Document: GRE-3-59-103

Nutrient export from two high mountain lakes to outflow streams: The importance of inflow hydrodynamics and in-lake processes.

BURKART, G.* ¹, W.WURTSBAUGH ¹, B.FLEENOR ² and C.LUECKE ¹

University, Logan, Utah 84322-5210 USA ¹
University of California-Davis, Davis, California 95616 USA ²

Abstract:
The way in which nutrients are transported into and through a lake may influence the availability of nutrients in the outflow. When the temperature of an inflow falls below that of the lake surface water, the inflow can plunge below the thermocline, delivering water and nutrients to the metalimnion. At higher inflow temperatures, however, the inflow will deliver water and nutrients directly to the epilimnion. For these two scenarios, depth-specific residence times and biological processes may lead to differences in the availability of nutrients in the outflow stream. We added water mass (NaBr and rhodamine) and biologically-active (¹⁵N) tracers to two mountain lakes via either a plunging inflow or a surface addition. Measurements of conservative tracers in the outflow over a four-week period following the additions indicate that water entering directly into the epilimnion was flushed to the outflow four times faster than water entering via the plunging inflow. Preliminary data suggests that sedimentation led to substantial losses of the biological tracer within the epilimnion and metalimnion of both lakes; however, ¹⁵N export to the outflow stream was greater when the nutrient tracer was added to the epilimnion than when it plunged into the metalimnion. Delta ¹⁵N values of periphyton in the two outflow streams support this hypothesis. Therefore, the interaction of inflow hydrodynamics and in-lake biological processes may have a significant influence on the amount of nutrients exported from a lake. These findings suggest that factors influencing inflow hydrodynamics, such as increases in temperatures, may influence nutrient export downstream from lakes.

Keywords: Nitrogen, Streams, Lakes, Tracers, Rhodamine, NaBr, 15N, Periphyton

This abstract is being presented at: 3:30 PM in session:
Poster Session #15: Nutrient Cycling.