Effect of elevations on soil properties of restored Louisiana salt marshes

Linda Benedict, Kongchum, Manoch  |  7/31/2013 11:51:35 PM

Manoch Kongchum

Research to restore and enhance deteriorating salt marshes in Louisiana has been ongoing for many years, with the primary goal of improving the long-term growth and sustainability of Louisiana’s coastal wetlands. A number of innovative sediment transportation and restoration design ideas – such as spray dredging, sediment transport through abandoned oil and gas pipelines, bay-bottom terracing and spoilbank mining – have potential for accelerating marsh creation. The prevailing marsh-restoration technique, however, uses sediment mining by suction dredging to create marsh.

Sediment addition is a preferred restoration method because vegetation health in coastal Louisiana is governed by relationships between marsh surfaces and water levels, which are affected by subsidence and submergence. Research has shown that adding sediment is effective in rehabilitating degraded marshes by increasing land elevations to levels that can support vegetation. However, mounting concern regarding distance, final elevation and cost is prompting researchers to investigate various sediment application techniques.

The hydraulic application of sediment slurries allows dredged material to travel an extended distance to the discharge pipe. The high water content and intense mixing reduces separation of particle sizes so they can be transported miles from the dredging location to otherwise unreachable interior marshes. Louisiana has readily available sediment material from the Mississippi River, Gulf of Mexico and frequently dredged navigable waterways.

An LSU AgCenter marsh restoration research project was funded by the Barataria-Terrebonne National Estuary Program during 2005-2009. The project was conducted at the Port Fourchon marsh creation and mitigation study site, which is about 270 acres of created mixed marsh immediately north of Port Fourchon, west of Pointe Fourchon and south-southwest of Leeville. The site is bordered on the south by Floatation Canal, on the west by a narrow strip of emergent marsh adjacent to and parallel to Bayou Lafourche, and on the north and east by open marsh.

One objective of the project was to assay the physical and chemical properties of dredged material used to raise the marsh surface. A spatial analysis was used to partition the restoration area into five discrete habitats: 

  • High habitat with an average elevation of 17.7 inches above ambient marsh elevation. 
  • Swale habitat, with an average elevation of 9.5 inches above ambient marsh. 
  • Upper-intertidal habitat, with an average elevation 3.95 inches above ambient marsh. 
  • Intertidal habitat, approximately the same elevation as ambient marsh. 
  • Shallow, open-water habitat (submerged) at approximately 3.95 inches below ambient marsh elevation

The number of soil samples taken within each habitat varied slightly, ranging from eight to 11 samples. Dry soils were sampled using an open-face auger; submerged soils were sampled using a specialized auger for sampling fluid soils. Samples were taken at 0-6 inches and 6-12 inches at each site. In 2005, 2007 and 2009, a total of 230 samples were taken at 115 sites within the five habitats. Soil samples were analyzed for a number of chemical and physical parameters, including soil pH, salinity, organic matter, particle size distribution and extractable nutrients such as phosphorus, potassium, calcium, magnesium and sulfur. In addition, soil oxidation-reduction was directly measured in each habitat.

Results showed that elevation has significant influence on most variables for both physical and chemical properties, except for soil pH, which was not affected by elevation. Particle size distribution (sand, silt and clay content) was highly related to marsh surface elevation. For example, higher elevations typically contained the greatest percentage of sand, whereas lower elevations were typically higher in clay content. The highest salinity was observed in isolated swales where water was captured during high tides or storms but wasn’t flushed out by daily tides. The percentage of organic matter was higher at lower elevations, and the calcium content was significantly greater in high-elevation habitats where oyster and clam shells were concentrated.

Variation across years was evident in fewer than half the variables tested, and in most cases the variations were small. Although year-to-year measurements did not show consistent variations in soil variables, year-to-year variations by habitat were usually significant – different habitats showing different kinds of variation. About half of the soil characteristics varied significantly between the two depths sampled.

Perhaps most marked among the soil variables were sodium concentrations, which influence salinity and conductivity and were disproportionately high in the swale habitat. Because these are as remained virtually unvegetated throughout the study, soil properties within the swales provided a reasonable explanation of their effect. Given the lack of success in growing vegetation in the swale areas, future efforts should minimize these midrange elevations.
If elevation diversity within site is a desired target, then short transit zones between the lower intertidal, upper-intertidal and elevated habitats should be designed. Midrange elevations will continue to have problems because of radiating heat, salt accumulation from water-table absorption and the absence of flushing by the normal tidal cycle. Based on an extensive vegetative-and-nutrient field trial, these midrange elevations will continue to be difficult to vegetatively propagate, either from natural sources or from human plant introductions.

Manoch Kongchum is an instructor in theSchool of Plant, Environmental & Soil Sciences.

(This article was published in the spring 2013 issue of Louisiana Agriculture magazine.)

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