Gracilaria vermiculophylla in the Virginia Coastal Bays: Documenting the Distributionand Effects of a Non-Native Species

Non-native species are of worldwide concern in both terrestrial and aquatic systems. Macroalgal introductions in coastal environments can have varied and often harmful effects, especially when surrounding habitats are altered by the invasion. Gracilaria vermiculophylla is a red macroalga that is native to East Asia and has been introduced to temperate estuaries around the world. It cannot be easily identified based on morphology alone, and is frequently mistaken for native congeners if genetic testing is not used. Mats of the macroalga can accumulate on subtidal and intertidal substrate within the Virginia coastal bays, USA and are held in place by tube decorating polychaetes on the order of months to years.
The broad goals of this dissertation were to determine how widespread the G. vermiculophylla invasion was in the Virginia coastal bays and to document potential effects of G. vermiculophylla mats on biogeochemistry, trophic cascades, and on public health in the region. I found that the introduction was widespread in both subtidal and intertidal habitats, with higher intraspecific genetic richness and diversity than currently documented in other invasions. In addition, I found that intertidal sediment, marsh cordgrass, and mudflat invertebrates all incorporated nitrogen of G. vermiculophylla origin, which indicates that the macroalga is an important mediator of nutrient transfers in the system. Work on intertidal mudflats showed that the presence of G. vermiculophylla in this system, at moderate densities, could increase oxic-anoxic heterogeneity in the sediment and thus increase coupled nitrification-denitrification. In addition, although G. vermiculophylla was associated with an overall increase in invertebrate biomass, shorebirds chose to forage on bare mudflats. Lastly, I found that G. vermiculophylla was a reservoir for the pathogenic bacteria, Vibrio parahaemolyticus and V. vulnificus which can cause gastroenteritis, severe wound infections, septicemia, and death in humans. In addition, oysters, sediment, and water collected in close proximity to mats of G. vermiculophylla had higher concentrations of both bacterial species when compared to samples collected on bare mudflats. Taken together, data collected within the Virginia coastal bays indicate that this widespread habitat modifier can have important effects on nitrogen availability, food web interactions, and shellfish sanitation.