Master’s Defense

Mangroves and salt marshes are highly productive coastal wetland ecosystems known for their disproportionately high rates of carbon sequestration given the small global surface area they occupy. Research along the Texas coast has shown that encroachment of the black mangrove (Avicennia germinans) into historical salt marsh vegetation benefits the quantity and stability of the soil organic carbon pool by increasing sedimentation rates and the supply of recalcitrant organic carbon. However, in February 2021, winter storm Uri hit Texas, dropping temperatures along the Gulf Coast below –7°C, killing many mangroves which have been slow to recover post-freeze. To assess how mass mangrove mortality impacts wetland soil carbon stability, in February and March of 2024, 30 cm deep soil samples were taken under patches of live mangrove, dead mangrove, smooth cordgrass (Spartina alterniflora), and mixtures of S. alterniflora and dead mangroves at three islands (Salt Island, Harbor Island, and Mud Island) in the Port Aransas area. Each core was analyzed for elemental organic carbon and nitrogen concentrations, δ¹³C, δ¹⁵N, mineral grain size composition, n-alkane and total hydrolyzable amino acid concentration and composition, and ²¹⁰Pb dating. Results from this study showed no signs of a significant loss in soil carbon stock between dead and living mangroves scaled the 1,215-ha area of mangrove wetland impacted by the freeze. However, the carbon and grain size composition did indicate initial signs of carbon degradation and removal from the soil, particularly in surface 10 cm. Major carbon losses will not be seen for approximately two decades according to the 19.23-year residence time of the soil organic carbon pool. Unless S. alterniflora were to reestablish in dead mangrove area, which has shown to drastically decrease soil carbon stores compared to live or dead mangroves. This study shows that mangrove soil organic carbon is relatively stable and would require harsh freezes frequent enough to prevent mangrove regrowth for 20 years for remove almost all mangrove-derived organic carbon from the soil.