Threshold of lugworm (<scp><i>Arenicola marina</i></scp>) densities for successful transplantation of eelgrass (<scp><i>Zostera marina</i></scp>)

The global decline of seagrasses has prompted numerous restoration efforts. However, these efforts are challenged by ecological feedbacks and the prevalence of stressors. This study investigated the effects of lugworms ( Arenicola marina ) in a large‐scale eelgrass ( Zostera marina ) transplanted area. Shoot densities of eelgrass transplants were monitored at a high temporal resolution at specific sites with naturally varying degrees of A. marina density. Arenicola marina density effects and threshold limits on eelgrass net shoot production (performance) were estimated based on shoot counts, revealing a sharp decline in transplant performance with increasing A. marina densities. Transplant performance was negatively impacted even at low A. marina densities, with a threshold of 29 ind. m −2 (22–42 ind. m −2 , 95% CI). Transplanted eelgrass performance displayed a high within‐site (10–100 m) variation due to the heterogenic spatial distribution of A . marina , suggesting that actively targeting areas with low A. marina densities can improve transplantation success. Monitoring A. marina fecal cast densities proved to be an effective proxy of the negative impact from sediment reworking caused by A. marina' s feeding strategy on eelgrass transplants and is an important parameter to consider during site‐selection procedures before restoring eelgrass.