More than 5,200 ha of seagrass were lost off Adelaide between ~ 1940 and 2002, primarily due to nutrient inputs. Improved wastewater management since has reduced nitrogen inputs from 2,400 to 600 tonnes p.a., leading to some... [ view full abstract ]
More than 5,200 ha of seagrass were lost off Adelaide between ~ 1940 and 2002, primarily due to nutrient inputs. Improved wastewater management since has reduced nitrogen inputs from 2,400 to 600 tonnes p.a., leading to some natural recovery (~ 4 %). To improve recovery, traditional seagrass rehabilitation methods of transplants and seedling propagation were tested and largely failed due to relatively strong hydrodynamic forces and extensive bioturbation. A fortuitous and novel combination of simple engineering and a biological peculiarity, however, produced encouraging results, whereby hessian bags were used to entangle the ‘grappling hook’ apparatus possessed by seedlings of the seagrass Amphibolis antarctica. While highly successful at small scales, this technique is not currently applicable to a broad range of other seagrass species. Initial trials implanting Posidonia seedlings into the bags have, however, also proved successful. Seedling survival over 2 ½ years ranged from 15-60% depending on fill type for bags seeded in early 2012, and after 3 years it was no longer possible to distinguish individual plants, which had spread through the production of rhizomes. Over this time period, plants grew to ~25-30 cm in height. Bags filled with 100% sand were the least successful, with a 50:50 mix of clay and sand the most successful. A repeat of this experiment in 2013 showed poorer survival (15-40%), but still good growth. Again the 50:50 mix appeared best, although not significantly so, and this time the worst mix was 70% sand 30% clay. Additional experiments showed little influence of organic matter addition (dried seagrass) on survival, although low rates (0 or 300 g per bag) produced the best growth. Large seeds showed the best survival, although seed size influences on growth were inconsistent. Finally, intermediate seedling densities resulted in the highest survival, but low seed densities produced the best growth. While successful, the major disadvantage of this technique for species such as Posidonia is that it requires divers to plant the newly sprouted seedlings into bags, which is time consuming. It remains to be determined if bags can be pre-seeded prior to deployment, which would remove the need for divers.
