Carbon distributions in Spartina alterniflora dominated salt marshes in Galveston, Texas: The role of elevation, relative sea level history, and land cover conversions

Abstract:

Coastal wetlands, including salt marshes, are considered to be large carbon sinks. Yet, there is little knowledge about how the terrain and land cover of these environments are related to carbon distribution. An understanding of the spatial and temporal patterns of carbon held in both the biomass and soil, and the factors that influence its distribution, will be necessary to allow coastal managers to initiate and verify “Blue Carbon” projects. In this study, we attempt to understand: 1) the temporal changes in salt marsh distributions as affected by marsh submergence, vertical accretion and land cover conversions; 2) patterns of soil carbon across different depths of the soil profile; and 3) to evaluate how changes in relative water level governs the spatial and temporal variability of salt marsh carbon storage ability. Our results indicate that over the study period (1954 to present) a considerable portion of salt marsh extents were submerged, while at the higher terrains these salt marshes indicated a landward shift in response to the sea level rise. Soil carbon measured in the soil profile, revealed a gradual depletion of soil carbon with depth. However, both the soil bulk density and the percent carbon indicated an abrupt and significant change at a depth of 15cm (p=0.05), which we interpreted as distinct of two different environments. As evidenced by historical aerial imagery (1954, 1969), the first (15-30 cm depth) coincided with an unvegetated salt flat at the sample locations, which were then overlain by lower bulk density and higher carbon Spartina alterniflora low marsh (0-15 cm depth) that migrated upslope in response to rapid relative sea level rise. However, within each of these two environments separately, carbon distribution followed a unique pattern with respect to elevation. Our results further point to two different processes, each acting at a different time scale (daily tides versus relative sea level rise), and each results in distinct spatial patterns of carbon deposition with respect to elevation. Thus, local and regional Blue Carbon projects or management actions, and global scale accounting of soil carbon, will need to consider both elevation and sea level history to predict carbon distribution.