Impacts of an abundant introduced ecosystem engineer within mudflats of the southeastern US coast

Invasive ecosystem engineers can have far-reaching effects on systems, especially if they provide structure where none was before. The non-native seaweed Gracilaria vermiculophylla has proliferated on estuarine mudflats throughout the southeastern US, including areas (South Carolina and Georgia) that historically were extremely low in seaweed biomass. Quantitative field surveys across 150 km of high salinity estuaries revealed that the density of the native onuphid polychaete Diopatra cuprea and the aboveground height of its biogenic tubes, which Diopatra decorates with drifting debris and seaweed, positively influenced Gracilaria biomass. The abundance of Gracilaria epifauna, composed primarily of amphipods and small snails, increased with Gracilaria biomass at many locations in our field surveys. To examine whether epifauna were facilitated by Gracilaria we experimentally manipulated Gracilaria biomass in two locations. Consistent with the field surveys, we found that increasing Gracilaria biomass facilitated epifauna, particularly amphipods and snails. Epifaunal densities on Gracilaria were higher than on a biologically-inert structural mimic of Gracilaria (plastic aquarium alga), indicating that epifauna colonize Gracilaria because Gracilaria provisions both physical structure and a biological resource. We also quantified the seaweed's net rate of productivity and decomposition. Primary production of Gracilaria was variable, but massive in some areas (up to 200 % net biomass increase in 8 weeks). The seaweed rapidly degraded upon burial in silty sediments (79 % loss in mass within 10 days) and thus may represent an important new addition to detrital foodwebs. As a copious, novel source of primary production, detritus, and desirable habitat for epifauna, Gracilaria has the potential to transform southeastern US estuaries.