Spatial and temporal exploration of lake-water humic DOM quality - from the Northern hemisphere

A widespread increase in the level of dissolved organic matter (DOM) in lakes has been documented across the northern hemisphere. Interestingly, a corresponding increase has not been seen in elements that are typically strongly linked to DOM (e.g., nitrogen, phosphorous, mercury, iron, and aluminium). This has been hypothesized to result from changes in the quality of the DOM. Here, we explore the link between DOM quality and temporal change in selected water chemical parameters  (generalised additive model). The study sites cover 400 lakes across Norway, providing large gradients in climate, atmospheric deposition, land cover, and primary productivity. At two timepoints (years 1995 and 2019) the lakes were sampled and the water chemistry determined, while the quality of DOM was described in 2019 only. DOM qualitative parameters covered fluorescence parallel factor analysis (PARAFAC) model components, fluorescence indices (BIX, HIX), light absorption indices and slope ratios (e.g., sUVa, A250_364), and biodegradation rate constants derived from incubation experiments.   Temporal changes in the lake-water chemistry covered declines in elemental ratios of nitrogen-to-carbon, iron-to-carbon, and non-labile aluminium-to-carbon. For iron and nitrogen, strong regional variations were observed (de Wit et al., 2023). The three PARAFAC components explained 72% of the spatial variation in the 2019 dataset of selected catchment and water chemistry descriptors (partial least square regression analysis). The characteristics of the underlying fluorophores were identified to be 1) allochthonous humic matter of terrestrial origin, 2) autochthonous humic matter that had been microbially or photochemically altered, and 3) humic matter with larger size and more hydrophobic and aromatic character than component 1. This was found from matches with the OpenFluor database and further substantiated through strong correlations within the dataset. Preliminary investigations show strong links between temporal change in the nitrogen-to-carbon ratio and component 1 as well as to the fluorescence humification index (HIX), the light absorption slope ratio (A250_364), and to the fraction of forest in the catchment. For iron-to-carbon, the temporal change can be linked to component 2 as well as to the light absorption index, sUVa. These findings can support the hypothesis of changing DOM quality.