Engineered Continental‐Scale Rivers Can Drive Changes in the Carbon Cycle

Abstract

Rivers play an essential role in the global carbon cycle by exporting ∼1 Pg C/yr (1 Pg = 10 15 g) from land to ocean, outgassing ∼1.1 Pg C/yr into the atmosphere, and storing ∼0.6 Pg C/yr in lakes and reservoirs along their pathways in modern time (Aufdenkampe et al., 2011; Battin et al., 2009; Regnier et al., 2013; Tranvik et al., 2009). Organic carbon (OC) constitutes about half of the riverine carbon reaching the ocean as dissolved and particulate OC (DOC and POC, respectively). In ocean margins, POC dominates the buried terrestrial OC but with highly variable burial efficiency, ranging from <1% to nearly 100%, depending on its reactivity and integrated oxygen-exposure time, and broadly scaling with sediment accretion rate (Blair & Aller, 2012; Burdige, 2007). The burial of terrestrial OC in marine sediments is related primarily to sedimentation from large river systems (Burdige, 2005). River-ocean transfer of OC is a crucial process sequestering carbon from short-lived reservoirs, such as biomass and soils, into the reservoirs represented by marine sediments over geologic timescales (Berner, 1982).

The combined effects of land use and seasonal environmental factors on stream food web structure

“…The impervious surface cover is the main contributor to the loss of connectivity because they usually alter the water flow which usually travels longitudinally and laterally (Humphries et al, 2014;Jones et al, 2019). Our results could be attributed to the riverbank being concreted at urban sites, which is a common practice globally (Figure 2E; Boggs and Sun, 2011;Valle Junior et al, 2015;Shen et al, 2021). Unlike woodland sites, the river channel was disconnected from the floodplain at urban sites, the terrestrial sources in the high flow season were not fully available to consumers, and thus terrestrial sources contributed much less than autochthonous carbon throughout the year.…”

Unexpected Consequences of River Engineering on the Carbon Cycle

“…If riverine organic carbon is deposited in floodplains, it can be oxidized and released to the atmosphere, increasing atmospheric CO 2 levels (Scheingross et al, 2019). In this issue of AGU Advances, (Shen et al, 2021) demonstrate that engineered river bank stabilization may reduce the amount of organic carbon oxidized in floodplains and increase the amount of organic carbon transferred to oceans. This suggests that bank stabilization techniques may create more efficient pathways for removing CO 2 from the atmosphere.Since the first civilizations, humans have been drawn to rivers for their fresh water, navigable routes, and numerous ecosystem services.…”

“…Calculating the balance between floodplain oxidation and lateral input of vegetation will be key for determining whether bank stabilization enables more CO 2 drawdown from the atmosphere compared to natural rivers. Shen et al (2021) have made an important observation that reveals the impacts of river engineering on organic carbon fluxes. This is the first study to document that CO 2 release from floodplains can be reduced by engineered bank stabilization.…”

“…In this issue, (Shen et al, 2021) investigate how artificial levees and other methods of stabilizing river banks have changed the pathways of carbon through the Lower Mississippi River. Artificial levees were first built along the Mississippi River in the early 1700s, and they now restrain more than 2,500 km of its main channel.…”

“…River engineering techniques, like levees that prevent flooding, change the way carbon is cycled through rivers, but it is difficult to identify and quantify these changes. Shen et al (2021) were able to quantify changes in river organic carbon fluxes induced by bank stabilization in the Mississippi River. The authors compared the age distribution of organic carbon in modern river sediment and old river sediment deposited before the construction of extensive levee systems.…”