Carbonate chemistry in the Great Barrier Reef
A new AIMS custom-built instrument measures how organisms modulate the carbonate chemistry of the sea water as it travels over a reef flat of the GBR. Photo: Eric Matson, AIMS
For open oceans, changes in the carbonate chemistry from rising atmospheric carbon dioxide (CO2) are relatively well understood. This is not the case in nearshore and shallow marine environments such as the Great Barrier Reef where conditions are more variable. For example, dense seagrass meadows or algal mats found in coastal waters deplete the CO2 in the water during the day due to the plants’ photosynthesis, but enrich it at night due to respiration.
AIMS research has started documenting substantial variability in the exposure of different reef tracts to CO2 at all scales: from within coral colonies, to gradients in CO2 as the water crosses reef flats, and to all the way across and along the large continental shelf.
Photo: Eric Matson, AIMS
Scientists are investigating to what extent the observed variability of CO2 is caused by biological activities (e.g. respiration), currents pushing water in from the Coral Sea, warming, storms, rainfall and river discharges of freshwater, nutrients and sediments, and how these interact with anthropogenic ocean acidification.
This is a burgeoning field of investigation, as little is known to date about how this greater variability will change the resilience of these ecosystems to ocean acidification. Difficulties also exist as a result of limited baseline data from such nearshore and shallow marine environments to provide insight to seasonal or regional differences.
Previous limitations have been overcome by a range of innovative approaches to data collection.
For example, the AIMS research vessel RV Cape Ferguson is equipped with instruments to constantly measure carbonate chemistry wherever the vessel goes.
In addition, IMOS (Integrated Marine Observing System) has deployed stable high-precision pH sensors for longer-term measurements of fluctuations in the carbonate chemistry on reef flats and in lagoons.
Under funding from the Great Barrier Reef Foundation, AIMS has also installed a pCO2 monitoring station at Davies Reef in the central Great Barrier Reef to look at conditions at mid-shelf reefs.
Furthermore, AIMS at its Cape Cleveland site is one of the places in the world where atmospheric CO2 concentrations are being measured.
The aim is to understand how the natural levels of carbonate chemistry vary across the Great Barrier Reef - from cooler southern reefs to warmer northern ones, from coastal reefs with high coastal influences to clear water oceanic reefs. Changes in carbonate chemistry over time scales from daily tidal cycles, seasonal changes to longer term trends is also of value. Collectively, this data gives a natural baseline in which the experimental and CO2 seep data can be contextualised
Ultimately, the data will feed into detailed models of carbon budgets, and may provide a better understanding of differences in the vulnerability of individual reef tracts to future ocean acidification.