Coral records of reef-water pH across the central Great Barrier Reef, Australia: assessing the influence of river runoff on inshore reefs
The boron isotopic (δ<sup>11</sup>B<sub>carb</sub>) compositions of long-lived <i>Porites</i> coral are used to reconstruct reef-water pH across the central Great Barrier Reef (GBR) and assess the impact of river runoff on inshore reefs. For the period f...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-02-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/12/1223/2015/bg-12-1223-2015.pdf |
| Summary: | The boron isotopic (δ<sup>11</sup>B<sub>carb</sub>) compositions of
long-lived <i>Porites</i> coral are used to reconstruct reef-water pH
across the central Great Barrier Reef (GBR) and assess the impact of river
runoff on inshore reefs. For the period from 1940 to 2009, corals from both
inner- and mid-shelf sites exhibit the same overall decrease in δ<sup>11</sup>B<sub>carb</sub> of 0.086 ± 0.033‰ per decade,
equivalent to a decline in seawater pH (pH<sub>sw</sub>) of
~0.017 ± 0.007 pH units per decade. This decline is consistent
with the long-term effects of ocean acidification based on estimates of
CO<sub>2</sub> uptake by surface waters due to rising atmospheric levels. We also
find that, compared to the mid-shelf corals, the
δ<sup>11</sup>B<sub>carb</sub> compositions of inner-shelf corals subject to
river discharge events have higher and more variable values, and hence higher
inferred pH<sub>sw</sub> values. These higher δ<sup>11</sup>B<sub>carb</sub>
values of inner-shelf corals are particularly evident during wet years,
despite river waters having lower pH. The main effect of river discharge on
reef-water carbonate chemistry thus appears to be from reduced aragonite
saturation state and higher nutrients driving increased phytoplankton
productivity, resulting in the drawdown of <i>p</i>CO<sub>2</sub> and increase in
pH<sub>sw</sub>. Increased primary production therefore has the potential to
counter the more transient effects of low-pH river water (pH<sub>rw</sub>)
discharged into near-shore environments. Importantly, however, inshore reefs
also show a consistent pattern of sharply declining coral growth that
coincides with periods of high river discharge. This occurs despite these
reefs having higher pH<sub>sw</sub>, demonstrating the overriding
importance of local reef-water quality and reduced aragonite saturation state
on coral reef health. |
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| ISSN: | 1726-4170 1726-4189 |