In open oceans, changes in the carbonate chemistry from rising atmospheric CO2 are relatively stable, and well understood. In contrast, conditions are more variable in nearshore and shallow marine environments such as the Great Barrier Reef. 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.
Understanding these variations in coastal marine ecosystems allows us to better predict, through measurement and modelling, the differences in vulnerability of individual reef tracts to future ocean acidification.
From corals to the continental shelf – understanding seawater acidification at different scales.
AIMS research is 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.
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. Knowing changes in carbonate chemistry over time scales from daily tidal cycles, seasonal changes to longer term trends are also of value.
By linking data from the eReefs models to those from the AIMS long-term reef monitoring data, AIMS researchers have shown that, all else being equal, reefs in areas of the Great Barrier Reef where ocean acidification is greater have fewer crustose coralline algae, more seaweed and fewer coral recruits than other reef sections where CO2 concentrations in the seawater are lower. These findings are in agreement with the observations from the CO2 seeps.
Scientists are now investigating to what extent the observed variability of CO2 is caused by biological activities (e.g. respiration and photosynthesis), 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.