ADVECTION AT THE SEDIMENT-WATER INTERFACE AFFECTS THE HYDROLOGY, GEOCHEMISTRY, AND ECOLOGY OF THE SHINGOBEE RIVER WATERSHED, NORTHERN MINNESOTA

Flow of water, solutes, and nutrients across the sediment-water interface at a fen, a stream and two lakes in northern Minnesota influences the local- and watershed-scale hydrology, geochemistry and ecology. At Little Shingobee Fen, ground water discharges at a break in slope between a steep-sloping upland and the edge of the fen. Ground-water discharge areas can be identified based on the distribution of marsh marigold. On a larger scale, the resultant stable hydrologic conditions in the fen allow rare insectivorous pitcher plants and several species of orchids to exist during drought conditions, and relatively constant discharge of water from the fen via small streams also helps to minimize water-level declines in an adjacent small lake during dry periods. Flow into and out of the nearby Shingobee River streambed affects nitrification and denitrification on a local scale, and possibly the distribution of Elodea canadensis mats growing there. At the corridor scale, discharge of ground water to the stream, either directly or via bankside seeps, provides a stable nutrient source for aquatic and riparian plants. At Williams Lake, near the headwaters of the Shingobee River watershed, temporal reversals in direction of flow between ground water and surface water are related to hydrogeologic setting and timing and magnitude of rainfall. Locally, flow reversals impact the hydraulic and chemical gradients in shallow sediments beneath the sediment-water interface. Longer-term changes in hydraulic gradient may also impact the lake-stage response to significant precipitation events. At Shingobee Lake, ground-water discharge is focused at springs, where algal and bacterial mats displace rooted macrophytes that dominate in littoral sediments where springs are absent. On a lake-wide scale, ground-water discharge provides a much higher inorganic-carbon content in the lake than would be predicted based on streamflow, which may be related to greater lake productivity in Shingobee Lake relative to Williams Lake.