Critical Nitrogen Deposition Loads in High-elevation Lakes of the Western US Inferred from Paleolimnological Records

Abstract

Critical loads of nitrogen (N) from atmospheric deposition were determined for alpine lake ecosystems in the western US using fossil diatom assemblages in lake sediment cores. Changes in diatom species over the last century were indicative of N enrichment in two areas, the eastern Sierra Nevada, starting between 1960 and 1965, and the Greater Yellowstone Ecosystem, starting in 1980. In contrast, no changes in diatom community structure were apparent in lakes of Glacier National Park. To determine critical N loads that elicited these community changes, we modeled wet nitrogen deposition rates for the period in which diatom shifts first occurred in each area using deposition data spanning from 1980 to 2007. We determined a critical load of 1.4 kg N ha −1 year −1 wet N deposition to elicit key nutrient enrichment effects on diatom communities in both the eastern Sierra Nevada and the Greater Yellowstone Ecosystem.

Climate‐induced changes in lake ecosystem structure inferred from coupled neo‐ and paleoecological approaches

“…The relative abundances of this species increased in both sediment cores at some point during the last two centuries (after 1850 AD in Hidden and 1940 AD in Siskiwit). At present, both lakes have comparable, moderate, nitrate concentrations, and their sediment records do not reveal increases in diatom taxa indicative of N enrichment over the 20th century (Saros et al 2011), hence we infer that the primary driver of changes in the abundance of this species in these two phosphorus-limited lakes is mixing depth. We suggest that nitrate concentrations and the presence of N indicator taxa in sediment records should be determined when making inferences from C. comensis.…”

“…A 30-cm gravity core was collected from the deepest spot in Siskiwit Lake in July 2010. Both cores were subsampled in the field in continuous 0.5-cm increments, and dated from 210 Pb activities counted by gamma ray (Hidden) or alpha (Siskiwit) spectroscopy methods, as described in Saros et al (2011). Chronology was based on the constant rate of supply (CRS) model.…”

“…However, our results suggest that nutrient conditions are an important component in the response of this species to changes in lake mixing depths. Interestingly, D. stelligera is either absent or has declined over the last century in sediment records from other small lakes in Glacier National Park without glaciers present within their watersheds (Saros et al 2011), and hence lacking associated nitrogen subsidies ).…”

Critical Loads of Acid Deposition for Wilderness Lakes in the Sierra Nevada (California) Estimated by the Steady-State Water Chemistry Model

“…The nitrogen content and changes in community composition of lichens (Fenn et al 2010;Geiser et al 2010) and terrestrial plants have been used as the basis for eutrophication critical loads. Fossil diatom assemblages have been used to infer critical loads for acidification (Holmes et al 1989) and eutrophication in mountain lakes (Saros et al 2010), but this approach relies upon very detailed studies of only a few individual lakes. For large regional assessments of lake status, simpler methods are often employed which rely on trends in concentrations of ANC, base cations, NO 3 − , and SO 4 2− in surface waters (Baron 2006;Baron et al 2011).…”

Effects of Stock Use and Backpackers on Water Quality in Wilderness in Sequoia and Kings Canyon National Parks, USA

“…These nutrient additions may cause changes in diatom species assemblages of N-and P-limited alpine lakes and streams, many of which are already receiving elevated levels of nutrients from atmospheric deposition (Sickman et al 2003b;Nanus et al 2012;Clow et al 2003;Saros et al 2010).…”