Articles | Volume 12, issue 4
Research article
25 Feb 2015
Research article |  | 25 Feb 2015

Coral records of reef-water pH across the central Great Barrier Reef, Australia: assessing the influence of river runoff on inshore reefs

J. P. D'Olivo, M. T. McCulloch, S. M. Eggins, and J. Trotter

Abstract. The boron isotopic (δ11Bcarb) compositions of long-lived Porites 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 δ11Bcarb of 0.086 ± 0.033‰ per decade, equivalent to a decline in seawater pH (pHsw) 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 CO2 uptake by surface waters due to rising atmospheric levels. We also find that, compared to the mid-shelf corals, the δ11Bcarb compositions of inner-shelf corals subject to river discharge events have higher and more variable values, and hence higher inferred pHsw values. These higher δ11Bcarb 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 pCO2 and increase in pHsw. Increased primary production therefore has the potential to counter the more transient effects of low-pH river water (pHrw) 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 pHsw, demonstrating the overriding importance of local reef-water quality and reduced aragonite saturation state on coral reef health.

Short summary
The boron isotope composition in the skeleton of massive Porites corals from the central Great Barrier Reef is used to reconstruct the seawater pH over the 1940-2009 period. The long-term decline in the coral-reconstructed seawater pH is in close agreement with estimates based on the CO2 uptake by surface waters due to rising atmospheric levels. We also observed a significant relationship between terrestrial runoff data and the inshore coral boron isotopes records.
Final-revised paper