Declining Posidonia meadows
Posidonia australis is a beautiful and slow-growing seagrass that makes extensive underwater meadows all over southern Australia, from Wallis Lake in New South Wales to Shark Bay in Western Australia.
Unfortunately, seagrasses like Posidonia have become severely threatened by human activities and have been declining at an alarming rate. On average, seagrasses are declining at the same rate as coral reefs, that is – one soccer field every half hour!
In eastern Australia, Posidonia meadows are found in sheltered bays, which are also preferred sites for us humans: this is where we often choose to live, work and play. Increased coastal development and pollution in these sheltered estuaries has led to major Posidonia declines over the last few decades.
The declines of Posidonia meadows in the central parts of NSW (where most people live) have been so severe that six meadows have been formally listed as ‘endangered’ by both the Australian Commonwealth Government (EPBC Act) and the NSW government. There’s a very real risk that this species may become locally extinct from some of these estuaries within the next 15 years unless new conservation actions reverse current trends.
To reverse this decline and prevent further losses, there are now strict regulations that limit coastal development near seagrass meadows, and water quality has greatly improved in the last couple of decades, as we’ve become better at managing waste and reducing pollution.
There are still some human activities that continue to be a problem, however, and are leading to the continued decline of Posidonia in some estuaries. Boat moorings are one of the most obvious current disturbances to seagrass habitats, as the sheltered bays where this seagrass naturally thrives are also the most ideal locations for people to moor their boats.
The problem with traditional, fixed block-and-chain boat moorings is that they scar the seafloor and remove seagrass shoots, causing the formation of bare patches that fragment the meadow and destabilise the sediment. After some time, as the mooring scars get larger, bare patches start to merge and this leads to more Posidonia losses.
Mapping has shown that mooring scars can be very large, form quickly and can take decades to recover fully once a mooring is removed.
Mooring scars are clearly visible from the water’s surface and have been increasing as the number of moorings has raised. Across all of NSW, currently leased moorings are causing losses of around 130,000 m2 (that is more than 26 football fields!), but the real loss is even greater, as many estuaries contain old scars that remain after the relocation of moorings.