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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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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).

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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.

Page, HN, Courtney TA, DeCarlo EH, Howins NM, Koester I, Andersson AJ.  2019.  Spatiotemporal variability in seawater carbon chemistry for a coral reef flat in Kane'ohe Bay, Hawai'i. Limnology and Oceanography. 64:913-934.   10.1002/lno.11084   AbstractWebsite

Coral reef community composition and ecosystem function may change in response to anthropogenic ocean acidification. However, the magnitude of acidification on reefs will be modified by natural spatial and temporal variability in seawater CO2 chemistry. Consequently, it is necessary to quantify the ecological, biogeochemical, and physical drivers of this natural variability before making robust predictions of future acidification on reefs. In this study, we measured temporal and spatial physiochemical variability on a reef flat in Kane'ohe Bay, O'ahu, Hawai'i, using autonomous sensors at sites with contrasting benthic communities and by sampling surface seawater CO2 chemistry across the reef flat at different times of the day during June and November. Mean and diurnal temporal variability of seawater CO2 chemistry was more strongly influenced by depth gradients (0.5-10 m) on the reef rather than benthic community composition. Spatial CO2 chemistry gradients across the reef flat reflected the cumulative influence from benthic metabolism, bathymetry, and hydrodynamics. Based on graphical assessment of total alkalinity-dissolved inorganic carbon data, reef metabolism in November was dominated by organic carbon cycling over inorganic carbon cycling, while these processes were closely balanced in June. Overall, this study highlights the strong influence of depth on reef seawater CO2 chemistry variability through its effects on benthic biomass to seawater volume ratio, seawater flow rates, and residence time. Thus, the natural complexity of ecosystems where a combination of ecological and physical factors influence reef chemistry must be considered when predicting ecosystem biogeochemical responses to future anthropogenic changes in seawater CO2 chemistry.