Methods

Study Sites and Pier Descriptions

Piers were visited in all three counties along the Mississippi (MS) coast in 2021 and data was compared to prior sampling. For the round of sampling conducted previously in the summer of 2006, 30 total sites were visited. Two thirds of the original pier sites were located in Jackson County with a lesser amount in Harrison County (Figure 3). Of the 20 sites visited in 2006 in Jackson County, only 10 remained in fair condition in 2021. All 10 of the previous sites in Harrison County were still intact or rebuilt. To expand on the geographic range and sample size, additional pier sites were selected with 15 in Jackson County, 15 in Harrison County, and 10 in Hancock County for a total of 40 sites for the 2021 sampling (Figure 4). 

A suite of attributes was recorded for each pier visited in summer 2021, as well as for the surrounding marsh. The total length of the pier if accessible was measured. The width at the section of pier being studied was measured. The construction material of the decking on the pier was classified as either “wood” or “composite”. Compliancy with current pier dimension regulations was determined based on USACE regulations for private piers and docks in coastal MS at the time of 2021 sampling.

Light Transects

To identify shading transect locations at a pier site, heights from the substrate to the decking of the pier were identified in 1-foot increments and were both within the marsh zone and accessible to sample. Light as photosynthetically available radiation (PAR) was measured using paired LI-COR meters and sensors with cross-calibration by obtaining a simultaneous reading when they were side-by-side on top of the pier. Readings were recorded simultaneously from the top of the pier and at each respective transect sampling point under the pier, so that there was a “paired” incident light measurement for every shading measurement below the pier deck. Light data were collected between approximately 10:00 A.M. and 3:00 P.M. Central Daylight Time, as periods outside of this range have solar zenith angles that can contribute to low light intensities.

With shading percent values for each point, profiles delineating the endpoints and centerline of shading from the overlying pier were determined for each point along each transect (Figure 5). The first endpoint of the shading profile was identified at the point where the irradiance percent value fell below 75%, and the second endpoint was identified at the last point before the shading percent value returned to 75% or higher. The center line was determined based on the midpoint (or average of midpoints with less than 5% irradiance) between these two endpoints.

Marsh Description

To describe the marsh habitat surrounding each pier, first the length of pier that crossed directly over marsh habitat was measured and recorded. The marsh vegetation community for each site was then assessed by describing each marsh species present within approximately 15 meters from the edges of the pier. The vegetation community present directly underneath the pier was also described by recording each species present. To assess marsh plant presence, plant species were recorded along the transect using a point-intersect method. If more than one species was present in this area, each species was recorded. For areas without any species present, the point was recorded as “bare”. Non-metric multidimensional scaling (nMDS) was conducted to determine if the plant community both underneath and in the general area around the piers were significantly different based on factors sampled in this study.

Estimating Pier Density

Using Geographic Information Systems (GIS) software and images from Google Earth, the number of piers along the MS coast potentially affecting salt marsh was analyzed. Satellite imagery from Google Earth was used to count all pier structures similar in appearance to pier sites that were manually identified. Using the inverse distance weighting (IDW) function in ArcMap, an estimated pier density map was created for the bounds of the sampling area in the three coastal counties. Densities were applied to wetland inventories to estimate total number of structures in the coastal counties.

The National Oceanographic and Atmospheric Administration (NOAA) Environmental Sensitivity Index (ESI) shoreline data in ESRI ArcMap, a pier density estimate based on shoreline length was produced. Only ESI shorelines categorized as “sheltered vegetated low banks” (9b) and salt- and brackish-water marshes” (10a) were selected to calculate the total marsh shoreline in MS. Second, the United States Fisheries and Wildlife Service (USFWS) National Wetland Inventory (NWI) data. From the attribute table, the estimated number of piers for each intersected “Estuarine and Marine Wetlands” polygon was calculated.