Seasonal patterns of organic matter stoichiometry along a mountain catchment

Organic matter (OM) nutrient content is expected to change as OM is transported from upstream forested reaches downstream to open land, and these changes could depend on seasonal patterns. The aim of this study was to analyze OM carbon nitrogen and phosphorus content, from headwater reaches to open reaches downstream. We sampled four sites along a mountain catchment over the course of one year and quantified fine particulate organic carbon (FPOC) in transport and dissolved organic carbon (DOC) concentration and elemental content. Both FPOC and DOC concentration decreased downstream, and changes in OM stoichiometry depended on the season. Stoichiometry of FPOM and DOM along the catchment was similar in autumn, with higher C:P and C:N ratios upstream, which could be associated with the relative importance of allochthonous versus autochthonous OM origin. Conversely in spring, C: N was very low in the forest sites. Our results highlight that headwaters running through pristine forests are a source of POM and DOM for downstream ecosystems, and that the quality of such resources depends on season.