How Seagrasses mitigate against climate change

Main Club March 2020

Our speaker was Paul Lavery, Professor of Marine Ecology at the Centre for Marine Ecosystems Research at Edith Cowan University. His area of research is benthic marine ecosystems, especially seagrasses and how they respond to human pressures.

Paul began by pointing out that seagrasses are not seaweeds (which are macro-algae), but angiosperms (flowering plants). Despite their name, they are not grasses. Being angiosperms, they have flowers with pollen, fruit with seeds and roots, stems and leaves. The flowers are inconspicuous, however, because their pollen is dispersed by currents, not pollinators.

Around 450 million years ago plants invaded the land and then, at a mere 60 million years ago, several groups of land plants evolved back to life in the oceans. Because each of these unrelated groups had to adapt to underwater conditions, they ended up looking quite similar (convergent evolution). There are 84 species of seagrass worldwide (all continents except Antarctica), in four separately evolving lineages. These are the Zosteracea, the Zanichelliaceae, the Hydrocharitaceae and the Cymodoceaceae lineages. Australia is rich in species (50 spp), while our South West is particularly diverse. At Point Peron, for example, Paul said you can see 7 or 8 species.

When fruit is shed, in several species it floats and disperses and then the seed drops to the bottom. In Amphibolus, the seedling develops while attached to the parent plant. This seedling develops a “sea anchor” to give it a chance to take root once it has been shed. Seagrasses are usually host to an array of epiphytes growing on the leaf blades. Too many of these will shade the leaf, so fresh, clean leaves must continually grow from the rhizome below.

Seagrasses provide 3D physical structure and/or persistent habitat on areas of the sea floor. This structure varies from large, long-lived, persistent forms such as Posidonia australis that spread largely by rhizomes, to small, short-lived colonising forms like tiny Halophila species that reappear by seeding.

Paul described a multitude of ways that seagrasses provide “ecosystem services”. They are high primary producers, supporting a great many food webs and high biodiversity. Dugongs and rock lobster harvesting are just two examples of this, as well as the food chains that occur in seagrass wrack (accumulations of dead leaves). They also filter the water. They stabilise sediments and protect coastlines and reefs from erosion. And very importantly, they sequester carbon.

“Blue carbon” is the term used for the sequestering of CO2 by seagrasses, mangroves and tidal marshes, as opposed to “green carbon” for CO2 captured by forests. The news is that blue carbon sequestration is much more effective than green carbon. Seagrass meadows slow water flow so that sand is deposited. This accelerates vertical growth of the seagrasses and accumulation of sediments. Old seagrass dies, is buried, and does not decompose because of the lack of oxygen deep in the sediment. There is no fire to remove it. Therefore, carbon is locked in the soil.

Cores can be extracted from below seagrass meadows, the carbon can be dated and the amount and rate of accumulation measured. Results show that seagrass sediment can be 8m deep, and carbon at the bottom can be up to 8000 years old. Blue carbon sequestration has been found to be 40 to 70 times more effective than green carbon sequestration. Paul brought us the good news that 1.5 billion tons of carbon is trapped in Australian seagrasses and that this is equivalent to eight years of Australia’s total carbon emissions. The bad news is that when we lose seagrass meadows, not only do we stop sequestering carbon, but the carbon from that removed seagrass goes back into the atmosphere as it decomposes and effectively becomes an emission.

Paul pointed out that Australia has a great opportunity to conserve seagrasses, since we have an exceptional amount of them. The Australian Government’s Emission Reduction Fund provides carbon credits for actions that conserve or enhance carbon trapped in ecosystems, though currently this applies solely to land-based schemes. He stressed the need to apply emissions trading schemes to blue carbon. Australian seagrasses are helping to fight global environmental change. “We need to look after our little green friends”, Paul said.

Mike Gregson