Fluvial carbon dioxide emission from the Lena River basin during spring flood 2

22 Greenhouse gas (GHG) emission from inland waters of permafrost-affected regions is one of the 23 key factors of circumpolar aquatic ecosystem response to climate warming and permafrost thaw. Riverine 24 systems of central and eastern Siberia contribute a significant part of the water and carbon (C) export to 25 the Arctic Ocean, yet their C exchange with the atmosphere remain poorly known due to lack of in-situ 26 GHG concentration and emission estimates. Here we present the results of continuous in-situ pCO 2 27 measurements over a 2600-km transect of the Lena River main stem and lower reaches of 20 major 28 tributaries (together representing watershed area of 1,661,000 km², 66% of the Lena’s basin), conducted 29 at the peak of the spring flood. The pCO 2 in Lena (range 400-1400 µatm) and tributaries (range 400-1600 30 µatm) remained generally stable (within ca. 20 %) over the night/day period and across the river channels. The pCO 2 in tributaries increased northward with mean annual temperature decrease and permafrost increase; this change was positively correlated with C stock in soil, the proportion of deciduous needle- leaf forest and the riparian vegetation. Based on gas transfer coefficients obtained from rivers of the 34 Siberian permafrost zone ( k = 4.46 m d -1 ), we calculated CO 2 emission for the main stem and tributaries. 35 Typical fluxes ranged from 1 to 2 g C m -2 d -1 (>99% CO 2 , < 1 % CH 4 ) which is comparable with CO 2 36 emission measured in Kolyma, Yukon and Mackenzie and permafrost-affected rivers in western Siberia.


45
Climate warming in high latitudes is anticipated to result in mobilization, decomposition and 46 atmospheric release of significant amounts of carbon (C) stored in permafrost soils, providing a positive 47 The peak of annual discharge depends on the latitude (Fig. 1) and occurs in May in the south 112 (Ust-Kut) and in June in the middle and low reaches of the Lena River (Yakutsk, Kysyr). From May 29 113 to June 17, 2016, we moved downstream the Lena River by boat with an average speed of 30 km h -1 114 (Gureyev, 2016). As such, we followed the progression of the spring and moved from the southwest to 115 the northeast, thus collecting river water at approximately the same stage of maximal discharge. Note 116 that transect sampling is a common way to assess river water chemistry in extreme environments (Huh 117 and Edmond, 1999; Spence and Telmer, 2005), and generally, a single sampling during high flow season  tributaries. During sampling, the sensor was left to equilibrate in the water for 10 minutes before 139 measurements were recorded. 140 The probe was enclosed and placed into a tube which was submerged 0.5 m below the water 141 surface. Within this tube, we designed a special chamber that allowed low-turbulent water flow around 142 the probe without gas bubbles. Previous studies (Park et   Sensor preparation was conducted in the lab following the method described by Johnson et al. and another probe used as a control and never employed for continuous measurements. We did not find 166 any sizable (>10%) difference in measured CO2 concentration between these two probes. 167 For CH4 analyses, unfiltered water was sampled in 60-mL Serum bottles and closed without air 168 bubbles using vinyl stoppers and aluminum caps and immediately poisoned by adding 0.2 mL of 169 saturated HgCl2 via a two-way needle system. In the laboratory, a headspace was created by displacing 170 approx. 40% of water with N2 (99.999%). Two 0.5-mL replicates of the equilibrated headspace were 171 analyzed for their concentrations of CH4, using a Bruker GC-456 gas chromatograph (GC) equipped with 172 flame ionization and thermal conductivity detectors. After every 10 samples, a calibration of the detectors 173 was performed using Air Liquid gas standards (i.e. 145 ppmv). Duplicate injection of the samples showed 174 that results were reproducible within ±5%. The specific gas solubility for CH4 (Yamamoto et al., 1976) 175 was used in calculation of total CH4 content in the vials and then recalculated to μmol L -1 of the initial 176 waters. The dissolved oxygen (CellOx 325; accuracy of ±5%), specific conductivity (TetraCon 325; 180 ±1.5%), and water temperature (±0.2 °C) were measured in-situ at 20 cm depth using a WTW 3320 181 Multimeter. The pH was measured using portable Hanna instrument via combined Schott glass electrode 182 calibrated with NIST buffer solutions (4.01, 6.86 and 9.18 at 25°C), with an uncertainty of 0.01 pH units.

183
The temperature of buffer solutions was within ± 5°C of that of the river water. The water was sampled   The exponent (Eqn. 2) is a coefficient that describes water surface (2/3 for a smooth water surface 209 regime while 1/2 for a rippled and a turbulent one), and the Schmidt number for 20°C in freshwater is 210 600. We used n = 2/3 because all water surfaces of sampled rivers were considered flat and had a laminar    River and 20 tributaries, respectively) and did not change appreciably along the main stem and among 267 the 20 tributaries ( Fig. 3 B). The DOC concentration did not demonstrate any systematic variations over of the Lena River ( Fig. 3 D), and pH decreased by 0.8 units downstream (Fig. 3 E).
Generally, the concentrations of DOC measured in the present study during the peak of the spring

Diurnal (night/day) pCO2 variations and spatial variations across the river transect 301
The diel continuous CO2 measurements of 3 tributaries (Kirenga, Tuolba and Aldan) and 14 sites 302 of the Lena River main channel showed generally modest variation with diurnal range within 10 % of 303 the average pCO2 ( Fig. 4 and Fig. S2). The observed variations in pCO2 between day and night were not 304 linked to water temperature (p > 0.05), which did not vary more than 1-2 °C between the day and night 305 period.

306
The spatial variations of hydrochemical parameters were tested in the upper reaches of the Lena 307 main stem and its largest tributary -the Aldan River (Fig. S3). In the Lena River, over a lateral distance 308 of 550 m across the river bed, the pCO2 and CH4 concentrations were equal to 569±4.6 µatm and 309 0.0406±0.0074 µmol L -1 , respectively, whereas the DIC and DOC concentrations varied < 15% (n = 5).  (Table 3). 326 Further assessment of landscape factor control on C parameters of the river water was performed 327 via a PCA. This analysis basically confirmed the results of linear regressions and revealed two factors 328 capable explaining only 12.5 and 3.5% of variability (Fig. S4). The F1 strongly acted on the sample   Fig. 1 B). stem along the sampling route (Fig. 2 A). Peaks shown on the diagram of the main stem are not 390 necessarily linked to CO2-rich tributaries, but likely reflect local processes in the main stem, including 391 lateral influx from the shores and shallow subsurface waters, which is typical for permafrost regions of 392 forested Siberian watersheds (i.e., Bagard et al., 2011). Given that the data were averaged over ~20-km 393 distance, we believe that these peaks are not artifacts but reflect local heterogeneity of the pCO2 pattern 394 in the main stem (turbulences, suprapermafrost water discharge, sediment resuspension and respiration).

395
Note that such a heterogeneity was not observed in the tributaries, at least at the scale of our spatial 396 coverage (see Fig. S1 B, S3). 397 The PCA demonstrated extremely low ability to describe the data variability (12% by F1 and only  (Fig. S6 A). Lack of such a correlation and absence of diurnal pCO2 variations imply that in-433 stream processing of dissolved terrestrial OC is not the main driver of CO2 supersaturation in the river 434 waters of the Lena River basin. Furthermore, a lack of lateral (across the river bed) variations in pCO2 435 does not support a sizable input of soil waters from the shore, although we admit that much higher spatial 436 coverage along the river shore is needed to confirm this hypothesis.

437
The role of underground water discharge in regulating pCO2 pattern of the tributaries is expected  (Fig. S6 B). Furthermore, for the Lena River main stem, the lowest CO2 concentrations were recorded in 442 the upper reaches (first 0-800 km) where carbonate rocks dominate. Altogether, this makes the impact of 443 CO2 from underground carbonate reservoirs on river water CO2 concentrations unlikely. This is further 444 illustrated by a lack of correlation between pCO2 and DIC or pH (Fig. S7 A of the Supplement). The pH 445 did not control the CO2 concentration in the main stem and only weakly impacted the CO2 in the 446 tributaries (Fig. S7 B). The latter could reflect an increase in pCO2 in the northern tributaries which 447 exhibited generally lower pH compared to the SW tributaries hosted within the carbonate rocks. Overall, 448 such low correlations of CO2 with DIC and pH reflected a generally low predictive capacity to calculate 449 pCO2 from measured pH, temperature and alkalinity (see section 3.4).

450
Therefore, other sources of riverine CO2 may include particulate organic carbon processing in the Overall, the present study demonstrates highly dynamic and non-equilibrium behavior of CO2 in the river waters, with possible hot spots from various local sources. For these reasons, in-situ, high spatial  Acknowledgements. 539 We acknowledge support from an RSF grant 18-17-00237_P, Competing interests. 551 The authors declare that they have no conflict of interest.