High-Resolution Insights into Climate-Eutrophication-Anoxia Interactions in Late-Glacial Soppensee

Climate warming is projected to intensify the eutrophication and deoxygenation of lakes globally, exacerbating the already severe consequences for society and ecosystems. Sedimentary archives record the complex interplay between climate, lake mixing, algal production, nutrient dynamics, anoxia and related chemical feedback. However, eutrophication phases could be short-lived, and relevant changes may happen at sub-decadal timescales. Often, eutrophic or anoxic phases are preserved in a few centimetres of sediment and cannot be studied in detail with conventional methods. Imaging techniques aid in investigating eutrophication and deoxygenation in the past by providing continuous, high-resolution and very large data sets for statistical analysis. This study utilises high-resolution techniques — hyperspectral imaging and X-ray fluorescence — to analyse hysteresis behaviour, leads and lags among drivers and responses, and temperature-eutrophication-anoxia relationships in the well-dated Late-Glacial lacustrine sediments of Soppensee (Switzerland). 

Soppensee got eutrophic and developed anoxia during the second half of the Bølling. Phosphorus (P) was efficiently recycled (reductive dissolution) further fuelling eutrophication. Eutrophication lagged warming by 300 years suggesting that warm temperatures were pre-conditional to eutrophication and anoxia while not directly triggering it; Instead, eutrophication responded non-linearly to vegetation dynamics and tree cover around the lake, exhibiting a threshold at 75% tree pollen (TP) and showing hysteresis behaviour. At the onset of eutrophication, the response of anoxia was immediate. Cyanobacteria bloomed with a lead of ~50 years before other phototrophic primary producers, highlighting their potential role as early ecosystem pioneers.

When eutrophication decreased due to landscape opening (TP < 75%) caused by a cold interval (GI1d, Older Dryas), the lake became well-mixed, and P started to become sequestered in sediments, further remediating lake eutrophication. During the warm phases of the Allerød, two more eutrophic and anoxic phases occurred. However, their amplitudes further decreased due to the effective sequestration of P (suppressed chemical feedback). Eutrophication disappeared at the start of the Younger Dryas when the landscape opened, and the lake was well mixed.

This high-resolution study demonstrates the potential of sedimentary imaging techniques to detect short-lived events, rapid regime shifts, leads and lags between forcing and responses in ancient ecosystems.