Collaborative Research: Arctic Oases - How does the delayed release of winter discharge from aufeis affect the ecosystem structure and function of rivers?

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This is a preliminary study to assess the potential effects of aufeis on the structure and function of arctic river-floodplain ecosystems. Aufeis are massive accumulations of ice common to many arctic rivers, particularly in northeastern Siberia and the North Slope of Alaska. River aufeis form during freeze-up when thickening surface ice and permafrost restrict river discharge, causing water to overflow existing ice. Successive cycles of overflowing and freezing of water from upstream groundwater sources accumulate enormous quantities of ice, with late-winter aufeis on some rivers attaining thicknesses of 3 to 5 meters, covering up to 20 square kilometers in some Alaskan fields (even larger in Siberia), and storing as much as a third of the annual discharge of the river in the form of ice. In addition, aufeis maintain an unfrozen base throughout winter due to the release of latent heat as water is converted to ice, which warms the underlying sediments, and a thick insulating layer of ice above. Although aufeis have been studied by hydrologists and are known to be required for winter and spawning habitat for important local fish species, understanding of their ecology is poor. In recognition of this, these investigators will conduct a targeted assessment of the ecological importance of a single aufeis field on the Kuparuk River, Alaska as a first step toward developing a hypothesis-driven research program. The investigators propose that aufeis be viewed as winter oases due to their wet bases that supply water and heat to otherwise frozen habitats. Aufeis also function as summer oases by providing a stable supply of water to riverbed and downstream habitats. This preliminary study will focus on assessing 1) river flow and nutrient chemistry above, within, and below the aufeis field, and 2) the potential for year-round, unfrozen habitat beneath the aufeis field. The investigators will collect water samples for nutrient analysis in transects extending 2.5 km upstream and downstream from the aufeis field. They will collect hourly data on river stage, temperature, and electrical conductivity along each transect. They will also install pairs of piezometers to depths of 1 meter and 2 meters to measure the vertical hydraulic gradient between the underlying aquifer and the surface channel, and dissolved oxygen, temperature, and electrical conductivity of hyporheic waters at these points. Measurements of vertical hydraulic gradient will determine whether river reaches are generally upwelling or downwelling. To estimate volume and the potential seasonal contributions of the aufeis to river discharge they will use precise digital elevation models created from low-altitude vertical photogrammetry. Year-round temperature and electrical conductivity records will provide data for simple end-member mixing analyses to determine the proportion of aufeis meltwater to the river during the thaw season. During freeze-up and into the winter, these records will provide estimates of potential downstream influences of the spring-water downstream of the aufeis if subsurface flow is maintained. In order to assess the vertical influence of aufeis, the investigators will instrument nine transects with digital temperature loggers. They anticipate that loggers directly under the aufeis will never record temperatures below freezing but that those outside the aufeis footprint will. The investigators also propose August and April ground-penetrating radar surveys to determine thaw depths. Finally, PVC wells penetrating about one meter below the surface will be used to sample small invertebrate animals living in river sediments and below aufeis. The project will support two masters-level graduate students and will involve two undergraduate field assistants in the summer fieldwork.