Strategies for Successful Mangrove Living Shoreline Stabilizations in Shallow Water Subtropical Estuaries

Abstract

By combatting erosion and increasing habitat, mangrove living shorelines are an effective alternative to hard-armoring in tropical and subtropical areas. An experimental red mangrove living shoreline was deployed within Mosquito Lagoon, Florida, using a factorial design to test the impact of mangrove age, breakwater presence, and mangrove placement on mangrove survival within the first year of deployment. Mixed mangrove age treatments were included to identify if seedling (11-month-old) survival could be enhanced by the presence of transitional (23-month-old) and adult (35 to 47-month-old) mangroves. Environmental factors were monitored to detect possible causes of mangrove mortalities. Approximately half (50.6%) of mangroves died, and of those, 90.7% occurred within the annual high-water season, and 88.9% showed signs of flooding stress. Planting seedlings haphazardly among older mangroves did not attenuate enough wave energy to significantly increase seedling survival. Breakwaters alleviated stress through a reduction in water velocity and wave height, increasing the odds of survival by 197% and 437% when mangroves were planted in the landward and seaward rows, respectively. Compared to seedlings, deployment of adult mangroves increased survival odds by 1087%. Collectively, our results indicate that sites with a high-water season should utilize a breakwater structure and mangroves with a woody stem.

Quantitative Analysis of Methodological and Environmental Influences on Survival of Planted Mangroves in Restoration and Afforestation

“…Unfortunately, there was not sufficient data to evaluate changes in survival over time. Overall, this type of additional information will complement global-scale mangrove studies, providing open and transparent data to increase the update and impact of large-scale mangrove research and conservation [84].…”

Hydrodynamic Limitations to Mangrove Seedling Retention in Subtropical Estuaries

“…As L. racemose inhabit areas that are comparatively less geomorphologically active [33], this species was not tested. Active shoreline restoration is ongoing in both locations, incorporating living shoreline techniques of emergent halophytic grass and mangrove plantings to combat widespread shoreline erosion [17]. To test the Type I removal thresholds of mangrove seedlings, study sites were chosen from within mature reference-condition mangrove forests (2 forest patches in CANA, 3 forest patches in DSNM) where evidence of erosion (e.g., scarping and slumping, exposed roots of vegetation) was not observed.…”

“…Mangroves inhabit mechanically challenging environments characterized by complex biophysical feedbacks [15,16]. In addition to eco-physiological stressors of salinity, temperature [17] and water level fluctuation [16], fringe forests on channel margins and lagoon shorelines are exposed to varied hydrodynamic forcings that create a dynamic morphological environment. For instance, hydrodynamic forces may range from tidal and riverine currents to tropical-storm-force winds and waves [1,18].…”

“…Recruitment occurs when mangrove propagules end their free-floating dispersal phase by rooting into the substrate, becoming seedlings with single, flexible stems and few leaves [24]. While the early establishment phase is characterized by rapid root and stem growth [25], seedlings are particularly vulnerable to physical stressors, including inundation and dislodgement by hydrodynamic forces [17,26]. Dislodgement of vegetation by hydrodynamic force is classified according to the specific geomorphic mechanism of dislodgement [27]: Type I removal occurs instantaneously when the hydrodynamic drag force exceeds the plant anchoring force.…”

The Use of Non-Plastic Materials for Oyster Reef and Shoreline Restoration: Understanding What Is Needed and Where the Field Is Headed

“…Oyster habitat enhancement is often a component of living shoreline designs, where materials are deployed as breakwaters to protect plants along the water's edge also become living reefs following natural oyster recruitment [33]. One common approach involves creating breakwater structures out of Naltex™ Duronet ® aquaculture-grade polyethylene mesh netting and filling bags with 20-30 pounds of recycled oyster shell [34]. The bags are laid perpendicular to the shore so they do not roll or dislodge when met with wave energy.…”