Results

Pier Descriptions

In general, transects sampled under piers in 2006 (1.11 m average) were less tall (shorter) than transects sampled in 2021 (1.42 m average). Similarly, transects sampled in Jackson County in 2021 were more likely to be shorter than transects sampled in Harrison or Hancock County. Pier width was greater in Harrison County when compared to Jackson and Hancock Counties. This width distribution was consistent among years. Board spacing was considerably larger in 2006 (2.09 cm) versus 2021 (1.28 cm) across counties. The percentage of North-South oriented piers was equal to or greater than the percentage of East-West oriented piers. The 4-foot height above the marsh was the median value for all collected heights (Figure 6). Widths were designated as Narrow (<1.83 m) or Wide ( ≥ 1.83 m) represented by the median value (Figure 7).

Pier ownership (public vs. private) was mixed among Jackson County for both year groups. Jackson County piers sampled in this project were 40% public and 60% private in 2006, but 60% public and 40% private in 2021. In both years Harrison County piers were almost entirely publicly owned (100% in 2006 and 87% in 2021), while in Hancock County in 2021 they were mostly private (90%). The overwhelming majority of piers sampled in this project from Jackson and Harrison County in both years were non-compliant, failing to meet the construction codes for 2021. For Jackson County, 95% of piers were non-compliant in 2006, and 100% were non-compliant in 2021. For Harrison County, 100% were non-compliant in 2006, and 80% were non-compliant in 2021. Hancock County (2021), however, had an even mix of non-compliant (50%) vs compliant (50%) piers sampled.

Light Analysis

Percent shading under pier structures showed that height, width, and board spacing all had a significant effect. Further, marsh under the grate-decking had less shading (more light) than the board-decking; and narrow board spacing gaps of less than 1.27 cm (0.5 inches) created more shade than those with wider gaps. 

Taller pier heights resulted in greater mean daily irradiance percentage (MDIP) values, for instance 0.62 m (2’) tall piers ranged from 7-35% MDIP, while 1.86 m (6’) tall structures were more consistent between 55-65% MDIP (Figure 8). Furthermore, taller pier heights appear to mitigate the shading effects of narrower board spacing. Larger spacing gaps (3.8 cm) and grating are more effective at allowing light to pass through at shorter pier heights, but had less of a positive enhancement effect as height increased (Figure 8). Narrower pier widths resulted in higher MDIP values. Pier widths over 1.22m (4’) show that board spacing has apparent effects on irradiance values (Figure 9). A threshold seems to exist at the 1.22m (4’) width, where board spacing gaps (excluding grate piers) no longer have an effect on shading. Excluding grate piers, MDIP values can differ as much as 15% (6 and 8 foot widths) based on a range of board spacing gaps, while the MDIP differs less than 5% for the 1.22m (4’) height with the same spacing gaps (Figure 9).

Vegetation

Vegetation data was used to create dominant plant species rankings and diversity indices. Jackson County sites were dominated by S. alterniflorus, J. roemarianus, and Spartina patens. Harrison county was dominated by less saline-tolerant marsh plants such as Sagittaria lancifolia and Spartina cynosuroides, among others. Hancock County had high occurrence of high-salinity species such as S. alterniflorus and low salinity species such as those of the genus Schoenoplectus. Plant species diversity was lower underneath piers than in the general area around the pier. Harrison County had the highest vegetation species diversity across both years sampled. There was no clear delineation in the nMDS plots for pier dimensions or other factors tested, suggesting overall vegetation similarity accross sampling piers.

Pier Density

In Jackson County, there was an average count of 8.87 marsh piers and 10.27 total piers within the sampling area. Harrison County averaged 12.93 and 25.8 piers respectively. Hancock County averaged only 3.5 marsh piers, but 16 total piers. Across counties, there were average counts of 9.05 marsh piers and 17.53 total piers within the 1.61 km (1 mile) circular buffer area. There were differences in pier densities among counties, with Harrison County having significantly higher pier density than both Jackson and Hancock Counties. 

Using ESI shoreline lengths categorized as “sheltered vegetated low banks” (9b) and salt- and brackish-water marshes” (10a), the number of piers was estimated at 11,145 over 1,981 km of shoreline. Using the NOAA index for total shoreline length, the number of piers was estimated at 3,249 across 578 km. The estimated number of piers on NWI classified “Estuarine and Marine Wetlands” polygons totaled 591 piers over approximately 189 square kilometers within the sampling bounds.