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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.

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.

2013
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.