Pore-Water and Substrate Quality of the Peat Marshes at the Barataria Preserve, Jean Lafitte National Historical Park and Preserve, and Comparison with Penchant Basin Peat Marshes, South Louisiana, 2000-2002

Swarzenski, Christopher; Doyle, Thomas; Hargis, Thomas

Pore-water and substrate quality of the peat marshes at the Barataria Preserve of Jean Lafitte National Historical Park and Preserve were measured from January 2000 to June 2002. Thesix major plant communities, including those dominated by Panicum hemitomon, were studied. The data were compared with pore-water and substrate quality of peat marshes dominatedby Panicum hemitomon in the Penchant Basin, an area of coastal Louisiana that has functioned as the hydrological and ecological equivalent of a freshwater diversion for over 30 years. Surface-water quality between the two areas also was compared. The comparisons were made to evaluate possible long-term responses of the peat marshes in the Barataria Preserveto the Davis Pond Freshwater Diversion, which began operations in 2003, and is introducing Mississippi River water to Barataria Preserve waterways and marshes. The marshes sampled at the Barataria Preserve align along a gradient of increasing exchange with adjacent surface water and proximity to the southern portion of the Barataria Preserve, where pulses of higher salinity water from the Gulf of Mexico first enter waterways. The gradient influences pore-water salinity and sulfide concentrations. Panicum hemitomon occurred as the dominant marsh community only at the two innermost habitats, which are better protected from saltwater intrusion than areas closer to waterways. These two sites had significantly lower median chloride concentrations (about 380-670 mg/L [milligrams per liter]) than the four other habitats (about 1,300-1,800 mg/L). Pore-water concentrations of inorganic nutrients, potassium, sulfate, and sulfide increased from the innermost sites to the southernmost sites, marsh communities dominated by Spartina patens and Schoenoplectus americanus. Median filtered ammonium concentrations ranged from about 0.022 to 1.95 mg/L as nitrogen; median sulfide concentrations ranged from about 0.07 to over 11.0 mg/L. The substrate of thePanicum hemitomon dominated communities was more fibric and less decomposed than that at the remaining four sites. The soil at the southernmost community, with the highest nutrientand sulfide concentrations, was the most highly decomposed. Environmental conditions in the Barataria Preserve may represent the extremes under which peat can accumulate and peatmarshes can sustain themselves in coastal Louisiana. Flow of river water across wetlands and shallow ponds in the Penchant Basin resulted in the transformation and uptake of nutrients, sulfate, and atrazine. Pore-water chloride concentrations in Panicum hemitomon dominated marshes were significantly lower, and average inorganic nutrient and sulfide concentrationswere significantly higher in the Penchant Basin than in the Barataria Preserve. Median pore-water ammonium concentrations were about 0.26 mg/L as nitrogen, and median sulfideconcentrations were about 0.33 mg/L in the Penchant Basin. The peat substrate of marsh communities dominated by Panicum hemitomon was more decomposed in the PenchantBasin than the substrate of any of the six marsh communities in the Barataria Preserve. Elevated concentrations of inorganic nutrients and sulfide in pore water and a more decomposed peat substrate represent a substantial deterioration of the Panicum hemitomon habitat. Long-term exposure of a peat substrate to the regular influx of nitrate and sulfate may initiate and sustain biogeochemical reactions in the organic matter that result in thedeterioration of the peat substrate. Such reactions could include the reduction of nitrate and sulfate, as well as the inhibition of nutrient uptake by the plants resulting from elevated sulfideconcentrations. In this view, freshwater diversions may compromise the long-term sustainability of highly organic marshes in coastal Louisiana.

Pore-Water and Substrate Quality of the Peat Marshes at the Barataria Preserve, Jean Lafitte National Historical Park and Preserve, and Comparison with Penchant Basin Peat Marshes, South Louisiana, 2000-2002

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