Nitrogen Transport, Transformation and Cycling through a Mountain Lake, Bull Trout Lake, Idaho, USA

The effects of a mountain lake on nitrogen dynamics in a sub-alpine watershed were examined via watershed monitoring, mesocosm experiments, microcosm experiments, and enzymatic assays during spring and summer of a single year. Our study addressed the questions: (1) How does hydrologic transport through the lake affect the net fluxes of dissolved nitrogen (N) species? (2) What are the net effects of the littoral zone biota on dissolved N fluxes? (3) What are the seston and benthic uptake rates of nitrate? (4) What is the magnitude of N retention in littoral zone sediments? (5) What role does microbial hydrolysis of amino-groups from organic matter play in the uptake of dissolved nitrogen, relative to rates of nitrate uptake? Our study found a net positive flux of total dissolved N and dissolved organic N (DON), and a net negative flux of nitrate through the lake. During snowmelt, when the majority of nutrients are transported in this watershed, DON was retained in the lake. Several experiments were run to more closely examine the mechanisms behind these observations. Experiments in 2.1 m3 mesocosms in June and July measured rates of DON flux from the littoral zone sediments into the water column that were similar to increments measured in the lake. 15N-nitrate mesocosm and microcosm tracer experiments quantified benthic and pelagic nitrate uptake and retention of that nitrate in the benthic sediments. Areal nitrate uptake was 65-times greater in the sediments than in the water column seston and the turnover rate (half life) of the newly input nitrate pool in the sediments was 33-64 days. Finally, the prevalence of DON relative to dissolved inorganic N (DIN) and high measured rates of enzymatic amino acid hydrolysis suggest the importance of DON as a source of N for this aquatic system