The role of vegetation in the development and resiliency of the coastal freshwater deltaic system of Wax Lake Delta
Thursday, 18 December 2014
The world's coastal deltaic wetlands are threatened by relative sea level rise. Protecting these ecosystems requires understanding deltaic growth and few studies have focused on how vegetation influences this growth. Here we explore the ecogeomorphic evolution of Wax Lake Delta (WLD) using a remote sensing database consisting of 1083 Landsat 5 and 7 images (30 m resolution) from 1983 to 2012 and 23 high-resolution images (1.5 m resolution) from 2001 to 2012. We calculate each Landsat image to the normalized difference vegetation index (NDVI), which indicates the relative above ground vegetation biomass. We also use the high-resolution images and spectral signatures from Landsat images to classify land cover into vegetation, sediment, and water. Only imagery from peak biomass season, August and September for this subtropical ecosystem, was analyzed to control for the effects of interannual variability in growing season and growth rates on the estimates of vegetated areas. Additional analyses of extreme events and fluctuating water levels is needed to correctly estimate land area and areal vegetation coverage. Our results show from 1984 to ~1995 WLD experiences a period of emergence where total delta area and the vegetated percent (at peak biomass) increases rapidly. After 1995, the vegetated percent of WLD levels off, but fluctuates from 70% to 90%. During winter months this value drops to ~10%. Accordingly, the bare sediment percent decreases as one minus the vegetated percent. At individual island scale vegetated percent at peak biomass levels off at different times and values. Proximal (or older) islands level off as soon as 1995, fluctuating from 80% to nearly 100% vegetated, while distal (or younger) islands do so at 2000 ranging from 20% to 80%. It is unclear why this difference occurs, but we conjecture it is caused by the lower elevation of the distal islands or their aggrading slower than proximal ones as the incoming sediment volume spreads over a larger deltaic area. Our analysis suggests that vegetation can most effectively enhance delta sedimentation if sediment delivery occurs during peak biomass season.