COS 124-2 - Centennial to decadal-scale changes in algal production and community composition in Santa Fe Lake, NM

Background/Question/Methods

As sentinel ecosystems, mountain environments serve as early warning signals for global change, including the impacts of extreme weather conditions, climate change, and airborne pollutants. Significant human-induced alterations including fossil fuel combustion, anthropogenic nitrogen-fixation, industrial agriculture, human population growth, and land-use change have altered Earth’s biogeochemical cycling and defined the Anthropocene. Imprints of these changes can be determined from lake sediments that accumulate sequentially over time. Recent work using lake sediments (paleolimnology) has documented increased carbon, nitrogen, and heightened productivity indicative of eutrophication in remote mountain lakes. Our study contributes to the growing body of research documenting ecological change in mountain lake systems by documenting the recent conditions of Santa Fe Lake, NM, the southernmost subalpine lake in the Rocky Mountain Range. We seek to capture and identify regional and global anthropogenically driven change and question how these signals relate to in-lake productivity trends over time. Sediments from a 49-cm long core were dated using ²¹⁰Pb activities and analyzed for percent C and N, δ¹³C, δ¹⁵N, algal pigments representative of total biomass, chlorophytes, cryptophytes, diatoms, and other primary producers from Santa Fe Lake.

Results/Conclusions

The core from Santa Fe lake provided a ca. 250-year record of change in phototrophic communities. Lake production (as total algal biomass, %C, %N) was found to increase during the latter half of the 20^th century. Cryptophytes (as alloxanthin) and chlorophytes (Lutein-Zeax) increased throughout most of the record. Diatoms (diatoxanthin) declined at the beginning of the record, increased until the late 20^th century, and declined thereafter. Although Santa Fe Lake exhibited increases in total algal biomass (pheophytin a) and other pigments, these changes were an order of magnitude lower than those observed within lakes in the Southern Rocky Mountains (Rocky Mountain National Park) and did not reflect the exponential increases observed post-1950 in lakes within the Sierra Nevada and Southern Rocky Mountains. Despite exponential increases observed in C and N, along with declining ∂¹³C and ∂¹⁵N values, and C:N ratios, total algal biomass remained relatively stable over time. Our findings highlight the heterogeneity of lakes’ responses to changing environmental conditions associated with the Anthropocene and point to the impact of internal (lake depth, benthic biomass) and external processes (atmospheric deposition) in regulating productivity.