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

• Published in: Biogeosciences
• Main Authors: J. P. D'Olivo, M. T. McCulloch, S. M. Eggins, J. Trotter
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2015
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-12-1223-2015; https://doaj.org/article/f23bb5825a5243c78f8687ca833af234

The boron isotopic (δ 11 B carb ) 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 δ 11 B carb of 0.086 ± 0.033‰ per decade, equivalent to a decline in seawater pH (pH sw ) 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 2 uptake by surface waters due to rising atmospheric levels. We also find that, compared to the mid-shelf corals, the δ 11 B carb compositions of inner-shelf corals subject to river discharge events have higher and more variable values, and hence higher inferred pH sw values. These higher δ 11 B carb 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 p CO 2 and increase in pH sw . Increased primary production therefore has the potential to counter the more transient effects of low-pH river water (pH rw ) 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 sw , demonstrating the overriding importance of local reef-water quality and reduced aragonite saturation state on coral reef health.