Tolerance of tropical seagrasses Zostera muelleri and Halophila ovalis to burial: toward an understanding of threshold effects

Seagrass meadows are critically important coastal habitats, but they are undergoing rapid global declines linked to degradation of coastal catchments and the expansion of development in coastal zones, which can lead to increased deposition of terrigenous sediments onto seagrass meadows. In the Asia-Pacific region, which includes the Great Barrier Reef, direct sedimentation effects are recognised as the major threat to seagrass growth and survival. However, tolerance to burial varies among species. Understanding thresholds for burial tolerance at the species-level is therefore critical to effective management of human impacts on seagrass meadows. This research examined the effects of sedimentation on two tropical seagrass species, Zostera muelleri and Halophila ovalis in an experimental mesocosm and field setting. The research suggests that shoot density in these species declines significantly at burial depths of 5-7.5 mm in a mesocosm setting, with very low levels of growth observed above 10 mm burial depth. Fine sediments (including fine sands, silts and clays) appear to have a greater negative effect on shoot density in both species, compared to coarser sandy sediments. In situ, the effect of burial is less pronounced and is likely to be mitigated by a range of factors, including clonal integration. These species also demonstrate different adaptations to burial disturbance, including by resisting burial effects (Z. muelleri) or by regenerating from the buried rhizome after disturbance (H. ovalis)