Estuaries play key role in carbon capture

Australian estuaries may be more important than previously thought in capturing and storing carbon from the atmosphere, according to new research from Southern Cross University’s Centre for Coastal Biogeochemistry Research.

Dr Damien Maher and centre director Professor Bradley Eyre published a paper in Global Biogeochemical Cycles that measured carbon flows and CO₂ fluxes in three temperate Australian estuaries.

“Knowing carbon fluxes from these ecosystems is important in understanding what will happen in the future as far as atmospheric CO₂ concentrations are concerned,” Dr Maher said.

“Global warming and other climate change issues like ocean acidification make carbon cycling an important topic to study.”

In order to get a range of different conditions, the study examined carbon fluxes in three estuaries along the mid NSW coast. Hastings River near Port Macquarie was the northern-most estuary, followed by Camden Haven south of Port Macquarie. The third estuary was Wallis Lake, 156 km north of Newcastle.

“These estuaries were selected because they represent a natural gradient in estuarine types in terms of geomorphology, river flow and habitat coverage,” Dr Maher said. “This enabled us to determine how these factors control the carbon cycle at the estuary scale.”

The results from the study point to these estuaries being different in how they process carbon when compared to similar estuaries found throughout the world.

“In terms of carbon cycling, estuaries are normally heterotrophic, which means that they release CO₂ to the atmosphere,” Dr Maher said.

“These three estuaries are unique in that they are autotrophic, meaning they absorb CO₂ from the atmosphere. We believe this is driven by the high primary production associated with the extensive seagrass beds found in these ecosystems.”

Dr Maher said the study showed that coastal Australian ecosystems may be helping to absorb CO₂ put into the atmosphere through human activities. The study also showed seasonal differences in CO₂ absorption, with more CO₂ being absorbed in the summer than during other seasons.

“These estuaries act naturally to absorb CO₂ from the atmosphere on a yearly basis,” he said.

Dr Maher said the study also looked at whether the carbon was released back into the oceans or buried, keeping it out of the atmosphere.

“The estuaries showed differences in their ability to bury the carbon, with the Wallis Lake and Camden Haven burying a larger percentage of the carbon as compared to Hastings River.

“This is probably because of the different residence times of water in the estuaries. Basically, the longer the water spends in the estuary the more of a chance it has to be buried,” he said.

Professor Eyre said the study demonstrated the important role of coastal estuaries.

“This paper illustrates the need to study more of these ecosystems, which will help to gain an understanding of how these ecosystems affect global carbon cycling,” he said.