Matching planting methods to species and site conditions is key in seed-based intertidal seagrass restoration

Seagrass meadows continue to be lost and degraded globally. Restoration is one promising and emerging conservation strategy to combat such losses and place seagrass on a pathway to net gain. However, successful restoration methods remain limited to a few species, and geographically constrained, with few experimental trials comparing planting methods across species and seagrass bioregions. This study trialled three seed-based seagrass restoration planting methods in two seagrass bioregions (the temperate north Atlantic and temperate southern oceans). Using two seagrass species Zostera marina and Zostera muelleri this research investigated seed-based planting methods and their influence on the likelihood of seedling emergence, shoot emergence, and seedling growth (i.e. leaf length). Seagrass emergence was observed at 50 % of the experimental sites, with the likelihood of seagrass emergence largely influenced by local site conditions. Each planting method performed variably in relation to species and environmental conditions. Dispenser injection seeding resulted in the highest shoot emergence efficiency of the three methods for Z. marina while biodegradable planting pots and hessian bags were the more favourable methods for use with Z. muelleri seeds. Despite all chosen sites deemed suitable for restoration from habitat suitability models, low seedling emergence suggests that site conditions including wind fetch, redox boundary depth and mud- dominant sediments present specific bottlenecks to seed germination and retention. This work demonstrates the importance of matching seed planting methods to site conditions and species life history traits and highlights the need for greater understanding of mechanisms to overcome germination and emergence bottlenecks in seed-based restoration.