Seagrass restoration off the Adelaide Coast via facilitating natural recruitment

Key words: Amphibolis, nanotechnology, seagrass loss, eutrophication

Jamie Quinton

Over the past 50+ years, more than 5,000 ha of seagrass has been lost along the Adelaide metropolitan coast, largely due to excessive nutrient inputs. Following substantial investment in reducing nutrient inputs, limited natural recolonisation has been observed. This is possibly due to sediments now mostly being too mobile for seagrasses to colonise.

The trial. An experiment was carried out to stabilize areas adjacent to existing seagrass meadows, deploying hessian sand bags to provide a stable substrate for the recruitment of Amphibolis seedlings. These seedlings are produced viviparously by the parent plant and have a ‘grappling hook’ for attachment to the substrate.

Fig. 1. Hessian sand bag covered by an outer layer of coarse-weave hessian

Various types of sand bag were trialed, with the most effective being a standard hessian bag covered with an outer layer of coarse-weave hessian. These bags can simply be dropped over the side of a boat, and do not require divers to deploy them, thus substantially reducing the coast of restoration. Densities of recruits averaged 150-350 seedlings per m² for different treatments, with individual sand bags attracting densities up to ~900 seedlings per m². Long-term survival (i.e. 3 years or more) occurred for up to 72 individuals per m², although many treatments failed over this duration. Preliminary estimates suggest that restoration costs could be less than $10,000 per ha, provided that long-term success and eventual meadow formation can be achieved. Initial studies of the bags suggested that spatial configuration was not important.

Fig 2. Hessian sand bag with Amphibolis recruits after six months

WATCH VIDEO Fig. 3. Video of a 3-year-old and adjacent 4-year-old zone where hessian sand bags were deployed showing seagrass colonisation in the older seagrass.

UPDATE FEBRUARY 2016: Re-established Ampibolis at 3 years remains interspersed with patches of bare, highly bioturbated, sand.  The first ~45 seconds of the video in Figure 3 is  a section ~3 years old, where seagrass has not colonised the space between the bags that were deployed, while the remainder is ~4 years old, and the spaces between bags have been colonised.  Natural recruits of Zostera species are prominent in the first 20 seconds. In other areas, Posidonia is recruiting, indicating that biodiversity is building over time as a result of the treatments.

Lessons learned and future directions: The key issue so far has been the inconsistent quality of the hessian used for the bags – some batches deteriorate rapidly leading to loss of all seedlings, whereas others last longer. The focus is now on trialling nanotechnology to improve the hessian longevity, while still retaining its long-term biodegradability. There is some suggestion that half-buried bags also perform better, possibly due to water flow through the sand in the bags, and this needs further study.

Stakeholders and funding bodies: SA Department of Environment & Natural Resources, SA Water, Adelaide & Mount Lofty Ranges Natural Resource Management Board, Australian Research Council, South Australian Research & Development Institute, Flinders University

UPDATE

Contact information: Dr Jason Tanner, Principal Scientist – Marine Environment & Ecology, SARDI Aquatic Sciences, PO Box 120, Henley Beach, SA. 5022. Tel: +61 8 8207 5489 Email: jason.tanner@sa.gov.au