The release of treated wastewater into natural wetlands has been shown in many cases to improve water quality, increase wetland accretion rates through enhanced vegetation productivity, and provide financial savings for municipalities employing this technique. However, understanding how altering key environmental factors, such as hydrologic regime and nutrient loading, may in turn affect vegetation growth and species composition of wetland habitats receiving wastewater effluent is critical to understanding their long-term sustainability. Point au Chene swamp is dominated by Taxodium distichum and Nyssa aquatica and is located approximately 10 km southwest of Thibodaux, LA (29° 45’ N, 90° 52’ 30” W). The city of Thibodaux, LA has been releasing treated municipal effluent into the Point au Chene swamp since 1992 at an average rate of 11,546 m -³ day effluent, 14.44 mg/L/day total nitrogen, and 1.52 mg/L/day total phosphorous, and provides an excellent opportunity to assess the effects of long-term wastewater effluent discharge on the sustainability of T. distichum-dominated swamps. Reference and effluent-receiving study plots have been established to assess impacts of wastewater effluent release on natural recruitment of T. distichum seedlings, survival and growth of planted seedlings, soil decomposition, and herbaceous understory vegetation community composition.
Results/Conclusions
Preliminary data analysis indicates no significant difference between control and treatment sites for T. distichum sapling survival or growth; however, a trend towards higher T. distichum growth in the control sites can be visually discerned. Percent of mass remaining in leaf litter decomposition bags was significantly higher at reference sites than sites receiving effluent, indicating higher decomposition at effluent receiving sites. Ordination of herbaceous understory vegetation delineated a clear gradient from reference to effluent receiving sites in community composition, with cover of Pontedaria cordata and Polygonum punctatum positively associated with reference sites and Bidens laevis and Hydrocotyle spp. positively associated with sites receiving effluent. These preliminary analyses suggest that those sites receiving wastewater effluent have experienced alterations in key vegetation characteristics that will alter their long-term sustainability. Continued monitoring of these vegetation characteristics with incorporation of natural recruitment surveys and hydrologic analyses will contribute to a greater understanding of the sustainability of assimilation wetlands.