Stan Jonah Schouten

Predicting the trajectory of aquatic deoxygenation under global warming requires a mechanistic understanding of lacustrine responses to rapid climate shifts. We investigated how climate-driven changes in catchment vegetation and local iron-rich lithology regulated lake stratification and ecosystem resilience in the maar lake Holzmaar (Central Europe). We focused on the Late Glacial, specifically on transitions during Dansgaard-Oeschger Event 1 (DOE-1; ca. 14 690–11 700 cal yr BP), a period of rapid natural warming and cooling that serves as an analogue for future high amplitude climate variation and for modern Arctic lakes undergoing rapid climate-driven transitions. Combining non-destructive hyperspectral imaging (HSI) of sedimentary pigments with high-resolution XRF geochemistry, we resolved parts of the ecosystem trajectory during DOE-1. The primary producer community shifted from an oligotrophic cyanobacterial and low-light Pleniglacial assemblage to a stable, stratified Allerød assemblage, characterized by the planktonic diatom Stephanodiscus minutulus and anoxygenic purple sulphur bacteria (PSB) in the photic zone. While regional warming (mean summer temperature increased ∼ 2.8 °C) provided the physical potential for lake stratification, our data suggest that intense anoxia was primarily triggered by the expansion of Betula in the watershed. This afforestation stabilized the water column through wind shielding. The termination of the anoxic phase coincided with the onset of the Younger Dryas cooling and increased aridity, which effectively destabilized the existing stratification. While the shift from Betula to Pinus forest may have caused a change in the terrestrial-aquatic linkage, the primary driver of the transition was the physical forcing (lake mixing) of the climatic shift (cooling). Geochemically, the lake exhibited remarkable resilience. Unlike carbonate-dominated systems prone to internal phosphorus loading, Holzmaar efficiently sequesters nutrients via a dual mechanism of reactive iron binding (authigenic vivianite) and stable mineral burial. The phosphorus trap prevents nutrient release by permanently sequestering P in the sediment, allowing rapid ecosystem recovery without delay once the specific climate and vegetation drivers shift. Our findings demonstrate that in volcanic maar lakes, catchment vegetation characteristics and local lithology can modulate, and even override, the direct effects of climate warming on aquatic anoxia.

In the North Atlantic domain, the Late Glacial period (19–11.8 cal ka BP) is characterized by rapid, high-amplitude climatic changes. The amplitude of temperature change in the Central Mediterranean region remains debated because reconstructions are limited and often contradictory. We present a chironomid-inferred temperature record from Lago di Mezzano, Central Italy, covering the Late Glacial. The chironomid record is contextualized with high-resolution multi-proxy data to elucidate the effects of changing primary production, sediment composition, and oxygen availability on chironomid assemblages. During the Oldest Dryas (>14.8 cal ka BP), reconstructed July air temperatures averaged ~12 °C. Temperatures increased in two steps, reaching ~16.8 °C (14.5–14.1 cal ka BP) and, finally, a plateau at ~17.5 °C during the Allerød and the Younger Dryas (13.9–12.0 cal ka BP). This warming is gradual and is paralleled by increasing primary production and the gradual extension of hypolimnetic anoxia. During the Younger Dryas, we observe low primary production, an increase in the siliciclastic fraction, and a shift in chironomid assemblages. This chironomid assemblage shift does not translate into significant summer cooling, suggesting that, while the Younger Dryas climatic reorganization affected catchment hydrology and erosion (Ti, K), the impact of the associated AMOC-slowdown on Central Italian summer temperatures was insignificant. Reconstructed absolute temperatures appeared “too cold” considering the geographical location of Mezzano. This discrepancy may stem from extensive cold, deep lake habitats in Mezzano and the edge effects of the applied transfer function. Habitat preferences of most chironomid taxa matched trophic and anoxic gradients during the Late Glacial.

Combined effects of climate warming and anthropogenic nutrient loadings lead to lake eutrophication and anoxia globally. Because of chemical feedbacks, lakes under multiple stressors often respond in non-linear ways. However, it remains unclear whether climate change alone can lead to non-linear lake responses in the absence of anthropogenic nutrient disturbances. Here, we investigate the interactions between climate variability, nutrient cycling and trophic state changes, mixing regimes, anoxia and related chemical feedback in a small kettle-hole lake in Switzerland during Late Glacial times (15.2–12.6 cal ka BP), a period known for high-amplitude climate change in pre-anthropogenic times. After its formation during Heinrich Stadial 1 (>15 cal ka BP), Soppensee was oligotrophic and well-mixed. Soppensee became eutrophic and developed anoxia at 14.25 cal ka BP. Phosphorus (P) was released from sediments through the reductive dissolution of Fe-oxyhydroxides, fuelling eutrophication. Eutrophication lagged the Bølling warming (14.65 cal ka BP) by 400 years, suggesting that rising temperatures were not the trigger for eutrophication. Instead, eutrophication responded non-linearly to forest closure (threshold at 76 % arboreal pollen AP), which shielded Soppensee from wind mixing, enhancing lake stratification, anoxia and P release, intensifying eutrophication. These conditions ended during the 200-years cold period of the Aegelsee Oscillation (GI-1d, ca. 14.0 cal ka BP) when the landscape regionally opened (AP<76 %); the lake became well-mixed, oxygenated and P was efficiently sequestered. Throughout the Allerød (13.9–12.8 cal ka BP), enhanced Fe input prompted diagenetic vivianite formation, sequestering P in sediments, naturally remediating lake eutrophication despite closed forests, warm temperatures, lake stratification and anoxia.

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.

This protocol provides a hands-on operational workflow for acquiring hyperspectral imaging (HSI) data using the Specim PFD-CL-65-V10E hyperspectral scanner operated through ChemaDAQ. It is designed for high-resolution sediment core scanning and outlines the complete acquisition procedure, including scanner preparation, calibration, scan configuration, white reference acquisition, data handling, and system shutdown.