Unravelling climate change impacts from other anthropogenic influences in a subalpine lake: a multi-proxy sediment study from Oberer Soiernsee (Northern Alps, Germany)

Andrea M Hofmann,Juergen Geist,Christoph Mayr,Nathalie Dubois,Uta Raeder,Wolfgang Kuefner

doi:10.1007/s10750-021-04640-8

Abstract

Mountain lakes are increasingly impacted by a series of both local and global disturbances. The present study reveals the eutrophication history of a remote subalpine lake (Oberer Soiernsee, Northern Alps, Germany), triggered by deforestation, alpine pasturing, hut construction, tourism and atmospheric deposition, and identifies the intertwined consequences of on-going global warming on the lake’s ecosystem. The primary objective was to disentangle the various direct and indirect impacts of these multiple stressors via down-core analyses. Our multi-proxy approach included subfossil diatom assemblages, carbon and nitrogen stable isotope ratios and subfossil pigments from dated sediments. Shifts within the diatom assemblages were related to variations in trophic state, lake transparency, water temperature and thermal stratification. The organic carbon isotope (δ13Corg) records, the diatom valve density and the pigment concentrations documented the development of primary production and composition. Total nitrogen isotope values (δ15N) are more likely to reflect the history of atmospheric nitrogen pollution than lake-internal processes, also mirrored by the decoupling of δ15N and δ13Corg trends. The composition of sedimentary pigments allowed a differentiation between planktonic and benthic primary production. Concordant trends of all indicators suggested that the lake ecosystem passed a climatic threshold promoted by local and long-distance atmospheric nutrient loadings.

Highlights

Mountain regions are sensitive to climate change
Assuming that the 137Cs peak at 1.5 cm corresponds to the Chernobyl disaster in AD 1986, the sedimentation rate has been very low during recent times (0.05 cm year-1). Since this does not correlate with our results of a stable isotope and pigment analysis that indicate enhanced in-lake productivity, we attributed this to the use of floristic foam to seal the core after sampling, as it destroys the topmost core section
The results of our study show that lake responses to climatic and human influences are complex, multidimensional, and often indirectly mediated through watershed processes

Summary

Introduction

Mountain regions are sensitive to climate change. They are hotspots of biodiversity (Korner et al, 2016) and very valuable drinking water reservoirs (Beniston, 2006). Recent global warming is affecting mountain lake ecosystems by a series of direct and indirect effects, altering the dynamics of these extremely sensitive habitats External drivers such as climate-induced changes of catchment soil and vegetation cover may affect the chemical characteristics of the lakes (Fritz, 2008; Rosbakh et al, 2014). Changes in precipitation patterns affect the water balance of lakes, especially in karstified catchment areas, leading to significant changes in water levels or enhanced seasonal water level fluctuations (Lauber & Goldscheider, 2014) All these complex impacts of climate change have been superimposed during the Anthropocene by simultaneous fertilisation via a diffuse deposition of atmospheric nitrogen and by local anthropogenic disturbances, e.g. mountain huts, forestry or alpine pastures (McMaster & Schindler, 2005; Jeppesen et al, 2010)

Objectives

Methods

Results

Discussion

Conclusion