Nitrogen flow pathways through an alpine lake

Background/Question/Methods Lakes have historically been underappreciated and neglected as critical nutrient processors when viewed at the watershed scale. Alpine lake ecosystems may rely heavily upon spring nutrient inputs from their upper watersheds to fuel production through the year. In order to better understand nitrogen dynamics in a stream-lake-stream linkage, isotopically enriched nitrate (15NO3) was injected for 10 days at multiple sites within a watershed in the Sawtooth Mountains of Idaho during spring runoff. Subsequently, the flow of the heavy isotope (15N) was followed through the lake ecosystem throughout the summer.  Here we report on nitrogen (N) cycling in the seston, epiphytes, sedimenting particles, zooplankton and benthic invertebrate pools in Bull Trout Lake (BTL). Bull Trout Lake is a 0.30 km² alpine system linked with 2^nd-order inflow and outflow streams.

Results/Conclusions

Our results indicate that a large proportion of added 15N was taken up by pelagic photoautotrophs in the lake before being transferred to primary consumers and benthic sediments (via sedimentation). The greatest increases in delta 15N values were seen in the epilimnetic seston and zooplankton. Nine days after the start of the injection 3-m seston del 15N increased 624‰_. Zooplankton enrichment peaked later at 544‰ above background, 15 days after the start of the injection. Epiphytes growing on submergent macrophytes did not become isotopically labeled as high as seston and peaked at 94‰, however, due to the abundance of submergent macrophytes in BTL, N uptake by epiphytes may be quite significant. Particles sedimenting out of the water column were not labeled nearly as high as seston and zooplankton; likely due to dilution by sedimenting particles from the metalimnion. Epilimnetic enrichment values were highest early in the season, however, with time deeper samples became 30% more enriched than those in the epilimnion; consistent with the development of a deep chlorophyll layer below 6 m. Aquatic insects sampled from littoral macrophytes and sediments became labeled with 15N within one week of the injection (4.5‰ above background), however peak values were not reached until mid-summer (depending on taxa and feeding group) with Ephemeroptera (38‰) peaking in late July and Odonata (42‰) and Amphipoda (10‰) in mid August. This research, in conjunction with results from the stream enrichment study, will improve understanding of nutrient flow in mountain watersheds. We demonstrate the value of whole-ecosystem stable isotope analyses as a tool for understanding nutrient flow pathways through complex ecosystems.