Mangrove expansion in the Gulf of Mexico with climate change: Implications for wetland health and resistance to rising sea levels

Abstract

Black mangroves ( Avicennia spp.) are hypothesized to expand their latitudinal range with global climate change in the 21st century, induced by a reduction in the frequency and severity of coastal freezes, which are known to limit mangrove colony extent and individual tree size. The Gulf of Mexico is a prime candidate for population expansion to occur because it is located at the northward limit of black mangrove habitat. This may come at the expense of existing coastal saline wetlands that are dominantly Spartina spp. marsh grasses. The present study was conducted to focus on the implications of a marsh to mangrove transition in Gulf wetlands, specifically: (1) wetland resistance to accelerating eustatic sea level rise (ESLR) rates; (2) resistance to wave attack in large storms (increased cyclonic storm frequency/intensity is predicted with future climate warming); and (3) organic carbon sequestration and wetland soil geochemistry. Field sites of adjacent and inter-grown Avicennia germinans mangrove and Spartina marsh populations in similar geomorphological setting were selected in back-barrier areas near Port Aransas and Galveston, TX. Elevation surveys in the more mature Port Aransas site indicate mangrove vegetated areas are 4 cm higher in elevation than surrounding marsh on an average regional scale, and 1–2 cm higher at the individual mangrove scale. 210Pb and 137Cs accumulation rates and loss on ignition data indicate that mineral trapping is 4.1 times higher and sediment organics are 1.7 times lower in mangroves at Port Aransas. This additional mineral trapping does not differ in grain size character from marsh accumulation. Elevation change may also be effected by soil displacement of higher root volumes in mangrove cores. Port Aransas porosities are lower in mangrove rooted horizons, with a corresponding increase in sediment strength, suggesting mangrove intervals are more resistant to wave-induced erosion during storm events. Port Aransas mangroves exhibit higher pore water redox potentials and salinities over entire core depths and depressed pH over rooted intervals, suggesting a distinct diagenetic environment exists relative to marsh sites. The rooting network, which introduces oxygen into the sediment and focuses evapo-transpiration and salt exclusion within this zone, may prove advantageous when competing with grasses by elevating salinities to levels that are toxic for Spartina. Trends observed in the more mature systems of Port Aransas are generally absent in Galveston, suggesting the youth and physically shorter stature of these systems means they have not yet established a unique sediment signature.