Uncertainties about the role of terrestrial dissolved inorganic carbon and alkalinity loads in buffering the Great Barrier Reef lagoon against ocean acidification

Abstract

Terrestrial dissolved inorganic carbon (DIC) and total alkalinity (TAlk) loads have contrasting effects on the pH and carbonate chemistry of the coastal ocean. While TAlk can buffer against ocean acidification, elevated exports of free CO2 can further exacerbate ocean acidification. In this study, we estimate terrestrial DIC and TAlk loads from rivers and mangrove floodplains from six bioregions and over different flow years to assess their influence on the buffering capacity of the Great Barrier Reef (GBR) lagoon in Australia.Overall, we find a large range of terrestrial (river + mangrove) DIC (-0.91 to 2.60 Tg C yr -1 ) and TAlk (-0.04 to 2.62 Tg C yr -1 ) loads that were higher in high flow years and in the tropical wet bioregions. Importantly, we show that a net buffering effect on the GBR is possible based on terrestrial loads that include short-term mangrove TAlk export rates that are higher in TAlk compared to DIC. However, when we consider long-term mangrove TAlk production via pyrite formation, which is low in TAlk, we find that terrestrial loads can acidify the GBR lagoon. Because of the large range associated with current measurements, it remains unclear if terrestrial inorganic carbon loads buffer or acidify the GBR lagoon. Ongoing monitoring will be essential to improve predictions of the conditions and drivers of the carbonate chemistry of the GBR coastal waters with immediate consequences on the health of the reef ecosystem.