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

Abstract. The boron isotopic (δ¹¹B_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 δ¹¹B_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 CO₂ uptake by surface waters due to rising atmospheric levels. We also find that, compared to the mid-shelf corals, the δ¹¹B_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 δ¹¹B_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 pCO₂ 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.