Environmentally stratified sampling design for the development of Great Lakes environmental indicators

Abstract

Understanding the relationship between human disturbance and ecological response is essential to the process of indicator development. For large-scale observational studies, sites should be selected across gradients of anthropogenic stress, but such gradients are often unknown for apopulation of sites prior to site selection. Stress data available from public sources can be used in a geographic information system (GIS) to partially characterize environmental conditions for large geographic areas without visiting the sites. We divided the U.S. Great Lakes coastal region into 762 units consisting of a shoreline reach and drainage-shed and then summarized over 200 environmental variables in seven categories for the units using a GIS. Redundancy within the categories of environmental variables was reduced using principal components analysis. Environmental strata were generated from cluster analysis using principal component scores as input. To protect against site selection bias, sites were selected in random order from clusters. The site selection process allowed us to exclude sites that were inaccessible and was shown to successfully distribute sites across the range of environmental variation in our GIS data. This design has broad applicability when the goal is to develop ecological indicators using observational data from large-scale surveys.

Development of ecological indicators for the U.S. Great Lakes coastal region – A summary of applications in Lake Huron

“…Study sites were selected across gradients of anthropogenic stress using a stratified random design as part of the larger sampling design for the entire U.S. Great Lakes coastal region (Danz et al, 2005); over 88 sites in coastal wetlands, uplands, estuaries/bays, and high-energy shoreline were selected in the Lake Huron basin (Figure 1). For site selection, land-based stress was quantified in a geographic information system (GIS) for 762 coastal segmentsheds that encompassed the entire U.S. basin; each segment-shed consisted of the land area that drained into a segment of coastline extending in either direction from 2nd-order or larger streams to one-half the distance to the adjacent stream.…”

“…For site selection, land-based stress was quantified in a geographic information system (GIS) for 762 coastal segmentsheds that encompassed the entire U.S. basin; each segment-shed consisted of the land area that drained into a segment of coastline extending in either direction from 2nd-order or larger streams to one-half the distance to the adjacent stream. Since the original selection of the sites by Danz et al (2005), and also described in detail by Johnston et al (2007), more detailed watersheds were delineated specific to each sampled site ). Sampled sites were represented within the GIS by polygons encompassing the sampling points for all GLEI indicator groups at a selected locale.…”

“…For each type of watershed summary, over 200 environmental variables in seven categories of environmental variation (Danz et al, 2005) were summarized. Principal components analysis (PCA) within each category of environmental variation was used to reduce dimensionality and derive overall gradients .…”

“…proportion of cultivated land within 100 m, 500 m, 1 km, and 5 km of wetland center), wetland attributes (e.g. proportion wetland area within 100 m, 500 m, 1 km, and 5 km), and eight principal component scores from the Danz et al (2005) analysis (e.g. Figure 1.…”

“…Coastal sample locations were selected as described in detail by Danz et al (2005) (Figure 1) and diatom assemblages were collected from surface sediments and other substrates; sampling and processing protocols are described in detail by Reavie et al (2006). A set of water quality measurements was concurrently collected at each sample location to be related to diatom assemblages (see .…”