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

摘要

The boron isotopic (δ11B_carb) compositions oflong-lived Porites coral are used to reconstruct reef-water pHacross the central Great Barrier Reef (GBR) and assess the impact of riverrunoff on inshore reefs. For the period from 1940 to 2009, corals from bothinner- 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 consistentwith the long-term effects of ocean acidification based on estimates ofCO₂ uptake by surface waters due to rising atmospheric levels. We alsofind that, compared to the mid-shelf corals, theδ11B_carb compositions of inner-shelf corals subject toriver discharge events have higher and more variable values, and hence higherinferred pH_sw values. These higher δ11B_carbvalues of inner-shelf corals are particularly evident during wet years,despite river waters having lower pH. The main effect of river discharge onreef-water carbonate chemistry thus appears to be from reduced aragonitesaturation state and higher nutrients driving increased phytoplanktonproductivity, resulting in the drawdown of pCO₂ and increase inpH_sw. Increased primary production therefore has the potential tocounter the more transient effects of low-pH river water (pH_rw)discharged into near-shore environments. Importantly, however, inshore reefsalso show a consistent pattern of sharply declining coral growth thatcoincides with periods of high river discharge. This occurs despite thesereefs having higher pH_sw, demonstrating the overridingimportance of local reef-water quality and reduced aragonite saturation stateon coral reef health.