Discussion

Pier Characteristics

We were able to assess what a typical pier for MS would look like. Based on all transects, average height was 1.26 m (4.13ft), average width was 1.87 m (6.14 ft), and average board spacing was 1.54 cm (0.61 in). Likely as a result of simple geography and a predominantly East-West oriented shoreline on the MS Gulf Coast, an average pier will most likely be oriented North-South. Approximately 73% of all piers sampled were oriented North-South.

After inquiry about records of pier permits from the state Department of Marine Resources, data was provided from permits issued from years 2002-2022. Across counties over a considerable time period, the permitting data shows pier widths remain consistent, averaging approximately 1.76 m (5.77 ft). This value is very close to the 1.87 m (6.14 ft) average across years and counties that we found from field sampling.

Light Factors

Both height and width were shown to have significant effects on the amount of light able to reach the marsh substrate. Lower pier heights and greater deck widths were shown to create more shaded environments underneath those piers. Both board spacing and decking material were also found to impact light threshold heights and widths for dominant marsh plant species. 

The gap spacing between decking boards as a factor affecting shading was shown to have an impact in certain circumstances. Under piers of lower heights and greater widths, the board spacing is more likely to have an impact on irradiance underneath than is the case for taller or narrower piers. In contrast, at piers with greater heights and lesser widths, board spacing did not play a major role in irradiance reductions. Grate decking was shown to more positively affect light passage when compared to board decking irrespective of board spacing.

Aside from direct shading impacts, tall and/or wide piers can have greater environmental impact footprints. Not only do they require extra material, but they also require heavier equipment to construct. Heavy equipment operation in a saltmarsh can be detrimental to its integrity. The plants are often laid over, and the root structure is destroyed when run over repeatedly by construction equipment.

No clear conclusions can be drawn from the analysis conducted about pier orientation’s effects on irradiance underneath the pier. The study design used here was not optimized for detecting orientation affects. There was not a large sample size of transects available for E-W oriented piers as almost 3/4 of the sampled piers are oriented in a N-S direction. 

Plant Success

Underneath most piers the vegetation is not only less diverse, but is also generally more sparse. This may be a result of multiple different effects of piers. Shading may affect the diversity by way of simply reducing abundance, therefore reducing the likelihood of a particular species being present. Another very likely way shading can affect diversity is shade-tolerance of different plant species. Shade-intolerant plants would be less likely to succeed underneath a pier. Another observed effect is the elevation difference underneath and around the piers. Lower elevations in the salt marsh are more often inundated, excluding many of the high marsh species. The most obvious explanation for the elevation difference is the heavy equipment use left after construction. Another construction technique that can lower elevation around a pier is the pile-driving process. This can destabilize the sediment and promote erosion. A less obvious explanation for the elevation difference is the loss of vegetation underneath the pier due to shading, and the subsequent erosion that can occur underneath the pier.

Scope of Potential Impacts

The attempts to estimate how many piers occur on the Mississippi Gulf Coast and have the potential to impact salt marshes through shading did not provide as clear of an answer. Using aerial/satellite photography to count piers presented two main issues. First was the difficulty of determining piers that spanned only over marsh habitat. The second issue was determining what spatial unit and associated wetland data to use when trying to account for pier density. Both shoreline length and marsh area estimates were used in these attempts.

Using the ESI shoreline length of habitat types 9b and 10a to extrapolate pier counts almost certainly overestimated the amount of marsh piers (Marsh pier estimate = 11,145). The pier estimate from using the NOAA shoreline value may have also overestimated marsh piers (Marsh pier estimate = 3,249). Using the pier density by area technique and NWI wetland data may have been closest to reality (Marsh pier estimate = 591). The proportion of marsh piers to total piers multiplied by the total number of permits issued by the MDMR provided another estimate of total marsh piers. Using this method, the total marsh pier estimate for all counties was 2,647 piers.

Management Implications

Current pier permit regulations are under the jurisdiction of the Mobile District of USACE. In coastal Mississippi, permits are submitted to- and enforced by MDMR. The width maximum started at 4 feet in 2007, then increased to 5 feet in 2013, and increased again in 2018 to 6 feet. The minimum height has remained consistent at greater than or equal to the width of the respective pier. The board spacing minimum was set at 0.5 inches in 2007, but that stipulation was not made in subsequent years. Maximum length started at the lesser distance between 25% of the water body or 300 feet in 2007. Currently, the maximum length is the lesser between 25% of the water body or 1,000 feet. There was no requirement for any year regarding pier orientation. The dimension requirements for newly constructed pier structures over salt marsh habitat have become more relaxed over time. Based on piers sampled combined with height and permitting requirements in effect right now, there may be ineffective enforcement. Even considering the amount of public piers sampled, the percentage of sampled piers that are non-compliant is alarming. This is an issue that may not be isolated to the Mississippi Gulf Coast, as Shafer et al. (2008) found similar non-compliance issues in Florida and Puerto Rico.

Conclusions

It is apparent that pier shading can have a negative impact on salt marsh plants. The most important factors considered here were height and decking material. Taller pier heights allowed more ambient light to reach the substrate during mid-day and direct light to reach the substrate for a greater amount of the daylight hours. Grate decking material allowed for greater amounts of light to pass through the deck to the substrate when compared to traditional board decking. Other factors such as width and board spacing were shown to have significant impacts, but were likely influenced by pier height. Narrower widths and greater board spacing gaps promoted higher irradiance values beneath the piers. Differences found amongst decking material were particularly important, as they suggested grate decking may have an increased effect at the lower latitudes of the Mississippi Gulf Coast versus the higher latitudes where much of previous research has been conducted.

Although the estimates of number of piers over saltmarsh made here vary widely, it is suspected that the lower estimates probably more accurately represent reality. It is estimated that the potential salt marsh area lost as a result of pier shading in South Carolina totaled approximately 0.1% or less of the state-wide saltmarsh area. As large areas of Mississippi’s saltmarsh habitat mostly lack pier development, it is likely that the total area affected by pier shading is very small as well. Coastal waterfront properties, particularly those with easy access to the water, demand a premium price relative to those without water access. For a state with below average incomes (U.S. Census Bureau 2021) and limited appeal, coastal Mississippi’s blue-economies rely on attractive waterfront properties to bring wealth in. Aside from economic values, piers provide important interaction services with the estuarine ecosystem.