Lignin-derived phenolic compounds in the particulate organic matter of a French Mediterranean river: seasonal and spatial variations

Samples from the Tech river (French Mediterranean river) were analysed for particulate organic carbon (POC) and lignin phenols, to characterise spatial and seasonal variations. A decreasing gradient of lignin phenols (from ca. 0.8 to 0.5 mg 100 mg OC−1) was observed downstream, together with an increase of p-hydroxyl (H) phenols. Seasonal variations in total and lignin–phenols composition were very noticeable and seemed to be connected directly to rainfall variations. Lignin-phenol concentrations were higher in the wet periods (up to 12.1 μg/l) while the dry periods were characterised by the predominance of H phenols over lignin-phenols (Λ) (H/Λ ratios reaching values of 245 during summer). This predominance of H phenols was also associated with high chlorophyll a concentrations, which may indicate an autochthonous (and fresher) source of these phenols to the water column. Mean vanillic acid/vanillin ratios were higher than those of fresh plant material (0.4 for POM against 0.2 for fresh plant tissues), indicating that the lignin component had already been degraded before entering the river. The syringyl/vanillyl (S/V) and cinnamyl/vanillyl (C/V) ratios indicate a predominance of angiosperm+gymnosperm woody material in the lower sector and the presence of some angiosperm non-woody material in the mountain sector, corresponding to alpine and sub-alpine vegetation.