A study involving the collaboration between researchers at The University of Western Australia and the Spanish National Research Council has shown that warming of Mediterranean seawater over this century, under a moderately optimistic scenario of greenhouse gas emissions, is likely to cause the functional extinction of these seagrass meadows.
The international study examined the trajectory of the density of seagrass (Posidonia oceanica) meadows in the western Mediterranean during the 21st century under estimated warming based on ten global climate models and two regional models. Researchers used the relationships between the annual mortality rate of P. oceanica and the maximum annual temperature to predict annual seagrass mortality rates. The result was a decrease in shoot density by 90 per cent at mid-century.
All models predict a rapid warming of surface seawater along the 21st century, leading to an increase in the frequency and intensity of heat waves. The models predict the average surface temperature of seawater during the summer would be 3.4 °C warmer by the end of this century compared to now. The models predict that from 2050 onwards, seawater temperature will exceed 28 °C, the threshold temperature triggering mortality of P. oceanica, every summer.
Posidonia meadows in the Mediterranean are in decline not only because of higher temperatures but also due to local disturbances, such as pollution and destruction of the prairie for anchors.
Co-author and Director of the UWA Oceans Institute Winthrop Professor Carlos Duarte says that the paper, Mediterranean seagrass vulnerable to regional climate warming, published online in the journal Nature Climate Change, reveals actions to mitigate local impacts, while beneficial, are not enough to increase seagrass resistance to warming.
"To assess whether local disturbances could increase the vulnerability of P. oceanica to warming, the researchers examined the trajectory of the abundance of P. oceanica under three scenarios of mitigation of local stresses; the immediate removal of local stresses, the mitigation of local perturbations by 2030, and ‘business as usual.'
"The most swift action - the mitigation of local disturbances by 2010 - delayed the functional extinction of the meadow by a decade, but only two years if mitigation of local stresses is only achieved by 2030.
"Therefore, measures to mitigate local stresses, while beneficial, increase the resistance of P. oceanica to ocean warming only modestly," Winthrop Professor Duarte said.
The researchers concluded the study demonstrates that rapid international action to reduce emissions of greenhouse gases at levels well below those considered in this study is the only solution capable of ensuring that this ancient ecosystem persists throughout the twenty-first century.
P. oceanica is an extremely slow-growing seagrass species endemic to the Mediterranean, where it forms extensive meadows between 0 and 40 m deep including clones that have been reported to exceed 100,000 years in age. Posidonia meadows provide important ecosystem services including; the burial of CO2, nutrient recycling, coastal protection from erosion and increased biodiversity, and are therefore considered one of the most valuable biomes on the planet. The meadows of P. oceanica are highly vulnerable to small increases in mortality due to its slow growth, which results in rapid loss of vegetation. This is evident in the Mediterranean due to excessive discharges of nutrients and organic matter to the coast, and mechanical impacts.
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