The experimental flow to the Colorado River delta: Effects on carbon mobilization in a dry watercourse

Abstract

Here we report on the effects of an experimental flood on the carbon cycling dynamics in the dry watercourse of the Colorado River in Mexico. We observed post‐flood differences in the degree of decay, age, and concentration of dissolved organic carbon (DOC), as well as dissolved CH4 and CO2 concentrations throughout the study site. Our results indicate that this flooded waterway was a limited source of CH4 and CO2 to the atmosphere during the event and that DOC age increased with time of flooding. Based on our findings, we suggest that the interplay between storage and mobilization of carbon and greenhouse gases in arid and semiarid regions is potentially sensitive to changing climate conditions, particularly hydrologic variability. Changes in the radiocarbon age of DOC throughout the flooding event suggest that organic matter (OM) that had been stored for long periods (e.g., millennial) was mobilized by the flooding event along with CO2. The OM residing in the dry riverbed that was mobilized into floodwaters had a signature reflective of degraded vascular plant OM and microbial biomass. Whether this microbial OM was living or dead, our findings support previous work in soils and natural waters showing that microbial OM can remain stable and stored in ecosystems for long time periods. As human appropriation of water resources continues to increase, the episodic drying and rewetting of once natural riverbeds and deltas may fundamentally alter the processing and storage of carbon in such systems.

Fundamental drivers of dissolved organic matter composition across an Arctic effective precipitation gradient

“…Precipitation and snowmelt may also recharge wetlands, which export high concentrations of aromatic DOM (Aitkenhead and McDowell ; Spencer et al ). Evaporation alone would increase DOC concentration concurrent with SpC, as evaporation in arid regions acts as a perennial control of solute concentrations in the absence of flooding (Wiklund et al ; Bianchi et al ). The extent to which evaporation can influence DOC concentration depends on the water residence time (Jones et al ).…”

Dry phase conditions prime wet‐phase dissolved organic matter dynamics in intermittent rivers

“…On the contrary, the reestablishment of the surface water flow by heavy rains after a long uninterrupted dry phase would flush the entire load of concentrated DOM and nutrients into downstream aquatic ecosystems in a single event. In conclusion, the frequency of rain events during the dry phase and its duration may regulate the load of carbon and nutrients flushed during flow resumption and thus have clear consequences for C processing and CO 2 emissions from intermittent rivers (Gallo et al ; Bianchi et al ; Datry et al ).…”

“…This could translate to an increased export of recalcitrant DOM from open intermittent riverbeds (Fig. a), with potentially noticeable implications for C cycling in far‐downstream receptor systems (Hladyz et al ; Bianchi et al ). With respect to carbon cycling, open intermittent rivers might thus be considered rather active “abiotic reactors” during the dry phase but more conservative “pipes” in the aquatic phase following rewetting.…”

“…Released nutrients and DOM can result in hot moments (McClain et al ) of microbial activity after rewetting. These are short periods of time with intense biogeochemical processing that can be pivotal for stream ecosystem functioning (Gallo et al ; Bianchi et al ; Datry et al ). Understanding these dynamics requires knowledge of processes occurring before flow resumption, in particular, understanding how environmental conditions during the dry phase can modulate the chemical composition of accumulated organic materials and hence, the biodegradability of their leachates.…”

Representing the function and sensitivity of coastal interfaces in Earth system models

“…The dominant chronic stressors on coastal ecosystems are sea-level rise 15,16 , temperature increases 69 , ocean acidification 70 (Fig. 4), land use conversion (e.g., urbanization), and long-term alterations to water flow (e.g., river impoundment and water extraction) and coastal-estuarine circulation 71 (Fig. 4).…”