Deltaic floodplain wetland vegetation dynamics along the sediment surface elevation gradient and in response to disturbance from river flooding and hurricanes in Wax Lake Delta, Louisiana, USA

Abstract

Herbaceous wetland vegetation species percent cover data collected over five growing seasons (2007–2011) was used to define unique species assemblages along the elevation gradient of deltaic islands in the actively prograding deltaic floodplain wetlands in the Wax Lake Delta, a major distributary of the Mississippi River. The passage of Hurricanes Gustav and Ike in September 2008 as well as a major river flood in spring of 2011 provided an opportunity to quantify the response and recovery of the vegetation community following disturbances of varying intensities. Two significant persistent species assemblages were observed from 2007 through 2011. The higher elevation species assemblage occurred above mean low water (MLW; –0.04 m NAVD88). While the lower elevation species assemblage occurred at elevations ranging from –0.47 to 0.30 m NAVD88. These unique lower and higher elevation species assemblages overlap at intertidal elevations. Hurricanes Gustav and Ike drastically reduced aboveground vegetation and percent cover immediately following the storms particularly for lower elevation species. No aboveground cover was observed for Sagittaria latifolia, Nelumbo lutea, Potamogeton nodosus, or Submerged Aquatic Vegetation (SAV) in any of the sampling stations following the hurricanes, compared to pre-hurricane aboveground cover mean ranging from 2 to 5%. Sagittaria platyphylla aboveground cover decreased from a pre-hurricane mean of 12.5% to <1% following the hurricanes. In subsequent years the community composition for all species returned to pre-disturbance cover, with the significant species assemblages returning within two years. In 2011 the major river flood resulted in disturbance that reduced species richness within both assemblages, likely related to differences in flood tolerance of vegetation species, with Nelumbo lutea cover expanding from a mean of 10% cover in the year prior to the flood to 22.5% following the flood. Vegetation community composition was shown to be robust to major river floods and hurricane storm surge passage.