Re-attachment and viability of vegetative propagules of subtropical intertidal seagrasses

Seagrass meadows are under increasing pressure from anthropogenic activities, which are contributing to global declines. Seagrass vegetative fragment dispersal can assist natural meadow recovery and can be influenced by winds, currents and marine animals (e.g. bioturbation). Fragment viability can also affect dispersal, settlement and establishment, ultimately impacting successful recruitment to an area, as well as resilience and metapopulation dynamics. Here, we investigated the viability of Zostera muelleri and Halophila ovalis fragments sourced from the water column and experimentally detached fragments collected from intertidal beds by mimicking megaherbivore foraging behaviours. We evaluated fragment settlement and root establishment, and viability and growth rates for 1 mo in an outdoor mesocosm experiment. Plant morphometrics were measured before and after the experiment to evaluate their influence on fragment settlement and root establishment. H. ovalis settlement was influenced by rhizome length, and Z. muelleri settlement was influenced by the number of shoots and shoot length. Root establishment was limited for both species, with no relation to plant morphometrics. Experimentally grazed fragments of both species showed a faster rhizome elongation rate than naturally floating fragments, and H. ovalis exhibited the fastest rhizome elongation rate. Naturally floating and experimentally grazed fragments also had similar viability. Ultimately, our results suggest that root establishment in the sediment is the rate-limiting step in fragment reattachment. Fragments actively grew fresh rhizomes and were equally viable for both species, indicating they are ecologically functional propagules. Root establishment potential requires further study, as it determines the success of natural seagrass reestablishment from vegetative fragments.