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Using Aquatic Invertebrates to Delineate Seasonal and Temporary Wetlands in the Prairie Pothole Region of North America

Methods


Within each study wetland, we established six transects radiating from the center (defined as the lowest elevation in the basin) and extending 5 m beyond the wetland edge along evenly distributed compass bearings (0°, 60°, 120°, 180°, 240°, and 300°). For purposes of sample collection, the wetland edge was defined as the point along each transect where the elevation exceeded the elevation of maximum possible pool level by 15 cm. Maximum possible pool level was defined as the elevation beyond which water would flow overland into an adjacent basin. We collected a soil sample each meter along the entire length of each transect and measured the elevation at each point where samples were collected. Soil samples consisted of 500 ml of soil collected with a trowel from the top 5 cm after removal of loose debris. All elevation measurements were obtained using a Spectra-Physics Model 650 Laserplane.

Along each transect, we also delineated the wetland edge using methods provided by the U.S. Army Corp of Engineers (1987). For this delineation (hereafter referred to as "standard delineation"), we often relied exclusively on indicators of hydric soils (U.S. Department of Agriculture 1996) and hydrophytic vegetation (Reed 1988) due to the absence of hydrologic indicators (e.g., watermarks, drift lines, sediment deposits, visual observation of saturation) in cropland wetlands. The distance from the wetland center to the wetland edge identified by the standard delineation and the relative elevation of the standard delineation were measured. We sampled each study wetland when their basins were dry and, in the case of the cropland wetlands, had been tilled or planted to an agricultural crop. Sampling of temporary wetlands was conducted between June 1 and July 26, 1995. Due to above-normal precipitation in 1995, most of our seasonal wetlands were flooded, so we delayed sampling of seasonal wetlands until the summer and fall (July 31 and October 1) of 1996 when they were dry.

We processed each soil sample by first sieving the sample through a 0.5-mm screen to concentrate invertebrate remains. We then sorted the remains to determine the presence or absence of aquatic invertebrate remains (Table 1) in each concentrated sample. Not all collected samples were processed. Instead, we processed every fifth sample along a transect to allow us to identify the area of interest (the wetland edge) quickly. We then processed all samples collected within 5 m of the last sample that contained aquatic invertebrate remains and the first sample that did not contain remains. For our delineation using invertebrate remains (hereafter referred to as "invertebrate delineation"), we defined the wetland/upland edge to be the location of the last soil sample along a transect that contained remains of aquatic invertebrates. We then compared distance from the wetland center and relative elevation of the invertebrate delineations to the standard delineations.

Table 1.  Types of aquatic invertebrate evidence used in delineating the wetland/upland edge of seasonal and temporary wetlands in the prairie pothole region.
Taxon Type of evidence
Planorbidae Shells
Lymnaeidae Shells
Physidae Shells
Cladocera Resting Eggs
Ostracoda Shells
Trichoptera Cases

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