Recruitment facilitation as a means of seagrass rehabilitation

Over recent decades, the Adelaide (Australia) metropolitan coast has lost more than 5,200 ha of seagrass habitat due to increasing anthropogenic pollution and coastal development. While typical seagrass rehabilitation techniques rely on transplanting adults or planting out seedlings, these are time consuming and expensive.  Here we present an alternative method of rehabilitation, the facilitation of natural recruitment.  This method relies on the fact that some local seagrasses in the genus Amphibolis release seedlings with small “grappling hooks” at their base, rather than the more typical seeds.  A range of biodegradable hessian bags, strips and mats in various configurations were deployed at two sites along the Adelaide metropolitan coast in September 2004. Approximately five weeks after deployment a total of 16,514 seedlings, or the equivalent of 157.2 seedlings m^-2, had recruited. Significant differences in seedling recruitment were observed between the two sites and are likely to reflect the composition of the surrounding seagrass beds, depth and local hydrodynamic regime. The retention of seedlings on the experimental units declined over time, but after a one-year period 31.4% of seedlings remained. Surviving seedlings grew considerably over this time, as was evident by a 2.6 fold increase in above ground-biomass and a 6.4 fold increase in below-ground biomass. However, mortality rates over the second year were high (>90%), as was mortality of recruits on a new series of bags.  Thus it appears that while environmental conditions were favourable for recruitment and growth in the first year of the study, they were unfavourable in the second year.  Future work will address the causes of this.