Seagrasses are important foundation species in shallow coastal ecosystems that provide critical ecosystem services including stabilizing sediment, sequestering carbon and nutrients, and providing habitat and an energy source for a diverse fauna. We followed the recovery of functional (primary productivity, carbon and nitrogen sequestration, sediment deposition) and structural (shoot density, biomass, plant morphometrics) attributes of Zostera marina (eelgrass) meadows in replicate large plots (0.2 to 0.4 ha) restored by seeding in successive years, resulting in a chrono sequence of sites from 0 (unvegetated) to 9 yr since seeding. Shoot density was the structural metric that changed most significantly, with an initial 4 yr lag, and a rapid, linear increase in plots 6 to 9 yr after seeding. Changes in Z. marina aerial productivity, sediment organic content, and exchangeable ammonium showed a similar trend with an initial 4 yr lag period before differences were observed from initial bare sediment conditions. After 9 yr, Z. marina meadows had 20× higher rates of areal productivity than 1 to 3 yr old meadows, double the organic matter and exchangeable ammonium concentrations, 3× more carbon and 4× more nitrogen, and had accumulated and retained finer particles than bare, unvegetated sediments. These results demonstrate the reinstatement of key ecosystem services with successful large-scale restoration, although none of the parameters reached an asymptote after 9 yr, indicating that at least a decade is required for these attributes to be fully restored, even in an area with high habitat suitability. Survivorship along a depth gradient showed that ~1.6 m (mean sea level) is the maximum depth limit for Z. marina, which matches the 'tipping point' for survival predicted for this system from a non-linear hydro dynamic/seagrass growth model.

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.

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