Hydrodynamic, Seasonal, and Climatic Factors Affecting the Terrestrial-to-Aquatic Export of Plant-Derived Biomarkers

Abstract

Source-specific lipid biomarkers, such as n-alkanes and n-fatty acids, serve as valuable tracers of past and contemporary exchanges of vegetation-derived organic matter across different carbon pools. While substantial research has explored the incorporation and transformation of vascular plant debris within soils, our understanding of the transfer of biospheric carbon from terrestrial to aquatic systems and along the fluvial continuum remains incomplete. To address this knowledge gap, we conducted a 30-month high-resolution time series study in the subalpine Sihl River watershed, monitoring the abundance and composition of higher plant-derived lipids in the suspended load to examine the response of long-chain n-alkanes and n-fatty acids to hydrological variability, seasonal changes, and climatic disturbances.

Our study reveals that over 80% of plant-derived lipids are mobilized during storm-driven events. Although concentrations of vascular plant lipids, when normalized to filtered water volume, increase during high-flow events, both lipid concentrations normalized to sediment dry weight and those normalized to organic carbon decrease. This decline can be attributed to the input of carbon-poor sediments and the concomitant mobilization of non-lipid plant material. Analysis of bulk organic carbon isotopic compositions and n-alkanes and n-fatty acid proxies indicates a shift in organic matter sourcing and transport pathways during storms. These events promote the mobilization of contemporary plant detritus and surface soils, notably from deciduous forests in the lower watershed, driven by surface runoff and flooding. In contrast, under baseflow conditions, plant lipid signals are dominated by coniferous forests in the upper Sihl River basin, where hillslopes directly connect with the aquatic system.

Seasonal variations in n-alkane and n-fatty acid concentrations transported within the suspended load correspond to plant growth cycles, with peaks during fall and winter due to increased plant litter input. The temporal delay between peak n-fatty acid concentrations in fall and peak n-alkane concentrations in winter highlights their distinct chemical properties, decomposition rates, and responses to external influences, providing insights into their dynamics in aquatic environments. This consistent pattern was disrupted in 2018 due to a severe drought, resulting in widespread vegetation dieback and reduced biomass, subsequently affecting biomarker fluxes and homolog chain length distributions.

Our findings provide novel insights into the temporal dynamics of plant biomarker export from subalpine fluvial catchments, revealing a strong influence of seasonal and climatic factors. These findings advance our understanding of hydrological processes, vegetation dynamics, and environmental controls on plant-derived proxy signals and their source-to-sink transfer. Moreover, we highlight potential biases in sediment records. Understanding these dynamics is essential for interpreting sedimentary records accurately and gaining insights into past ecological changes, shedding light on the role of plant-derived organic matter in riverine systems.