Roman-driven cultural eutrophication of Lake Murten, Switzerland

Mischa Haas,Franziska Baumann,Daniel Castella,Negar Haghipour,Anna Reusch,Michael Strasser,Timothy Ian Eglinton,Nathalie Dubois

doi:10.1016/j.epsl.2018.10.027

Abstract

Land cover transformations have accompanied the rise and fall of civilizations for thousands of years, exerting strong influence on the surrounding environment. Soil erosion and the associated outwash of nutrients are a main cause of eutrophication of aquatic ecosystems. Despite the great challenges of water protection in the face of climate change, large uncertainties remain concerning the timescales for recovery of aquatic ecosystems impacted by hypoxia. This study seeks to address this issue by investigating the sedimentary record of Lake Murten (Switzerland), which witnessed several phases of intensive human land-use over the past 2000 years.Application of geophysical and geochemical methods to a 10 m-long sediment core revealed that soil erosion increased drastically with the rise of the Roman City of Aventicum (30 CE). During this period, the radiocarbon age of the bulk sedimentary organic carbon (OC) increasingly deviated from the modeled deposition age, indicating rapid flushing of old soil OC from the surrounding catchment driven by intensive land-use. Enhanced nutrient delivery resulted in an episode of cultural eutrophication, as shown by the deposition of varved sediments. Human activity drastically decreased towards the end of the Roman period (3rd century CE), resulting in land abandonment and renaturation. Recovery of the lake ecosystem from bottom-water hypoxia after the peak in human activity took around 50 years, while approximately 300 years passed until sediment accumulation reached steady state conditions on the surrounding landscape. These findings suggest that the legacy of anthropogenic perturbation to watersheds may persist for centuries.

Highlights

Human activity has become an important geological agent, influencing key biogeochemical cycles by modifying the Earth’s surface (Olofsson and Hickler, 2008)
During this time period the C/N ratio has a mean of 12 and the δ13COC and δ15N isotopic signature of the organic matter remains relatively stable around −30h and 5h respectively. δ13COC values are in the range of that of terrestrial plants and soils (Stuiver, 1975), pointing towards predominantly terrestrial input of OM
During Roman times (15 before the Common Era (BCE)–3rd century Common Era (CE)) rapid agricultural expansion and the growing influence of the ancient city Aventicum led to increased runoff of nutrient-rich soil materials, resulting in eutrophication of Lake Murten and the accumulation of varved sediments

Summary

Introduction

Human activity has become an important geological agent, influencing key biogeochemical cycles by modifying the Earth’s surface (Olofsson and Hickler, 2008). Agricultural and urban areas have increased dramatically in the last decades to ensure food security and living space, land cover transformations are not a recent phenomenon, and are intimately linked with human history. During deforestation and land cultivation, nutrients of the catchment soils, such as phosphorus (P), nitrogen (N) and iron oxides (Fe oxides), are washed out following hydrological pathways, exerting strong influence on key geochemical cycles in aquatic ecosystems within corresponding watersheds (Ekholm and Lehtoranta, 2012). Increased nutrient export from catchment soils due to urbanization is nowadays – and was in the past – a key cause of eutrophication, acidification, and bottom-water hypoxia of lakes (Jenny et al, 2016).

Methods

Results

Conclusion