Do small-scale saltmarsh planting living shoreline projects enhance coastal functionality? A case study in the Northern Gulf of Mexico.

Human coastal occupation often leads to the degradation of the structural properties and environmental functions of natural coastlines. . Much research has been done on the cost-effectiveness of various living shorelines designs, however more work is needed for simple, small-scale designs that are typically adopted in waterfront residential or recreational properties. To contribute to this gap, we planted small-scale plots of black needlerush (Juncus roemerianus) in two sites, one in a residential property and another one in a recreational property in the Northern Gulf of Mexico that experienced significant wave energy. Plots were planted at two different densities (50% or 100% initial cover) or left unplanted (controls) and, along with monitoring the evolution of the planted salt marsh, we measured a number of functional metrics including soil slope, abundance of nekton within and in front of the plots, and cover of submerged aquatic vegetation (SAV) in front of the plots monthly over two years. In one of the sites plant cover decreased precipitously, and in the other site we did not observe any significant changes in plant cover over time (i.e. the initial 50% and 100% plantings remained at that level throughout the experiment) despite protecting the planted salt marsh with coir logs. We did not find any changes in soil slope or nekton abundance between planted and control plots. SAV growth was restrained in front of planted plots in relation to control plots, possibly due to deleterious impacts by the coir logs. Overall, the results suggest the protection against wave energy attained in this experiment is insufficient for adequate saltmarsh establishment and growth, thereby encountering decreasing or stationary plant density and no significant differences in soil slope or nekton abundance between planted and non-planted plots. Our results indicate the adoption of small-scale saltmarsh planting to reduce erosion and enhance coastal functionality needs to ensure that wave energy is sufficiently dampened for adequate saltmarsh growth and, concomitantly, the conceived saltmarsh protection mechanism does not negatively impact adjacent SAV.

Large-scale variation in wave attenuation of oyster reef living shorelines and the influence of inundation duration.

One of the paramount goals of oyster reef living shorelines is to achieve sustained and adaptive coastal protection, which requires meeting ecological (i.e., develop a self-sustaining oyster population) and engineering (i.e., provide coastal defense) targets. In a large-scale comparison along the Atlantic and Gulf coasts of the United States, the efficacy of various designs of oyster reef living shorelines at providing wave attenuation was evaluated accounting for the ecological limitations of oysters with regard to inundation duration. A critical threshold for intertidal oyster reef establishment is 50% inundation duration. Living shorelines that spent less than one-half of the time (<50%) inundated were not considered suitable habitat for oysters, however, were effective at wave attenuation (68% reduction in wave height). Reefs that experienced >50% inundation were considered suitable habitat for oysters, but wave attenuation was similar to controls (no reef; ~5% reduction in wave height). Many of the oyster reef living shoreline approaches therefore failed to optimize the ecological and engineering goals. In both inundation regimes, wave transmission decreased with an increasing freeboard (difference between reef crest elevation and water level), supporting its importance in the wave attenuation capacity of oyster reef living shorelines. However, given that the reef crest elevation (and thus freeboard) should be determined by the inundation duration requirements of oysters, research needs to be refocused on understanding the implications of other reef parameters (e.g., width) for optimizing wave attenuation. A broader understanding of the reef characteristics and seascape contexts that result in effective coastal defense by oyster reefs is needed to inform appropriate design and implementation of oyster-based living shorelines globally.