Using mangroves to stabilize coastal historic sites: deployment success versus natural recruitment

Native Americans created shell middens along estuarine shorelines, and these archeologically significant sites are now threatened by erosion and sea level rise. We worked with Canaveral National Seashore to apply living shoreline techniques to protect middens using stabilized oyster shell, Spartina alterniflora, and Rhizophora mangle. Deployed mangroves were successful in terms of survival, growth, and reproduction, but natural recruitment following stabilization was minimal. After evaluating natural mangrove recruitment at stabilized and neighboring shorelines, we found propagule and seedling counts were significantly lower at stabilized shorelines. Third, we experimentally tested effects of varying sediment grain sizes on R. mangle. Overall, survival and growth were high in all treatments, suggesting that seedlings were not inhibited by the presence of large oyster shell fragments. Combined, this suggests that the retention and transition of the R. mangle propagule into a seedling is being limited in the field. Large shell potentially acts as a barrier for initial anchoring in the sediment or increasing likelihood of dislodgement during movement by boat or wind wakes. The human-altered substrate at shell middens may prevent natural recovery of vegetated shorelines, highlighting the need for on-going management for long-term protection of these archeological sites.