The river as a chemostat: fresh perspectives on dissolved organic matter flowing down the river continuum

TLDR

DOC and DOM dynamics along river gradients are poorly understood, necessitating better insight into how hydrological and biogeochemical processes control their concentrations from headwaters to large rivers. The study proposes that rivers tend toward chemostasis, with a downstream shift from hydrologic to biogeochemical controls driving preferential loss of aromatic DOM and gain of aliphatic DOM. The authors analyzed approximately 100,000 USGS National Water Information System measurements of DOC concentration and DOM composition across many river sites in the United States. Quantile regression revealed downstream homogenization of DOC concentrations and a shift from aromatic to aliphatic DOM, while DOC–discharge relationships at each site trended toward a zero slope, indicating chemostasis.

Abstract

A better understanding is needed of how hydrological and biogeochemical processes control dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) composition from headwaters downstream to large rivers. We examined a large DOM dataset from the National Water Information System of the US Geological Survey, which represents approximately 100 000 measurements of DOC concentration and DOM composition at many sites along rivers across the United States. Application of quantile regression revealed a tendency towards downstream spatial and temporal homogenization of DOC concentrations and a shift from dominance of aromatic DOM in headwaters to more aliphatic DOM downstream. The DOC concentration–discharge (C-Q) relationships at each site revealed a downstream tendency towards a slope of zero. We propose that despite complexities in river networks that have driven many revisions to the River Continuum Concept, rivers show a tendency towards chemostasis (C-Q slope of zero) because of a downstream shift from a dominance of hydrologic drivers that connect terrestrial DOM sources to streams in the headwaters towards a dominance of instream and near-stream biogeochemical processes that result in preferential losses of aromatic DOM and preferential gains of aliphatic DOM.