ACCRETING MUDFLATS AT THE MISSISSIPPI RIVER DELTA: 

 SEDIMENTATION RATES AND VASCULAR PLANT SUCCESSION 



David A. White 



Loyola University 



New Orleans, LA 70118 



ABSTRACT 



Geologically, the Mississippi River Delta is an extremely dynamic environment. Its interior 

 marshlands have subsided to form huge (1,000's ha), shallow (<1 m) ponds, where once (1940's- 

 1950's) vast freshwater marshes existed. The creation of crevasses in the levees which border these 

 ponds has allowed river water to flow into the ponds and consequently sediment deposition has 

 formed large "inner delta splays." I have been monitoring the rate of sedimentation and the 

 vascular plant succession on the developing mudflats at three sites for 4 years. Sedimentation rates 

 across the sites averaged 0.0189 cm/d or 6.9 cm/yr for 3 years. Vascular plant colonization and 

 succession were rapid; by the third year total live standing crop averaged 1,194 g/m 2 . A total of 

 62 vascular plant species have been collected at the sites. Scirpus deltarum (three-square grass) 

 became the dominant herb the second or third year on the lower regions of the mudflats. This 

 species is particularly significant for its value as a wildlife food. Salix nigra (black willow) was 

 prevalent on the highest flats. Several species of sedges and grasses made up the majority of the 

 remaining plant biomass. The amount of belowground organic material (live and dead) increased 

 to 1,212 g/m 2 by the third year. Herbivory by large mammals did affect succession. 



INTRODUCTION 



The coastal marshes of Louisiana are one of the most productive habitats in North America. 

 Here the production of fish and wildlife is directly related to the abundance and diversity of 

 photosynthetic plants (Chabreck 1982). Louisiana's wetlands are winter habitat for the largest 

 percentage of the North American duck and goose populations. The wetlands also produce the 

 largest fur harvest in North America, principally muskrat and nutria. The continued high 

 production of these primary consumers requires freshwater marshes since only in these marshes 

 are their food plants abundant. Unfortunately the entire State of Louisiana is losing freshwater 

 marshes from land loss and saltwater intrusion (Fruge 1982; Baumann and DeLaune 1982) at an 

 alarming rate (Craig et al. 1979; Wicker 1980). The management of the remaining marshes for 

 wildlife is of paramount importance (Weller 1978). 



The present Mississippi River Delta encompasses over 1,400 km 2 and is a composite of numerous 

 lobes formed during the past 1,000 years (Figure 1). The delta's biotic character exists because of 

 sheet flow of freshwater towards the saline Gulf of Mexico waters, creating a horizontal 

 stratification of salinity. Plant species distributions (marsh types) at the delta must be governed 

 by water depth and salinity as Penfound and Hathaway (1938), Chabreck (1972), and Weiss et aL 

 (1979) found in other Louisiana marshes. Therefore, the central portion of the delta supports 

 tremendous areas of freshwater marshes, while the peripheral portions support saline marshes. 



The freshwater marshes of the Mississippi River Delta are not unique in that they, too, have 

 been disappearing at a rapid rate. Due to human-induced factors, between 1956 and 1978 nearly 

 400 km 2 of wetlands have been lost here (U.S. Fish and Wildlife Service 1982). Levees have 



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