CO2 partial pressure and CO2 emission along the lower Red River (Vietnam)

The Red River (Vietnam) is representative of a south-east Asian river system, strongly affected by climate and human activities. This study aims to quantify the spatial and seasonal variability of CO2 partial pressure and CO2 emissions of the lower Red River system. Water quality monitoring and riverine pCO2 measurements were carried out for 24 h at five stations distributed along the lower Red River system during the dry and the wet seasons. The riverine pCO2 was supersaturated relative to the atmospheric equilibrium (400 ppm), averaging about 1589± 43 ppm and resulting in a water–air CO2 flux of 530.3± 16.9 mmol m−2 d−1 for the lower Red River. pCO2 and CO2 outgassing rates were characterized by significant spatial variation along this system, with the highest values measured at Hoa Binh station, located downstream of the Hoa Binh Dam, on the Da River. Seasonal pCO2 and CO2 outgassing rate variations were also observed, with higher values measured during the wet season at almost all sites. The higher river discharges, enhanced external inputs of organic matter from watersheds and direct inputs of CO2 from soils or wetland were responsible for higher pCO2 and CO2 outgassing rates. The difference in pCO2 between the daytime and the night-time was not significant, suggesting weak photosynthesis processes in the water column of the Red River due to its high sediment load.


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The percentage of dissolved oxygen (% DO) varied from 50.5 % to 70.7 % with an average 217 value of 64.3 % ( Table 2). The mean values were the highest for the Yen Bai station (70.1 %) in the 218 wet period, and 69.5 % for the Ba Lat station in the dry period. The lowest values were observed at the 219 Hoa Binh station in both periods (55.0 % in the wet period, 51.4% in the dry period) ( Table 2). DO 220 showed the seasonal and spatial variations but no clear day-night difference was observed (p<0.05). 221 Salinity values at the four upstream sites were under the detection limit both in the rainy and 222 dry seasons , but in the estuary downstream river at the Ba Lat station, values up to 8.75 were measured 223 during the dry season (Table 2). Conductivity followed the same trend as salinity, and was close to 224 0.2±0.0 mS cm -1 for the 4 upstream sites, and reached up to 6.6±3.4 mS cm -1 at Ba Lat (Table 2). 225 Total alkalinity ranged from 84.3 ± 1.9 to 152.9 ± 6.6 mg L -1 , with higher values measured in 226 the dry season than in the rainy season (p< 0.05), except at Vu Quang station. The difference of total 227 alkalinity was spatially recorded but no clear variation appeared between values in day and night times 228 at 5 sites (p<0.05). 229 Chlorophyll a was quite low during the two sampling campaigns, ranging from 0.23 to 2.77 230 g L -1 , with an average of 1.61 g L -1 . Higher values in the rainy season than in the dry season were 231 observed but no clear day-night difference was observed at almost sites (p< 0.05). From Yen Bai to Ba 232 Lat, Chl-a concentrations in the main axe (at Yen Bai and Hanoi stations) were higher than in the two 233 tributaries Da and Lo (Table 2), even under the higher values of turbidity. 234

Carbon concentrations of the lower Red River 235
During the two sampling campaigns, DOC concentrations ranged from 0.5 to 4.6 mgC L -1 , averaging 236 1.5 mgC L -1 . Higher values were observed during the rainy season (2.0 mgC L -1 vs.1.5 mgC L -1 during 237 the dry season), and the highest value was recorded at Hanoi station (Table 2). POC concentrations 238 varied from 0.4 to 4.6 mgC L -1 . Among the 5 sites, POC concentrations in the main reach of the Red 239 River (Yen Bai, Hanoi and Ba Lat sites) were higher than in the two tributaries Da and Lo, where dams 240 were constructed. Spatial and seasonal variations of DOC and POC were observed but no clear 241 difference in day -night time was found (p< 0.05) ( Table 2, table SM1). 242 DIC concentrations at the five sites fluctuated between 16.7 and 32.9 mgC L -1 , averaging 23.8 243 mgC L -1 . Lower values were measured in the rainy season (22.3 mgC L -1 ) than in the dry season (25.3 244 mgC L -1 ) and the difference of DIC was noted for the 5 sites (p<0.05) ( Table 2). 245 246 3.3. Comparisons of the pCO 2 results obtained by the two methods 247 pCO 2 along the lower Red River (Vietnam) in the dry and the wet seasons were determined by two 8 same trend of spatial and seasonal variations (R 2 = 0.77, Fig. 2; Table 2). Lower values of the 252 calculated pCO 2 in this study may be caused by the analytical errors in pH or under-estimation of total 253 alkalinity. Similarly, the CO 2 outgassing rates which were calculated from measured pCO 2 from 254 equilibrator were higher than the ones derived from the calculated pCO 2 from CO 2 -SYS, however they 255 are in the same orders and have similar variation trends (Table 3, Fig 2). 256 Below, we use the results of pCO 2 (and fCO 2 ) from direct measurements to discuss the spatial and 257 seasonal variations of pCO 2 (and fCO 2 ) of the lower Red River. 258

Relations between pCO 2 and water chemistry variables 259
The riverine water pCO 2 was supersaturated with CO 2 in contrast to the atmospheric equilibrium (400 260 ppm), averaging 1,589 ± 43 ppm for all sites observed. In general, the results did not show a clear 261 variation in pCO 2 between the day and night, except higher values at the night time at the Ba Lat site 262 and higher value in daytime at Vu Quang in dry season (p< 0.05) ( Table 2). This leads to the same 263 trends of CO 2 outgassing rates: no clear difference between daytime (548.9 ± 17.9 mmol m -2 day -1 ) and 264 night time (551.8 ± 15.9 mmol m -2 day -1 ) (p< 0.05) ( seasons whereas the lowest one was observed at the Yen Bai site. Spatial variations of both pCO 2 and 268 fCO 2 flux for all 5 sites were observed (p<0.05). Higher values of pCO 2 in the wet season than in the 269 dry season were observed at almost all the sites (p<0.05) ( Table 2). 270 CO 2 outgassing rates of the 5 stations of the lower Red River showed seasonal and spatial 271 variations (p<0.05). The highest value was recorded at Hoa Binh site in both the rainy and the dry 272 season, averaging 1447.5 ± 27.4 mmol m -2 d -1 and the lowest value was observed at Ba Lat site, 273 averaging 54.6 ± 6.5 mmol m -2 d -1 . CO 2 outgassing rates were higher in the wet season than in the dry 274 season at all sites (Table 3,  The Hoa Binh station has highest flows that correlate with the pCO 2 and CO 2 flux. 285 The Pearson correlation coefficient showed a strong negative correlation between pCO 2 and 9 and DOC was found (r  0.15) ( Table 4). However, the pCO 2 is positively correlated with the flow of 288 the river (r = 0.3). Consequently, we included that the pCO 2 are the results of a combination of multiple 289 parameters, rather than a single one, such as the flow of river, season (including precipitation and 290 temperature), dam construction, population density, geomorphological characteristics of the catchment. have a strong influence on circadian variation of pCO 2 or CO 2 outgassing, since this process consumes 300 CO 2 during the day (Linn and Doran, 1984). 301 Concerning the lower Red River, water temperature did not show clear variation between the 302 day and the night. In addition, low Chl-a concentrations were measured, from 0.5 to 3.1 µg L -1 , 303 probably as a result of the high turbidity limiting light penetration in the water column. Thus, 304 phytoplankton activity had a low influence on C dynamic in the lower Red River system. should also induce higher weathering rates, leading to higher DIC export. Increase in temperature 311 decreases CO 2 solubility but increase OM decomposition processes, which produce CO 2 . These 312 processes may partly explain the higher pCO 2 of the lower Red River during the hot and rainy season. 313 However, direct relationship between temperature and pCO 2 was not evidenced during the rainy 314 season, probably because riverine inputs were the dominant factor driving pCO 2 . Conversely during the 315 dry season, pCO 2 clearly increased with temperature, suggesting that metabolic rate controlled pCO 2 316 when adjacent soils inputs are limited (Fig. 5). 317 Another important factor that impacted pCO 2 seasonal variations in the lower Red River was 318 the river discharge. Indeed, during the monsoon season, the Red River discharges were about 2 to 3 319 times higher at all the sites (p<0.05) ( Table 1). Higher pCO 2 and CO 2 flux values in wet season were 320 observed at almost all sites (p < 0.05). CO 2 flux varied from 54.6 ± 6.5 mmol m -2 d -1 (at Ba Lat) in the 321 dry season to 1447.5 ± 27.4 mmol m -2 d -1 (at Hoa Binh) in the wet season. The higher pCO 2 and CO 2 322 flux values observed during the wet season may reflect the influence of soil organic matter inputs to the 323 riverine water column, evidenced by the higher values of DOC and POC in the rainy seasons measured 324 in our study (p< 0.05). In tropical regions, the wet season usually experienced higher pCO 2 than the dry 325 season because the intense rainfall induced higher OM inputs into the river ( (Table 5). However, very high pCO 2 value, up to 11,000 ppm, was also observed for some other Asian  (Table 5). Thus, the high alkalinity, pCO 2 and fCO 2 in the Red River in this study can be 358 partly explained by wide distribution of carbonate-silicate rocks in the upper Red River drainage area, 359 especially during high water discharge as observed for other Asian rivers. 360 361

Influence of hydrological characteristics
showed the clear difference of pCO 2 , CO 2 flux and river discharges in the rainy and the dry seasons for 371 the lower Red River. 372 Regarding the Ba Lat site, which is situated in the Red River estuary and thus in a very low 373 and flat land, pCO 2 values were lower than in Hanoi. It is interesting to observe that the river water 374 discharge at Hanoi site (3,296 ± 86 and 1,915 ± 149 m 3 s -1 ) was about 3 times higher than the one at Ba 375 Lat (1,269 ± 93 and 453 ± 31 m 3 s -1 ) in both wet and dry seasons respectively (Table 1)

Influence of population density on pCO 2 and CO 2 emission 430
Previous studies demonstrated very high value of pCO 2 in river estuaries as a result of different human 431 activities. For instance, pCO 2 up to ∼25,000 ppm was measured in the Rhine estuary (Kempe, 1982) or 432 up to ∼15,200 ppm in the Scheldt estuaries due to high discharge of pollutants (Borges and 433 Frankignoulle, 2002). 434 Concerning the Red River, from the upstream to the downstream part of the main axe, pCO 2 435 together with CO 2 outgassing flux slightly increased from Yen Bai (993 ± 14 ppm and 364.9 ± 10.3 436 mmol m -2 d -1 respectively) to Hanoi (1,275 ± 17 ppm and 304 ± 7.3 mmol m -2 d -1 ), whatever the 437 season. However, it is worth to note that the Hanoi station was located within the city itself and at this 438 station, the river has not yet received the wastewater discharge of the whole city. Consequently, the 439

Conclusions 447
This work presented the spatial and seasonal variability of pCO 2 along the lower Red River system. 448 The riverine water was supersaturated with CO 2 in contrast to the atmospheric equilibrium (400 ppm), 449 with pCO 2 values averaging about 1589 ± 43 ppm, resulting thus in a water-air CO 2 ux of 550.3± 16 450 mmol m -2 d -1 from the lower Red River system. The pCO 2 from the water surface of the lower Red 451