Drought-induced discontinuities in the source and degradation of dissolved organic matter in a Mediterranean river

Abstract

The composition of dissolved organic matter (DOM) in rivers results from the different sources and in-stream transformations along the land to ocean aquatic continuum. Riverine DOM sources are highly dependent on the hydrological connection between the river channel and the surrounding terrestrial ecosystems, but how the lack of this connectivity (e.g., during drought episodes) affects the sources and biodegradation of DOM in rivers remains unclear. Here we identified the DOM sources as well as the different DOM pools that are respired along a Mediterranean river during drought by combining absorbance-fluorescence spectroscopy, size-exclusion chromatography, biodegradation assays, and stable and radiocarbon isotopes. DOM composition was highly heterogeneous along the river in response to different sources and in-stream processes in each distinct aquatic environment (i.e., isolated water pools, running waters, and impounded waters in weirs). The reduced hydrological connectivity with terrestrial ecosystems promoted the influence of autochthonous DOM sources. Still, tree leaves from overhanging canopies stood out as an important terrestrial DOM source, especially in sites where water residence time was high such as isolated pools and weirs. Degradation of leaf leachates was a relevant process in these sites, whereas autochthonous DOM and groundwater millennial DOM (>1300 year B.P.) seemed to be degraded in running waters. Overall, our results highlight that the drought-induced hydrological disconnection entails a great spatial heterogeneity in the sources of DOM, which at the same time determines the different DOM pools that are respired in each environment along the riverThis research was funded by the Spanish Ministry of Economy and Competitiveness through the project CARBONET (CGL2011-30474-C02-01). J. P. Casas-Ruiz and Ll. Gómez-Gener were additionally supported by FPI predoctoral grants (BES-2012-059655 and BES-2012-059743), and D. von Schiller by a “Juan de la Cierva” postdoctoral grant (JCI-2010-06397). N. Catalán held a Wenner-Gren foundation stipend (Sweden). Authors also acknowledge the support from the Economy and Knowledge Department of the Catalan Government through Consolidated Research Group (2014 SGR 291)—Catalan Institute for Water Researc

Retention and release of nutrients and dissolved organic carbon in a nutrient-rich stream: A mass balance approach

“…We observed a consistent net DOC release at baseflow and with high stream water temperature in Las Flores. This pattern contrasts with the general assumption that stream reaches behave as reactors under low flow, promoting active DOM processing (Raymond et al, 2016;Casas-Ruiz et al, 2016). Median net release was 860 mg DOC/(m 2 •d) in Las Flores, which contrast with significant DOC net retention observed in streams and rivers from other regions (Sirivichi et al, 2011;Kaushal et al, 2014;Butturini et al, 2016).…”

“…The time that DOC spends in the system (i.e. the residence time) may be one of the main factors determining in-stream DOC processing (Battin et al, 2008;Casas-Ruiz et al, 2016). Low flow velocities in Pampean streams allow the development of dense epiphytic and benthic biofilms (Giorgi et al, 2005) and may provide physical opportunities for microorganisms to metabolize organic carbon (Battin et al, 2008).…”

Review: Water quality in non-perennial rivers (with erratum)

“…The extent to which redox-driven chemical transformations will occur in a N-PR is likely to depend on the length of time pools have been hydrologically isolated (Mosley, 2015). The diffusion rate of oxygen between air and water has a marked effect on the oxygen content of water and this in turn is strongly influenced by turbulence, which is a function of current speed and stream bed characteristics (Campbell, 1982), and wind. Slowly flowing or stagnant waters, on the other hand, are prone to anoxia.…”

Influence of Hydrological Perturbations and Riverbed Sediment Characteristics on Hyporheic Zone Respiration of CO2 and N2

“…As a zone that links the river and aquifer compartments, hyporheic zones represent a transitional region important for a multitude of ecosystem services (J. W. Harvey & Gooseff, ; Larned et al, ). Hyporheic zones support heterotrophic microbial aerobic respiration (AR), anaerobic denitrification (DN), and nitrification (NI) (Gomez‐Velez et al, ; J. W. Harvey et al, ), processes that transform a significant percentage of terrestrial organic matter (Casas‐Ruiz et al, ; Findlay, ; Rode et al, ). Here we define the hyporheic zone as the portion of the sediment where biogeochemical conditions are influenced by the bidirectional supply of nutrients from the stream or aquifer across steep redox gradients (Figure ).…”