## 2 Materials and Methods  
This study was conducted in the Lower Mississippi River at Baton Rouge, Louisiana, USA (30\({}^{\circ}\)26\({}^{\prime}\)16.6\({}^{\prime\prime}\) N, 91\({}^{\circ}\)11\({}^{\prime}\)31.6\({}^{\prime\prime}\) W, Figure 1), approximately 369 river km upstream of the river's outlet to the Gulf of Mexico. The Mississippi is the largest river in North America, draining 3.2 million km\({}^{2}\) of land and discharging approximately 680 km\({}^{3}\) of freshwater into the Gulf annually, in combination with the Atchafalaya River [24]. Due to the vast drainage size and diverse land use, the Mississippi River carries dissolved inorganic and organic carbon (DIC and DOC) from various sources, with an average DIC and DOC flux rate of 12.6 Tg yr\({}^{-1}\) and 4.1 Tg yr\({}^{-1}\), respectively, at Baton Rouge during the past three years [25]. DIC fluxes over recent years are very similar to those reported in the Mississippi River at this location approximately one decade ago [26], however, recent DOC fluxes are nearly double some past estimates [27; 28; 29]. The large fluxes of diverse organic materials [30] and nutrients [31] provide the resources necessary for in-situ respiration, or production of CO\({}_{2}\). The river reach in our study area is approximately 750 m wide by 15 m deep, depending on the river stage, with a minimal slope [32], providing a slow-moving environment conducive of in-situ biological processing. The climate in southcentral Louisiana is considered subtropical, characterized by short, mild winters paired with humid, hot summer months. Based on these meteorological, physical, and biogeochemical characteristics of the river at our study site, we believe this portion of the river is biologically productive, especially as [PERSON] and others [33] found high rates of both in-situ photosynthesis and respiration in the river approximately 40 km downstream of our study site. We are only aware of one previous study [29] analyzing calculated \(p\)CO\({}_{2}\) and CO\({}_{2}\) outgassing values in the Lower Mississippi River, which suggested that the river was regularly super-saturated in \(p\)CO\({}_{2}\) with respect to the atmosphere due to a combination of in-situ and terrestrial respiration of organic material. Two other studies [34; 35] have reported a significant amount of carbon processing in the Lower Mississippi River's coastal plume near the river's mouth to the Gulf of Mexico. In May and August 2018, we conducted in-situ measurements at 3-h intervals over a span of 24 h starting at 6:00 Central Standard Time (CST) in the U.S. and ending on 6:00 CST the following day for two days in spring (11-12 May and 17-18 May) and two days in summer (13-14 August and 15-16 August). The field schedule was designed to capture high and low flow conditions as well as two different water temperature regimes, which typically occur in the Mississippi River during spring and summer months. During each field trip, in-situ measurements of partial pressure of carbon dioxide (\(p\)CO\({}_{2}\)) (C-Sense\({}^{\text{TM}}\)\(p\)CO\({}_{2}\) sensor, Turner Designs, San Jose, CA, USA), water temperature, dissolved oxygen (YSI 556 multi-probe meter, YSI Inc., Yellow, Springs, OH, USA), pH (Orion Star A221, Thermo Scientific, Beverly, MA, USA), and turbidity (T100, Oakton Instruments, Vernon Hills, IL, USA) were collected. The C-SenseTM measures \(p\)CO\({}_{2}\) in-stream by gas diffusion across a hydrophobic  
Figure 1: Mississippi River Basin with a vast drainage area of 3.2 million km\({}^{2}\) and the locations of the sampling site and U.S. Geological Survey gauge station (USGS #07374000). membrane into an isolated headspace chamber, where the wavelength of \(p\)CO\({}_{2}\) is absorbed using infrared sensors. The sensor was calibrated at 20 \({}^{\circ}\)C at average sea-level atmospheric pressure less than six months prior to the field campaign using a manufacturer developed equation to convert sensor voltage output (V) to gaseous CO\({}_{2}\) concentration (ppm).