The objective of Project 5.4 was to determine whether recovery of seagrass in the Pilbara following disturbance is by sexual (recruitment from seeds) or asexual (vegetative regrowth from rhizome extension) means, and the relative importance of each, thereby determining the capacity, timeframes and mechanisms of recovery from light and sediment deposition effects. This was achieved through an experiment in which plots cleared of seagrass were enclosed by a barrier or left open, and changes in cover were compared to unmanipulated control (and procedural control) plots. The experiment was done at two sites, a 2-m deep (the 'shallow' site) and a 6-m deep (the 'deep' site). The experiment was intended to run for 6 months, but after 5 months disturbance caused by Tropical Cyclone (TC) Olwyn reduced seagrass cover at both sites, and removed the experimental apparatus at the shallow site. Nevertheless, results yielded by surveys prior to TC Olwyn unambiguously showed strong evidence for recovery through vegetative regrowth (full recovery in cleared plots with no barriers, plus a pattern of increasing cover from the edges of the plots) and no evidence for recovery through recruitment from seeds (no seagrass was ever recorded in any cleared plot with a barrier). The main species of seagrass present was Halophila ovalis, which is widespread throughout northwestern Australia. The transferability of inferences from this study to other places in the Pilbara is hampered by the substantial variation in abundance and species composition from place to place - different species might have different mechanisms of recovery. The nature of the experimental disturbance (complete removal of all seagrass, including roots and rhizomes) is a reasonable facsimile of a severe dredging-induced disturbance, but the spatial extent of the experimental clearances (~0.5 m2) is orders of magnitude smaller than the spatial extents of dredging-induced mortality induced elsewhere: it is plausible that recovery of patches within meadows is more easily achieved through rhizome extension than recovery that encompasses spatial extents of hectares.
