Metabolism, Gas Exchange, and Carbon Spiraling in Rivers

Abstract

Ecosystem metabolism, that is, gross primary productivity (GPP) and ecosystem respiration (ER), controls organic carbon (OC) cycling in stream and river networks and is expected to vary predictably with network position. However, estimates of metabolism in small streams outnumber those from rivers such that there are limited empirical data comparing metabolism across a range of stream and river sizes. We measured metabolism in 14 rivers (discharge range 14-84 m 3 s -1 ) in the Western and Midwestern United States (US). We estimated GPP, ER, and gas exchange rates using a Lagrangian, 2-station oxygen model solved in a Bayesian framework. GPP ranged from 0.6-22 g O 2 m -2 d -1 and ER tracked GPP, suggesting that autotrophic production supports much of riverine ER in summer. Net ecosystem production, the balance between GPP and ER was 0 or greater in 4 rivers showing autotrophy on that day. River velocity and slope predicted gas exchange estimates from these 14 rivers in agreement with empirical models. Carbon turnover lengths (that is, the distance traveled before OC is mineralized to CO 2 ) ranged from 38 to 1190 km, with the longest turnover lengths in high-sediment, arid-land rivers. We also compared estimated turnover lengths with the relative length of the river segment between major tributaries or lakes; the mean ratio of carbon turnover length to river length was 1.6, demonstrating that rivers can mineralize much of the OC load along their length at baseflow. Carbon mineralization velocities ranged from 0.05 to 0.81 m d -1 , and were not different than measurements from small streams. Given high GPP relative to ER, combined with generally short OC spiraling lengths, rivers can be highly reactive with regard to OC cycling.

Hydromorphological restoration stimulates river ecosystem metabolism

“…Rates of ER mirrored those of GPP, suggesting that autotrophic respiration largely drove temporal patterns in ER, despite an overall ratio of GPP : ER < 1 and a slightly negative NEP during most of the measurement period. Similar patterns were found in streams in the US (Beaulieu et al, 2013;Hall et al, 2016). The average GPP : ER ratio was significantly higher downstream of the restored reaches in our study (0.86 and 0.97, respectively) and in the combined restored reach (1.16) than in the upstream degraded river (0.66), indicating an increase in autotrophic processes following restoration.…”

“…The majority of stream ecosystem metabolism work has investigated natural changes, such as effects of floods and droughts (e.g., Uehlinger, 2000), seasonal or interannual changes (e.g., Uehlinger, 2006;Beaulieu et al, 2013), interbiome differences (e.g., Mulholland et al, 2001), or landuse change (e.g., Gücker et al, 2009;Silva-Junior et al, 2014). The majority of these studies have focused on smaller streams, while only few studies have measured metabolism of larger streams and rivers (e.g., Uehlinger, 2006;Dodds et al, 2013;Hall et al, 2015Hall et al, , 2016. The response of stream metabolism to hydromorphological changes, e.g., through river widening, is almost unknown, especially for larger rivers (but see Colangelo, 2007).…”

Carbon dioxide degassing at the groundwater-stream-atmosphere interface: isotopic equilibration and hydrological mass balance in a sandy watershed

“…In addition, the amount of carbon that originally leaves the terrestrial biosphere is much larger than the amount of terrestrial carbon that ultimately reaches the ocean (Cole et al, 2007). The CO 2 dissolved in riverine waters originates from two different sources and processes (Hotchkiss et al, 2015): (1) internal, i.e., resulting from heterotrophic decomposition (e.g., Hall et al, 2016) and photooxidation (e.g., Moody and Worrall, 2016) of organic matter in the aquatic system itself, or (2) external, i.e., resulting from inputs of groundwater enriched in CO 2 , which comes from plant root and microbial respiration of terrestrial organic matter in soils and groundwater. However, sources of and processes controlling CO 2 emissions change with the size of streams and rivers (Hotchkiss et al, 2015).…”

“…This could be due to the hotspot character of CO 2 evasion and the very fast degassing at the groundwater-stream interface that Another important question that must be carefully considered when comparing the two methods is the contribution of in-stream CO 2 production (i.e., respiration and photooxidation) to degassing. Indeed, when groundwater DOC enters the superficial river network through drainage, part of it might be rapidly recycled by photooxidation (e.g., Macdonald and Minor, 2013;Moody and Worrall, 2016) and by respiration within the stream (e.g., Roberts et al, 2007;Hall et al, 2016). Method 1 is based on the mass balance calculation and assumes that all the CO 2 originates from the groundwater, whereas method 2 is based on gas transfer velocity and accounts for all the CO 2 outgassed from the streams: the CO 2 from the groundwater and the CO 2 produced by in-stream CO 2 production (Battin et al, 2008;Hotchkiss et al, 2015).…”

From land to lake: Contrasting microbial processes across a Great Lakes gradient of organic carbon and inorganic nutrient inventories

“…This unique hydrology forms land margin ecosystems that are hot spots for degradation, sedimentation, and export of organic matter along freshwater tributary systems, and could be an underestimated piece of the carbon cycle puzzle. Modeling studies show that freshwater ecosystems are very important sites of global carbon cycling (Cole et al, 2007;Hall et al, 2016) respiring about half of the terrestrially derived carbon that passes through them and delivering the remainder to oceans. Thus, understanding the range of metabolic processes along the varied waterways of a freshwater land-to-lake gradient is important to fully demonstrating this large contribution to global carbon cycling.…”