Hurricanes Katrina and Rita: The Role of Species-species Interactions in the Recovery of Coastal Marshes Subjected to Flooding and Sedimentation Disturbance

Hurricanes Katrina and Rita devastated massive areas of coastal marsh, barrier islands, swamp forests, and submerged sea grasses in Louisiana by uprooting, excessive sedimentation, erosion, and mortality of dominant and keystone species. After burial by sediments, the reestablishment of vegetation and wetland processes may depend on the presence of invertebrates, as suggested by prior studies of invertebrates on dredge spoil marshes in the region (Egerova et al. 2003; Chiasson in press; Proffitt and Chiasson unpublished) and in other coastal wetlands (Proffitt and Devlin 2005). Sediment input into marshes impacted by Hurricane Katrina/Rita is 4-14 cm or more, so that burrowing by invertebrates could play a key role in making the deposited sediment more amenable to plant establishment. Macroinvertebrates may also play a key role in degrading standing dead plants and wrack (accumulations of plant debris), which will impact both revegetation rates and patterns and nutrient cycling in these marshes. Knowledge of the importance of invertebrates in these systems for vegetation re-establishment is critical, and the window for observing the initial process may be quite short following a hurricane (0-6 months).

Coastal Louisiana may be increasingly vulnerable to storms, particularly because of the high rates of marsh loss there; some of this loss was associated with the massive salt marsh die-off in 2000, as well as historical losses due to the natural deltaic cycle of the Mississippi River and human activities. Studies of the role of invertebrates in vegetation recovery are rare, and should be done along with studies that explore the magnitude of the losses of coastal marsh habitat and the rates, extents, and the nature and trajectories of recovery of the different communities and species. Analyses of the dominant and keystone species in wetland ecosystems are critical to predict responses of these wetlands and species to future disturbance events. There is an urgent need to begin study of invertebrate-vegetation interactions before the critical moment is lost and the patterns are obscured by subsequent marsh development.

Species-species facilitation may be very important following sedimentation after hurricanes because the oxygen level in the sediment is very low immediately after vegetation burial. We observed in created dredge spoil marshes that the crab Uca enhances the seagrass Spartina alterniflora growth probably by oxygenating the soil via its burrows, a sediment situation not unlike burial of vegetation by sediments during hurricanes. After Hurricane Katrina at Pearl River in a coastal marsh, we observed buried and top killed vegetation under anaerobic soil conditions, but with a number of invertebrate species present. Many invertebrate populations probably suffered losses during Katrina, and their densities in the wetlands may be variable. Therefore it is likely that the level of facilitation of vegetation development by invertebrates may vary in coastal wetlands depending on the densities of surviving invertebrates. The Uca themselves are important to these coastal ecosystems because Uca larvae are an important source of nutrition for estuarine fish. In addition, gastropods such as species of Littoraria and Melampus affect plant species abundances (Silliman and Zieman 2001; Pennings and Silliman 2005) and the rates of decomposition of dead plant material, which is now in abundance after Katrina (Proffitt and Devlin 2005).

Permanent plots were established to look at vegetation production and sedimentation rates (SET, feldspar markers) in brackish, intermediate (oligohaline) and freshwater marshes before Hurricane Katrina in March 2005 (BM and KM). Pre-hurricane elevations were measured using SET baseline markers in April 2005 at all sites, and again immediately before Hurricane Rita at one brackish marsh site.