Proceedings of the 2022 Mississippi Water Resources Conference

Year: 2022 Authors: Turnage G., Sample A.

Barrier island lagoons may act as refugia for seagrass species; many of which are vital components of marine ecosystems in the Gulf of Mexico and the Mississippi Sound. Seagrass beds serve as feeding and spawning habitat for many species of marine fauna (i.e., fish and invertebrates) and reduction of seagrass beds can have a cascading effect on ecosystem processes (e.g., fish feeding) that can affect economic opportunities by reducing commercially and recreationally important fish species (e.g. speckled trout) that utilize this habitat. Unfortunately, lagoons may also be invaded by aquatic invasive plant species AIS capable of surviving brackish water environments (i.e., torpedograss, common reed, and Eurasian watermilfoil) that can form dense plant beds that shade out and kill native plant species. Loss of ecosystem function has been documented in many systems that have AIS present, including brackish and saline habitats, suggesting that AIS invasion could further reduce seagrass abundance in island lagoons such that these sites no longer function as nursery populations that replenish seagrass beds in the Mississippi Sound. The goal of this project was to survey the aquatic and marine vegetation in select lagoons on the four major barrier islands (Petit Bois, Horn, Ship, and Cat) of Mississippi to 1) determine the aquatic/marine plant community of each lagoon and 2) determine if AIS were present in these systems. Islands were surveyed using the point-intercept method in the fall of 2020 under research permit #GUIS 00283. Twelve lagoons over 0.4 ha (1 ac) in size were identified from historical satellite imagery. Six reference sites in the Mississippi Sound were identified adjacent each island and also surveyed to generate baseline density of seagrasses, against which seagrass densities from individual lagoons were statistically compared. All lagoons had vegetation present with Shoal grass (native) and torpedograss (AIS) being the most prevalent species. Seagrass was recorded in 58% of lagoons with 70% of those populations potentially serving as nursery populations for offshore sites adjacent to each island and approximately 40% serving as nursery sites across the Mississippi Sound. However, AIS were present in 66% of lagoons and co-occurred with seagrass populations in 25% of lagoons. To our knowledge, this is the first work to document AIS co-occurring with seagrasses on the Mississippi Barrier islands. Future reduction of AIS in lagoons may allow for subsequent colonization by seagrasses which could further increase the abundance of nursery seagrass populations in the Mississippi Sound.