Macroalgae play an important role in coastal ecosystems by taking up nutrients from the water column and providing
habitats and food for epifauna. Invasion of alien macroalgae can displace nativemacroalgae and cause habitat
modification. This may result in changes in the capacity of the community to remove nutrients and the
structure of epifaunal assemblages in the area. In Hawaii, the alien invasive macroalgae, Gracilaria salicornia
and Acanthophora specifera have been the focus of removal projects in recent years, but little has been studied
about their ecological impacts and functions. We investigated potential impacts of these invasive algae by comparing
their capacity for ammonium uptake and their epifaunal community structure to those of the native
alga Padina thivyi. The capacities of G. salicornia and P. thivyi to remove ammonium as individuals (per gram nitrogen
in algae) were comparable. However, ammonium uptake as a community was, on average, higher in the
invasive-dominated canopies than in the native-dominated canopies, probably due to a higher density (biomass
per sediment surface area) of G. salicornia than P. thivyi. Invasive canopies harbored higher abundances and species
richness of epifauna (per sediment surface area) than native canopies.Most of the numerically dominant epifauna
in the present study were detritivores and were associated with G. salicornia. Amphipod grazers were the
only numerically dominant taxon whose abundance was positively correlated with the biomass of P. thivyi.
A. specifera had no apparent effects on epifaunal distributions. This study showed facilitative effects of
G. salicornia, but the difference in the epifaunal community structure (i.e. detritivores in the invasive canopies
vs. herbivores in the native canopies) should be treated as a potential ecological concern and calls for further investigations
of how nutrients taken up by these algal species are cycled in the food web.