Effects of hurricane storm surge and sediment deposition on coastal plant communities

Background/Question/Methods

Coastal ecosystems along the northern Gulf of Mexico are subject to chronic sea level rise and acute incursions of the sea when hurricanes make landfall.  We hypothesized that hurricane-generated storm surge disassembles landward communities dominated by species intolerant of salinity pulses and burial by sediment inputs, thereby enabling species from seaward positions to establish farther inland.  In 2009, five transects were established at points spanning East River, a tidal river ecosystem in northwest Florida.  Each transect was surveyed for elevation and extended perpendicular to the river into upland habitat to include 1m, 2m, and 3m changes in relative elevation from the river’s edge.  In September 2010, five 0.6m x 0.6m plots were established in each of the zones along the five transects (n=75).  Following an initial plant composition survey, four of the five plots in each zone received a saline storm surge (28 ppt) and sediment slurry application consisting of marine sand and river silt collected from East River.  Control and treatment plots were re-surveyed for species composition in September 2011 and 2012.  Ordination and Permutational multivariate analysis of variance were used to compare compositional trends across the estuarine gradient and in response to the treatment. 

Results/Conclusions

With the exception of several areas that are inundated periodically by East River, most plots remained covered by a 5 cm sediment layer.  Communities at the seaward end of the estuarine gradient did not exhibit significant changes in response to the treatment, suggesting that they are somewhat conditioned to these kinds of disturbances.  In contrast, inland communities had pronounced compositional shifts and appeared to be quite sensitive to the storm surge treatment.  Shifts were driven initially by marked losses in species; many small-stature plant species buried by sediment had died by the end of the 2010 growing season.  Over time, a few of those species recovered.  Establishment of “new” species into these plots was rare, and most communities with high initial mortality remained depauperate.  Many plots still had little or no live vegetation by the end of 2012.  These results partially support our hypothesis in that landward communities were more disassembled than their seaward counterparts.  We did not, however, find evidence that disassembly triggered establishment by seaward species in the short term.  The footprint of this “double disturbance” may be long-lived, thus providing an extended window for propagule interception and colonization by species from other parts of the coastal landscape.