Epifaunal community structure and ammonium uptake compared for the invasive algae, Gracilaria salicornia and Acanthophora specifera, and the native alga, Padina thivyi

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 native macroalgae 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.