Divergent responses of alpine lakes to regional and global change drivers since 1800

Background/Question/Methods

Alpine lake sediments preserve markers of envinonmental change that can be interpreted to infer variation in both drivers -- including climate, atmospheric deposition, vegetation, and disturbance -- and responses -- including primary productivity, species assemblages, and nutrient cycling. Recent work demonstrates that shallow alpine lakes are undergoing eutrophication worldwide, despite their remote locations. Lakes within the southern Rocky Mountains exhibited increased productivity and biogeochemical changes starting ca. 1950 and accelerating with regional warming trends since 1980. In light of these findings we collected new sediment cores from additional high elevation sites and surveyed literature from three continents to quantify the prevalence of historical “greening” of alpine lakes in the western US and other mountain ranges. Short sediment cores (25-50 cm) were collected from two lakes in the Sierra Nevada (CA) and one in northern New Mexico. Sediments were dated with 210Pb and sections were analyzed for percent C and N, δ13C, δ15N, and a suite of algal pigments that represent total biomass, chlorophytes, diatoms, cyanobacteria, and other primary producers. Pigment or biogeochemical data from 16 other published studies augmented our data set.

Results/Conclusions

Analyses revealed relative increases in production during the 20th century in all lakes ca. 1950 coincident with the onset of the Anthropocene, but lake-specific variation in regulatory processes were evident. Rates of change in algal production increased in all lakes to present day, a phenomenon attributed to increasing temperature trends ca. 1980. Despite there common changes lakes differed substantially in the magnitude of variation in markers of biogeochemical cycling and algal composition, as determined from analysis of sedimentary pigments and sub-fossil diatoms. In general, lakes with the greatest change in primary production were associated with elevated atmospheric N deposition, whereas phototrophic response was muted in lakes in regions of low N deposition. Diatom remains, fossil pigments, and biogeochemical markers (elemental composition, stable isotope values) suggested that historical variation in species assemblages and lake chemistry may reflect region-specific disturbances including fire, P inputs associated with dust deposition, drought, lake acidification, and food-web alterations. Together, sediment records suggest lake production is increasing across alpine regions from a combination of global (temperature, reactive N deposition) and regional scale processes (fires, fish stocking, land use change).