Lake anoxia, primary production, and algal community shifts in response to rapid climate changes during the Late Glacial

Abstract. Lakes around the world are facing growing threats from climate change and human impacts. Rising temperatures and increased nutrient levels are causing eutrophication and deoxygenation, harming freshwater resources and the essential ecosystem services they provide. While modern impacts are well studied, knowledge on the responses of lake ecosystems to climate change in pre-anthropogenic times is still sparse. Current studies often rely on models or short observation time series, making it challenging to isolate the effects of warming from other factors. Lake sediments provide long-term records to study these effects in times prior to anthropogenic impact. We investigate the responses of aquatic primary production, lake stratification, and deoxygenation to rapid climate change during the Late Glacial (18–11 ka cal BP) using hyperspectral imaging, pigment extractions, XRF, and sequential extraction of redox-sensitive P, Mn, and Fe in a small kettle hole lake (Amsoldingersee, Switzerland). Our record reveals that ice cover was the primary driver of hypolimnetic anoxia, while the availability of nutrients determined the composition of algal communities. Four anoxic phases occurred in cold periods with prolonged ice cover: (i) Heinrich 1 (ca. 16.1 ka cal BP), (ii) the Aegelsee Oscillation, (iii) the Gerzensee Oscillation, and (iv) the Younger Dryas. Aquatic primary production and algal communities already responded to initial relatively weak warming during Heinrich 1 (16.1 ka cal BP) long before the rapid Bølling warming and synchronously to rapid climatic changes during Late Glacial times. Responses of the algal community to temperature were strongly modulated by nutrient limitations (P, N, and Si), which have varying importance over time, with dust and volcanic tephra (Laacher See) being major nutrient sources. Anoxic phases changed the algal communities, but these shifts were found to be reversible once the anoxia disappeared. Further, the sediments of Amsoldingersee provide a continuous record of atmospheric dust deposition (Ti, Zr, Si) covering the entire Late Glacial period. The similarity with the NGRIP dust record supports the view that the same large-scale atmospheric circulation regime controlled central Europe (Switzerland) and Greenland.