Travel time and source variation explain the molecular transformation of
dissolved organic matter in an Alpine stream network
Abstract
Streams and rivers are important components of the carbon cycle as they
simultaneously transport and transform terrigenous dissolved organic
matter (DOM). The time DOM spends in a stream network is an important
constraint on the biogeochemical processes that act upon DOM. We used
high-resolution Fourier-Transform Ion Cyclotron Resonance Mass
Spectrometry (FT-ICR MS) to study the spatial distribution of DOM at the
molecular level at more than 100 sites within an Alpine stream network
during summer and winter baseflow. We developed a model approximating
the time DOM spent in the fluvial network. Discharge-weighted travel
time (DWTT) explained the compositional changes of DOM, which differed
markedly in summer and winter. We attribute the seasonal differences to
differences in source material. Hydrological mixing at confluences was
an important driver of the spatial dynamics of DOM. From the spatial
patterns of individual DOM compounds we inferred the distribution of
sources within the catchment, which differed seasonally. Finally, we
estimated the apparent mass transfer coefficients of individual DOM
compounds at the network level and identified the oxidative state of DOM
as an important factor explaining uptake efficiency. This work
contributes to our understanding of the spatial processes, temporal
constraints and chemical properties of DOM that regulate the
transformation and diagenesis of DOM at the fluvial network scale.