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Wetland Symposium

Use of Macroinvertebrates to Identify and Delineate Temporary and Seasonal Wetlands in the Prairie Pothole Region


U.S. Fish and Wildlife Service, Northern Prairie Wildlife Research Center, Route 1, Box 96C, Jamestown, ND 58401-9736

In wetland classification systems presently used, any one of three wetland characteristics (hydrology, hydrophytic vegetation, or hydric soils) may be used to define wetlands. Using those characteristics, however, it is often difficult to identify and delineate temporary and seasonal wetlands because wetland basins may be dry and tilled, soils may not be classified or mapped, and vegetation may be absent or may consist of facultative species that also occur in uplands. The problem is exacerbated during drought years due to increased agricultural activity within wetland basins. Recalcitrant remains such as shells, chitinous exoskeletons, head capsules, decay-resistant eggs, and aestivating adults or immatures of obligate wetland macroinvertebrates may offer a new approach to identify, classify, and delineate wetlands.

In 1992, we initiated a study to test the feasibility of using macroinvertebrate remains as a classification and identification tool in the Prairie Pothole Region. Ninety-six temporary and seasonal wetlands within agricultural fields were selected at random from 10.4-km2 plots used by the U.S. Fish and Wildlife Service to provide input for the mallard model. We considered only plots from the drift prairie in North Dakota, South Dakota, and Minnesota because of intensive agricultural development and high density of seasonal and temporary wetlands in that ecoregion. We collected 12 soil samples from random sites for each wetland, six from the deepest portion of the wetland basin and six from the adjacent upland. Soil samples were frozen until processed in our laboratory.

We examined the soil samples in two ways. First, we inspected three wetland and three upland soil samples from each wetland for recalcitrant remains of invertebrates under a low-magnification dissecting scope, after concentrating remains by sieving through a 0.5-mm mesh screen. Second, we incubated remaining soil samples in aquaria under standardized light (12-hour daylength), specific conductance (70 mS•cm-2), and temperature regimes (four weeks at 8oC and four weeks at 22oC for each sample).

We considered six response variables in our preliminary analysis: (1) number of species, (2) total count of macroinvertebrates, and (3) log (total count + 1); the three remaining variables were modifications of variables 1-3 weighted by a score that corresponded to the habitat affinity of each invertebrate taxon (i.e., obligate wetland taxa were weighted by one and obligate upland taxa were weighted by 0; three categories of facultative taxa were identified and assigned intermediate scores).

The habitat affinities and corresponding weights assigned to each invertebrate taxon were determined a priori from scientific literature on autecological relationships. Comparisons were made for each wetland class to determine if upland sites could be separated from wetland sites on the basis of macroinvertebrates.

Preliminary data analysis indicates that uplands and wetlands are clearly separable using our hand-sorting technique. The mean number of species in seasonal and temporary wetlands was three times greater in our wetland samples relative to our upland samples. After weighting according to habitat affinities, the separation was even greater, with the mean number of species occurring in our wetland samples being eight times greater than our upland samples from both wetland classes. Remaining variables followed the same trend with our weighted variables separating upland from wetland to a greater extent than our unweighted variables. For all variables considered, wetland sample means were 3-92 times greater than our upland sample means. The separation between wetland and upland was even greater in our aquaria incubation experiments. The taxa that were most important in contributing to these differences were gastropods, cladocerans (including ephippia), fairy shrimp, calanoid copepods, and chironomids.

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