Abstract
We analyzed data from 967 randomly selected wetland sites across the conterminous United States (US) as part of the 2011 National Wetland Condition Assessment (NWCA) to investigate the relative and attributable risk of various stressors on wetland vegetation condition. Indicators of stress included six physical stressors (damming, ditching, filling/erosion, hardening, vegetation removal, and vegetation replacement) and two chemical stressors (soil phosphorus and heavy metals) that represent a wide range of human activities. Risk was evaluated nationally and within four aggregate ecoregions and four aggregate wetland types. Nationally, all of the stressors except soil heavy metals and phosphorus had a significant relative risk but values were always < 2 (a relative risk of two indicates that it’s twice as likely to have poor vegetation condition when the stressor is present relative to when it is absent). Among the different ecoregions or wetland types, no one stressor was consistently riskier; all of the stressors were associated with poor vegetation condition in one or another of the subpopulations. Overall, hardening had the highest attributable and relative risks in the most different subpopulations. Attributable risks above 25% were observed for vegetation removal in the Coastal Plain, hardening and ditching in the West, and hardening in Estuarine Woody wetlands. Relative risks above 3 were noted for heavy metals and soil phosphorus in the Interior Plains, and vegetation removal, vegetation replacement, and damming in Estuarine Woody wetlands. Relative and attributable risk were added to the data analyses tools used in the NWCA to improve the ability of survey results to assist managers and policy makers in setting priorities based on conditions observed on the ground. These analyses provide useful information to both individual site managers and regional-national policy makers.
Highlights
The United States (US) Environmental Protection Agency (USEPA), in partnership with the States, is tasked with making assessments of all US surface waters
Half of this wetland area is located in the Coastal Plain (CPL), 32% is in the Eastern Mountains and Upper Midwest (EMU), and only 18% in the Interior Plains (IPL) and West (W) ecoregions combined (Table 1)
The vast majority (69%) of the estimated wetland area in the National Wetland Condition Assessment (NWCA) target population is in the palustrine, riverine, or lacustrine–woody (PRLW) wetland type
Summary
The United States (US) Environmental Protection Agency (USEPA), in partnership with the States, is tasked with making assessments of all US surface waters. The use of sample survey approaches for characterizing water resources began in the late 1980s with the National Surface Water Survey to assess acidic deposition impacts (Landers et al 1988; Kaufmann et al 1991). The NWCA uses a probability design and is one of the National Aquatic Resource Surveys (NARS) conducted by the USEPA to assess the nation’s waters. The NARS were designed to make quantitative estimates of the condition of surface waters throughout the US. To address this objective, a large number (~ 1000) of randomly selected lakes, streams, rivers, wetlands, or near coastal sites are visited each year during a defined index period. The NARS include streams and rivers, lakes, near coastal areas, and wetlands (e.g., USEPA 2016c; USEPA 2016d)
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