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Vegetation dieback is an important component of wetland loss in low salinity marshes of coastal Louisiana. A field experiment was conducted to determine the factors responsible for vegetation dieback within oligohaline marshes of Louisiana. Sections of marsh, dominated by Sagittaria lancifolia L., were transplanted into one of four locations depending on the treatment: (1) increased submergence—sods were lowered 15 cm below the donor marsh surface, (2) increased salinity—sods were transplanted into a higher salinity marsh and adjacent dieback pond, (3) increased salinity and submergence—sods were transplanted into a higher salinity marsh and adjacent dieback pond at 15 cm below the marsh surface, and (4) control—sods were exhumed and replaced at the ambient elevation of the donor marsh. Plant biomass and edaphic characteristics were measured after 5 mo. An increase in submergence caused decreased plant growth of the S. lancifolia‐dominated marsh community. An increase in salinities to 4–5 g/kg were not detrimental to plant growth. Although saltwater intrusion alone did not cause decreased growth of the S. lancifolia‐dominnled plant community, the combination of saltwater intrusion and increased plant submergence caused the greatest decrease in plant growth due to increased toxic sulfides and a likely reduction in the uptake of NH4‐N by the wetland vegetation. This illustrates that the dieback of oligohaline marsh vegetation can be alleviated by decreasing plant submergence even at salinities as high as 4.6 g/kg.

Vegetation dieback is an important component of wetland loss in low salinity marshes of coastal Louisiana. A field experiment was conducted to determine the factors responsible for vegetation dieback within oligohaline marshes of Louisiana. Sections of marsh, dominated by Sagittaria lancifolia L., were transplanted into one of four locations depending on the treatment: (1) increased submergence—sods were lowered 15 cm below the donor marsh surface, (2) increased salinity—sods were transplanted into a higher salinity marsh and adjacent dieback pond, (3) increased salinity and submergence—sods were transplanted into a higher salinity marsh and adjacent dieback pond at 15 cm below the marsh surface, and (4) control—sods were exhumed and replaced at the ambient elevation of the donor marsh. Plant biomass and edaphic characteristics were measured after 5 mo. An increase in submergence caused decreased plant growth of the S. lancifolia‐dominated marsh community. An increase in salinities to 4–5 g/kg were not detrimental to plant growth. Although saltwater intrusion alone did not cause decreased growth of the S. lancifolia‐dominnled plant community, the combination of saltwater intrusion and increased plant submergence caused the greatest decrease in plant growth due to increased toxic sulfides and a likely reduction in the uptake of NH4‐N by the wetland vegetation. This illustrates that the dieback of oligohaline marsh vegetation can be alleviated by decreasing plant submergence even at salinities as high as 4.6 g/kg.

Oil spill cleanup operations in wetlands remain a critical issue. Oil spills in four Phragmites marshes provided an excellent opportunity to document the effect of spilled oil and different intensities of cleanup activity on ecological recovery and oil degradation in wetlands. Results showed that the intensity of oil spill cleanup and initial oiling differentially affected marsh vegetation and oil removal. Neither intensive nor light cleanup adversely affected the plant stem density, shoot height, and aboveground biomass of the dominant marsh plant, Phragmites australis. In contrast, these parameters in the oiled marsh that did not receive cleanup (no-cleanup site) were significantly lower than that for the reference marshes and the intensive and light cleanup sites. A thick floating oil layer on the surface was most likely the primary cause of mortality of the marsh vegetation at the no-cleanup site. Soil petroleum hydrocarbon concentrations were a function of the initial volume of oil spilled and the intensity of the cleanup. Although the intensive cleanup obviously removed floating and surface oil (as evidenced by the visual absence of surface oil at the beginning of this investigation), this cleanup activity may initially increase the oil incorporated into the sediment. Overall, oil concentration in the sediment of the intensive cleanup site decreased with time; however, sediment oil concentration increased slightly at light cleanup sites and increased substantially at the no-cleanup site. Containment of spilled oil within booms at the light and no-cleanup sites appeared to be the primary cause for increased oil incorporation into the sediment because the floating oil inside the boom likely served as an oil source for incorporation into the sediment. The authors conclude that a no-response option in heavily oiled Phragmites wetlands where oil pools cannot naturally dissipate is not a satisfactory option.

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