Relating seagrass habitat to geomorphology and substrate characteristics around Ship Island, MS

Abstract

Coastal vegetation, including marsh and submerged aquatic vegetation (SAV), is threatened in many places across the United States and globally. Human activity, sediment starvation, and subsidence, among other factors, put these ecosystems at risk (Kirwan and Megonigal 2013; Orth et al. 2006). One type of SAV, seagrass, is of particular ecological importance, providing food and critical habitat for numerous coastal species. However, seagrass is currently in decline globally due to the aforementioned factors (Eleuterius 1987). Identifying the necessary conditions to promote seagrass recovery and growth is highlighted as one of the "important yet essentially unanswered questions" globally in the U.S. Geological Survey's (USGS) report on seagrass coverage in the Mississippi Sound from 1940-2002 (Moncreiff 2007). In sandy barrier island systems, where seagrass habitat has been related to barrier island stability and length, there is a distinct need to quantify in detail the conditions that will promote seagrass recovery and growth (Carter et al. 2011; Eleuterius 1987; Pham et al. 2014). This report introduces a new methodology for quantifying the relationship between seagrass habitat and its physical environment, including substrate and geomorphological characteristics. BACKGROUND: Studies relating seagrass growth to physical conditions tend to focus on finegrained estuarine environments, where wave exposure and light are identified as primary limiting factors for growth (Boer 2007; Koch 2001; Livingston et al. 1998). Important seagrass ecosystems also occur in sandy, geomorphologically dynamic barrier island systems, where controlling factors such as wave energy and rapid deposition or erosion can be much more complex (Pham et al. 2014). Seagrass Ecosystems. Barrier island systems are ecologically unique. They form the boundary between high-energy marine environments and low-energy back-barrier marshes, bays, or sounds. Barrier islands are composed of unconsolidated sediments, and are therefore dynamic. Storms, sea-level rise, and sediment supply are the primary controlling factors of their formation,

Environmental factors affecting coastal and estuarine submerged aquatic vegetation (SAV)

“…Slow and limited deposition and erosion: Established SAV beds can tolerate deposition and erosion that occurs at an average rate slow enough for the SAV to adjust to over time, but SAV can be buried and killed by sedimentation caused by shifting coastal geomorphology or rapid deposition caused by storm events (Short and Neckles 1999;Short et al 2007). SAV along the back-barrier platform of Ship Island, Mississippi, has been observed to tolerate ~5 cm/yr of sand deposition, but SAV was unable to establish in zones of frequent overwash that experience higher average depositional rates (Eisemann et al 2019). SAV slowly repopulates with different species along the Chandeleur Islands, Louisiana, changing SAV community dynamics and potentially reducing diversity (Franze 2002;Poirrier and Handley 2007).…”

“…LiDAR elevations can reveal static characteristics like the location and elevation of islands and shoals as well as how these features change (Anderson, Carter, and Funderburk 2016;Eisemann et al 2018b). This type of information, coupled with information about local SAV growth, can reveal how these factors are related (Eisemann et al 2019).…”