THE ROLE OF GROUNDWATER IN THE DEVELOPMENT AND SUSTENANCE OF GLACIAL-RELICT, BOREAL FEN ECOSYSTEMS OF THE NEBRASKA SAND HILLS

The Nebraska Sand Hills are a grass stabilized sand sea that receives on average only 430 to 585 mm of rainfall each year. Yet this arid region presents a curious paradox in that it contains some of the more unique and productive wetland ecosystems in the world. Within the region’s interdunal valleys, four hydrogeomorphic conditions can be identified - dry valleys, wet meadows, fens, and lakes. The formation of each is dependent on a combination of factors, including: adjacent dune topography, depth of erosion within the valley, depth to water table, degree of development of a local groundwater flow system within the valley, extent of surface drainage within the valley, and in some instances, dune blockage of the local drainage network. Of these four valley types, the fens, often underlain by several meters of saturated, generally undegraded peat, are perhaps the most unique, and contain a number of plant species such as marsh marigold, cotton grass, buckbean, flat-top aster, and Canada rush, for example that are typically found only in more boreal regions of North America. Recently, an interdisciplinary project was initiated to study the complex interactions between hydrology, water/nutrient chemistry, geology and ecology at two representative fen sites within the region. We sought to understand how these unique ecosystems were formed and how they evolved over the past 15,000 years under varied climatic and hydrologic conditions. Results indicate that the fens are fed both by groundwater seeps along their edges, issuing from local flow systems developed beneath the adjacent dunes, and from older up-welling discharge along more regionally extensive flow paths. The combined influx of groundwater is substantial, maintaining water levels near the surface through the year, transporting nutrients to the fen, and fueling evapotranspiration that in turn creates a microclimate above the fen capable of sustaining the ecosystem during drought periods. Sand layers within peat cores taken at the fen suggest that there have been periods in the past where the fens have dried out, yet they survived to thrive again when water levels rebounded. Understanding the reasons for such hydrologic changes is critical in developing long-term conservation strategies for the fens and for predicting how these ecosystems might respond to future climatic changes.