Contrasting Seasons and Land Uses Alter Riverine Dissolved Organic Matter Composition

Abstract Different sources and fates control riverine dissolved organic matter (DOM) composition in catchments of contrasting land use, and climate. However, assessing the changes in DOM composition together with nutrient forms along rivers exposed to these gradients remains rare. Here we quantified the spatial and temporal patterns in DOM components and nutrient forms along the mainstem of a 5th order river through sequential forested, urban, and agricultural reaches during low flow moments in summer and winter, and two contrasting springs, one with a historically rare flooding event. There were widespread abrupt shifts in the composition of DOM in low flow seasons that coincided with changes in land use whose sources could be inferred by endmember samples and nutrient changes. Compared to summer, DOM pools considered bio- (microbial-like) and photo-labile (Peak C) tended to dominate during the winter, along with reactive nutrient forms (ammonium, dissolved phosphorus). This implied higher processing potentials during summer, where microbial-humic-like DOM and nitrate dominated, and accumulation of reactive forms during winter. DOM composition remained relatively stable under typical flow conditions with reduced retention time and processing, but major shifts were observed during an extreme flood year, pointing to unusual loadings of highly labile sources of DOM. Overall we found that, despite relatively small changes in the quantity of DOM flowing in this north temperate river, there were major spatial and temporal shifts in its composition and associated nutrients that reflected contrasting loading and processing potentials depending on land use and seasonal patterns in temperature and hydrology.