Nitrogen transport through linked stream-lake systems measured with 15 N tracer Wayne Wurtsbaugh Koren Nydick Michelle Baker Bob Hall Stream-Lake Interaction Project (SLI) Utah State University University of Wyoming

Stream-Lake Interaction (SLI) Program Question: How do complex patterns of lakes and stream influence ecosystem function? Boulder Chain Lakes Watershed, White Cloud Mountains, Idaho A B D C E ?

Boise Sawtooth Mountains, ID

Tracer Addition Experiments Bull Trout Watershed Sawtooth Mountains, Idaho Lake: 28 ha 16 m deep

Deep Chlorophyll Layers Chl a 21 Jun August 2003 °C

Tracer Addition Experiments Bull Trout Watershed Sawtooth Mountains, Idaho Drip 1800 m

Tracer Addition Experiments Bull Trout Watershed Sawtooth Mountains, Idaho Lake: 28 ha 16 m deep 0.2  M NO 3 Solution containing 70 g 15 N-NO kg Br - added continuously for 14 d - June Aug 2003 Drip 1800 m

Tracer Addition Experiments Bull Trout Watershed Sawtooth Mountains, Idaho Lake: 28 ha 16 m deep 0.2  M NO 3 Solution containing 70 g 15 N-NO 3 55 kg Br - added continuously for 14 d - June Aug 2003 Streams 15 N Dissolved N Benthic (3) Seston export Drip 1800 m

Tracer Addition Experiments Bull Trout Watershed Sawtooth Mountains, Idaho Lake: 28 ha 16 m deep Drip 1800 m m Solution containing 15 N-NO 3 and Br added continuously for 14 d - June Aug 2003 Streams 15 N Dissolved N Benthic (3) Seston export Lake Dissolved Seston Epiphytes Epipelic (in progress) Sedimentation Zooplankton

Discharge & Residence times

2002 Inflow Outflow May Jun Jul Aug Sep NO 3 – N (  M) Inflow Outflow Spring Runoff Baseflow addition Biota N or N & P co-limited Stream

Stream Benthos & Seston Lake Epiphytes Dissolved 15 N Lake Seston, Zooplankton, Sediment Traps

Stream N15 budgets Spring Runoff (2002) Base Flow (2003) Retained Exported (32%) Retained Export to Lake (73%)

I know I’ll find that tracer in here somewhere…

Rhodamine Plunging Stream Inflows SUMMER

Excess 15 N (seston) Spring runoffBaseflow

Day (48%) Seston Standing Stock Sed- imented Material Epiphytes Zooplankton Nitrate- 15 N = 0.03 % Seston Export Day Sedimented Material Epi- phytes Zooplankton Seston Export Day (62 %) Seston Nitrate- 15 N = undetectable 2002: Budget – 15 N in Lake

N-15 Pathways during Snowmelt Drip Seston Epiphytes Shallow plunging inflow = more N15 in upper layers of lake Sedimentation Seston Export Short hydraulic residence time: seston export = sedimentation NO 3 + little seston High discharge = connectivity to floodplain = high nitrate uptake by inflow wetland High discharge & low temps = little uptake in stream channel Warmer water temps, abundant filter- feeders, and efficient seston retention

Predicted N-15 Pathways during Base Flow NO 3 + more seston Low discharge = lack of connectivity to floodplain = little? nitrate uptake by inflow wetland Low discharge & warmer temps = somewhat more NO 3 uptake in stream channel Deep plunging inflow = more 15 N in deeper layers of lake Seston Epiphytes Sedimentation Seston Export Longer hydraulic residence time: seston export << sedimentation

NO 3 0.1% Benthos 2% Seston 13% Hyporheic* 28% Seston 27% Profundal Sedimentation 4% Littoral** Epiphytes Sedimentation 26% Inflow Lake Stream Outflow 2002 Ecosytem Budget Spring Runoff Day 18

HAVE YOU HUGGED A LIMNOLOGIST TODAY ? Reaching Out to Other Aquatic Environments

National Science Foundation (DEB ) U.S. Forest Service Jim Haefner Joel Moore Agnes Chartier Matt Townsend Jessie Decker Amy Marcarelli Phil Brown Chris Craemer Julia Nielsen Lisa Jeffs Acknowledgements

Abstract ABSTRACT In mountainous regions, lakes and streams are often closely linked in complex spatial patterns. These spatial patterns interact to modify nutrient transport in both lakes and streams, in turn influencing ecosystem function. Despite these coupled interactions, streams and lakes are usually studied in isolation. We used 15N-nitrate additions to a nitrogen-limited subalpine system to help understand coupling and functioning within a stream-lake-stream triplet. During spring runoff, the stream channel retained little tracer, but 30% was retained in the flood plain. The lake incorporated all of the nitrate tracer that reached it and exported only organic nitrogen. During a late summer tracer addition, significantly 3X more tracer was retained in the coupled stream-floodplain system. The lake again retained the remaining dissolved tracer, and over 60% was held in the deep chlorophyll layer and periphyton, thus reducing the flushing of nitrogen to the lake outflow. The timing of nutrient flux through the system, as well as the configuration of streams and lakes in a watershed, will thus have marked influence on watershed retention of nitrogen.

15 N : Br - ratios