In the Mississippi River Deltaic Plain, there are over 100,000 ha of floating marsh communities.  These floating ecosystems support a diverse array of plant species, all of which are dependent upon the floating, low bulk density mat bound together by an extensive fibrous root system.  Floating marshes (flotant) are usually found in freshwater habitat, but they can sometimes be inundated with saline water during regional drought, canal intrusion, or tropical storm surge.  During the 2008 season, there were two hurricanes, Hurricanes Ike and Gustav, which affected coastal Louisiana marshes.  We used 80 permanent vegetation monitoring plots to examine the effects of hurricane disturbance on wrack necromass, plant biomass, diversity, and community composition in these marshes.  We predicted that wrack would increase, diversity and biomass would decrease, and community composition would shift in response to the salinity pulse from the sequential storm surges.    We compared aspects of the pre- and post- disturbance vegetation data using t-tests, Non-metric multidimensional scaling (NMDS) ordination, and Analysis of Similarity (ANOSIM).

Results/Conclusions

Our hypotheses were only partially supported by the results.  Overall biomass and necromass did not differ between the two years.  Species richness did, however, decrease significantly following the storms.  This decline was driven primarily by loss of species that were initially low in abundance and apparently very sensitive to changes in salinity.  These changes in richness were further evident in the multivariate community analyses.  NMDS ordinations, based on species cover and biomass data, suggested a compositional shift in the herbaceous community between the two years.  ANOSIM confirmed that this trend was statistically significant based on cover data (R = 0.1818, p < 0.0001) as well as species-specific biomass (R = 0.0222, p = 0.0130).  In most cases, compositional shifts resulted from species dropping out of the community, presumably as a result of increased salinity from the surge, rather than establishment of new species not present prior to the storms.  This study documents short-term changes in the floating marsh community in response to the sequential storm surges of Hurricanes Gustav and Ike.  Long-term studies will be necessary to determine whether the changes set into motion by these events are ephemeral or lasting and if the community exhibits additional shifts over time.