Abstract
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.
Highlights
Coral reefs are under threat, from the global effects of CO2-driven climate change and from direct local impacts, in particular disturbed river catchments and degraded water quality (McCulloch et al, 2003; Brodie et al, 2010b)
The present study reveals that decadal-scale wet periods with increased terrestrial runoff in the central Great Barrier Reef (GBR) coincide with periods of reduced inner-shelf coral growth (Fig. 5)
The reconstructed pHsw values calculated from the coral δ11Bcarb indicate that, in their natural environment, massive Porites up-regulate pH of the calcifying fluid (pHcf) by ∼ 0.4 units
Summary
Coral reefs are under threat, from the global effects of CO2-driven climate change and from direct local impacts, in particular disturbed river catchments and degraded water quality (McCulloch et al, 2003; Brodie et al, 2010b). Changing land-use practices since the arrival of European settlers to the central Queensland region has produced increased discharge of terrestrial material from the Burdekin River into the Great Barrier Reef (GBR; McCulloch et al, 2003; Lewis et al, 2007). Changes in water quality within inner-shelf environments of the GBR have been linked to a decrease in coral calcification (D’Olivo et al, 2013), coral biodiversity (De’ath and Fabricius, 2010), decreased coral cover (Sweatman et al, 2011), and crown-of-thorns starfish outbreaks (Brodie et al, 2005). Despite the mounting evidence for the negative impacts of increased terrestrial discharge into the GBR, the effect of river flood plumes on the carbonate status of reef waters, a fundamental property controlling calcification, remains largely unknown. It is commonly assumed that, because both the salinity and pH of plume waters (pHpw) are Published by Copernicus Publications on behalf of the European Geosciences Union
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