Operation Posidonia
When we think of conservation horticulture or restoring native vegetation, we usually imagine it all happens above ground or, at most, on the edges of rivers, lakes or other water bodies. However, some plants grow completely underwater. Some types of seaweed (green algae) are a familiar example. There are flowering plants (i.e. angiosperms) that grow underwater too. One example is Posidonia australis, that goes under the banal common name, seagrass. It’s a monocot but not a true grass. It produces flowers and (sometimes!) fruits and seeds and does this while completely submerged.
There are nine species of Posidonia, with P. australis the only one occurring in NSW and seven of the others elsewhere in Australia. Posidonia communities are found along the Australian coast southwards from Wallis Lake near Forster in NSW, then around the entire southern coast and northwards to Shark Bay in WA, taking in northern Tasmania as well. In eastern Australia, P. australis grows in sheltered bays, which, ominously, are places humans spend a lot of time too! The species grows from the low tide line to a depth of 10 metres and deeper, if the water is clear enough. It can develop into large, dense stands (“meadows”) and grows along with other seagrasses like Zostera (eelgrass). It grows on a rhizome (i.e. an underground stem) in seabed sediment. The Posidonia communities in NSW do not reproduce from seedlings, although some communities in WA do. Scientists think the reasons may come down to a combination of environmental factors and genetics. Growing exclusively by vegetative means puts the NSW communities at a disadvantage. If the growing tips of the rhizomes are damaged, plants do not send out runners. It can then take up to 50 years for a clump to close a one metre wide gap. With the extent of damage done to Posidonia communities in NSW, it’s not likely those communities can re-establish themselves naturally in the medium term (source).
Why is Posidonia important?
Why is Posidonia worth talking about? Posidonia communities perform many important ecological functions. Now that these communities are becoming scarce, we’re realising their value and how important they are to preserve!
Posidonia plays an essential role in coastal marine ecosystems. No other species can create the habitat needed for other species living in this particular marine environment and so it helps to maintain biodiversity. There are many species of fish, molluscs and crustaceans that use Posidonia meadows for habitat, feeding and shelter. These animals include some other threatened species, such as White’s seahorse, which comes back into the story later. This is sounding very much like what groundcovers and small, dense shrubs do in our gardens on land. Posidonia also provides important ecosystem services, filtering water and removing and recycling nutrients, which improve water quality. It stabilises sediment on the seabed it grows in – underwater erosion control. It also removes carbon dioxide from the water for photosynthesis, capturing carbon, and releases oxygen. All of this is just what plants do in our gardens on land.
Posidonia meadows are classified as an endangered ecological community in some areas of coastal NSW and some of those communities may become extinct within 15 years. What have humans done that has decimated Posidoniacommunities? Most of these things are the underwater equivalent of what happens on land. Sediment from stormwater can smother seagrass and cause turbidity, blocking the light needed for photosynthesis. This stormwater can also bring in high levels of nutrients, which encourage excessive epiphyte growth on Posidonia leaves, reducing their ability to photosynthesise. Physically interfering with the plants through coastal development and dredging, for example, has also greatly contributed to the destruction of Posidonia communities. Two other serious ongoing sources of physical damage are boat propellers and moorings. A traditional mooring consists of a heavy block sitting on the seabed with a chain connecting it to a buoy that a boat is attached to. As the wind or tide changes, the boat swings around and drags the chain across the seabed, scouring the seagrass (source).
The ecological value of Posidonia ecosystems and the threats they face have clearly been recognised. What can be done to protect and re-establish them? We all know about restoring terrestrial ecosystems and the regeneration and revegetation work involved. Can land-based techniques be transferred to totally submerged environments? Underwater rugby might give us some clues (source). We can do underwater what we do on land, if we just adapt our equipment and techniques a little! These communities are being restored by painstakingly planting Posidonia fragments into the seabed just as (almost as, anyway) we’d plant small plants into our gardens. Vegetative methods have to be used, as Posidonia communities in NSW don’t produce seedlings.
Getting Posidonia fragments to replant poses a problem. Harvesting plants from existing populations would damage the rhizome tips of growing plants and greatly slow their continued growth, as mentioned above. So where do we get plant fragments from? Storms and large swells break off pieces of Posidonia and many of these pieces get washed up onto beaches, forming seagrass wrack.
This wrack itself supports a range of organisms, including insects and crustaceans. However, some of the pieces of Posidonia in the wrack can be suitable for replanting, and this is where material for replanting in NSW seagrass communities has come from. To collect this material, a community-based approach is being used, where anybody who uses beaches is encouraged to join a local “Seagrass Storm Squad.” There are two main collection points for the Posidonia fragments: Bundeena and Pittwater. There’s plenty of information about this on the Operation Posidonia website (Storm Squad page). These Posidonia fragments are then stored in tanks at the Sydney Institute of Marine Science at Clifton Gardens (SIMS) before they are planted out – storing them for several months substantially improves their survival rate after replanting.
Species in ecosystems depend on one another, and with the decline in Posidonia meadows, other species in that ecosystem have also suffered. One species that relies on Posidonia meadows, as well as sponge gardens and kelp on the NSW coast, is White’s seahorse, which is now listed as endangered. There is a co‑restoration project, along with Posidonia restoration, for White’s seahorse to boost its numbers in its natural habitat. Seahorses are bred and raised at the Sydney Institute of Marine Science and then released into their natural environment (Seahorse Hotels).
Endangered plant communities may occur in what may seem unusual places. Restoring them can involve similar techniques with some necessary modifications, such as using scuba gear! While very few of us can take part in the underwater gardening work to restore Posidonia meadows, playing underwater rugby is still open to everyone.
