Figure 1. Quantitative and qualitative aspects of DOM.

(a) Colour (as a₄₄₀) increases with DOC concentration. Dashed lines represent 95% confidence intervals. (b) Principal component analysis (PCA) based on the optical properties of the 20 streams distinguishes terrigenous from autochthonous DOM (PC1). Terrigenous DOM was characterized by high specific absorption at 254 nm (SUVA₂₅₄) and 440 nm (SA₄₄₀), and a high humification index (HIX). Fluorometry revealed two humic-like compounds of terrigenous origin (C1 and C2), while the third component (C3) was putatively associated with new biological production (Supplementary Information). High values of the freshness index (β/α) and the fluorescence index (FI) but also lower apparent molecular weight (indicated by a255/a365 and the slope ratio S_R) describe DOM with a more autochthonous character. Arrows are based on PCA structural coefficients.