Abstract
We used a stable isotope tracer to measure nitrogen (N) assimilation and transfer through Bull Trout Lake, a 0.3‐km 2 mountain lake in Idaho, specifically to explore the relative importance of pelagic and benthic producers. was added into the inflow stream above the lake during spring runoff and the resulting mass of tracer was measured within the various ecosystem compartments, including the outflow stream. Although a portion of the moved through the lake quickly due to a low hydraulic residence time during the addition, the tracer was also assimilated rapidly by seston in the water column and at a slower rate by benthic primary producers. By the end of the 10‐d injection, 10% of the tracer had left via outflow, 21% was within seston, and 17% was in epiphytes and macrophytes. However, 70 d after the termination of the injection, only ∼ 1% of the tracer remained within seston, whereas 10% was within the benthic primary production compartment as N was recycled within the benthic zone. Quantitative transfer of 15 N to invertebrate and fish consumers was low, but turnover in these compartments was slow. A conservative water mass tracer (bromide) indicated that the turnover rate for lake water was 1.8% d −1 , whereas 15 N turnover for the whole lake was only 0.7% d −1 , demonstrating how lakes exert drag on nutrients as they move through the watershed. Due to uptake and storage of nutrients, Bull Trout Lake strongly influenced the timing and magnitude of nutrient export from its watershed.