The Effect of Mississippi River Delta Lobe Development on the Habitat Composition and Diversity of Louisiana Coastal Wetlands

Abstract

The patterns of habitat changes that occur during the growth and decay of Mississippi River deltaic lobes were examined by comparing different-aged lobes using habitat maps constructed from aerial photographs. Mudflats and fresh marshes dominated young delta lobes (10-1000 years old). Intermediate-aged lobes (1000-2000 years old) contained higher proportions of brackish and salt marshes. Old lobes (2000-4000 years old) contained large areas of salt and brackish marshes and open water. The number of habitat types (N) and Shannon-Wiener index of habitat diversity (H') were lowest in the youngest lobe (N = 14, H' = 0.68), highest in a medium-aged lobe (N = 56, H'= 1.77), and intermediate in the oldest lobe (N = 44, H' = 1.38). A new cycle of vegetation change is initiated approximately every 1000 years when upstream river diversion reintroduces fresh water and sediment into an old lobe. INTRODUCTION This study examines the effect of the large-scale disturbance caused by major shifts in the course of the Mississippi River on the distribution of plant communities in the Mississippi River deltaic plain region of Louisiana. Land in the Mississippi River delta region was formed during the last 6000 years by sediment deposited in major deltaic lobes by the Mississippi River (Fig. 1). Together the delta lobes built a broad plain of approximately 3 million ha. The nature and timing of these deltaic deposits control the long-term dynamics and persistence of the habitats found in this predominantly wetland coastal region. The five delta lobes that today make up the Mississippi River deltaic plain range in age from 10-4000 years. New delta lobes begin to form approximately once every 1000 years in response to a major change in the course of the river (Fig. 1). Each delta lobe undergoes a cycle of geological development (Kolb and van Lopik, 1966; Frazier, 1967; Gagliano and van Beek, 1975) and each of the existing five lobes lobes represents a different stage of development (Fig. 2). The initial phase includes below-water (subaqueous) deposition and limited above-water (subaerial) development. This is followed by a second phase of sustained land building. Land building continues until the river's hydraulic gradient becomes too flat to efficiently transport water and sediment to the Gulf of Mexico. This sets the stage for an upstream diversion to a new stream course with a steeper gradient (Morgan, 1967). A third phase of prolonged subsidence, land erosion and lobe decay follows diversion of the river course. During this stage of delta abandonment, progressively less water and sediment are transported by the dying distributary. The reduced quantity of sediment at its mouth cannot compensate for subsidence and wave attack, and erosion becomes the dominant process acting on a delta lobe (Morgan, 1967). Salinity decreases as a delta lobe develops, is lowest during time of greatest land building because of large riverine freshwater input, and increases again as freshwater input declines (Fig. 2). Land deposition and erosion and changes in salinity during this geologic cycle pro'Present address: Department of Forestry and Wildlife Management, University of Massachusetts, Amherest, 01003