Seagrass meadows, better than forests

I've written before about how amazing seagrass is (part I and part II). A recent paper in Nature Geoscience reviews the published literature on the carbon storage capacity of seagrass meadows. And what it concludes is quite surprising. Per hectare they are better at storing carbon than terrestrial forests despite the insignificant amount of carbon in the plants themselves.

Over time seagrass meadows accumulate large amounts of organic material in soils beneath the growing plants. Part of the soil is formed by dead seagrass, but typically half of the organic material is from other sources that collect in the meadows. The amount of soil that accumulates can be substantial, sometimes forming a layer several meters thick.

The paper looked at published and unpublished data on the organic carbon content of the soils beneath seagrass meadows. They found that the seagrass soils stored almost twice as much carbon as terrestrial soils. Principally this is because the break down of organic matter in seagrass soils occurs anaerobically, which releases hydrogen sulphide, rather than carbon dioxide released in the aerobic break down occuring in terrestrial soils.

Another impressive finding of the review is that carbon stored in the seagrass soils can be stable over thousands of years. This is far longer than carbon can be stored in terrestrial systems, where fires and decomposition eventually liberate the stored carbon back to the atmosphere. However, over the last century nearly 30% of of seagrass meadows have disappeared, most likely causing the release of the sequestered carbon.

Protection and restoration of terrestrial forests is often advocated as a tool for mitigating the effects of climate change. The conclusions of the review paper suggest that restoration and greater protection of seagrass meadows might also be an important tool for reducing the amount of carbon dioxide in the oceans and atmosphere. Intriguingly, deforestation is a threat to seagrass meadows.

Seagrass meadows are exposed to a number of anthropogenic threats. Poor land-use practices, such as deforestation, that increase the silt and nutrient loads that reach seagrass meadows are the most common cause of meadow degradation and loss. Increased turbidity, caused by both silt and nutrient loads, reduces the amount of light penetrating to seagrass meadows. In addition, high nutrient loads can cause the increased growth of epiphytic organisms.

The capacity of marine ecosystems to help mitigate against the effects of climate change is increasingly being looked at. And they are showing that they are not only important pieces in the puzzle, but that they could be more important than terrestrial systems.

James W. Fourqurean, Carlos M. Duarte, Hilary Kennedy, Núria Marbà, Marianne Holmer, Miguel Angel Mateo, Eugenia T. Apostolaki, Gary A. Kendrick, Dorte Krause-Jensen, Karen J. McGlathery & Oscar Serrano (2012). Seagrass ecosystems as a globally significant carbon stock Nature Geoscience, 5 (7), 505-509 DOI: 10.1038/ngeo1477