Historical reconstruction of ocean acidification in the Australian region

Abstract

Abstract. The ocean has become more acidic over the last 200 years in response increasing atmospheric carbon dioxide (CO 2 ) levels. Documenting how the ocean has changed is critical for assessing how these changes impact marine ecosystems and for the management of marine resources.Here we use present-day ocean carbon observations, from shelf and offshore waters around Australia, combined with neural network mapping of CO 2 , sea surface temperature, and salinity to estimate the current seasonal and regional distributions of carbonate chemistry (pH and aragonite saturation state). The observed changes in atmospheric CO 2 and sea surface temperature (SST) and climatological salinity are then used to reconstruct pH and aragonite saturation state changes over the last 140 years . The comparison with data collected at Integrated Marine Observing System National Reference Station sites located on the shelf around Australia shows that both the mean state and seasonality in the present day are well represented, with the exception of sites such as the Great Barrier Reef. Our reconstruction predicts that since 1870 decrease in aragonite saturation state of 0.48 and of 0.09 in pH has occurred in response to increasing oceanic uptake of atmospheric CO 2 . Large seasonal variability in pH and aragonite saturation state occur in southwestern Australia driven by ocean dynamics (mixing) and in the Tasman Sea by seasonal warming (in the case of the aragonite saturation state). The seasonal and historical changes in aragonite saturation state and pH have different spatial patterns and suggest that the biological responses to ocean acidification are likely to be non-uniform depending on the relative sensitivity of organisms to shifts in pH and saturation state. This new historical reconstruction provides an important link to biological observations that will help to elucidate the consequences of ocean acidification.

Technical note: Continuous fluorescence-based monitoring of seawater pH in situ

“…This process was repeated at least three times, with approximately 30 min between each sampling time. Within 30 min of collection, pH was measured spectrophotometrically with m-cresol purple indicator dye (Acros Organics, Lot # A0321770) using a custom-built automated system with a USB4000 fibre optic spectrophotometer (Ocean Optics) following SOP 6b (standard operating procedure) from Dickson et al (2007) and calculations from Liu et al (2011). Spectrophotometric measurements of pH T were frequently validated using certified reference seawater (Dickson Standard Batch 145).…”

“…All data were saved on board and downloaded to PC after retrieval of the instrument. Alkalinity was calculated from salinity data according to a linear salinity-alkalinity relationship for Australasian waters described by Lenton et al (2016). Drift and offset corrections were made to the raw data as required (described above), and final pH, salinity, temperature and alkalinity values were used (with equilibrium constants from Mehrbach et al, 1973 refit by Dickson and Millero, 1987) to calculate Ca and Ar in CO2Sys (V2.1) (Dickson and Millero, 1987).…”

“…We incorporated a commercially available fluorescence-based pH sensing system (Presens GmbH) in a submersible continuous-monitoring device (Aquation Pty Ltd) to measure short-term changes in seawater pH in surface waters in Sydney Harbour over two multi-day intervals in austral autumn and summer. Electrical conductivity and temperature were also measured, and an approximation for alkalinity was derived from a salinity-alkalinity relationship reported for the Australasian region (Lenton et al, 2016). We acknowledge that the relationship between salinity and alkalinity is subject to terrestrial influence, and consequently our estimate of alkalinity remains an approximation.…”

Continuous fluorescence-based monitoring of seawater pH in a temperate estuary

“…We 45 incorporated a commercially available fluorescence-based pH sensing system (Presens GmbH) in a submersible continuous monitoring device (Aquation Pty Ltd) to measure short term changes in seawater pH in surface waters in Sydney Harbour over two multi-day intervals in Autumn and Summer. Electrical conductivity and temperature were also measured, and alkalinity was derived from a salinity-alkalinity relationship reported for the Australasian region (Lenton et al 2016). With these data we estimated changes in seawater carbonate chemistry during the deployments.…”

“…Australasian waters described by Lenton et al (2016). Corrections were made to the raw data if required, and final pH, pCO2, temperature and alkalinity values were used to calculate Ca and Ar in CO2Sys (V2.1) (Dickson and Millero 1987).…”