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 the impacts of these stressors are well-documented on modern lakes, a critical gap remains in our understanding of how lake ecosystems responded to climate change in pre-anthropogenic conditions. Most studies investigating the interplay of climate warming, eutrophication and hypolimnetic anoxia rely on models or short-term observations, 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 in a small kettle hole lake (Amsoldingersee, Switzerland) to rapid climate change during the Late-Glacial (18–11 ka BP) using hyperspectral imaging, pigment extractions, XRF and sequential extraction of redox-sensitive P, Mn and Fe.

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 (Greenland Stadial GS-2a, ca. 16.1 ka BP), (ii) the Greenland Interstadials GI-1d and (iii) GI-1b, and (iv) the Younger Dryas (GS-1). Aquatic primary production and algal communities already responded to initial relatively weak warming during Heinrich-1 (16.1 ka 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) as a 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.