Impacts of the invasive grass Spartina anglica on benthic macrofaunal assemblages in a temperate Australian saltmarsh

Reported impacts of the invasive saltmarsh grass Spartina anglica on benthic macrofaunal assemblages around the world vary considerably, and there is little understanding of the reasons for this variation. We compared macrofaunal assemblages and sediment characteristics among patches of S. anglica and adjacent uninvaded habitats (bare mudflat and native saltmarsh) in southeastern Australia. Invaded patches showed reduced species richness (by 50%) and diversity compared to both uninvaded habitats. Macrofaunal abundance in S. anglica patches was also lower than in native marsh (by 60%), but not different from mudflat. There were no differences in biomass among habitats. Ordination clearly separated the species assemblage of invaded patches from uninvaded habitats, suggesting a unique community in the Spartina habitat. Molluscs and crustaceans were the most depleted in S. anglica patches, while the polychaete Nephtys australiensis was enhanced. Infauna and epifauna were both depleted in S. anglica, although the mechanisms for these impacts should differ. Burrowing by infauna in S. anglica patches was likely impeded by dense roots and rhizomes, because the below-ground plant biomass was 72% greater than in native saltmarsh. Epifauna were likely depleted in S. anglica patches due to shadinginduced inhibition of microphytobenthos growth, consistent with measured reductions of porewater salinity and increased mud content. Salinity and mud content were the sediment parameters that correlated most strongly with macrofaunal assemblage composition. These results, combined with a synthesis of published S. anglica impacts, suggest predictions of when S. anglica facilitates or inhibits macrofauna, considering infauna and epifauna separately.