Abstract

We analyzed data from 1138 wetland sites across the conterminous United States (US) as part of the 2011 National Wetland Condition Assessment (NWCA) to investigate the response of indicators of wetland quality to indicators of human disturbance at regional and continental scales. The strength and nature of these relationships in wetlands have rarely been examined over large regions, due to the paucity of large-scale datasets. Wetland response indicators were a multimetric index of vegetation condition (VMMI), percent relative cover of alien plant species, soil lead and phosphorus, and water column total nitrogen and total phosphorus. Site-level disturbance indices were generated from field observations of disturbance types within a circular 140-m radius area around the sample point. Summary indices were calculated representing disturbances for ditching, damming, filling/erosion, hardening, vegetation replacement, and vegetation removal. Landscape-level disturbance associated with agricultural and urban land cover, roads, and human population were based on GIS data layers quantified in 200, 500, and 1000-m circular buffers around each sample point. Among these three buffer sizes, the landscape disturbance indicators were highly correlated and had similar relationships with the response indictors. Consequently, only the 1000-m buffer data were used for subsequent analyses. Disturbance-response models built using only landscape- or only site-level disturbance variables generally explained a small portion of the variance in the response variables (R2 < 0.2), whereas models using both types of disturbance data were better at predicting wetland responses. The VMMI was the response variable with the strongest relationship to the disturbances assessed in the NWCA (national model R2 = 0.251). National multiple regression models for the soil and water chemistry and percent alien cover responses to disturbance indices were not significant. The generally low percentage of significant models and the wide variation in predictor variables suggests that stressor-response relationships vary considerably across the diversity of wetland types and landscape settings found across the conterminous US. Logistic regression modeling was more informative, resulting in significant national and regional models predicting site presence/absence of alien species and/or the concentration of lead in wetland soils above background.

Highlights

  • There is an increasing demand for information that can enhance understanding of the ecological quality of the world’s wetland resources beyond status and trends in wetland extent or qualitative indicators of wetland function (e.g., Fennessy et al 2007; Wardrop et al 2013)

  • We focused our water chemistry analysis on total nitrogen (TN) and total phosphorus (TP) concentrations, which were measured in the laboratory by acid persulfate digestion and colorimetry

  • We considered areas coded as national parks, trails, and landmarks, as well as recreation management areas, historic/cultural areas, state parks, and local recreation areas to be BRecreational.^ Hydrologic modification was defined as the total length of canals or ditches in the buffer as represented in the National Hydrography Dataset Plus (NHDPlus) version 2 (McKay et al 2012)

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Summary

Introduction

There is an increasing demand for information that can enhance understanding of the ecological quality of the world’s wetland resources beyond status and trends in wetland extent or qualitative indicators of wetland function (e.g., Fennessy et al 2007; Wardrop et al 2013). Recent years have seen attention given to development of quantitative, field-based methods in support of wetland management and protection These efforts have resulted in progress on development of new assessment methods, definition of reference condition, and design of protocols for obtaining a representative sample of wetlands (e.g., Fennessy et al 2007; Stevens Jr. and Jensen 2007; Wardrop et al 2007b; Whigham et al 2007). Many studies have illustrated the effects of different land uses and other disturbances on wetland condition on a local or basin scale (e.g., Mensing et al 1998; Houlahan and Findlay 2004; Hychka et al 2007) The expectation that such relationships exist has formed the basis for the design of many

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