Abstract
Rivers continually integrate terrestrial organic matter (OM) into their waters, in a process that transfers 1.9 Pg C yr-1, as the primary linkage between oceanic and terrestrial carbon cycles. Yet rivers are not simple, conservative OM integrators. Patchy local land uses (wetlands, bogs, agriculture) release OM that can disproportionately alter river biogeochemistry and overprint upstream carbon. These releases are quantifiable at the plot scale but remain unpredictable across river reaches and watersheds, critically inhibiting our ability to scale up terrestrial-aquatic linkages to regional/global carbon cycling models. We evaluated OM overprinting distance along a human-influenced watershed to quantify river integration of terrestrial OM and to bridge the quantification gap between habitats and waterway biogeochemistry. We investigated changes in dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition (lignin phenols, excitation-emission spectra using parallel factor analysis [PARAFAC], and the relative fraction of optically active DOM [EEMDOC]). DOC concentrations increased continually (p<0.001) downstream, from median 1.0 mg L-1 at 30 km (headwaters) to 3.3 mg L-1 at the river mouth. This rate of increase corresponded to a DOC overprinting distance—the longitudinal distance over which DOC concentrations double—of 13 km. Mainstem DOC overprinting distance ranged from 8 km (winter, rainy season) to 21 km (summer, dry / irrigation season), highlighting stronger overprinting during increased hydraulic connectivity. Stronger overprinting also correlated to higher EEMDOC (p<0.001). Overprinting distance effectively quantifies river integration of DOM along the terrestrial-aquatic interface, helping to refine bottom-up carbon cycle estimates, inform upscaling of site-specific fluxes, and track land use and climate influence on river biogeochemistry.
Highlights
Rivers act as biogeochemical integrators across their entire drainage basin (Hedges, 1981; Ertel et al, 1986), embodying— at the chemical level—the foundational concept from stream ecology in which the terrestrial environment determines river characteristics (“In every aspect, the valley rules the stream”) (Hynes, 1975)
We tested for incremental, directional change in organic matter concentration and composition across the Willow Slough system using multiple parameters
Reviewing dissolved organic carbon (DOC) concentrations and how they change along the watershed helps to better identify the magnitude of Parallel Factor Analysis (PARAFAC) component potential transformation of organic matter as it passes through the watershed
Summary
Rivers act as biogeochemical integrators across their entire drainage basin (Hedges, 1981; Ertel et al, 1986), embodying— at the chemical level—the foundational concept from stream ecology in which the terrestrial environment determines river characteristics (“In every aspect, the valley rules the stream”) (Hynes, 1975). This integration process is globally responsible for the capture of about 1.9 Pg yr−1 of carbon from the terrestrial environment (Cole et al, 2007), rendering rivers as the primary linkage between oceanic and terrestrial carbon cycles. Directly quantifying the control that local sources exact over river DOM composition remains elusive, highlighting critical uncertainties surrounding the terrestrial-aquatic interface and its influence on carbon cycling at local to global scales
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