Characterizing ambient nutrient concentrations and potential warning levels for surface water in natural forested wetlands in the Piedmont and Coastal Plain of North Carolina, USA

Abstract

Nearly thirty years ago the U.S. EPA directed states to begin developing water quality standards specific to wetlands; however, progress has been limited. This study presents an overview of ambient wetland water quality and the first step towards the development of numeric nutrient standards for forested wetlands in North Carolina. Water quality samples were collected in 16 natural wetlands across the state for three years. The sites span a range of anthropogenic disturbance. The data was combined with historical data collected at these and other sites from 2005 to 2013. Concentration levels in wetlands without significant anthropogenic disturbance are likely higher than streams for TN, TP, similar for NH4+, and likely lower than streams for NO3–-N. A method developed by the U.S. EPA to establish reference levels was modified to define draft nutrient warning levels that may indicate anthropogenic impacts. These warning levels were calculated as 0.06 mg/L, 0.14 mg/L, 0.34 mg/L and 2.8 mg/L for NO3–-N, NH4+-N, TP and TN, respectively. NO3–-N and NH4+-N concentrations were the best indicators of disturbance. Overall, despite the limitations imposed by the sampling frequency and inherent variability in wetland ecosystems, this research methodology presents a reasonable first attempt at defining nutrient warning levels for undisturbed natural forested wetlands in N.C., and may serve as a starting point in developing numeric nutrient criteria for wetlands.