Dynamics of recruitment and establishment of the invasive seaweed within an eelgrass habitat.

Knowledge of the potential distribution (i.e. abundance and spatial extent) of an invasive species is important to estimating its potential impacts on recipient communities. Most previous studies have focused on the potential spatial extent of invasive species populations at regional scales, but little is known on how species successfully recruit and establish at more local scales. In this study, we examined how recruitment of the green alga ssp. (hereafter ) can vary spatially and the environmental factors associated with establishment in eelgrass () beds. Standardized recruitment blocks (65 blocks in a 720 × 240 m grid) were used to monitor the number of recruits, juveniles and adults over 2 years. Environmental factors (depth, relative water flow, light and temperature) and attributes of the surrounding macrophyte assemblage (eelgrass density, eelgrass length, biomass) were also measured. Recruitment occurred on all blocks or nearby artificial structures (i.e. buoys) and mainly originated from button stages (i.e. female gametes or utricles). Contrary to other studies, the abundance of (recruits, juveniles and adults) was best predicted by the density of the native canopy-forming species, , which highlights a positive interaction between native and non-native canopy-forming species. Seasonal variation in recruitment was observed; it was lower during the summer. Recruitment did not show any distinct spatial pattern (e.g. gradient or patch), but the same spatial pattern of recruitment was observed every sampling date, suggesting that there are "hotspots" for recruitment. In general, the total number of fronds observed on a block at the end of the experiment was positively correlated with the cumulative number of recruits. However, recruitment occurred on some blocks but recruits never grew, suggesting that some environmental factors limit distribution and abundance in eelgrass beds. Overall, the assessment of recruitment over 2 years showed that the colonization of suitable locations by within seagrass beds may take several years and that some factors may not only limit, but also inhibit expansion within eelgrass beds.