Characterising morphological variation and mapping seagrass upper boundaries to inform restoration outcomes

Seagrass meadows located in the intertidal and subtidal zones across the world provide a wide range of socio-economic and ecological services. Their global decline due to anthropogenic pressures, have forced marine scientists to invest in restoring degraded seagrass meadows using a variety of transplantation techniques. Seed and shoot transplantation are among the most globally successful restoration techniques. Despite their advancements, there are many challenges and knowledge gaps when mapping the appropriate tidal boundary to transplant seagrass transplants. Seagrasses are adapted to short periods of exposure during low tides. New transplants are more susceptible to desiccation damage and early mortality when grown in an area of the intertidal that is too high and where seagrass typically does not grow. Understanding the upper boundary limits of where seagrass can survive due to aerial exposure and seagrass morphological variation in different environments along the intertidal gradient is valuable information to maximise restoration success and avoid future loss of seagrass transplants. In this study I explored the upper seagrass limits and variation in morphology of the intertidal seagrass Zostera muelleri, which has seen significant losses in Western Port, Australia. Using data collected from timelapse videos integrated with local tidal models, upper boundary limits and maximum aerial exposure duration of Z. muelleri was determined for 12 sites in Western Port. Morphological variation within 12 sites across the upper and middle tidal boundaries was also characterised. Results indicate that the upper boundary varied between some site from a minimum of 0.84 m at Flinders to a maximum of 1.76m at Woolleys Beach. Woolleys Beach and Rhyll had upper seagrass boundary heights significantly higher (and therefore longer aerial exposure times) than other sites. Multivariate analysis revealed significant variation in seagrass morphology between sites and upper boundary intertidal heights. Major 4 differences in seagrass morphology were detected between upper and middle tidal boundaries of Woolleys Beach and Rhyll that also had the highest upper intertidal heights. These results indicate that donor seagrass used for restoration needs to be carefully matched to the appropriate intertidal height at a restoration site as they may be preadapted to a narrow aerial exposure duration and morphology for that intertidal height. These findings have important implications for restoration of this species moving forward and further studies testing the matching of phenotypes to environments should be undertaken.