Results

Hydrographic features

Wave gauge data was used to determine the energy exposure from the average wave power resulting in the high and low energy groupings. Average wave power was significantly different among the six sites. The average wave power indicated two significantly different groups: (1) the high energy sites at HC, ST, AL, and the low energy sites at CW, OS, GB. The average wave power for the high energy sites was over five times greater than the average at the low energy sites.

Turbidity was significantly higher in the winter than the summer. The turbidity data was influenced by both site specific and energy group variables. Turbidity may also be influenced by relative exposure and wave power, as well as sediment grain size at the site, with finer-grained sediments resulting in more frequent and intense turbidity.

Geomorphic features

The relative exposure at each of the six sites was influenced by their orientation to the dominant seasonal wind direction. Relative exposure, therefore, differed by fetch distance and dominant wind direction, with sites exposed to the dominant seasonal winds across a long fetch experiencing the highest relative exposure. There were significant differences due to site and energy groups, however, there was no significant effect of shoreline type or season on relative exposure, despite the different shoreline orientations and seasonal wind directions.

Erosion rates and Slope profiles

Shorelines with high relative exposure also experienced more rapid erosion rates. The average annual erosion rate at that the shoreline erosion at NS (M = 0.70 m/yr, SE = 0.13 m/yr) averaged over the six sites had the highest mean annual erosion rate. In comparison the LS (M = 0.25 m/yr, SE = 0.06 m/yr) had intermediate mean annual erosion rates, and the HS (M = -0.02 m/yr, SE = 0.01 m/yr) had the lowest annual rate of erosion. The high rate of shoreline retreat at the various NS indicates retrograding facies are common in the study region and suggests marsh edge erosion is a frequent problem. Erosion rates of natural shorelines may be influenced by relative exposure and wave power.

The mean slope at the three shoreline types were also significantly different. The HS (M = 24.99 cm, SE = 2.01 cm) had a significantly steeper slope than either the NS (M = 12.84 cm, SE = 1.43 cm) or LS (M = 10.62 cm, SE = 1.37 cm). Shoreline slope is often affected by manmade structures especially in the HS and in some LS sites. Shoreline slope and mean erosion rates may in turn affect sediment composition, turbidity, and vegetation found at the site.

Sediment Characteristics

Bulk density (BD) and organic matter were significantly different among the three shoreline types. The NS had the most porous sediments followed by the LS. The HS had the least porous sediments, which can stunt root growth for vegetation. This shows that the shoreline type plays a role in the BD, a proxy for porosity, of the intertidal sediments. Bulk density may be influenced by the type of shoreline and the amount of energy that shoreline receives. In the higher energy sites, the NS and LS act the same as the NS at the low energy sites, having more porous sediment and allowing water and roots to migrate within it, so it may be expected for these shorelines to have finer sediment particles.

Low organic matter (OM) at HS indicates that there is little organic content in the sediment, while LS had over five times as much organic matter and NS had over six times as much organic matter as the HS in this study. This indicates that the OM content of the sediment may be positively influenced by having marsh vegetation present (NS and LS). A scatterplot shows that BD and OM have an inverse relationship with BD increasing as OM declines. The HS had higher BD and lower OM, the LS had the most variability, with ST having the highest OM, and the NS were clustered with lower BD and higher OM.

Sediment depth in the top 30 cm significantly affected grain size composition, with the percent sand declining and silt/clay increasing with depth; percent pebbles and coarse sand was low (<10%) and not influenced by depth. These results suggest sand grains were more abundant in the upper portion of the sediment cores, potentially because of wave energy winnowing the finer silt and clay particles, which then tend to accumulate at deeper depths (>10cm) or are transported to lower energy conditions in deeper water depths offshore. Similar trends were observed with higher sediment OM found in the deeper core depths. Percent sand was the dominant sediment grain size in both HS and LS except at OS, which had slightly more silt than sand. The percent sand exhibited an inverse relationship to the percent silt and clay in both energy groups, except for the HS, with the low energy sites having a higher percent of silt and clay than the LS. 

Sediment grain size data indicated that the sand vs. slit/clay fractions were the major change observed in the sediment composition across all sites and shoreline types. There was an inverse relationship between these two grain size classes, with higher sand content resulting in less silt/clay and vice versa. The sand fraction was higher in the shallow (<10cm) portion of the sediment core when averaged across all six sites. The sand fraction was greater in the HS and reduced at the LS and NS, with the NS shoreline type on average having the largest silt/clay fraction. Finally, high energy sites tended to have more sandy sediments than low energy sites.

Vegetation abundance

A total of 39 plant species were found within the 180 quadrats. Vascular plant species richness was not significantly different among the six sites or among the shorelines. Species richness was significantly different between the two energy groups. The species richness for the low energy sites was ~50 % higher than the high energy sites. Species richness may be influenced by the energy groups, with low energy tending to increase the number of species found.

Of the 39 species found, nine of them are considered dominant marsh species in Mississippi by Eleuterius (1972). There was a significant difference between the shoreline types of the percent dominant marsh species present. The NS had the highest percent of dominant marsh species (M = 72.83%, SE = 9.06%), which was not significantly different from the LS (M = 55.77%, SE = 14.07%), but both were significantly different from the HS.

Vegetation percent cover was significantly different among the three shorelines. The NS (M = 67.37%, SE = 2.93%) had the highest average percent cover and was similar to the LS (M = 65.01%, SE = 3.27%), but the HS (M = 46.07%, SE = 5.20%) was significantly lower. The low energy sites (M = 66.76%, SE = 2.46%) had a higher percent cover of vegetation than the high energy sites (M = 52.21%, SE = 3.91%). These results show that both site and energy may affect the percent cover of vegetation.

A Bray Curtis dissimilarity matrix was plotted using NMDS (k = 2, stress = 0.079) to represent the relationship of the different species of vegetation between the different sites and shoreline types.

Multivariate Analyses

To visualize data interactions among the different factors (hydrographic, geomorphic, and vegetation) an NMDS and a PCA were conducted. Hydrographic features that were included in both the MDS and the PCA were the average wave power and turbidity. The geomorphic features included were relative exposure, average erosion rate, average slope, percent of sand, and organic matter. The vegetative features included were species richness, percent of dominant species, and percent cover. The MDS and the PCA show a similar pattern in the dataset. The axis NMDS1 shows a greater separation for type of shoreline, while the axis NMDS2 shows separation by the different energy groups.

Approximately 75% of the estimated variance is explained by the first three axes of the PCA, the first axis (PC1) explains 35.9% and the second axis (PC2) explains 31.0%, and the third axis (PC3) explains 8.70% of the estimated variance in the total dataset. The PCA plots show that the LS and NS are similar while they have a strong separation from the HS. There is strong separation amongst the high energy sites, while the low energy sites are more clumped together. PC1 represents the percent of dominant vegetative species percent cover of vegetation, and the average slope of the shoreline. This axis shows a pattern of low energy groups’ LS and NS to high energy groups’ HS. PC2 represents the turbidity, erosion rate, relative exposure, and average wave power. This axis mostly represents energy and shape of the shorelines, it shows a gradient between high and low energy groups as well as from HS to LS to NS. The PCA results help to analyze the main driving features in the data collected from the six study sites. Areas with high turbidity, erosion rates, wave power and relative exposure have steeper slopes and a higher percent of sand, but lower percent cover and percent of dominant vegetative species.