Landscape Patterns of Streams & Lakes in Montane Watersheds Determine Water Temperatures & Nutrient Transport: Watershed Analyses and N Tracer Experiments

In glaciated mountains, lakes are interspersed through watersheds and connected by streams. Although lakes or streams are frequently studied as individual water bodies, studying them as integrated functional units provides considerable insight on temperature patterns, nutrient transport and other functions. In the Sawtooth Mountains (Idaho), inter-lake distance averages 2.8 km. In summer, lakes are solar collectors, and warm outflow streams as much as 10 C, thus increasing metabolic rates. These streams seldom cool to equilibrium temperatures before encountering another lake, where the waters tend to overflow and mix into the epilimnion. With overflow, N-15 tracer experiments demonstrated that water and nutrients pass quickly through the epilimnion to the outflow stream where they can be taken up by periphyton. In contrast, in watersheds without upstream lakes, cold stream inflows plunge into the metalimnia and bring nutrients to the deep chlorophyll layer where they are retained, thus reducing downstream transport to other lakes and stream reaches as much as 5-fold. A landscape approach is consequently necessary to understand how the lakes and streams in these watersheds function