The effects of breakwaters on salt marsh nitrogen cycling in Mobile Bay, AL

Coastal land managers increasingly utilize wetland restoration practices to decrease shoreline erosion, support biodiversity, and enhance resilience of coastal communities. In these restoration efforts, practitioners often incorporate engineered structures such as breakwaters, hardened structures placed parallel to shoreline, to reduce wave height and improve shoreline protection. While breakwaters alter shoreline hydrodynamic conditions, it is unclear how these changes impact ecosystem functioning such as nitrogen cycling. Here, we evaluated how breakwaters affected salt marsh physicochemical conditions and potential nitrate (NO 3 - ) reduction rates (i.e., denitrification and dissimilatory nitrate reduction to ammonium (DNRA)) along a shoreline in Mobile Bay, AL. We sampled nine sites, three controls without breakwaters, three with breakwaters established in 2012 (old), and three with breakwaters established in 2017 (new), to assess breakwater age effects on NO 3 - reduction rates. Within each site, we collected sediments from two locations—vegetated marshes and mudflats— to assess plant-mediated feedbacks of breakwaters on NO 3 - reduction rates. Across sites, we did not detect differences in NO 3 - reduction rates or most physicochemical conditions. Within sites, we found differences in physicochemical conditions, but we did not observe differences in NO 3 - reduction rates between vegetated marshes and mudflats. The vegetated marshes tended to have over 60% finer sediment relative to mudflats regardless of the presence of a breakwater, which may indicate that marshes are better at slowing incoming water velocity than breakwaters alone. This study adds to a growing body of research on how engineered structures like breakwaters may impact wetlands and their ecosystem functions.