Interactions among science, environmental policy and public perception in a Mississippi River Delta assimilation wetland.

In November 2006, the City of Hammond, Louisiana, began discharging secondarily treated, disinfected municipal effluent through a distribution pipeline to a 14,000 ha emergent wetland complex referred to as the East Joyce Wetlands. The primary goals were to improve local water quality by taking effluent that would normally be discharged to local waterways and directing it to wetlands that were hydrologically isolated and degraded by saltwater intrusion. During the 2007 growing season there was robust growth of marsh vegetation with a near doubling of productivity compared to controls, however, during winter 2007 there was a massive decline in vegetation and transition to open water in the 121 ha of wetlands in the direct path of effluent discharge. Herbivory by nutria was found to be the cause of the decline, but not before several other causes were hypothesized and largely accepted by the public, specifically that the cause of the vegetation decline was due to excess nutrients that increased rates of soil decomposition and limited root growth of plants. Studies show these hypotheses were flawed due to scale and methodology issues. Intensive nutria removal began in the spring of 2008 and continued through the winter of 2008–2009, with approximately 2000 nutria killed, and vegetation recovery began during the spring of 2009, which was most pronounced and consistent nearest the discharge pipe. Vegetative cover increased dramatically from the near complete loss after nutria grazing to near 80% by 2015, and wetland recovery has continued to the present. • Treated municipal effluent was discharged into 121 ha of emergent wetlands. • Within two years, the entire 121 ha wetland turned to open water. • Two main hypotheses emerged of why: increased decomposition and nutria herbivory. • Herbivory by nutria was found to be the cause based on several lines of evidence. • Increased decomposition hypothesis rejected due to a lack of supportive evidence. [ABSTRACT FROM AUTHOR]