THE WETLAND CONTINUUM IN THE PRAIRIE POTHOLE REGION

The continuum of the hydrologic cycle in space and time provides a framework for integrating predictable and observable biological features of prairie wetlands. In landscapes characterized by isolated depressions containing wetlands, such as the prairie pothole region, the hydrologic continuum in space is defined by the ground-water system. The wetlands are surface-water expressions of larger ground-water watersheds, in which wetlands serve recharge and discharge functions with respect to ground water. The interaction of these wetlands with ground water, although a small part of their water budget, provides the primary control on delivery of solutes to and from the wetlands. Temporal variability of these interactions is controlled largely by precipitation and evaporation, the dominant components of their water budget. The Cottonwood Lake area in North Dakota has provided a field laboratory for developing and evaluating the wetland continuum concept because the area has experienced many years of relatively normal hydrologic conditions followed by an extreme drought and subsequent flood. The hydrologic continuum of prairie wetlands in space and time provides the necessary framework for integrating predictable and observable biological features of prairie wetlands in the same sense as that proposed for streams by the river continuum concept. In contrast to the river continuum, however, the continuum for prairie wetlands reflects strong gradients in both the spatial hydrologeologic characteristics among hydrologically interconnected wetlands as well as the temporal events that modify the biological productivity of individual wetlands as they cycle between wet and dry periods. Ecosystem studies at the Cottonwood Lake area over the past 30 years illustrate how spatial and temporal hydrologic gradients affect the biota of prairie pothole wetlands throughout wet and dry cycles. Data from the site were used to develop a conceptual model for the wetland continuum proposed here to facilitate valid comparisons among studies, thereby advancing the science and management of prairie pothole wetlands.