Environmental conditions in high mountain lakes containing toxic benthic cyanobacteria

Summer-dry headwater streams provide an important interface between aquatic and terrestrial environments. Six summer-dry streams differing in flow duration and exposure were studied in western Oregon. On a temporal and a spatial scale, nitrate patterns in such systems reflect the close connection to subsurface flow and nitrification/denitrification processes in the soil. Retention efficiency for sediment generated from a forest road was high. In ephemeral streams, 60–80% of suspended sediment (1.6 μm < suspended sediment < 53 μm) was removed from the water column over a 75 m stretch at moderate input levels. During injection trials solute removal was largely due to groundwater exchange. Exchange rates between stream water and subsurface flow were estimated at 0.75 and 0.8% per meter of channel. Particularly high removal of nitrate in a meadow stream indicated biological uptake. This revised version was published online in July 2006 with corrections to the Cover Date.