Complex interactions of in-stream dissolved organic matter and nutrient spiralling unravelled by Bayesian regression analysis
Authors: Pucher Matthias , Flödl Peter , Graeber Daniel , Felsenstein Klaus , Hein Thomas , Weigelhofer Gabriele 
Published: May 2021

Abstract. Uptake and release patterns of dissolved organic matter
(DOM) compounds and co-transported nutrients are entangled, and the current
literature does not provide a consistent picture of the interactions between
the retention processes of DOM fractions. We performed plateau addition
experiments with five different complex DOM leachates in a small
experimental stream impacted by diffuse agricultural pollution. The study
used a wide range of DOM qualities by including leachates of cow dung, pig
dung, corn leaves, leaves from trees, and whole nettle plants. We measured changes
in nutrient and dissolved organic carbon (DOC) concentrations along the
stream course and determined DOM fractions by fluorescence measurements and
parallel factor (PARAFAC) decomposition. To assess the influences of
hydrological transport processes, we used a 1D hydrodynamic model. We developed a non-linear Bayesian approach based on the nutrient spiralling
concept, which we named the “interactions in nutrient spirals using Bayesian
regression” (INSBIRE) approach. This approach can disentangle complex
interactions of biotic and abiotic drivers of reactive solutes' uptake in
multi-component DOM sources. It can show the variability of the uptake
velocities and quantify their uncertainty distributions. Furthermore,
previous knowledge of nutrient spiralling can be included in the model using
prior probability distributions. We used INSBIRE to assess interactions of
compound-specific DOM and nutrient spiralling metrics in our experiment. Bulk DOC uptake varied among sources, showing decreasing uptake velocities
in the following order: corn > pig dung > leaves > nettles > cow dung. We found no correlations between
bulk DOC uptake and the amounts of protein-like compounds or co-leached
soluble reactive phosphorus (SRP). The fastest uptake was observed for SRP
and the tryptophan-like component, while the other DOM components' uptake
velocities more or less resembled that of the bulk DOC. Almost all DOM
components showed a negative relationship between uptake and concentration,
known as efficiency loss. Furthermore, we observed a few negative and (weak)
positive interactions between the uptake and the concentration of different
components, such as a decreased uptake of protein-like compounds at high
concentrations of a high-molecular-weight humic-like compound. We also found an
influence of the wetted width on the uptake of SRP and a microbially derived humic substance, which indicates the
importance of the sediment–water interface for P and humic C cycling in the
studied stream. Overall, we show that bulk DOC is a weak predictor of DOC uptake behaviour
for complex DOM leachates. Individual DOM compound uptake, including
co-leached nutrients, is controlled by both internal (quality-related) and
external (environmental) factors within the same aquatic ecosystem. We
conclude that the cycling of different C fractions and their mutual
interaction with N and P uptake in streams is a complex, non-linear problem,
which can only be assessed with advanced non-linear approaches, such as the
presented INSBIRE approach.