Barrier Islands

As the outermost land exposed to hurricanes, barrier islands often lose significant areas of beach and marsh. Storm waves associated with cold fronts and tropical storms continuously alter the shapes of these islands, but large storms like hurricanes can cause significantly more erosion in one event than several years of cold front passages. Other disruptive forces such as subsidence, sea-level rise, inadequate sediment supply, and human disturbance work in concert with tropical and winter storms to degrade these islands. (Figure 3)

Among the first habitats of coastal Louisiana to experience the devastating effects of Hurricane Andrew were the barrier islands, Isles Dernieres. Over the past 130 years, nearly 78 percent of the land area in the Isles Dernieres chain had already been lost. The continuous island arc present in 1853 had deteriorated into a series of five narrow islands. Recent photointerpretation has documented that between 1990 and 1992 after Hurricane Andrew passed near them, the Isles Dernieres lost an additional 30 percent of their land area.

Hurricane Andrew’s impact on Isles Dernieres varied considerably, depending on the position of an island relative to the storm’s path. Islands farther to the west and closer to the path of the storm suffered greater alterations in shape and size than did islands on the eastern end of the chain. The westernmost islands of the chain, Whiskey and Raccoon, were severely eroded. (Figure 4)

On Trinity Island, in the eastern part of the chain, large breaches of water up to 2 km (1 mi) long opened across the island. The shoreline receded 68 m (223 ft) in some areas, and there were significant overwashing and deposition of sand on back barrier wetlands. It is also likely that the entire island was submerged by the accompanying storm surge.

Overwash and sand movement from Hurricane Andrew damaged island plant communities that protect the underlying layers of sand from eroding, but in some areas the land was scoured and breached by waves, and the plant communities were essentially stripped away. In other areas, substantial overwash and sand deposition of 50-100 cm (20-39 inches) partially or completely buried plants.

Deposited sand partially or completely covered plant communities of Isles Dernieres and other Louisiana barrier islands. Low island marsh communities, dominated by smooth cordgrass, as well as marshes at higher elevations characterized by a mixture of mangroves, saltwort, saltgrass, glassowrt, smooth cordgrass, and wiregrass were buried. In some cases, only the tops of the mangroves were still exposed to the air.

Because plants on barrier islands are generally adapted to sand movement, salt spray, and low levels of nutrients, it is not suprising that, despite burial by sand and exposure to the storm surge, the vegetation on Trinity Island is rapidly recovering. Throughout the recovery, both total plant cover and number of species have incresed. The reestablishment and growth of these plant communities will help to stabilize the new sand surfaces created by the strong storm winds, which continuously eroded and transported the sandy dune and swale soils.

When Hurricane Andrew carried beach sand across Trinity Island and deposited it on the back barrier wetland plant communities, it created habitats with environmental conditions different from those present before the storm. Changes in salinity and elevation resulted in a redistribution of plant species. For instance, increased elevation from sand deposition caused a shift in species dominance from smooth cordgrass to wiregrass. Mangroves disappeared from higher sites but became established in the saltier lower sites.

Many barren areas had still not recovered from the hurricane three years later. Most recovery occurred when new shoots grew from the runners of adult plants that had survived the storm. Because runners have to grow from adult plants, barren areas distant from surviving vegetation will take a long time to regain plant cover. In addition, damage to new growth by nutria has been noted. This herbivore can damage new shoots and may play a major role in the future establishment of vegetation. (Figure 5)

Dispersal and germination by seeds on Trinity Island were minimal, as a persistent bank of seeds has not yet been established. The new sand deposits are barren surfaces swept by wind that continually resuspends the sands, making it difficult for seeds to accumulate and contribute to vegetation recovery. Only in areas with existing adult plants were germinated seeds found, because these plants provided a windbreak that allowed both sediments and seeds to accumulate.

Barrier island beaches are also important habitats for benthic invertebrates, including ghost shrimp. Although not seen in the seafood markets of Louisiana, these burrowers are an important food for wading birds and are agents in nutrient cycling and other sediment development processes. These shrimp suffered mass mortalities during the passage of Hurricane Andrew near Isles Dernieres.

Within two years of the hurricane, shoreline populations of ghost shrimp had reestablished to prestorm levels, as had the populations of wading birds feeding in these habitats. Surprisingly, revegetation on the bay side of the islands was strongly correlated with the presence of shrimp burrows, suggesting that the shrimp influence the entrapment and growth of plant propagules. (Figure 6)

Understanding how vegetation recovers from storms is important for land managers. The state of Louisiana began a beach restoration and nourishment operation to reestablish the portions of Raccoon Island swept away by Hurricane Andrew. The long-term stability of these newly restored areas may be significantly improved by the establishment of vegetative cover. (Figure 7a,b,c)

Natural resource managers must balance restorative sediment deposition with possible further decimation of reestablished ghost shrimp, which are important in the barrier island ecosystem. For the greatest success, there must be a combination of active management, including the planting of adult plants, and careful monitoring of benthic shrimp populations.

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