Prolonged buoyancy and viability of Zostera muelleri Irmisch ex Asch. vegetative fragments indicate a strong dispersal potential

Abstract

The establishment of clonal marine plant populations, including the seagrass Zostera muelleri (Zosteraceae), may be more dependent on the availability of viable vegetative fragments rather than seed. New populations may establish through long-distance dispersal of viable vegetative fragments, potentially increasing genetic diversity and resilience to anthropogenic or naturally occurring disturbance. A number of activities can dislodge vegetative fragments of Z. muelleri (leaves, rhizomes and roots) from the sediment. These fragments can remain positively buoyant, floating on the surface of the water. As the time since dislodgement increases, buoyancy may become reduced, causing fragments to move lower into the water column. However, what is not known is how long these fragments remain buoyant and potentially viable for recolonization. To address this knowledge gap, we collected wrack samples (n=125) of Z. muelleri from four Victorian estuaries. Fragments were floated in outside aquaria for up to ten weeks, with subsamples tested for metabolic activity using tetrazolium violet. Porosity of seagrass rhizomes was also investigated to understand the influence of lacunae (large air filled spaces within plant tissues) on the flotation of vegetative fragments. The average proportion of potentially viable fragments collected in wrack ranged from 3.6% (SD=2.23) to 11.2% (SD=5.9). While there was a steady decline in the buoyancy of fragments across the ten-week period, initial buoyancy was relatively high, with approximately 50% of the fragments remaining positively buoyant for the initial five weeks. The viability of fragments following flotation was high. One hundred percent of fragments (n=25 per assay) remained viable after floating for three weeks, with only a marginal decline (=96% viability) occurring after five weeks. When considered in conjunction with the highly porous nature of seagrass rhizomes (lacunae accounted for 45.2% of total volume), our findings indicate that the species may be capable of prolonged periods of transport dispersal within the marine environment.