HYDROGEOLOGIC AND VEGETATIVE GRADIENTS ACROSS A WETLAND TRANSECT IN SOUTH CENTRAL WISCONSIN

Restoration of disturbed wetland systems is studied and practiced widely, yet there remains uncertainty about the controls on undisturbed wetlands. In order to design more effective restoration strategies and reestablish native plant communities in disturbed wetlands, it is imperative to understand undisturbed systems.

Cherry Island of Cherokee Marsh located in Madison, WI, is surrounded by a relatively undisturbed area of wetland consisting of plant communities common within the prairie landscape including a fen, sedge meadow, and shallow marsh. These distinct communities are found within an area of minimal topographic relief yet transitions from one community to the next occur over short distances. The objective of this project was to characterize the geologic, hydrologic, and chemical gradients that may explain these shifts in vegetation.

Methods employed included vegetation surveys, continuous recording of water levels in shallow wells, analyses of major ions, and stable isotopes of oxygen and hydrogen. In depth investigation of the subsurface using a soft sediment coring method and grain size analysis was also completed.

Vegetation analysis revealed a transition of dominant sedge species, which appears to correspond to changes in hydrology from a groundwater to surface water dominated system. Along the same vegetation transect, the water chemistry changes from ambient groundwater composition to waters high in dissolved solids and high in dissolved iron relative to the rest of the site. The results of hydrograph analysis in combination with species survey data indicate that the presence of wetland plant communities is related to distinct patterns of water level rise and fall (hydroperiods) associated with evapotranspiration and rainfall events. Subsurface coring results show a heterogeneous composition of peat and till with significant lateral variations in stratigraphy. A silt (82.67%) and clay (15.78%) unit underlying the emergent marsh exclusively can be correlated with degraded water chemistry and lower species diversity.

The results of this research indicate the importance of thorough, interdisciplinary investigation of wetlands emphasizing the importance of a systems approach.