Quantifying Ecological Shift and the Fate of Blue Carbon in Coastal Wetlands of the Western Gulf of Mexico

Patrick Louchouarn, Texas A & M University @ Galveston, loup@tamug.edu (Presenter)
Anna R Armitage, Texas A & M University @ Galveston, armitaga@tamug.edu
Wes Highfield, Texas A & M University @ Galveston, highfiew@tamug.edu
Matthew Norwood, Texas A & M University, matthew2005@tamu.edu
Sam Brody, Texas A & M University @ Galveston, brodys@tamug.edu
Karl Kaiser, Texas A & M University @ Galveston, kaiserk@tamug.edu
Amanda Sterne, Texas A & M University @ Galveston, fay155@tamu.edu

Landscape-level shifts in plant species distribution and abundance can fundamentally change the structure and services of an ecosystem. Such shifts are occurring within mangrove-marsh ecotones of the U.S., where over the last few decades, relatively mild winters have led to mangrove expansion into areas previously occupied by salt marsh plants. On the Texas (USA) coast of the western Gulf of Mexico, most cases of mangrove expansion have been documented within specific bays or watersheds. Based on this body of relatively small-scale work and broader global patterns of mangrove expansion, we hypothesized that there has been a recent regional-level displacement of salt marshes by mangroves affecting the carbon sequestration balance of the region. Here we present the synthesis of 3 years of multidisciplinary work performed by an ecosystem ecologist, a geospatial analyst, and urban planner, and an environmental geochemist to quantify ecosystem shifts at the regional scale and transcribe these shifts into carbon sequestration mass balances. We classified Landsat-5 Thematic Mapper images using artificial neural networks to quantify shifts in areal coverage of black mangrove (Avicennia germinans) and salt marsh (Spartina alterniflora and other grass and forb species) over 20 years across the Texas Gulf coast region. Between 1990 and 2010, mangrove area grew by 16 km^2, a 74% increase. Concurrently, salt marsh area decreased by 78 km^2, a 24% net loss. Only 6% of that loss was attributable to mangrove expansion; most salt marsh was lost due to conversion to tidal flats or water, likely a result of relative sea level rise. On the Texas Coast, above and below ground carbon loads are statistically similar for the two ecotones. Only 46% of the total loss of above ground carbon (~7.0·10^9 g), due almost exclusively to saltmarsh loss, is offset by saltmarsh expansion (2.0·10^9 g) and mangrove expansion (1.8·10^9 g) over the study period. Concurrently, the net loss in saltmarsh coverage leads to a loss in below ground carbon accumulation capacity of 2.0·10^9 g/yr, whereas the net but small expansion of mangrove wetlands leads to an added below ground carbon accumulation capacity of 0.4·10^9 g/yr. Climate change is expected to accelerate both sea level rise and mangrove expansion; these mechanisms are likely to interact synergistically and contribute to salt marsh loss and shifts in carbon storage in coastal wetlands.

Presentation Type:  Plenary Talk

Session:  Theme 2: Landscapes to coasts: understanding Earth system connections

Presentation Time:  Mon 3:36 PM  (22 minutes)