Ecomorphodynamic feedbacks and barrier island evolution, Virginia Coast Reserve, USA

Ecomorphodynamic feedbacks play an important role in barrier island response to disturbance. Dune-building grasses like Ammophila breviligulata can restore areas of high relief after overwash events (resisting disturbance); however, if overwash recurs before dunes have reestablished, overwash-adapted "maintainer" species like Spartina patens (upright var.)--which preserve low, flat topography--may preferentially survive, thereby increasing the likelihood of future overwash (reinforcing disturbance). Over time, this positive feedback may lead to overwash persistence. We explore the potential influence of the maintainer feedback on two morphologically distinct islands in the Virginia Coast Reserve (VCR), located on the U.S. mid-Atlantic coast. Combined topographic and vegetation surveys show that on Hog Island (high-relief, rotating, infrequently disturbed), where dunes dominated by A. breviligulata are ubiquitous, overwash is currently limited in extent and related to beach width rather than dominance by S. patens. Historical aerial photos and stratigraphic evidence (ground-penetrating radar, cores) indicate that gradual recovery has taken place after overwash events on Hog Island, except where the beach is narrow and eroding. Conversely, on Metompkin Island (low-relief, transgressing, frequently disturbed), overwash is widespread and dominated by S. patens, particularly along the rapidly migrating northern half of the island (where shell armoring is also prevalent). Results also suggest that spatially heterogeneous disturbance patterns on Metompkin Island may increase vegetative compositional variability relative to Hog Island. Finally, overwash has generally been more prevalent and persistent over time on Metompkin Island than on Hog Island. In aggregate, our findings suggest that within barrier island systems like the VCR in which both dune-building grasses and overwash-adapted maintainer species are common, the maintainer feedback is likely to be a more important dynamic on islands that are already susceptible to frequent disturbance due to physical and geological factors. The maintainer feedback therefore has the potential to accelerate large-scale shifts from dune-dominated to overwash-dominated barrier morphologies as overwash becomes more frequent as a result of climate change-induced increases in storm intensity and sea level rise.