Abstract
In the wake of the Deepwater Horizon oil spill, large-scale breakwater projects have been constructed to restore and conserve marshes across the northern Gulf of Mexico. These breakwater projects are often termed living shorelines due to the perceived increase in secondary productivity around the breakwaters and within the fringing marsh shoreward of these structures. However, evaluations of the effectiveness of breakwaters at preserving natural shorelines are limited. To evaluate the effectiveness of large-scale breakwaters at protecting or restoring marshes in high wave energy environments, we conducted experimental plantings and a shoreline monitoring program landward of eight-year-old breakwaters (OBW) and reference no breakwater sites (NBW) along Bon Secour Bay, AL. The OBW and NBW complexes cover 0.6km and 1.2km of consecutive shoreline, respectively. In both the OBW and NBW sites, eight replicates of planted vegetation (nursery-grown Spartina alterniflora sods), natural vegetation, and no vegetation treatments were established along the shoreline. Vegetation monitoring took place over three years and evaluated plot percent cover and species composition. Additionally, the perimeters of all the natural S. alterniflora patches were field mapped using an RTK GPS across breakwater treatments. Results from fixed-plot monitoring showed a positive impact of large-scale breakwaters on natural fringing marshes and no impact on planted marshes. The RTK monitoring showed no discernable impact of breakwaters on S. alterniflora extent, but did show that S. alterniflora behind breakwaters experienced little to no upland migration while those in no breakwater areas retreated significantly. Cumulatively, these results suggest that large-scale breakwaters could have an impact on preserving fringing marsh vegetation in high wave energy environments.
