Growth and recovery after small-scale disturbance of a rapidly-expanding invasive seagrass in St. John, U.S. Virgin Islands

Abstract

Seagrass meadows evolved in the presence of disturbances and have a wide variance in recovery rate that largely follow a trajectory of re-colonization and succession to pre-disturbance levels. Invasive species may gain a foothold within native seagrass beds after space is opened by disturbances that reduce seagrass cover. Here we use shoot density monitoring data to describe growth of the invasive seagrass Halophila stipulacea in Caribbean bays over a 5-year period and conduct a series of experiments using a combination of mesocosm, seagrass removal plots, and field measurements to quantify vegetative fragment survivorship and recovery after benthic disturbance. Shoot density increased rapidly in invaded bays, reaching average densities of approximately 600 shoots/m2 within 5 years of establishment. Seagrass cover was estimated to recover to pre-disturbance level 17–31 weeks after removal from vegetative propagation of neighboring plants outside the treatment plot. Mesocosm experiments found floating and settled fragments as small as 2 cm in length and containing at least one vertical shoot survived the 4-day trial and increased in both length and biomass. Empirical evidence here demonstrates that H. stipulacea is highly resilient to small-scale disturbance which we suggest is having a transformative impact on Caribbean seagrass communities.