Soil Shear Strength Losses in Two Fresh Marshes with Variable Increases in N and P Loading

We measured soil shear strength (SSS) from 2009 to 2018 in two hydrologically distinct freshwater marshes dominated by Panicum hemitomon after nitrogen (N) and phosphorous (P) were applied to the surface in spring. The SSS averaged over 100-cm depth in the floating and anchored marshes declined up to 30% throughout the profiles and with no apparent differences in the effects of the low, medium, and high N + P dosing. Plots with only N or P additions exhibited significant changes in SSS at individual depths below 40 cm for the anchored marsh, but not the floating marsh. The average SSS for the anchored marsh over the entire 100 cm profile declined when N and P were added separately or together. At the floating marsh, however, the SSS decreased when N and P were added in combination, or P alone, but not for the N addition. Increasing nutrient availability to these freshwater marsh soils makes them weaker, and perhaps lost if eroded or uplifted by buoyant forces during storms. These results are consistent with results from multi-year experiments demonstrating higher decomposition rates, greenhouse gas emissions, and carbon losses in wetlands following increased nutrient availability.

Responding to Oil Spills in Louisiana's Coastal Floating Marshes: Ecology, Oil Impact, and Response Alternatives

ABSTRACT Freshwater marshes cover 4000 square kilometers of the Louisiana coastal zone and are the most abundant marsh habitat type. Many of these marshes actually float as organic mats on underlying water. Some estimates suggest as much as 70% of Louisiana's coastal freshwater marsh are of the floating variety. The slow flow of water characteristic of these environments generally transports very little sediment. As a result, the marsh substrate is composed of primarily live and dead organic matter (peat formation) rather than mineral sediments. Since floating marshes are structurally different than intertidal marsh habitats, many traditional oil spill response options are ineffective or inappropriate. Access to the marsh is often limited since there is no open water ingress and the marsh structure cannot support the weight of equipment. Oil spill response options are further complicated when the source of the oil is a pipeline leak located below the floating marsh mat; spilled oil is free to travel at the interface of the underlying water and mat. Protection booming is impossible. Oil impacts often result in the death of all the living plants that are integral to the formation and sustainability of the habitat. This paper reports on two oil spills in a floating marsh near Paradis, Louisiana that occurred eight years apart. Both spills were spatially close to each other, which provided an excellent comparison for assessing potential long-term impact from oil spills in floating marshes. During both oil spill responses, unique response techniques were developed to recover spilled oil and enhance marsh recovery. An effective technique was to rake away and remove the dead oil-contaminated surface plant debris from the site and employ sorbent recovery. Lessons learned from these responses were used to develop mitigation guidance for future responses.

The role of environmental variables on interannual variation in species composition and biomass in a subtropical minerotrophic floating marsh

Floating marshes supporting emergent vascular vegetation occur in expansive areas in many parts of the world. We analyzed the long-term variability in species composition and related plant biomass to environmental variables in a subtropical minerotrophic floating marsh, Louisiana, U.S.A. Panicum hemitomon was the dominant plant species, representing 76% of the total mean end of season aboveground dry weight of 840 g∙m−2. Multivariate analyses showed that community structure in the Lake Boeuf floating marsh has changed little during the 11 years included in this study. Individual species occurring in varying frequency with the dominant, Panicum hemitomon, form two marginally distinct assemblages. Mean live end of season biomass varied from a low of 602 g dry wt∙m−2 to a high of 1173 g dry wt∙m−2 during the period of the study. Ninety-nine percent of the variation in total aboveground biomass can be predicted by environmental variables related to temperature, precipitation, evaporation, and water level. Mapping of the area for the years 1945, 1952, 1981, and 1992 show that a net loss of about 4% of marsh has occurred between 1945 and 1992. Key words: vegetation, stability, freshwater floating marsh, Panicum hemitomon, Louisiana, climate.