Vegetation and Soil Dynamics of a Louisiana Estuary Receiving Pulsed Mississippi River Water Following Hurricane Katrina

We monitored wetland biomass, decomposition, hydrology, and soil porewater chemistry at the Breton Sound estuary, which receives Mississippi River water from the Caernarvon river diversion structure. The estuary was in the direct path of hurricane Katrina in 2005, which caused a dramatic loss of wetlands in the upper basin. From March 2006 to October 2007, we made duplicate measurements at three distance classes from the diversion structure along the estuarine gradient as well as at a reference area, designated Near (N1&2), Mid (M1&2), Far (F1&2), and Ref (R1&2). Above- and belowground live biomass, porewater nutrients (NOx, NH4, and PO4), salinity, sulfide, and soil Eh were measured every 2 months. Water level was monitored with gauges. Above- and belowground decomposition was measured using the litterbag (both) and cotton strip (belowground only) methods. Analysis of porewater parameters showed that stress factors affecting biomass production (porewater salinity, sulfide, flooding, and redox potential) were generally low to moderate, while measurable porewater nutrient concentrations occurred at all sites. Aboveground end of season live (EOSL) standing crop in October ranged from 423 g/m2 at site M2 to 1,515 at site F1, and was significantly greater at site N1 than at sites N2, M1, or M2. Aboveground EOSL biomass during this study was significantly lower than previously measured in 1999, 2000, and 2001. Peak belowground biomass ranged from 8,315 g/m2 at site R2 to 17,890 g/m2 at site N1, which is among the highest reported in the literature, and there were significant increases throughout the study, suggesting recovery from hurricane Katrina. The decomposition bag data did not indicate any significant differences; however, the cotton strip decomposition rate was significantly lower at the lowest depth. Wetland surface vertical accretion ranged from 0.49 cm/year at N2 to 1.24 cm/year at N1, with site N1 significantly greater than N2, M1, F2, and R1, and site N2 significantly less than all other sites except site R1. These findings show that marsh productivity and stability is related to a number of factors and no one factor can explain the impacts of the hurricanes.