Distribution and ecological role of the non-native macroalga Gracilaria vermiculophylla in Virginia salt marshes

Intertidal salt marshes are considered harsh habitats where relatively few stress-resistant species survive. Most studies on non-native species in marshes describe terrestrial angiosperms. We document that a non-native marine macroalga, Gracilaria vermiculophylla, is abundant throughout Virginia's Atlantic coastline. We sampled eight marshes, characterized by low slopes and by the presence of the tube-building polychaete Diopatra cuprea on adjacent mudflats, which have been shown previously to be associated with G. vermiculophylla. G. vermiculophylla was found in 71% of the sampled quadrats on the border between the mudflat and tall Spartina alterniflora, 51% within the tall S. alterniflora zone, and 12% further inland. We also tagged G. vermiculophylla from two habitats: (1) unattached G. vermiculophylla within marshes and (2) G. vermiculophylla 'incorporated' onto D. cuprea tubes on the adjacent mudflats. Of the incorporated thalli, 3-9% ended up in the marsh, demonstrating connectivity between habitats. In addition, 21% of unattached thalli remained for 2 weeks within the marsh, suggesting that entanglement around marsh plants reduces tidal drift. Growth experiments in mesh bags indicate that most of the G. vermiculophylla transferred from the lagoon to the marsh decomposed there, potentially enhancing local nutrient levels. Finally, we document that G. vermiculophylla in marshes had a reduced associated flora and fauna compared to G. vermiculophylla on the adjacent Diopatra mudflats. In conclusion, unattached G. vermiculophylla is abundant along marsh borders in the tall S. alterniflora zone in Virginia, and we hypothesize that this non-native species has significant impacts in terms of marsh habitat complexity, species abundance and diversity, nutrient dynamics, productivity, and trophic interactions.