Is climate warming causing eutrophication and anoxia? Lessons learned from Late-Glacial sediments of Lakes Amsoldingen and Soppen, Switzerland.

Many lakes nowadays experience eutrophication, which poses significant threats to ecosystem stability and people who depend on lakes for freshwater. Lake hypoxia is a state with low dissolved oxygen and often associated with external nutrient additions to the lake. Hypoxia typically deteriorates lake water quality by 1) changing the chemistry of the lake water, and 2) challenging heterotrophic organisms but promoting growth of bacterial autotrophs that are adapted to anoxia and may produce harmful toxins. Little is known about how external factors (e.g., nutrients, climate) and algal/bacterial community dynamics compounded into the chemical deterioration of lake water and shaped lake ecology. We hypothesize that, at times without human disturbance, trophicity and hypoxia may have been driven by rapid climatic shifts (e.g., Dansgaard-Oeschger Events, DOE) with a rate and extend comparable to or faster than current global warming. To gain insights into the driving processes of natural eutrophication and recovery phases, we studied the sedimentary records of two comparable Swiss lakes (Soppensee and Amsoldingersee) focussing on their (bio-)geochemistry during the Last Glacial Maximum and Late Glacial (17.0-11.6 cal. kyr. BP). The chronology of the cores was obtained using the Laacher See Tephra, a set of C-14 dated macrofossils, and bio-stratigraphic markers. We combined sequentially extracted data on phosphorous (P), iron (Fe), and manganese (Mn) to elaborate on redox-induced changes within the P, Mn, and Fe cycles. We reconstructed the changes in past primary producer communities using coloured biomarkers chloro-pigments and carotenoids inferred by HPLC as proxies. Using hyperspectral imaging, we assessed bulk pigment groups for leads and lags between primary producer groups on a sub-millimetre resolution. Both lakes experienced similar large-scale forcings and have similar catchment properties. According to our results, the lakes both record algal blooms and anoxia in the Late Glacial, yet there are, surprisingly, significant differences in the timing of these eutrophication phases and anoxia events between the lakes. The Soppensee pigment record responded to the initial Blling warming (14.6 cal. kyr. BP) by developing eutrophic conditions in a stratified lake with hypolimnetic anoxia and redissolution of redox sensitive phosphorous, iron and manganese. In Amsoldingersee, pigment data shows clear anoxic events that pre-date the Blling warming and relate consistently to the colder phases within the Late-Glacial (GS-2/Heinrich Event 1, GI-d, GI-c3, and GS-1). In contrast to Soppensee, total chlorophyll, and carotenoids peaked when the climate was cool and dry, advocating for substantial aquatic production during cold periods. However, the rate of compositional change (RoC) was highest during the three major climatic transitions (DOE-1, Onset Younger Dryas, Onset Holocene), and not during the anoxic phases. From ordination experiments, we further infer that algal/bacterial communities indeed recovered from their anoxic states. In addition, we noticed a surprisingly high pigment diversity throughout the Oldest Dryas (GS-2). Our data add to the view of a dynamic landscape evolution during the Oldest Dryas (Heinrich Event 1) which was previously assumed to be a stable cold phase in the peri-alpine area.