Coral reefs feed millions of people worldwide, provide coastal protection and generate billions of dollars annually in tourism revenue1. The underlying architecture of a reef is a biogenic carbonate structure that accretes over many years of active biomineralization by calcifying organisms, including corals and algae2. Ocean acidification poses a chronic threat to coral reefs by reducing the saturation state of the aragonite mineral of which coral skeletons are primarily composed, and lowering the concentration of carbonate ions required to maintain the carbonate reef. Reduced calcification, coupled with increased bioerosion and dissolution3, may drive reefs into a state of net loss this century4. Our ability to predict changes in ecosystem function and associated services ultimately hinges on our understanding of community- and ecosystem-scale responses. Past research has primarily focused on the responses of individual species rather than evaluating more complex, community-level responses. Here we use an in situ carbon dioxide enrichment experiment to quantify the net calcification response of a coral reef flat to acidification. We present an estimate of community-scale calcification sensitivity to ocean acidification that is, to our knowledge, the first to be based on a controlled experiment in the natural environment. This estimate provides evidence that near-future reductions in the aragonite saturation state will compromise the ecosystem function of coral reefs.
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We thank R. Dunbar for the use of his laboratory and D. Mucciarone for laboratory assistance; the Australian Institute of Marine Science for scientific and technical support; and the following people for their support in the field and/or laboratory: M. Byrne, T. Hill, L. Caldeira, R. Johnson, D. Ross and the staff of the One Tree Island Research Station. Expedition and staff support was provided by the Carnegie Institution for Science. Additional support for staff, but not expedition expenses, was provided by the California Academy of Sciences and the Fund for Innovative Climate and Energy Research. This work was permitted by the Great Barrier Reef Marine Park Authority under permit G14/36863.1.
The authors declare no competing financial interests.
Reviewer Information Nature thanks H. Kayanne, J. Lough and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
a–f, Relationships between alkalinity and dye for a representative experiment day (n = 20 independent experiments, 29 September 2016 shown here) and control day (n = 10 independent experiments, 30 October 2016 shown here). a, b, Dye concentrations; c, d, alkalinities; e, f, alkalinity anomalies versus dye concentrations. Linear regressions were fit to alkalinity–dye data using least-squares residuals. d, On control days, the observed (measured) alkalinities closely agree with predicted values for each station. Comparing the upstream and downstream alkalinity–dye ratios provides an estimate of the effect of CO2 enrichment on NCC, as described in the Methods. e, On experiment days (n = 20 independent experiments), if CO2 suppresses NCC, the drawdown in alkalinity is smaller in areas with more CO2 (and more dye) than in areas with less CO2 (and less dye). This effect yields a positive correlation between dye and alkalinity (that is, a positive alkalinity–dye slope) that increases as the water mass moves across the reef—in other words, the alkalinity–dye slope at the downstream transect is greater than that of the upstream transect. f, On control days (n = 10 independent experiments), when dye but no CO2 was added, alkalinity and dye were not correlated, and the mean alkalinity–dye slopes for the upstream and downstream transects did not differ from zero.
Solid lines represent control days, and dashed lines represent experiment days.
a, b, Unique offsets by station (xs) for the upstream and downstream transects (mean ± s.e.m., n = 10). c, d, Magnitude of offsets by day (yd) for upstream and downstream transects. e, f, Alkalinity–dye ratios by day (rd), for upstream and downstream transects. g, h, Mean background alkalinities by day (âd) for upstream and downstream transects. In c–h, bars represent central values as calculated by the multivariate regression described in the ‘Mathematical explanation’ section of the Methods, and error bars represent s.e.m. (for c, e and h, n = 11; for d, f and h, n = 15).
Background and in-plume Ωarag and NCC values (mean ± s.e.m.) for experiment (n = 20 independent experiments) and control (n = 10 independent experiments) days. Error bars reflect underlying natural variability (that is, day-to-day, hour-to-hour), because Ωarag and NCC varied based on time of day and light availability.
Extended Data Figure 5 Relationship between the change in NCC and background NCC across all experiment days.
Linear regression using least-squares residuals (n = 20 independent experiments).
Change in NCC inside of the plume compared to background conditions (mean ± s.e.m.) for experiment (n = 20 independent experiments) and control (n = 10 independent experiments) days. On all 20 experiment days, we detected a statistically significant reduction in calcification within the plume (paired t-tests). On control days, in-plume NCC was higher than background NCC on five days, and lower than background NCC on five days.
Time series of environmental data from SeapHOx and SAMI sensors. Instruments logged at 10-min intervals over the duration of the study. Gaps in the data correspond to when the instruments were removed from the reef for maintenance. a, PAR; b, temperature; c, pressure; d, salinity; e, dissolved oxygen; f, pH.
This file contains the Computer Code. (PDF 754 kb)
Raw data for chemical and physical parameters across all days and station locations (measured and calculated). Details regarding measurements and associated errors are provided in the Methods. (XLSX 139 kb)
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Albright, R., Takeshita, Y., Koweek, D. et al. Carbon dioxide addition to coral reef waters suppresses net community calcification. Nature 555, 516–519 (2018). https://doi.org/10.1038/nature25968
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