Evaluating the Long-Term Efficacy of Breakwaters in Stabilizing Fringing Spartina alterniflora Marshes

Breakwaters are often installed to prevent or reduce shoreline and marsh retreat. Despite their widespread application, few studies have quantified their efficacy at decreasing marsh retreat, and even fewer studies have considered their long-term viability. To evaluate the ability of breakwaters to decrease marsh retreat, Spartina alterniflora was monitored from 2017 to 2024 on a shoreline containing two breakwater complexes, constructed 5 years apart, and along unprotected control sites. We expected breakwaters would decrease the marsh edge retreat of S. alterniflora relative to control sites and increase the patch area and the total area of S. alterniflora. Overall, we found that both breakwater complexes decreased S. alterniflora upland retreat relative to control sites. Mean S. alterniflora patch area and total S. alterniflora area did not differ between the two breakwater and the control sites, although the total area increased across all sites. These results demonstrate breakwaters can be effective at decreasing marsh retreat. However, marsh edge retreat still occurred in contrast to a previous study at this location, which found S. alterniflora seaward expansion behind the breakwaters. These results indicate that marsh retreat-expansion dynamics may change over time, demonstrating the need for regular, long-term monitoring to evaluate the efficacy of breakwaters and the importance of collecting data on additional environmental variables, which may provide a mechanistic understanding of marsh expansion-retreat dynamics.