal surface waters, although the spatial extent of this buffering is likely small.

MeQuaid, JB, Kustka AB, Obornik M, Horak A, McCrow JR, Karas BJ, Zheng H, Kindeberg T, Andersson AJ, Barbeau KA, Allen AE.  2018.  Carbonate-sensitive phytotransferrin controls high-affinity iron uptake in diatoms. Nature. 555:534-+.   10.1038/nature25982   AbstractWebsite

In vast areas of the ocean, the scarcity of iron controls the growth and productivity of phytoplankton(1,2). Although most dissolved iron in the marine environment is complexed with organic molecules(3), picomolar amounts of labile inorganic iron species (labile iron) are maintained within the euphotic zone(4) and serve as an important source of iron for eukaryotic phytoplankton and particularly for diatoms(5). Genome-enabled studies of labile iron utilization by diatoms have previously revealed novel iron responsive transcripts(6,7), including the ferric iron-concentrating protein ISIP2A(8), but the mechanism behind the acquisition of picomolar labile iron remains unknown. Here we show that ISIP2A is a phytotransferrin that independently and convergently evolved carbonate ion-coordinated ferric iron binding. Deletion of ISIP2A disrupts high-affinity iron uptake in the diatom Phaeodactylum tricornutum, and uptake is restored by complementation with human transferrin. ISIP2A is internalized by endocytosis, and manipulation of the seawater carbonic acid system reveals a second order dependence on the concentrations of labile iron and carbonate ions. In P. tricornutum, the synergistic interaction of labile iron and carbonate ions occurs at environmentally relevant concentrations, revealing that carbonate availability co-limits iron uptake. Phytotransferrin sequences have a broad taxonomic distribution(8) and are abundant in marine environmental genomic datasets(9,10), suggesting that acidification-driven declines in the concentration of seawater carbonate ions will have a negative effect on this globally important eukaryotic iron acquisition mechanism.

Eyre, BD, Cyronak T, Drupp P, DeCarlo EH, Sachs JP, Andersson AJ.  2018.  Coral reefs will transition to net dissolving before end of century. Science. 359:908-911.   10.1126/science.aao1118   AbstractWebsite

Ocean acidification refers to the lowering of the ocean's pH due to the uptake of anthropogenic CO2 from the atmosphere. Coral reef calcification is expected to decrease as the oceans become more acidic. Dissolving calciumcarbonate (CaCO3) sands could greatly exacerbate reef loss associated with reduced calcification but is presently poorly constrained. Here we show that CaCO3 dissolution in reef sediments across five globally distributed sites is negatively correlated with the aragonite saturation state (War) of overlying seawater and that CaCO3 sediment dissolution is 10-fold more sensitive to ocean acidification than coral calcification. Consequently, reef sediments globally will transition from net precipitation to net dissolution when seawater War reaches 2.92 +/- 0.16 (expected circa 2050 CE). Notably, some reefs are already experiencing net sediment dissolution.

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.

2017
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.

2016
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.

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.

Page, HN, Andersson AJ, Jokiel PL, Rodgers K’uleiS, Lebrato M, Yeakel K, Davidson C, D’Angelo S, Bahr KD.  2016.  Differential modification of seawater carbonate chemistry by major coral reef benthic communities. Coral Reefs. :1-15.   10.1007/s00338-016-1490-4   AbstractWebsite

Ocean acidification (OA) resulting from uptake of anthropogenic CO2 may negatively affect coral reefs by causing decreased rates of biogenic calcification and increased rates of CaCO3 dissolution and bioerosion. However, in addition to the gradual decrease in seawater pH and Ω a resulting from anthropogenic activities, seawater carbonate chemistry in these coastal ecosystems is also strongly influenced by the benthic metabolism which can either exacerbate or alleviate OA through net community calcification (NCC = calcification – CaCO3 dissolution) and net community organic carbon production (NCP = primary production − respiration). Therefore, to project OA on coral reefs, it is necessary to understand how different benthic communities modify the reef seawater carbonate chemistry. In this study, we used flow-through mesocosms to investigate the modification of seawater carbonate chemistry by benthic metabolism of five distinct reef communities [carbonate sand, crustose coralline algae (CCA), corals, fleshy algae, and a mixed community] under ambient and acidified conditions during summer and winter. The results showed that different communities had distinct influences on carbonate chemistry related to the relative importance of NCC and NCP. Sand, CCA, and corals exerted relatively small influences on seawater pH and Ω a over diel cycles due to closely balanced NCC and NCP rates, whereas fleshy algae and mixed communities strongly elevated daytime pH and Ω a due to high NCP rates. Interestingly, the influence on seawater pH at night was relatively small and quite similar across communities. NCC and NCP rates were not significantly affected by short-term acidification, but larger diel variability in pH was observed due to decreased seawater buffering capacity. Except for corals, increased net dissolution was observed at night for all communities under OA, partially buffering against nighttime acidification. Thus, algal-dominated areas of coral reefs and increased net CaCO3 dissolution may partially counteract reductions in seawater pH associated with anthropogenic OA at the local scale.

Lebrato, M, Andersson AJ, Ries JB, Aronson RB, Lamare MD, Koeve W, Oschlies A, Iglesias-Rodriguez MD, Thatje S, Amsler M, Vos SC, Jones DOB, Ruhl HA, Gates AR, McClintock JB.  2016.  Benthic marine calcifiers coexist with CaCO3-undersaturated seawater worldwide. Global Biogeochemical Cycles. 30:1038-1053.   10.1002/2015GB005260   Abstract

Ocean acidification and decreasing seawater saturation state with respect to calcium carbonate (CaCO3) minerals have raised concerns about the consequences to marine organisms that build CaCO3 structures. A large proportion of benthic marine calcifiers incorporate Mg2+ into their skeletons (Mg-calcite), which, in general, reduces mineral stability. The relative vulnerability of some marine calcifiers to ocean acidification appears linked to the relative solubility of their shell or skeletal mineralogy, although some organisms have sophisticated mechanisms for constructing and maintaining their CaCO3 structures causing deviation from this dependence. Nevertheless, few studies consider seawater saturation state with respect to the actual Mg-calcite mineralogy (ΩMg-x) of a species when evaluating the effect of ocean acidification on that species. Here, a global dataset of skeletal mole % MgCO3 of benthic calcifiers and in situ environmental conditions spanning a depth range of 0 m (subtidal/neritic) to 5600 m (abyssal) was assembled to calculate in situ ΩMg-x. This analysis shows that 24% of the studied benthic calcifiers currently experience seawater mineral undersaturation (ΩMg-x < 1). As a result of ongoing anthropogenic ocean acidification over the next 200 to 3000 years, the predicted decrease in seawater mineral saturation will expose approximately 57% of all studied benthic calcifying species to seawater undersaturation. These observations reveal a surprisingly high proportion of benthic marine calcifiers exposed to seawater that is undersaturated with respect to their skeletal mineralogy, underscoring the importance of using species-specific seawater mineral saturation states when investigating the impact of CO2-induced ocean acidification on benthic marine calcification.

Edmunds, PJ, Comeau S, Lantz C, Andersson A, Briggs C, Cohen A, Gattuso JP, Grady JM, Gross K, Johnson M, Muller EB, Ries JB, Tambutte S, Tambutte E, Venn A, Carpenter RC.  2016.  Integrating the effects of ocean acidification across functional scales on tropical coral reefs. Bioscience. 66:350-362.   10.1093/biosci/biw023   AbstractWebsite

There are concerns about the future of coral reefs in the face of ocean acidification and warming, and although studies of these phenomena have advanced quickly, efforts have focused on pieces of the puzzle rather than integrating them to evaluate ecosystem-level effects. The field is now poised to begin this task, but there are information gaps that first must be overcome before progress can be made. Many of these gaps focus on calcification at the levels of cells, organisms, populations, communities, and ecosystem, and their closure will be made difficult by the complexity of the interdependent processes by which coral reefs respond to ocean acidification, with effects scaling from cells to ecosystems and from microns to kilometers. Existing ecological theories provide an important and largely untapped resource for overcoming these difficulties, and they offer great potential for integrating the effects of ocean acidification across scales on coral reefs.

Hopner, F, Bender FAM, Ekman AML, Praveen PS, Bosch C, Ogren JA, Andersson A, Gustafsson O, Ramanathan V.  2016.  Vertical profiles of optical and microphysical particle properties above the northern Indian Ocean during CARDEX 2012. Atmospheric Chemistry and Physics. 16:1045-1064.   10.5194/acp-16-1045-2016   AbstractWebsite

A detailed analysis of optical and microphysical properties of aerosol particles during the dry winter monsoon season above the northern Indian Ocean is presented. The Cloud Aerosol Radiative Forcing Experiment (CARDEX), conducted from 16 February to 30 March 2012 at the Maldives Climate Observatory on Hanimaadhoo island (MCOH) in the Republic of the Maldives, used autonomous unmanned aerial vehicles (AUAV) to perform vertical in situ measurements of particle number concentration, particle number size distribution as well as particle absorption coefficients. These measurements were used together with surface-based Mini Micro Pulse Lidar (MiniMPL) observations and aerosol in situ and off-line measurements to investigate the vertical distribution of aerosol particles. Air masses were mainly advected over the Indian subcontinent and the Arabian Peninsula. The mean surface aerosol number concentration was 1717 +/- 604cm(-3) and the highest values were found in air masses from the Bay of Bengal and Indo-Gangetic Plain (2247 +/- 370cm(-3)). Investigations of the free tropospheric air showed that elevated aerosol layers with up to 3 times higher aerosol number concentrations than at the surface occurred mainly during periods with air masses originating from the Bay of Bengal and the Indo-Gangetic Plain. This feature is different compared to what was observed during the Indian Ocean Experiment (INDOEX) conducted in winter 1999, where aerosol number concentrations generally decreased with height. In contrast, lower particle absorption at the surface (sigma(abs)(520nm) = 8.5 + 4.2Wm(-1)) was found during CARDEX compared to INDOEX 1999. Layers with source region specific single-scattering albedo (SSA) values were derived by combining vertical in situ particle absorption coefficients and scattering coefficients calculated with Mie theory. These SSA layers were utilized to calculate vertical particle absorption profiles from MiniMPL profiles. SSA surface values for 550 nm for dry conditions were found to be 0 : 94 +/- 0 : 02 and 0 : 91 +/- 0 : 02 for air masses from the Arabian Sea (and Middle East countries) and India (and Bay of Bengal), respectively. Lidar-derived particle absorption coefficient profiles showed both a similar magnitude and structure as the in situ profiles measured with the AUAV. However, primarily due to insufficient accuracy in the SSA estimates, the lidar-derived absorption coefficient profiles have large uncertainties and are generally weakly correlated to vertically in situ measured particle absorption coefficients. Furthermore, the mass absorption efficiency (MAE) for the northern Indian Ocean during the dry monsoon season was calculated to determine equivalent black carbon (EBC) concentrations from particle absorption coefficient measurements. A mean MAE of 11.6 and 6.9m(2) g(-1) for 520 and 880 nm, respectively, was found, likely representing internally mixed BC containing particles. Lower MAE values for 880 and 520 nm were found for air masses originating from dust regions such as the Arabian Peninsula and western Asia (MAE(880 nm) = 5.6m(2) g(-1), MAE(520 nm) = 9.5m(2) g(-1)) or from closer source regions as southern India (MAE(880 nm) = 4.3m(2) g(-1), MAE(520 nm) = 7. 3m(2) g(-1)).

2015
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.

Yeakel, KL, Andersson AJ, Bates NR, Noyes TJ, Collins A, Garley R.  2015.  Shifts in coral reef biogeochemistry and resulting acidification linked to offshore productivity. Proceedings of the National Academy of Sciences of the United States of America. 112:14512-14517.   10.1073/pnas.1507021112   AbstractWebsite

Oceanic uptake of anthropogenic carbon dioxide (CO2) has acidified open-ocean surface waters by 0.1 pH units since preindustrial times. Despite unequivocal evidence of ocean acidification (OA) via open-ocean measurements for the past several decades, it has yet to be documented in near-shore and coral reef environments. A lack of long-term measurements from these environments restricts our understanding of the natural variability and controls of seawater CO2-carbonate chemistry and biogeochemistry, which is essential to make accurate predictions on the effects of future OA on coral reefs. Here, in a 5-y study of the Bermuda coral reef, we show evidence that variations in reef biogeochemical processes drive interannual changes in seawater pH and Omega(aragonite) that are partly controlled by offshore processes. Rapid acidification events driven by shifts toward increasing net calcification and net heterotrophy were observed during the summers of 2010 and 2011, with the frequency and extent of such events corresponding to increased offshore productivity. These events also coincided with a negative winter North Atlantic Oscillation (NAO) index, which historically has been associated with extensive offshore mixing and greater primary productivity at the Bermuda Atlantic Time-series Study (BATS) site. Our results reveal that coral reefs undergo natural interannual events of rapid acidification due to shifts in reef biogeochemical processes that may be linked to offshore productivity and ultimately controlled by larger-scale climatic and oceanographic processes.

Pickett, M, Andersson AJ.  2015.  Dissolution rates of biogenic carbonates in natural seawater at different pCO2 conditions: a laboratory study. Aquatic Geochemistry.   10.1007/s10498-015-9261-3   Abstract

The bulk dissolution rates of six biogenic carbonates (goose barnacle, benthic foraminifera, bryozoan, sea urchin, and two types of coralline algae) and a sample of mixed sediment from the Bermuda carbonate platform were measured in natural seawater at pCO2 values ranging from approximately 3000 to 5500 μatm. This range of pCO2 values encompassed values regularly observed in porewaters at a depth of a few cm in carbonate sediments at shallow water depths (<15 m) on the Bermuda carbonate platform. The biogenic carbonates included calcites of varying Mg content (2–17 mol%) and a range of specific surface areas (0.01–2.7 m2 g−1) as determined by BET gas adsorption. Measured rates of dissolution increased with increasing pCO2 treatment for all substrates and ranged from 2.5 to 18 μmol g−1 h−1. The highest rates of dissolution were observed for the bryozoans and the lowest rates for the goose barnacles. The relative ranking in dissolution rates between different substrates was consistent at all pCO2 levels, indicating that substrates dissolve sequentially and that some substrates will be more vulnerable than others to rising CO2 and ocean acidification. Furthermore, dissolution rates were found to increase with increasing Mg content, though the relative dissolution rates were observed to be a function of both Mg content and microstructure (surface area).

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.
2014
Venti, A, Andersson A, Langdon C.  2014.  Multiple driving factors explain spatial and temporal variability in coral calcification rates on the Bermuda platform. Coral Reefs. 33:979-997.   10.1007/s00338-014-1191-9   AbstractWebsite

Experimental studies have shown that coral calcification rates are dependent on light, nutrients, food availability, temperature, and seawater aragonite saturation (Omega (arag)), but the relative importance of each parameter in natural settings remains uncertain. In this study, we applied Calcein fluorescent dyes as time indicators within the skeleton of coral colonies (n = 3) of Porites astreoides and Diploria strigosa at three study sites distributed across the northern Bermuda coral reef platform. We evaluated the correlation between seasonal average growth rates based on coral density and extension rates with average temperature, light, and seawater Omega (arag) in an effort to decipher the relative importance of each parameter. The results show significant seasonal differences among coral calcification rates ranging from summer maximums of 243 +/- A 58 and 274 +/- A 57 mmol CaCO3 m(-2) d(-1) to winter minimums of 135 +/- A 39 and 101 +/- A 34 mmol CaCO3 m(-2) d(-1) for P. astreoides and D. strigosa, respectively. We also placed small coral colonies (n = 10) in transparent chambers and measured the instantaneous rate of calcification under light and dark treatments at the same study sites. The results showed that the skeletal growth of D. strigosa and P. astreoides, whether hourly or seasonal, was highly sensitive to Omega (arag). We believe this high sensitivity, however, is misleading, due to covariance between light and Omega (arag), with the former being the strongest driver of calcification variability. For the seasonal data, we assessed the impact that the observed seasonal differences in temperature (4.0 A degrees C), light (5.1 mol photons m(-2) d(-1)), and Omega (arag) (0.16 units) would have on coral growth rates based on established relationships derived from laboratory studies and found that they could account for approximately 44, 52, and 5 %, respectively, of the observed seasonal change of 81 +/- A 14 mmol CaCO3 m(-2) d(-1). Using short-term light and dark incubations, we show how the covariance of light and Omega (arag) can lead to the false conclusion that calcification is more sensitive to Omega (arag) than it really is.

Eyre, BD, Andersson AJ, Cyronak T.  2014.  Benthic coral reef calcium carbonate dissolution in an acidifying ocean. Nature Climate Change. 4:969-976.   10.1038/nclimate2380   AbstractWebsite

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

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.

Parson, RJ, Nelson CA, Carlson CA, Denman CC, Andersson AJ, Kledzik AL, Vergin KL, McNally SP, Treusch AH, Giovannoni SJ.  2014.  Marine bacterioplankton community turnover within seasonally hypoxic waters of a subtropical sound: Devil’s Hole, Bermuda. Environmental Microbiology.   10.1111/1462-2920.12445   Abstract

Understanding bacterioplankton community dynamics in coastal hypoxic environments is relevant to global biogeochemistry because coastal hypoxia is increasing worldwide. The temporal dynamics of bacterioplankton communities were analysed throughout the illuminated water column of Devil's Hole, Bermuda during the 6-week annual transition from a strongly stratified water column with suboxic and high-pCO2 bottom waters to a fully mixed and ventilated state during 2008. A suite of culture-independent methods provided a quantitative spatiotemporal characterization of bacterioplankton community changes, including both direct counts and rRNA gene sequencing. During stratification, the surface waters were dominated by the SAR11 clade of Alphaproteobacteria and the cyanobacterium Synechococcus. In the suboxic bottom waters, cells from the order Chlorobiales prevailed, with gene sequences indicating members of the genera Chlorobium and Prosthecochloris – anoxygenic photoautotrophs that utilize sulfide as a source of electrons for photosynthesis. Transitional zones of hypoxia also exhibited elevated levels of methane- and sulfur-oxidizing bacteria relative to the overlying waters. The abundance of both Thaumarcheota and Euryarcheota were elevated in the suboxic bottom waters (> 109 cells l−1). Following convective mixing, the entire water column returned to a community typical of oxygenated waters, with Euryarcheota only averaging 5% of cells, and Chlorobiales and Thaumarcheota absent.

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  
2013
McLeod, E, Anthony KRN, Andersson A, Beeden R, Golbuu Y, Kleypas J, Kroeker K, Manzello D, Salm RV, Schuttenberg H, Smith JE.  2013.  Preparing to manage coral reefs for ocean acidification: lessons from coral bleaching. Frontiers in Ecology and the Environment. 11:20-27.   10.1890/110240   AbstractWebsite

Ocean acidification is a direct consequence of increasing atmospheric carbon dioxide concentrations and is expected to compromise the structure and function of coral reefs within this century. Research into the effects of ocean acidification on coral reefs has focused primarily on measuring and predicting changes in seawater carbon (C) chemistry and the biological and geochemical responses of reef organisms to such changes. To date, few ocean acidification studies have been designed to address conservation planning and management priorities. Here, we discuss how existing marine protected area design principles developed to address coral bleaching may be modified to address ocean acidification. We also identify five research priorities needed to incorporate ocean acidification into conservation planning and management: (1) establishing an ocean C chemistry baseline, (2) establishing ecological baselines, (3) determining species/habitat/community sensitivity to ocean acidification, (4) projecting changes in seawater carbonate chemistry, and (5) identifying potentially synergistic effects of multiple stressors.

Regnier, P, Friedlingstein P, Ciais P, Mackenzie FT, Gruber N, Janssens IA, Laruelle GG, Lauerwald R, Luyssaert S, Andersson AJ, Arndt S, Arnosti C, Borges AV, Dale AW, Gallego-Sala A, Godderis Y, Goossens N, Hartmann J, Heinze C, Ilyina T, Joos F, LaRowe DE, Leifeld J, Meysman FJR, Munhoven G, Raymond PA, Spahni R, Suntharalingam P, Thullner M.  2013.  Anthropogenic perturbation of the carbon fluxes from land to ocean. Nature Geoscience. 6:597-607.   10.1038/ngeo1830   AbstractWebsite

A substantial amount of the atmospheric carbon taken up on land through photosynthesis and chemical weathering is transported laterally along the aquatic continuum from upland terrestrial ecosystems to the ocean. So far, global carbon budget estimates have implicitly assumed that the transformation and lateral transport of carbon along this aquatic continuum has remained unchanged since pre-industrial times. A synthesis of published work reveals the magnitude of present-day lateral carbon fluxes from land to ocean, and the extent to which human activities have altered these fluxes. We show that anthropogenic perturbation may have increased the flux of carbon to inland waters by as much as 1.0 Pg C yr(-1) since pre-industrial times, mainly owing to enhanced carbon export from soils. Most of this additional carbon input to upstream rivers is either emitted back to the atmosphere as carbon dioxide (similar to 0.4 Pg C yr(-1)) or sequestered in sediments (similar to 0.5 Pg C yr(-1)) along the continuum of freshwater bodies, estuaries and coastal waters, leaving only a perturbation carbon input of similar to 0.1 Pg C yr(-1) to the open ocean. According to our analysis, terrestrial ecosystems store similar to 0.9 Pg C yr(-1) at present, which is in agreement with results from forest inventories but significantly differs from the figure of 1.5 Pg C yr(-1) previously estimated when ignoring changes in lateral carbon fluxes. We suggest that carbon fluxes along the land-ocean aquatic continuum need to be included in global carbon dioxide budgets.

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.

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.