Discussion

Along the shores of the Gulf of Mexico erosion rates have begun to steadily increase in the past few decades. This has caused shorelines’ shape and slope to change as environmental pressures have increased. For example, shoreline slopes can be seen increasing dramatically due to natural marsh edge erosion resulting in cut banks instead of a more natural gradual shoreline slope. This can cause shoreline instability, with uprooting of marsh vegetation and thereby a further increase in erosion from this undercutting and the resulting lack of a natural vegetation buffer. This is a concern to not only natural systems, but also to human communities that live in coastal zones and who depend on the coast for income. 

Therefore, this study looked at natural, living, and hardened shorelines from two different energy groups to see how the hydrographic, geomorphic, and vegetative processes affect them. Predominantly high vs. predominantly low wave energy exerted on a shoreline influenced all the other variables studied. Site played a role in all the variables, except for species richness and the percent of dominant marsh species present. Shoreline type (NS, LS, HS) affected the erosion rate, slope, sediment variables, and the percent cover, percent of dominant marsh species present.

The average wave power shows the amount of impact a shoreline is receiving, but use of this method would depend on field sampling abilities and access to equipment. A suitable alternative to using the average wave power would be calculating relative exposure from fetch distance using Google Earth Pro. When using the relative exposure, it is important to be aware that currents and boat traffic do play a role in the energy impact the shoreline receives. The relative exposure was calculated using the method of La Peyre et al. (2014) to explain the wave power and turbidity found at each site based on fetch distance, wind speed, and wind direction. Sites with a maximum fetch distance less than 804.67 m (0.5 mi) were considered low energy and less likely to require human intervention, like construction of a living shoreline to protect against further erosion and shoreline retreat. 

Shorelines with steeper slopes tend to reflect the impacts of higher wave energy and have larger grain-size sediments and higher edge erosion rates. Steep slopes make it more difficult for vegetation to grow and the implementation of a breakwater structure like in a living shoreline may help facilitate conditions suitable for vegetation expansion. Sediment grain size can be affected by the energy that impacts the shoreline, which in turn can affect the ability of vegetation to thrive. Bulk density is important for the ability of plant roots to grow and expand into the sediments and tends to reflect the percent of sand in the sediment. The organic matter content in the sediment that is available for plants is also reflective of the silt/clay content because the pore space available allows for organisms to break down materials.

Vegetation can protect a shoreline from erosion, filter runoff, and provides both food and habitat for different organisms. Species richness was found to only be affected by the energy groups, with the low energy sites having higher richness. The NS and LS were found to have a higher total percent cover than the HS. Total percent cover was also found to be higher at the low energy sites. 

Low energy sites amongst the three different shoreline types had similar spatial patterns in response to vegetation qualities, energy, and shape of the shoreline. Based on the conceptual model (below) these results show that the LS and NS are indicative of shallow slopes with higher vegetation diversity and coverage. These shoreline conditions also tend to support higher organic matter and finer sediment composition. The presence of vegetation and the high sediment organic matter are part of what make a natural marsh a key habitat to many species. Based on the results in this study, the LS provide similar food and protective habitat to nearby NS, as had been found in other studies. 

The erosion rate of the coastline and its geographic shape were mostly affected by the wave energy groups. As shown in the conceptual model, the energy exerted on the shorelines affected the sediment, morphology and vegetation found at the different shoreline types. Shorelines that received high energy had fewer dominant marsh species and this is probably because they tended to have steeper slopes, meaning the vegetation present is not required to be salt or inundation tolerant.

A conceptual model representing the results found with the PCA. The ellipses are from the PCA and show the different types of shorelines: natural (blue), living (yellow), and hardened (red). Quadrant A represents high energy hitting a hardened structure with sandy sediment at the base and has no native vegetation. Quadrant B represents high energy with the less sand but features a steeper slope with native vegetation. Quadrant C is a low energy shoreline but with a steep slope, moderate sand content and less native vegetation. Quadrant D represents a low energy shoreline, with mostly silt/clay sediments but little sand, and lots of native vegetation.

According to the conceptual model, a living shoreline will do best at a site that is receiving high or low energy impacts as long as it has a gradual slope. The gradual slope does not have to be natural; many LS involve the creation of a more gradual slope. This conceptual model can be used to help predict where the implementation of a LS will help retain the shoreline and ecosystem at similar sites within the northern Gulf of Mexico.