From Ecosystem-Scale to Litter Biochemistry: Controls on Carbon Sequestration in Coastal Wetlands of the Western Gulf of Mexico

Abstract:

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. Here we present the synthesis of 3 years of multidisciplinary work to quantify ecosystem shifts at the regional scale, along the entire Texas (USA) coast of the western Gulf of Mexico, and transcribe these shifts into carbon (C) 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. Between 1990 and 2010, mangrove area expanded by 74% (+16 km²). Concurrently, salt marsh area experienced a net loss of 24% (-78 km²). Most of that loss was due to conversion to tidal flats or water, likely a result of relative sea level rise, with only 6% attributable to mangrove expansion. Although relative carbon load (per surface area) are statistically larger for mangrove wetlands, total C loads are larger for salt marsh wetlands due to their greater aerial coverage. The entire loss of above ground C (~7.0·10⁹ g), was offset by salt marsh expansion (2.0·10⁹ g) and mangrove expansion (5.6·10⁹ g) over the study period. Concurrently, the net loss in salt marsh coverage led to a loss in below ground C accumulation capacity of 2.0·10⁹ g/yr, whereas the net expansion of mangrove wetlands led to an added below ground C accumulation capacity of 0.4·10⁹ g/yr. Biomarker data show that neutral carbohydrates and lignin contributed 30-70% and 10-40% of total C, respectively, in plant litter and surface sediments. Sharp declines of carbohydrate yields with depth occur parallel to increases in lignin degradation ratios, indicating substantial decomposition of both the polysaccharide and lignin components of litter detritus. Further, biomarker data suggest that litter chemistry is the primary control of C preservation in these wetland ecosystems. This study shows that shifts in plant composition influence C sequestration potential from landscape to molecular levels.