Marine heatwave drives massive emissions from seagrass death

The loss of seagrass at Shark Bay after the 2010/11 marine heatwave released up to nine million metric tons of carbon dioxide (CO2) into the atmosphere over the following three years, according to Edith Cowan University (ECU), the University of Western Australia and the Department of Biodiversity, Conservation and Attractions.

This amount is roughly equivalent to the annual CO2 output of 800,000 homes, two average coal-fired power plants or 1,600,000 cars driven for 12 months.

The international research team suggests this could raise Australia’s annual estimate of national land-use change carbon emissions over the period by 21 per cent.

“Seagrasses have thrived for millennia in Shark Bay, with the two most significant banks – Wooramel Bank and Faure Sill – the result of approximately 8,000 years of continuous growth,” said Dr Oscar Serrano from ECU’s School of Science.

“Yet the widespread losses in the summer of 2010/11 were unprecedented, in both the above- and below-ground biomass of Amphibolis and to a minor extent Posidonia, the only two species forming large continuous beds.

“This is significant, as seagrass meadows are known as ‘blue carbon’ ecosystems, because they store carbon dioxide in their soils through biosequestration.

“So when you have an event such as the losses at Shark Bay, you not only lose the seagrass as a way of removing CO2, but the sequestered gas is released back into the atmosphere during seagrass matter decomposition.”

Researchers initially mapped 78 per cent of the Marine Park within the UNESCO World Heritage Site in 2014 and found a 20 per cent loss of seagrass habitat as compared to 2002 baseline, equivalent to a loss of 1,000 km2 of meadows if extrapolated to the entire seagrass extent.

What remained was sparser, with ‘dense’ seagrass areas declining from 72 per cent in 2002 to 46 per cent in 2014.

They then used soil modelling and in situ sampling from 50 sites to make their calculations of potential CO2 release.

Like local threats, such as overfishing and nutrient inputs, global pressures such as marine heat waves are challenging to manage and require good knowledge as a basis for planning.

“We need to develop strategies to deal with the impact of climate change and extreme weather events,” Dr Serrano said.

“We have seen how quickly losses can occur, and once destroyed, the capacity of seagrass meadows to recover is limited and slow, and largely depends on the arrival of seeds or seedlings.”

Plans for future catastrophes might include removing seagrass detritus, restoration of impacted areas through reseeding and repopulation of some areas with more resilient types of seagrass.

The research team was led by Ariane Arias-Ortiz from Universitat Autonoma de Barcelona and included scientists from Australia, Catalonia, Spain, Malaysia, the United States and the Kingdom of Saudi Arabia.

‘A marine heat wave drives massive losses from the world’s largest seagrass carbon stocks’ is published in Nature Climate Change.

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