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

Abstract

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.

Exotic macroalga Gracilaria vermiculophylla provides superior nursery habitat for native blue crab in Chesapeake Bay

“…However, Chesapeake Bay has experienced severe declines in Zostera marina, including mass defoliations (Orth & Moore 1984, Orth et al 2010, which may partly explain the inability of the blue crab population to recover from an 80% decline in biomass since the early 1990s (Lipcius & Stockhausen 2002). Thus, if Gracilaria vermiculophylla is effective as alternative nursery habitat, as Thomsen and colleagues have posited for invertebrates including the blue crab (Thomsen et al 2009, Thomsen 2010, then it may compensate for the detrimental effects of eelgrass loss on blue crab population recovery.…”

“…This is in keeping with the hypothesis that shelter-seeking species use structural complexity rather than evolutionary familiarity to determine their use of habitat-forming vegetation (Duffy & Baltz 1998, Jackson & Hobbs 2009, Martin & Valentine 2011, Schlaepfer et al 2011). In addition, prey items occurring in macroalgae such as G. vermiculophylla provide the necessary food quality to support juvenile crab growth (Epifanio et al 2003, Thomsen et al 2009, Byers et al 2012.…”

Foundation species' overlap enhances biodiversity and multifunctionality from the patch to landscape scale in southeastern United States salt marshes

“…Secondary foundation species such as oysters cemented to mangrove roots and epiphytes layered on trees are often among the organisms facilitated by these foundation species [15,[19][20][21][22] and have the potential to modify an ecosystem's multifunctionality through their own engineering activities and through their further enhancement of biodiversity by increasing niche space [21]. Indeed, recent research in cobble beach plant communities [20], mangroves [22], mudflats [23,24], savannahs [21], tropical forests [25] and temperate woodlands [26] has shown that 20 -80% of the species richness and abundance maintained by foundation species in these ecosystems can be attributed to the presence of secondary foundation species. Despite their well-documented, positive effects on community structure [15,19], studies have yet to experimentally investigate the importance of foundation species' overlap in driving ecosystem functioning and multifunctionality.…”

“…In being structured by dominant grasses and, often, bivalve and macroalgae secondary foundation species [20,23,30], salt marshes are a suitable system for investigating the relationships between foundation species' overlap, biodiversity and ecosystem multifunctionality. In the southeastern USA, Spartina alterniflora (hereafter, cordgrass) generates much of the three-dimensional structure of salt marsh habitats.…”

Research on mutualisms between native and non-native partners can contribute critical ecological insights

“…The native sediment dwelling polychaete Diopatra cuprea in the mid-Atlantic U.S. incorporates the invasive red alga Gracilaria vermiculophylla into its decorated tube caps (Thomsen and McGlathery 2005). The non-native alga is also found unattached, but the stable attachment point of Diopatra tube caps increases its longevity and growth rate (Thomsen et al 2009) and biomass (Byers et al 2012). Although the nature of the reciprocal benefits of G. vermiculophylla to D. cuprea have not been investigated, D. cuprea generally receives shelter and food from the algae it hosts and favors attaching G. vermiculophylla over other native algal species (Thomsen and McGlathery 2005).…”