Some Effects of Nutrient and Flooding Stress Manipulation on Coastal Louisiana Wetland Vegetation

Freshwater diversions are a relatively recently utilized tool in wetland loss mitigation that stimulate an organic accretion response in marsh vegetation, which is based in root production and thus belowground biomass. The effectiveness of freshwater diversions in slowing marsh loss probably varies across a gradient of the factors they supply: decreased salinity, increased nutrient concentrations, and increased inundation. Flooding stress is commonly thought to be the overriding factor limiting wetland vegetation growth, however its influence may vary across salinity and nutrients. Therefore, plugs of Spartina patens were planted in four “marsh organs” consisting each of 36 sediment-filled PVC pipes of incremental heights that created a range of growth elevation and thus flooding stress. The organs were located in mean salinities from 2.6-14.7 ppt and two of them fertilized at 22 g N m-2, 11 g P m-2, and 5.5 g K m-2. Twice during the growing season below- and aboveground biomass was harvested, sorted, dried, weighed, and analyzed for relationships between variables. Flooding stress was found to be primary, with a secondary interactive effect between salinity and fertilization that suggests fertilization may help plants resist relatively higher salinity levels. Variations in salinity and nutrient concentrations associated with freshwater diversions, which can differ in intensity and extent, might also be linked to increased nutria herbivory. Therefore, nutria preference for fertilized versus unfertilized vegetation was also tested in a controlled setting on Panicum hemitomon, Sagittaria lancifolia, and S. patens, representative dominant plant species of fresh, fresh-brackish, and brackish marsh. Nutria showed a significant preference for fertilized vegetation within species. It is therefore possible that benefits derived from nutrient-rich fresh floodwater could potentially be negatively impacted by increased preferential herbivory.