Estimating biotic integrity to capture existence value of freshwater ecosystems

Abstract

The US Environmental Protection Agency (EPA) uses a water quality index (WQI) to estimate benefits of proposed Clean Water Act regulations. The WQI is relevant to human use value, such as recreation, but may not fully capture aspects of nonuse value, such as existence value. Here, we identify an index of biological integrity to supplement the WQI in a forthcoming national stated preference survey that seeks to capture existence value of streams and lakes more accurately within the conterminous United States (CONUS). We used literature and focus group research to evaluate aquatic indices regularly reported by the EPA's National Aquatic Resource Surveys. We chose an index that quantifies loss in biodiversity as the observed-to-expected (O/E) ratio of taxonomic composition because focus group participants easily understood its meaning and the environmental changes that would result in incremental improvements. However, available datasets of this index do not provide the spatial coverage to account for how conditions near survey respondents affect their willingness to pay for its improvement. Therefore, we modeled and interpolated the values of this index from sampled sites to 1.1 million stream segments and 297,071 lakes across the CONUS to provide the required coverage. The models explained 13 to 36% of the variation in O/E scores and demonstrate how modeling can provide data at the required density for benefits estimation. We close by discussing future work to improve performance of the models and to link biological condition with water quality and habitat models that will allow us to forecast changes resulting from regulatory options.

Keywords: biological integrity; existence value; lakes; stated preference; streams.

Fig. 1.

The 2013/2014 National Rivers and Streams Assessment (NRSA) and 2007 National Lakes Assessment (NLA) sample sites. The EPA conducts surveys that include new sample locations on a 5-y cycle. NARS ecoregions: WMTNS = Western Mountains, PLNLOW = Plains and Lowlands, EHIGH = Eastern Highlands.

Fig. 2.

Distribution of sample sites with O/E scores from the (A) 2013/2014 NRSA and (B) 2007 NLA. Model interpolated O/E scores for NHD (C) streams and (D) lakes. Separate O/E assessments for streams and lakes were developed within three ecoregions: WMTNS = Western Mountains, EHIGH = Eastern Highlands, and PLNLOW = Plains and Lowlands. Dark gray areas in C and D represent streams or lakes that are outside of the sampling frame of the EPA NRSA or NLA, respectively, and were excluded from model interpolation. Despite using the full range of values for modeling, O/E scores >1 are ignored by NRSA and NLA and are truncated to 1 in A–D.